Whether you’re a geology student, preparing for a competitive exam, or just someone curious about how our planet works, understanding Plate Tectonics is essential. From the groundbreaking theory of Continental Drift to the dynamic process of Seafloor Spreading and the complex interactions at Plate Boundaries, this topic reveals the powerful forces constantly reshaping Earth’s surface.
In this post, we’ve compiled 100 multiple-choice questions—each with a clear answer and detailed explanation—to help you test your knowledge and deepen your understanding. These questions cover all major concepts, including tectonic plate movement, geological features, and the scientists behind the theories. It’s the perfect way to study smarter, not harder.
Let’s dive into the world beneath our feet and explore how continents drift, oceans expand, and mountains rise—one question at a time.
MCQs on Plate Tectonics with Answers
1. Which of the following lines of evidence was NOT used by Alfred Wegener to support his theory of Continental Drift?
(a) The jigsaw fit of the coastlines of South America and Africa.
(b) The presence of similar fossil flora and fauna on separated continents.
(c) Paleoclimatic evidence suggesting past glaciations in now-tropical regions.
(d) Paleomagnetic data showing the apparent polar wander of continents.
Answer: (d)
Explanation: While paleomagnetic data strongly supports plate tectonics, it was not available to Alfred Wegener when he proposed his theory of Continental Drift in the early 20th century. His evidence primarily relied on the geometric fit of continents, fossil correlations, geological similarities across continents, and paleoclimatic indicators.
2. The concept of Seafloor Spreading, proposed by Harry Hess, primarily relies on evidence from:
(a) The distribution of earthquakes and volcanoes along plate boundaries.
(b) The increasing age of the oceanic crust with distance from mid-oceanic ridges.
(c) The presence of transform faults offsetting mid-oceanic ridges.
(d) The subduction of oceanic plates beneath continental plates at trenches.
Answer: (b)
Explanation: Harry Hess’s theory of seafloor spreading was largely based on the observation that the oceanic crust is youngest at the mid-oceanic ridges and progressively older as one moves away from these ridges towards the continents. This age gradient, coupled with the relatively thin sediment layer on the ocean floor, suggested continuous creation of new crust at the ridges.
3. Which of the following geological features is LEAST likely to be associated with a divergent plate boundary?
(a) Mid-oceanic ridges.
(b) Rift valleys.
(c) Volcanic island arcs.
(d) Shallow-focus earthquakes.
Answer: (c)
Explanation: Volcanic island arcs are characteristic features of convergent plate boundaries where one oceanic plate subducts beneath another, leading to the melting of the mantle and the formation of volcanoes above the subduction zone. Divergent boundaries, where plates move apart, are associated with mid-oceanic ridges, rift valleys, and shallow-focus earthquakes due to tensional forces.
4. The Benioff zone is a seismically active area associated with:
(a) Transform plate boundaries.
(b) Divergent plate boundaries.
(c) Convergent plate boundaries where subduction is occurring.
(d) Hotspots located within tectonic plates.
Answer: (c)
Explanation: The Benioff zone is a dipping planar zone of earthquakes that traces the descent of a subducting oceanic plate beneath another plate (either oceanic or continental). The increasing depth of earthquakes along this zone indicates the angle and depth of the subducting slab.
5. The formation of the Himalayan mountain range is primarily a result of:
(a) Oceanic-oceanic convergence.
(b) Oceanic-continental convergence.
(c) Continental-continental convergence.
(d) Hotspot volcanism.
Answer: (c)
Explanation: The Himalayas are the result of the ongoing collision between the Indian and Eurasian continental plates. Neither plate is dense enough to significantly subduct beneath the other, leading to intense folding, faulting, and uplift of the crust, forming the towering mountain range.
6. Which of the following is a characteristic feature of a transform plate boundary?
(a) The creation of new lithosphere.
(b) The destruction of old lithosphere.
(c) The sliding of plates past each other horizontally.
(d) The formation of volcanic island arcs.
Answer: (c)
Explanation: Transform plate boundaries are characterized by plates sliding past each other horizontally. This movement doesn’t typically create or destroy significant amounts of lithosphere, but it can generate significant earthquakes due to the friction between the moving plates.
7. The driving force behind plate tectonics is primarily thought to be:
(a) Tidal forces exerted by the Sun and Moon.
(b) Gravitational forces causing continental landmasses to slide.
(c) Convection currents in the Earth’s mantle.
(d) The Earth’s rotation causing centrifugal forces.
Answer: (c)
Explanation: The prevailing theory suggests that convection currents within the Earth’s mantle, driven by heat from the Earth’s core and radioactive decay, exert drag on the overlying tectonic plates, causing them to move.
8. The East African Rift Valley is an example of:
(a) A mature convergent plate boundary.
(b) An active divergent plate boundary.
(c) A transform fault system.
(d) A volcanic hotspot.
Answer: (b)
Explanation: The East African Rift Valley is a series of interconnected rift valleys that represent a continental divergent plate boundary. The African plate is slowly splitting apart along this zone, leading to volcanism, faulting, and the formation of new crust.
9. Which of the following statements about oceanic trenches is INCORRECT?
(a) They are the deepest parts of the ocean floor.
(b) They are associated with subduction zones.
(c) They are sites where new oceanic crust is formed.
(d) They often have volcanic island arcs or continental volcanic arcs parallel to them.
Answer: (c)
Explanation: Oceanic trenches are formed at convergent plate boundaries where one oceanic plate is forced beneath another (subduction). This process leads to the destruction (recycling) of old oceanic crust back into the mantle, not the creation of new crust.
10. The San Andreas Fault in California is a classic example of:
(a) A mid-oceanic ridge.
(b) A subduction zone.
(c) A transform fault.
(d) A continental rift valley.
Answer: (c)
Explanation: The San Andreas Fault is a major transform fault where the Pacific Plate is sliding past the North American Plate. This movement is responsible for frequent earthquakes in the region.
11. The presence of ancient glacial tillites in present-day low-latitude regions of continents like South America, Africa, and India was a key piece of evidence supporting:
(a) The theory of seafloor spreading.
(b) The concept of subduction zones.
(c) Alfred Wegener’s theory of Continental Drift.
(d) The existence of mantle plumes and hotspots.
Answer: (c)
Explanation: Wegener noted that glacial tillites, which are deposits left by ancient glaciers, were found in regions that are now close to the equator. This suggested that these landmasses were once located at much higher latitudes where glaciation could occur, supporting the idea that continents have moved over time.
12. Which of the following statements accurately describes the process of seafloor spreading?
(a) Old oceanic crust is recycled at mid-oceanic ridges, and new crust is formed at subduction zones.
(b) New oceanic crust is generated at mid-oceanic ridges, and older crust is consumed at subduction zones.
(c) Oceanic plates slide past each other horizontally, resulting in the creation of new crust.
(d) Continental plates collide, leading to the upwelling of magma and the formation of new oceanic crust.
Answer: (b)
Explanation: Seafloor spreading is the process where molten rock (magma) rises from the mantle at mid-oceanic ridges, cools, and solidifies to form new oceanic crust. This new crust then moves away from the ridge, and the older crust eventually subducts (sinks back into the mantle) at convergent plate boundaries like oceanic trenches.
13. The Andes Mountains in South America are a direct result of:
(a) A continental-continental convergent plate boundary.
(b) An oceanic-oceanic convergent plate boundary.
(c) An oceanic-continental convergent plate boundary.
(d) A transform plate boundary.
Answer: (c)
Explanation: The Andes Mountains have formed due to the subduction of the dense Nazca oceanic plate beneath the less dense South American continental plate. This process, known as oceanic-continental convergence, leads to the melting of the mantle above the subducting slab and the subsequent rise of magma, resulting in the formation of volcanic mountain ranges.
14. Which of the following geological features is most commonly associated with a continental rift valley?
(a) Deep-sea trenches and volcanic island arcs.
(b) Fold mountains and reverse faults.
(c) Normal faults, volcanism, and grabens.
(d) Strike-slip faults and shallow earthquakes.
Answer: (c)
Explanation: Continental rift valleys form at divergent plate boundaries within continents. As the crust stretches and thins, normal faults develop, causing blocks of crust to subside and form grabens (down-dropped blocks). The thinning crust also allows magma to rise to the surface, leading to volcanic activity.
15. Paleomagnetic studies of volcanic rocks on the seafloor have provided strong evidence for seafloor spreading by showing:
(a) A random distribution of magnetic polarities across the ocean floor.
(b) A consistent pattern of normal magnetic polarity throughout the oceanic crust.
(c) Symmetrical bands of normal and reversed magnetic polarity parallel to mid-oceanic ridges.
(d) Magnetic anomalies that decrease in intensity with distance from mid-oceanic ridges.
Answer: (c)
Explanation: As new basaltic lava erupts and cools at mid-oceanic ridges, it records the Earth’s magnetic field at that time. The Earth’s magnetic field periodically reverses its polarity. This results in symmetrical bands of rocks with normal and reversed magnetic polarities on either side of the mid-oceanic ridge, providing a “magnetic tape recorder” of seafloor spreading.
16. The Mariana Trench, the deepest part of the world’s oceans, is located at:
(a) A divergent plate boundary in the mid-Atlantic Ocean.
(b) A transform fault zone in the eastern Pacific Ocean.
(c) A convergent plate boundary where the Pacific Plate subducts beneath the Philippine Plate.
(d) A hotspot location in the central Pacific Ocean.
Answer: (c)
Explanation: The Mariana Trench is formed at a convergent plate boundary where two oceanic plates collide, and the denser Pacific Plate is forced beneath the less dense Philippine Plate in a process called oceanic-oceanic subduction.
17. Which of the following is NOT considered a major driving force of plate tectonics?
(a) Slab pull.
(b) Ridge push.
(c) Mantle plumes.
(d) Tidal forces.
Answer: (d)
Explanation: Slab pull, the force exerted by the sinking of a dense oceanic plate at a subduction zone, and ridge push, the force exerted by the elevated mid-oceanic ridge pushing the plates away, are considered major driving forces of plate tectonics. Mantle plumes, while contributing to intraplate volcanism, are not considered a primary driver of large-scale plate motion. Tidal forces exerted by the Sun and Moon have a negligible effect on the movement of tectonic plates.
18. The Great Rift Valley system in East Africa is characterized by:
(a) Intense folding and thrust faulting.
(b) Extensive strike-slip faulting and crustal shortening.
(c) Normal faulting, volcanism, and the formation of elongated depressions.
(d) Deep-sea trenches and associated seismicity.
Answer: (c)
Explanation: As mentioned earlier, the East African Rift Valley is a continental divergent zone. The tensional forces cause the crust to fracture along normal faults, leading to the subsidence of blocks and the formation of elongated rift valleys (grabens). Magma rising along these fractures also results in significant volcanic activity.
19. Ophiolites, which are sequences of oceanic crust and upper mantle rocks exposed on land, provide evidence for:
(a) The process of seafloor spreading at mid-oceanic ridges.
(b) The subduction and obduction (overthrusting) of oceanic crust onto continental crust at convergent boundaries.
(c) The existence of transform faults and their role in crustal deformation.
(d) The activity of mantle plumes and the formation of oceanic plateaus.
Answer: (b)
Explanation: Ophiolites are thought to form when a piece of oceanic crust and the underlying upper mantle is thrust (obducted) onto a continental margin during a collision at a convergent plate boundary. Their presence on land provides direct evidence of past ocean basins and the processes of subduction and continental collision.
20. The Hawaiian Islands are an example of volcanism associated with:
(a) A mid-oceanic ridge.
(b) A subduction zone.
(c) A hotspot.
(d) A continental rift valley.
Answer: (c)
Explanation: The Hawaiian Islands are a chain of volcanic islands formed by a stationary mantle plume (hotspot) beneath the moving Pacific Plate. As the plate moves over the hotspot, magma rises to the surface, creating a series of volcanoes that become progressively older away from the active hotspot.
21. The Wilson Cycle describes:
(a) The cyclical pattern of magnetic reversals recorded in oceanic crust.
(b) The supercontinent cycle, involving the assembly and breakup of Earth’s continents over geological time.
(c) The continuous creation and destruction of oceanic lithosphere at plate boundaries.
(d) The cyclical variations in global sea level driven by tectonic activity.
Answer: (b)
Explanation: The Wilson Cycle is a model that describes the opening and closing of ocean basins and the associated formation and breakup of supercontinents through plate tectonic processes. It involves stages from continental rifting to ocean basin formation, subduction, continental collision, and eventually, the formation of a new supercontinent.
22. Which type of volcanic eruption is LEAST likely to occur at a divergent plate boundary?
(a) Effusive eruptions of basaltic lava flows.
(b) The formation of shield volcanoes.
(c) Highly explosive eruptions of viscous, silica-rich magma.
(d) Submarine volcanic activity along mid-oceanic ridges.
Answer: (c)
Explanation: Divergent plate boundaries, particularly mid-oceanic ridges and continental rifts, primarily involve the upwelling of mafic (basaltic) magma from the mantle. This magma is relatively low in silica and viscosity, leading to effusive eruptions and the formation of shield volcanoes or fissure eruptions. Highly explosive eruptions are more characteristic of convergent plate boundaries where the subducting plate releases water into the mantle wedge, lowering the melting point and generating more viscous, silica-rich magmas.
23. The presence of a deep-sea trench, a volcanic island arc parallel to the trench, and a zone of deep-focus earthquakes are characteristic features of:
(a) A transform fault boundary between two oceanic plates.
(b) A divergent boundary where two continental plates are rifting apart.
(c) A convergent boundary where an oceanic plate is subducting beneath another oceanic plate.
(d) A hotspot located beneath an oceanic plate.
Answer: (c)
Explanation: This combination of features is a classic signature of an oceanic-oceanic convergent plate boundary. The deep-sea trench marks the zone of subduction, the volcanic island arc forms from the melting of the mantle above the subducting slab, and the deep-focus earthquakes occur along the descending Benioff zone.
24. Isostatic rebound is a phenomenon observed when:
(a) Two continental plates collide, causing significant crustal thickening and uplift.
(b) The weight of a large ice sheet or mountain range is removed from the Earth’s crust, causing it to slowly rise.
(c) Oceanic lithosphere cools and becomes denser as it moves away from a mid-oceanic ridge, causing it to sink.
(d) The Earth’s magnetic poles shift over time, as recorded in the magnetic minerals in rocks.
Answer: (b)
Explanation: Isostatic rebound (or post-glacial rebound) is the slow upward movement of the Earth’s crust after the removal of a large weight, such as an ice sheet or a significant amount of eroded material from a mountain range. The crust, having been depressed by the load, slowly returns to its equilibrium position.
25. The age of the oldest oceanic crust is significantly younger than the age of the oldest continental crust because:
(a) Continental crust is less dense and therefore more resistant to subduction.
(b) Oceanic crust is continuously being created at mid-oceanic ridges and destroyed at subduction zones.
(c) Continental crust incorporates older, recycled oceanic crust through accretion.
(d) The radioactive decay rates in continental rocks are slower, making them appear older.
Answer: (b)
Explanation: Oceanic crust is constantly being generated at divergent plate boundaries (mid-oceanic ridges) and recycled back into the mantle at convergent plate boundaries (subduction zones). This continuous cycle limits the age of oceanic crust to typically less than 200 million years. Continental crust, on the other hand, is not subjected to this systematic recycling process and can therefore be billions of years old.
26. Deep-focus earthquakes, which can originate hundreds of kilometers below the Earth’s surface, are primarily associated with:
(a) Transform faults in continental crust.
(b) Divergent plate boundaries in oceanic crust.
(c) Subduction zones at convergent plate boundaries.
(d) Hotspots located beneath thick continental lithosphere.
Answer: (c)
Explanation: Deep-focus earthquakes occur within the subducting oceanic plate as it descends into the mantle. The increasing pressure and temperature at these depths can cause brittle failure and seismic activity. These earthquakes are a key indicator of subduction zones.
27. A volcanic arc formed on continental crust parallel to a subduction zone is called a:
(a) Mid-oceanic ridge.
(b) Volcanic island arc.
(c) Continental volcanic arc.
(d) Hotspot track.
Answer: (c)
Explanation: When an oceanic plate subducts beneath a continental plate, the melting of the mantle wedge above the subducting slab generates magma that rises to the surface and erupts, forming a chain of volcanoes on the continental crust. This is known as a continental volcanic arc, exemplified by the Andes Mountains. In contrast, a volcanic island arc forms when one oceanic plate subducts beneath another.
28. Magnetic anomalies on the seafloor are used to determine the:
(a) Depth of the ocean at different locations.
(b) Composition of the oceanic crust.
(c) Rate and direction of seafloor spreading.
(d) Age of the continental landmasses bordering the oceans.
Answer: (c)
Explanation: The symmetrical bands of normal and reversed magnetic polarity on either side of mid-oceanic ridges act like a “magnetic tape recorder” of seafloor spreading. By measuring the width of these bands and knowing the approximate frequency of magnetic reversals, scientists can calculate the rate at which the seafloor is spreading. The orientation of these bands also indicates the direction of plate movement relative to the ridge.
29. Which of the following types of stress is predominantly associated with transform plate boundaries?
(a) Tensional stress.
(b) Compressional stress.
(c) Shear stress.
(d) Hydrostatic stress.
Answer: (c)
Explanation: Transform plate boundaries are characterized by plates sliding past each other horizontally. This type of movement generates shear stress within the lithosphere along the fault zone, leading to earthquakes. Tensional stress is associated with divergent boundaries, and compressional stress is associated with convergent boundaries.
30. Accretionary wedges (or prisms) are formed at:
(a) Mid-oceanic ridges where new crust is being generated.
(b) Transform faults where plates are sliding past each other.
(c) Subduction zones where sediments and oceanic crust are scraped off the descending plate.
(d) Continental rift valleys where the crust is being pulled apart.
Answer: (c)
Explanation: At subduction zones, as the denser oceanic plate descends beneath another plate, sediments accumulated on the seafloor and fragments of oceanic crust are scraped off and piled up against the overriding plate, forming a wedge-shaped mass known as an accretionary wedge or accretionary prism.
31. Terrane accretion is the process by which:
(a) Oceanic crust is subducted beneath continental crust.
(b) Small crustal fragments with distinct geological histories are added to a continent.
(c) New oceanic crust is formed at mid-oceanic ridges.
(d) Continental rift valleys develop into new ocean basins.
Answer: (b)
Explanation: Terranes are crustal blocks that have been transported by plate tectonic movements and then added to the margin of a continent. These terranes often have different geological histories, rock types, and fossil assemblages compared to the adjacent continental crust, providing evidence of their distant origins.
32. Back-arc basins are often associated with:
(a) Divergent plate boundaries within continental lithosphere.
(b) Transform fault systems along mid-oceanic ridges.
(c) Convergent plate boundaries where an oceanic plate is subducting and causing extension in the overriding plate.
(d) Hotspots located near continental margins.
Answer: (c)
Explanation: Back-arc basins are formed on the landward side of volcanic island arcs or continental volcanic arcs associated with subduction zones. The subducting plate can cause extension in the overriding plate due to factors like slab rollback (the sinking slab retreating faster than the overriding plate advances), leading to the thinning of the crust and the formation of a new ocean basin.
33. Mantle plumes are thought to originate from:
(a) The asthenosphere directly beneath the lithosphere.
(b) The transition zone between the upper and lower mantle.
(c) The core-mantle boundary.
(d) Subducting slabs that have reached deep into the mantle.
Answer: (c)
Explanation: The prevailing hypothesis suggests that mantle plumes are narrow upwellings of hot, buoyant rock that originate deep within the Earth, likely at the core-mantle boundary. These plumes rise through the mantle and can cause volcanism at the Earth’s surface, forming hotspots.
34. A passive continental margin is characterized by:
(a) Frequent earthquakes and volcanic activity.
(b) A transition from continental to oceanic crust without a major plate boundary.
(c) The presence of deep-sea trenches and subduction zones.
(d) Extensive faulting and mountain building due to plate collision.
Answer: (b)
Explanation: Passive continental margins are found where the transition from continental crust to oceanic crust is not associated with an active plate boundary. These margins are typically broad, with features like continental shelves, slopes, and rises, and are characterized by relatively low levels of seismic and volcanic activity. Examples include the Atlantic coast of North America.
35. Reverse faults are most commonly associated with which type of plate boundary?
(a) Divergent plate boundaries.
(b) Transform plate boundaries.
(c) Convergent plate boundaries experiencing compressional stress.
(d) Hotspots causing uplift and fracturing of the crust.
Answer: (c)
Explanation: Reverse faults (and their low-angle counterparts, thrust faults) occur when compressional forces cause the hanging wall to move up relative to the footwall. These types of faults are characteristic of convergent plate boundaries where plates are colliding.
36. The distribution of earthquakes and volcanoes provides strong evidence for:
(a) The random nature of geological hazards.
(b) The locations and boundaries of tectonic plates.
(c) The uniformitarian principle in geology.
(d) The concept of isostatic equilibrium.
Answer: (b)
Explanation: Earthquakes and volcanoes are not randomly distributed across the globe. They are concentrated along the boundaries of tectonic plates where plates interact – diverge, converge, or slide past each other. Their distribution clearly delineates the edges of these massive lithospheric plates.
37. The breakup of the supercontinent Pangaea began with:
(a) The initiation of subduction zones around its margins.
(b) The development of extensive transform fault systems.
(c) The formation of large continental rift valleys.
(d) A global increase in volcanic hotspot activity.
Answer: (c)
Explanation: The breakup of Pangaea, which began around 200 million years ago, started with the development of large-scale continental rift valleys. These rifts eventually widened and deepened, leading to the formation of new ocean basins and the separation of the once-connected landmasses.
38. Which of the following is NOT a characteristic feature of a mid-oceanic ridge?
(a) High heat flow.
(b) Young volcanic rocks.
(c) Deep-sea trenches.
(d) Shallow-focus earthquakes.
Answer: (c)
Explanation: Mid-oceanic ridges are divergent plate boundaries where new oceanic crust is formed. They are characterized by high heat flow due to the upwelling of magma, young volcanic rocks (basalt), and shallow-focus earthquakes caused by the tensional forces pulling the plates apart. Deep-sea trenches, on the other hand, are associated with convergent plate boundaries where subduction occurs.
39. The concept of absolute plate motion refers to:
(a) The movement of one tectonic plate relative to another.
(b) The movement of a tectonic plate relative to a fixed reference frame, such as hotspots.
(c) The average speed of all tectonic plates over a geological timescale.
(d) The change in the size and shape of tectonic plates over time.
Answer: (b)
Explanation: Absolute plate motion describes how fast and in what direction individual tectonic plates are moving relative to a fixed point or a relatively stable feature, such as mantle plumes that create hotspot tracks. This is different from relative plate motion, which describes the movement of one plate with respect to another.
40. Hydrothermal vents, also known as black smokers, are commonly found:
(a) In deep-sea trenches associated with subduction zones.
(b) Along mid-oceanic ridges where volcanic activity occurs.
(c) In back-arc basins where the crust is undergoing extension.
(d) At passive continental margins with high sedimentation rates.
Answer: (b)
Explanation: Hydrothermal vents are fissures on the seafloor where geothermally heated water is discharged. They are commonly found along mid-oceanic ridges, where magma rises close to the surface, heating the surrounding seawater that circulates through the fractured basaltic crust. These vents are often associated with unique chemosynthetic ecosystems.
41. An ophiolite sequence typically consists of, from bottom to top:
(a) Sediments, pillow basalts, sheeted dikes, gabbro, mantle peridotite.
(b) Mantle peridotite, gabbro, sheeted dikes, pillow basalts, sediments.
(c) Gabbro, sheeted dikes, pillow basalts, sediments, mantle peridotite.
(d) Pillow basalts, sediments, mantle peridotite, gabbro, sheeted dikes.
Answer: (b)
Explanation: An ophiolite sequence represents a slice of oceanic crust and upper mantle that has been obducted (thrust) onto continental crust. The typical order from bottom to top reflects the structure of oceanic lithosphere: mantle peridotite (upper mantle), gabbro (oceanic crust formed at depth), sheeted dikes (representing magma conduits), pillow basalts (formed by lava erupting onto the seafloor), and finally, overlying sediments.
42. Transform faults along mid-oceanic ridges accommodate:
(a) The subduction of older oceanic crust.
(b) The spreading of the seafloor at different rates along the ridge segment.
(c) The upwelling of mantle plumes beneath the ridge.
(d) The collision of two oceanic plates.
Answer: (b)
Explanation: Mid-oceanic ridges are not continuous linear features but are often segmented. Transform faults are strike-slip faults that run perpendicular to the ridge axis and accommodate the differential spreading rates between adjacent ridge segments. They allow the segments to move past each other.
43. The formation of the Alps mountain range is primarily attributed to:
(a) Oceanic-oceanic subduction.
(b) Oceanic-continental subduction.
(c) Continental-continental collision.
(d) Hotspot volcanism.
Answer: (c)
Explanation: Similar to the Himalayas, the Alps formed due to the collision between the African and Eurasian continental plates. The immense compressional forces resulted in extensive folding, faulting, and uplift of the crust, creating the Alpine mountain system.
44. Sea level changes can be influenced by plate tectonics through:
(a) Changes in the volume of mid-oceanic ridges.
(b) The melting of glaciers and ice sheets due to volcanic activity at plate boundaries.
(c) The uplift or subsidence of continental margins due to tectonic forces.
(d) All of the above.
Answer: (d)
Explanation: Plate tectonics can influence sea level in several ways. The volume of mid-oceanic ridges affects the capacity of ocean basins (larger ridges displace more water). Volcanic activity can indirectly contribute to sea-level rise by influencing climate and ice melt. Tectonic uplift or subsidence of continental margins directly alters the relative sea level along coastlines.
45. Paleomagnetic studies can help reconstruct past continent configurations by analyzing:
(a) The chemical composition of ancient rocks.
(b) The fossil assemblages found in different continents.
(c) The declination (direction to magnetic north) and inclination (angle with the horizontal) of magnetic minerals in ancient rocks.
(d) The radiometric ages of rocks from different continents.
Answer: (c)
Explanation: Paleomagnetism relies on the fact that magnetic minerals in rocks align themselves with the Earth’s magnetic field at the time of their formation. The declination indicates the direction to the magnetic pole, and the inclination indicates the latitude of the rock’s formation. By studying these properties in rocks of different ages from different continents, scientists can reconstruct their past positions and movements.
46. Slab suction is a force that contributes to plate motion at:
(a) Divergent plate boundaries, pulling the plates apart.
(b) Transform plate boundaries, causing lateral movement.
(c) Convergent plate boundaries, where the sinking slab pulls the overriding plate towards the trench.
(d) Hotspots, causing the lithosphere to move over the mantle plume.
Answer: (c)
Explanation: Slab suction occurs at subduction zones. As the dense oceanic slab sinks into the mantle, it can induce a flow in the asthenosphere that pulls the overriding plate towards the trench, contributing to the overall convergence.
47. Continental flood basalts are often associated with:
(a) The initial stages of continental rifting and the upwelling of mantle plumes.
(b) Mature subduction zones with extensive arc volcanism.
(c) Transform fault systems within continental interiors.
(d) The final stages of continental collision and mountain building.
Answer: (a)
Explanation: Large Igneous Provinces (LIPs), including continental flood basalts, are massive outpourings of basaltic lava that can occur over relatively short geological timescales. They are often linked to the arrival of a mantle plume head beneath continental lithosphere, which can cause significant melting and initiate continental rifting.
48. The Moho discontinuity represents the boundary between:
(a) The Earth’s inner and outer core.
(b) The Earth’s crust and the mantle.
(c) The upper and lower mantle.
(d) The lithosphere and the asthenosphere.
Answer: (b)
Explanation: The Mohorovičić discontinuity, or Moho, is the boundary between the Earth’s crust (both oceanic and continental) and the underlying mantle. It is characterized by a significant increase in seismic wave velocity as waves pass from the crust to the denser mantle material.
49. In the mantle wedge above a subducting oceanic plate, melting occurs primarily due to:
(a) Increased pressure from the overlying plate.
(b) Decreased temperature due to the cold subducting slab.
(c) The release of volatiles (mainly water) from the subducting plate, which lowers the melting point of the mantle rocks.
(d) Frictional heating between the subducting plate and the overriding plate.
Answer: (c)
Explanation: As the oceanic plate subducts, it carries water-rich sediments and hydrated minerals into the mantle. At depth, these volatiles are released, migrating into the overlying mantle wedge. The addition of water lowers the melting point of the mantle rocks, leading to the generation of magma that rises to form volcanic arcs.
50. A stable triple junction occurs when:
(a) All three plate boundaries meeting at the junction are of the same type (e.g., ridge-ridge-ridge).
(b) The relative motions of the three plates are consistent and do not lead to the junction breaking apart or changing its configuration over time.
(c) The three plates involved are moving at the same speed.
(d) The junction is located far from any major mantle plumes or hotspots.
Answer: (b)
Explanation: A stable triple junction is a point where three tectonic plates meet, and the geometry and types of boundaries (ridge, trench, fault) remain consistent over time as the plates move relative to each other. The relative velocities of the three plates must be such that the junction maintains its configuration.
51. The formation of rift valleys is primarily associated with which type of stress regime?
(a) Compressional stress.
(b) Tensional stress.
(c) Shear stress.
(d) Hydrostatic stress.
Answer: (b)
Explanation: Rift valleys form where the Earth’s crust is being pulled apart, leading to tensional stress. This stress causes the crust to fracture along normal faults, resulting in the subsidence of blocks and the formation of elongated depressions bounded by fault scarps.
52. Forearc basins are sedimentary basins that typically develop:
(a) At divergent plate boundaries, parallel to mid-oceanic ridges.
(b) At transform plate boundaries, along major strike-slip faults.
(c) At convergent plate boundaries, between the volcanic arc and the accretionary wedge.
(d) Within continental interiors, due to crustal flexure from loading.
Answer: (c)
Explanation: At subduction zones, the descending oceanic plate scrapes off sediments and oceanic crust, forming an accretionary wedge. The weight of the overriding plate and the bending of the lithosphere can create a depression between the volcanic arc (formed by magma rising from the subducting slab) and the accretionary wedge. This depression often fills with sediments, forming a forearc basin.
53. Intraplate earthquakes:
(a) Are the most common and largest magnitude earthquakes globally.
(b) Occur along the boundaries of tectonic plates.
(c) Are relatively rare and occur within the stable interiors of tectonic plates.
(d) Are exclusively associated with hotspot volcanism.
Answer: (c)
Explanation: While the majority of earthquakes occur at plate boundaries, intraplate earthquakes occur within the interiors of tectonic plates, away from active margins. They are generally less frequent and often smaller in magnitude compared to interplate earthquakes, but they can still be significant and their causes are not always well understood, potentially related to ancient faults or stress buildup.
54. Satellite geodesy techniques like GPS (Global Positioning System) and InSAR (Interferometric Synthetic Aperture Radar) are used in plate tectonics to:
(a) Determine the composition of the Earth’s mantle.
(b) Measure the precise rates and directions of plate movements and crustal deformation.
(c) Image the magnetic anomalies on the seafloor.
(d) Analyze the heat flow patterns at mid-oceanic ridges.
Answer: (b)
Explanation: GPS and InSAR provide highly accurate measurements of ground positions and their changes over time. This allows scientists to directly observe the movement of tectonic plates, the deformation of the crust along plate boundaries, and even subtle movements associated with earthquakes and volcanic activity.
55. The concept of flexural isostasy explains:
(a) The global balance between the elevation of continents and the depth of ocean basins.
(b) The upward movement of the crust after the removal of a load like an ice sheet.
(c) The bending of the lithosphere under the weight of tectonic features like volcanic islands or mountain ranges.
(d) The lateral variations in crustal thickness due to plate tectonic processes.
Answer: (c)
Explanation: Flexural isostasy describes how the rigid lithosphere (crust and uppermost mantle) bends or flexes under the weight of overlying loads. This bending extends beyond the immediate area of the load, creating a broader response compared to simple isostatic adjustment where the crust directly beneath the load subsides.
56. In the evolution of a subduction zone, slab rollback can lead to:
(a) Compression and the formation of fold mountains in the overriding plate.
(b) Extension and the formation of back-arc basins in the overriding plate.
(c) The cessation of volcanism in the overriding volcanic arc.
(d) A decrease in the frequency and magnitude of earthquakes along the Benioff zone.
Answer: (b)
Explanation: Slab rollback occurs when the subducting oceanic plate sinks into the mantle at a steeper angle and retreats faster than the overriding plate advances. This can cause extension in the overriding plate, leading to the thinning of the crust and the formation of a back-arc basin behind the volcanic arc.
57. Oceanic plateaus are large, relatively flat areas of thickened oceanic crust that are thought to form due to:
(a) Slow, continuous spreading at mid-oceanic ridges.
(b) Extensive volcanism associated with mantle plumes.
(c) The accumulation of thick sediment layers over millions of years.
(d) The collision and amalgamation of microcontinents.
Answer: (b)
Explanation: Many oceanic plateaus are believed to be formed by massive outpourings of basaltic lava associated with the heads of mantle plumes erupting beneath oceanic lithosphere. These events can create vast areas of thickened crust that are buoyant and remain relatively high above the surrounding seafloor.
58. Seismic tomography is a technique used to:
(a) Measure the precise location and magnitude of earthquakes.
(b) Determine the age of oceanic crust based on magnetic anomalies.
(c) Create three-dimensional images of the Earth’s interior by analyzing the travel times of seismic waves.
(d) Map the distribution of different rock types in the Earth’s crust.
Answer: (c)
Explanation: Seismic tomography is analogous to medical CT scans but uses seismic waves instead of X-rays. By analyzing the travel times of seismic waves that pass through the Earth, scientists can create images of variations in seismic velocity, which can be interpreted in terms of temperature, density, and composition, providing insights into mantle convection and plate tectonics.
59. Flexural isostasy plays a significant role in the formation of:
(a) Mid-oceanic ridges.
(b) Deep-sea trenches.
(c) Volcanic island arcs.
(d) Ocean island chains associated with hotspots.
Answer: (d)
Explanation: As a tectonic plate moves over a stationary hotspot, it can create a chain of volcanic islands. The weight of these volcanic edifices causes the lithosphere to flex downward, creating a moat-like depression around the islands and potentially influencing the location and size of subsequent volcanoes in the chain.
60. The process of subduction is critical for the global carbon cycle because it:
(a) Releases large amounts of carbon dioxide into the atmosphere through volcanic eruptions at subduction zones.
(b) Transports carbon-rich sediments and altered oceanic crust into the mantle, where carbon can be stored for long periods.
(c) Facilitates the weathering of continental rocks, which removes carbon dioxide from the atmosphere.
(d) Leads to the formation of carbonate platforms in forearc basins, sequestering carbon.
Answer: (b)
Explanation: Subduction zones act as a major pathway for transporting carbon from the Earth’s surface into the deep mantle. Carbon-rich sediments and altered oceanic crust, including carbonates, are carried down with the subducting plate, effectively removing carbon from the surface system and influencing long-term carbon storage within the Earth.
61. Geochronology, the science of dating geological events, plays a crucial role in understanding plate tectonics by:
(a) Determining the chemical composition of tectonic plates.
(b) Measuring the present-day velocities of plate movements.
(c) Establishing the timing of continental breakup, seafloor spreading, and volcanic activity associated with plate boundaries.
(d) Mapping the distribution of earthquakes along fault zones.
Answer: (c)
Explanation: Radiometric dating techniques are essential for determining the ages of rocks and minerals, allowing scientists to reconstruct the history of plate movements, the timing of supercontinent cycles, the rates of seafloor spreading, and the ages of volcanic eruptions related to plate boundaries. This temporal framework is crucial for understanding the evolution of Earth’s surface.
62. The primary driving forces behind plate tectonics are thought to be:
(a) Tidal forces and the Earth’s rotation.
(b) Gravitational forces and magnetic field interactions.
(c) Convection in the Earth’s mantle, ridge push, and slab pull.
(d) Radioactive decay in the crust and atmospheric pressure gradients.
Answer: (c)
Explanation: While the exact contributions of each are still debated, the prevailing view is that convection currents in the mantle exert drag on the plates. Additionally, ridge push (gravitational sliding of plates off elevated mid-oceanic ridges) and slab pull (the sinking of dense oceanic lithosphere at subduction zones) are considered major forces driving plate motion.
63. Continental shelves at passive margins are characterized by:
(a) Steep slopes and deep-sea trenches.
(b) Extensive volcanic activity and frequent earthquakes.
(c) Gentle slopes and accumulations of sediments eroded from the adjacent continent.
(d) Highly deformed metamorphic rocks and complex fault systems.
Answer: (c)
Explanation: Passive continental margins, not being active plate boundaries, are characterized by a gradual transition from continental to oceanic crust. Continental shelves here are broad, gently sloping areas submerged beneath the ocean, underlain by thinned continental crust and covered by sediments derived from the erosion of the adjacent landmass.
64. Eustatic sea level changes refer to:
(a) Local changes in sea level due to tectonic uplift or subsidence.
(b) Global changes in sea level caused by variations in the volume of water in the oceans or changes in the volume of ocean basins.
(c) Sea level fluctuations caused by tidal forces.
(d) Changes in sea level due to atmospheric pressure variations.
Answer: (b)
Explanation: Eustatic sea level changes are global and are caused by factors such as the melting or growth of ice sheets and glaciers (changing the volume of water) or changes in the volume of the ocean basins themselves (e.g., due to the rate of seafloor spreading affecting the size of mid-oceanic ridges).
65. Paleobiogeography provides evidence for past continental connections through the study of:
(a) The magnetic orientation of ancient rocks.
(b) The distribution of similar fossil plants and animals across separated continents.
(c) The radiometric ages of rocks found on different continents.
(d) The chemical composition of ancient ocean sediments.
Answer: (b)
Explanation: The discovery of identical or very similar fossil species on continents now separated by vast oceans was a key piece of evidence supporting Continental Drift. It suggested that these landmasses were once joined together, allowing these organisms to disperse across them.
66. The formation of porphyry copper deposits is often associated with:
(a) Mid-oceanic ridges and hydrothermal vent activity.
(b) Subduction zones and associated magmatic arcs.
(c) Continental rift valleys and alkaline magmatism.
(d) Stable continental interiors and ancient shield regions.
Answer: (b)
Explanation: Porphyry copper deposits, which are major sources of copper and other metals, are typically formed in association with magmatic arcs above subduction zones. The interaction of rising magma with hydrothermal fluids leaches and concentrates metals in the surrounding rocks.
67. Plate tectonics has significantly influenced the evolution of life on Earth by:
(a) Creating stable, unchanging environments that promote slow evolution.
(b) Causing dramatic changes in climate, sea level, and the distribution of landmasses, leading to periods of both diversification and extinction.
(c) Preventing the mixing of gene pools by maintaining the isolation of continents.
(d) Reducing volcanic activity and its impact on atmospheric composition.
Answer: (b)
Explanation: The movement of tectonic plates has led to the formation and breakup of continents, the opening and closing of ocean basins, and the creation of mountain ranges. These geological changes have profoundly impacted global climate patterns, sea levels, and the connectivity of habitats, driving evolutionary change, speciation, and mass extinction events throughout Earth’s history.
68. True polar wander refers to:
(a) The apparent movement of the Earth’s magnetic poles over time, as recorded in rocks.
(b) The actual movement of the Earth’s spin axis relative to the solid Earth.
(c) The migration of continents across the Earth’s surface due to plate tectonics.
(d) The periodic reversals of the Earth’s magnetic field.
Answer: (b)
Explanation: While apparent polar wander curves reflect the movement of continents relative to the magnetic poles, true polar wander involves the shifting of the entire solid Earth (mantle and crust) relative to the Earth’s spin axis. This phenomenon is thought to be driven by imbalances in the distribution of mass within the Earth.
69. Large Igneous Provinces (LIPs) are often linked to:
(a) Slow, steady plate movements at convergent margins.
(b) The impact of large meteorites on the Earth’s surface.
(c) The activity of mantle plumes and their interaction with the lithosphere.
(d) Extensive erosion and sedimentation in continental basins.
Answer: (c)
Explanation: Many LIPs, characterized by massive outpourings of basaltic lava over relatively short geological periods, are thought to be caused by the arrival of the hot, buoyant heads of mantle plumes at the base of the lithosphere. The intense heat and melting associated with these plumes can lead to voluminous volcanic eruptions.
70. Seismic reflection and refraction surveys are used to study plate boundaries by:
(a) Measuring the magnetic properties of the rocks.
(b) Analyzing the way seismic waves are reflected and bent as they pass through different layers of the Earth’s crust and upper mantle, revealing their structure and composition.
(c) Determining the heat flow patterns along the boundary.
(d) Mapping the gravitational anomalies associated with the boundary.
Answer: (b)
Explanation: These geophysical techniques use controlled sources of seismic energy to generate waves that travel through the Earth and are recorded by seismometers. The way these waves are reflected and refracted at interfaces between different rock layers provides information about the depth, thickness, and properties of these layers, allowing scientists to image the structure of plate boundaries.
71. Orogenic collapse is a process that occurs:
(a) During the initial stages of continental collision, leading to rapid uplift.
(b) After significant crustal thickening and uplift due to continental collision, where gravitational forces cause the mountain range to spread laterally and subside.
(c) At divergent plate boundaries, as the crust thins and subsides to form rift valleys.
(d) Along transform faults, where strike-slip movement causes the uplift of fault blocks.
Answer: (b)
Explanation: Orogenic collapse is a post-orogenic process. After continental collision results in substantial crustal thickening and the formation of high mountain ranges, the elevated crust becomes gravitationally unstable. The weight of the mountains overcomes the strength of the lithosphere, causing the range to spread out laterally and the overall elevation to decrease through processes like normal faulting and ductile flow in the lower crust.
72. Aulacogens are geological structures characterized by:
(a) Long, linear mountain ranges formed at convergent plate boundaries.
(b) Three-armed rift systems where one arm fails to develop into a full-fledged ocean basin.
(c) Extensive volcanic plateaus formed by mantle plume activity.
(d) Deep-sea trenches associated with the subduction of old oceanic lithosphere.
Answer: (b)
Explanation: Aulacogens are failed rift arms that originate at a triple junction during the early stages of continental rifting. While two arms of the rift may continue to widen and eventually form a new ocean basin, the third arm often becomes inactive and fills with sediments, forming a linear sedimentary basin within the continental interior.
73. Blueschist facies metamorphic rocks are characteristic of:
(a) High-temperature, low-pressure environments associated with contact metamorphism.
(b) High-pressure, low-temperature environments found in subduction zones.
(c) Medium-temperature and pressure conditions typical of regional metamorphism during continental collision.
(d) Low-temperature and pressure conditions associated with burial metamorphism in sedimentary basins.
Answer: (b)
Explanation: Blueschist metamorphism occurs in subduction zones where oceanic crust is rapidly buried to significant depths but remains relatively cool. The high pressure and low temperature conditions lead to the formation of distinctive blue amphibole minerals like glaucophane, giving the rocks their characteristic blue color.
74. Mariana-type subduction zones are characterized by:
(a) A young, buoyant oceanic plate subducting at a shallow angle.
(b) An older, denser oceanic plate subducting at a steep angle, often leading to trench rollback and back-arc extension.
(c) The subduction of continental lithosphere beneath oceanic lithosphere.
(d) The absence of a volcanic arc due to a shallow subduction angle.
Answer: (b)
Explanation: Mariana-type subduction zones typically involve the subduction of old, cold, and dense oceanic lithosphere at a steep angle. This steep angle can cause the trench to retreat (rollback) oceanward, resulting in extension in the overriding plate and the formation of a back-arc basin with associated volcanism.
75. Transform faults in continental settings, such as the San Andreas Fault, are primarily associated with:
(a) The creation of new crust through volcanic activity.
(b) The destruction of old crust through subduction.
(c) Horizontal sliding of crustal blocks past each other, leading to significant earthquake activity.
(d) The uplift of large mountain ranges due to compressional forces.
Answer: (c)
Explanation: Continental transform faults occur where large crustal blocks on either side of the fault are moving horizontally relative to each other. This movement generates shear stress, which can build up over time and be released suddenly in the form of significant earthquakes.
76. Continental accretion is the process by which continents grow through:
(a) The gradual erosion of coastal areas and the deposition of sediments offshore.
(b) The upwelling of mantle plumes beneath continental interiors, adding new crust.
(c) The addition of smaller crustal fragments (terranes) to the margins of existing continents through tectonic collisions.
(d) The expansion of continental ice sheets during glacial periods.
Answer: (c)
Explanation: Continental accretion is a fundamental process in the evolution of continents. It involves the collision and amalgamation of various crustal blocks, island arcs, and microcontinents (terranes) with the edges of larger continental landmasses, gradually increasing their size and complexity.
77. The strength and rheology of the lithosphere (its ability to flow or deform) are significantly influenced by:
(a) The Earth’s magnetic field strength.
(b) The composition, temperature, and pressure of the rocks.
(c) The amount of solar radiation reaching the Earth’s surface.
(d) The rate of seafloor spreading at mid-oceanic ridges.
Answer: (b)
Explanation: The lithosphere’s behavior (whether it behaves rigidly or deforms ductilely) is strongly dependent on the materials it is composed of, the temperature (increasing temperature makes rocks weaker and more prone to flow), and the pressure (increasing pressure generally increases strength, but can also influence deformation mechanisms).
78. A key ongoing research question in plate tectonics involves:
(a) Whether the Earth’s continents are currently moving.
(b) The precise mechanisms that drive plate motion and the relative importance of mantle convection, ridge push, and slab pull.
(c) Whether the Earth’s magnetic field has reversed in the past.
(d) The existence of mid-oceanic ridges and subduction zones.
Answer: (b)
Explanation: While the fundamental principles of plate tectonics are well-established, the exact nature and relative contributions of the forces driving plate motion remain an active area of research. Scientists continue to investigate the complex interplay between mantle convection, ridge push, slab pull, and other potential factors.
79. Numerical modeling is used in plate tectonics research to:
(a) Directly observe the movement of tectonic plates in real-time.
(b) Create physical scale models of plate boundaries in the laboratory.
(c) Simulate plate tectonic processes using computer programs to test hypotheses and understand the dynamics of Earth’s lithosphere and mantle.
(d) Analyze the chemical composition of rocks collected from different plate tectonic settings.
Answer: (c)
Explanation: Numerical modeling involves using computational methods to simulate the complex physical processes involved in plate tectonics, such as mantle flow, heat transfer, and the deformation of the lithosphere. These models help scientists to understand how different factors influence plate motion, the formation of geological features, and the evolution of the Earth’s interior.
80. Plate tectonics plays a crucial role in the long-term stability of Earth’s climate through its influence on:
(a) The frequency of meteorite impacts.
(b) The rate of Earth’s rotation.
(c) The carbon cycle, including volcanic emissions and the weathering of silicate rocks.
(d) The intensity of solar radiation reaching the Earth’s surface.
Answer: (c)
Explanation: Plate tectonics is intimately linked to the carbon cycle. Volcanic eruptions release greenhouse gases like carbon dioxide into the atmosphere. Conversely, the weathering of silicate rocks, a process influenced by tectonic uplift and mountain building, removes carbon dioxide from the atmosphere. The subduction of carbon-rich materials also plays a role in long-term carbon storage.
81. Kimberlites, which are the primary source of diamonds, are typically found in:
(a) Young volcanic arcs associated with active subduction zones.
(b) Ancient, stable continental interiors (cratons) that have experienced deep mantle volcanism.
(c) Mid-oceanic ridges where high-pressure fluids circulate through the oceanic crust.
(d) Continental rift valleys characterized by alkaline magmatism.
Answer: (b)
Explanation: Kimberlites are rare, ultramafic igneous rocks that originate from great depths within the Earth’s mantle (sometimes over 150 km). They are typically found in old, stable parts of continents known as cratons, which have thick lithosphere that can preserve diamonds formed under high pressure and temperature conditions in the mantle.
82. Water plays a crucial role in subduction zones by:
(a) Increasing the viscosity of the mantle wedge, hindering magma generation.
(b) Decreasing the density of the subducting slab, making it more buoyant.
(c) Lowering the melting point of mantle rocks in the overriding plate, leading to the formation of volcanic arcs.
(d) Increasing the rate of heat transfer from the subducting slab to the overriding plate.
Answer: (c)
Explanation: As the subducting oceanic plate descends, it carries water-rich sediments and hydrated minerals. At depth, these volatiles are released into the overlying mantle wedge. The addition of water significantly lowers the melting temperature of the mantle peridotite, causing it to melt and generate magma that rises to form volcanic arcs on the overriding plate.
83. Seismic anisotropy, the variation of seismic wave speeds with direction, in the Earth’s mantle can provide insights into:
(a) The chemical composition of different mantle layers.
(b) The temperature gradients within the mantle.
(c) The alignment of minerals due to mantle flow and deformation associated with plate tectonics.
(d) The presence of partially molten zones in the mantle.
Answer: (c)
Explanation: Minerals in the mantle, such as olivine, can become preferentially aligned due to the stresses and deformation associated with mantle flow driven by plate tectonics. This alignment causes seismic waves to travel at different speeds depending on their direction of propagation, providing a way to study the patterns and direction of mantle flow.
84. Magnetic stripes on the seafloor provide strong evidence for:
(a) The स्थिर nature of the Earth’s magnetic poles over geological time.
(b) The episodic nature of volcanic eruptions at hotspots.
(c) The process of seafloor spreading at mid-oceanic ridges coupled with periodic reversals of the Earth’s magnetic field.
(d) The occurrence of major meteorite impacts throughout Earth’s history.
Answer: (c)
Explanation: As new basaltic magma erupts and cools at mid-oceanic ridges, it records the Earth’s magnetic field at that time. Because the Earth’s magnetic field periodically reverses, this process creates symmetrical bands (stripes) of rocks with alternating normal and reversed magnetic polarities parallel to the ridge axis. These stripes act as a “magnetic tape recorder” of seafloor spreading.
85. Pyroclastic flows, a significant volcanic hazard, are most commonly associated with:
(a) Effusive eruptions of basaltic lava at shield volcanoes.
(b) Explosive eruptions of viscous, silica-rich magma at composite volcanoes near convergent plate boundaries.
(c) Submarine volcanic eruptions along mid-oceanic ridges.
(d) The degassing of magma during periods of volcanic quiescence.
Answer: (b)
Explanation: Pyroclastic flows are fast-moving currents of hot gas and volcanic matter (ash, pumice, and blocks) that can travel down the flanks of volcanoes at high speeds. They are typically generated by the collapse of eruption columns or the explosive ejection of material from viscous, silica-rich magmas, which are common at composite volcanoes found near convergent plate boundaries.
86. Gravity and magnetic surveys can be used to study the subsurface structure of plate margins by:
(a) Directly imaging the boundaries between tectonic plates.
(b) Detecting variations in rock density (gravity) and magnetic susceptibility (magnetism) that can indicate the presence of features like subducted slabs, fault zones, or igneous intrusions.
(c) Measuring the precise rates of plate movement.
(d) Determining the age of the oceanic crust.
Answer: (b)
Explanation: Gravity anomalies (variations in the Earth’s gravitational field) can indicate differences in rock density, which can be associated with features like dense subducted slabs or buoyant mantle plumes. Magnetic anomalies (variations in the Earth’s magnetic field recorded in rocks) can help map the distribution of magnetic minerals and identify features like oceanic crust with its characteristic magnetic stripes or magnetic intrusions.
87. Isostatic compensation is the mechanism by which the Earth’s lithosphere:
(a) Generates heat through radioactive decay.
(b) Maintains a state of gravitational equilibrium by adjusting its elevation in response to changes in mass distribution.
(c) Undergoes brittle deformation during earthquake events.
(d) Transfers heat from the core to the surface through convection.
Answer: (b)
Explanation: Isostatic compensation is the principle that the lithosphere floats on the denser asthenosphere in a state of gravitational balance. If a load is added to the lithosphere (e.g., a mountain range or an ice sheet), it will subside until equilibrium is restored. Conversely, if a load is removed (e.g., through erosion or ice melting), the lithosphere will rebound upwards.
88. Oceanic core complexes (OCCs) are geological features found at:
(a) Fast-spreading mid-oceanic ridges, characterized by extensive sheeted dike complexes.
(b) Slow-spreading mid-oceanic ridges, where extensional faulting exposes lower crustal and upper mantle rocks on the seafloor.
(c) Subduction zones, associated with the uplift of forearc regions.
(d) Hotspots, where mantle plumes interact with oceanic lithosphere.
Answer: (b)
Explanation: Oceanic core complexes are distinctive seafloor structures found primarily at slow-spreading mid-oceanic ridges. They form due to large-scale detachment faulting that exposes deep-seated gabbroic and peridotitic rocks on the seafloor, often creating domed or corrugated surfaces. This process accommodates a significant portion of the plate divergence at these slow-spreading centers.
89. The distribution of geothermal energy resources is closely linked to:
(a) Stable continental interiors with high rates of radioactive decay.
(b) Areas with low tectonic activity and thick sedimentary basins.
(c) Active plate boundaries, particularly divergent boundaries (mid-oceanic ridges and rift valleys) and convergent boundaries with volcanism.
(d) Regions with high rates of erosion and sedimentation.
Answer: (c)
Explanation: Geothermal energy originates from the Earth’s internal heat. Active plate boundaries are regions where this heat is readily accessible near the surface due to volcanic activity, hydrothermal circulation along fault zones, and high heat flow associated with magma upwelling (at divergent boundaries) or mantle wedge melting (at convergent boundaries).
90. The study of plate tectonics has broader implications for understanding:
(a) The composition of the Earth’s atmosphere and oceans.
(b) The geological evolution and potential for tectonic activity on other terrestrial planets and moons in our solar system and beyond.
(c) The behavior of the Earth’s magnetic field.
(d) The processes driving climate change on Earth.
Answer: (b)
Explanation: By understanding the processes that drive plate tectonics on Earth, scientists can gain insights into the internal dynamics and surface evolution of other rocky bodies in our solar system and exoplanets. Comparing Earth’s tectonic activity (or lack thereof on other bodies) helps us understand the conditions necessary for plate tectonics and its role in shaping planetary landscapes and potentially influencing habitability.
91. Continental rift valleys are characterized by all of the following EXCEPT:
(a) Normal faulting and graben formation.
(b) Volcanic activity associated with magma upwelling.
(c) Compressional stress leading to significant crustal shortening.
(d) The potential for the eventual formation of a new ocean basin.
Answer: (c)
Explanation: Continental rift valleys form due to tensional stress, which pulls the crust apart, leading to normal faulting and the subsidence of blocks to form grabens. While volcanism is common due to the thinning crust and rising magma, compressional stress and crustal shortening are characteristic of convergent plate boundaries, not divergent rift zones. Rift valleys represent the initial stages of continental breakup and can eventually evolve into new ocean basins if rifting continues.
92. Which of the following is a key difference between oceanic-oceanic and oceanic-continental convergent plate boundaries?
(a) The type of faulting that predominates at the boundary.
(b) The depth of the earthquakes that occur along the Benioff zone.
(c) The nature of the volcanic arc that forms (volcanic island arc vs. continental volcanic arc).
(d) The presence or absence of a deep-sea trench.
Answer: (c)
Explanation: Both oceanic-oceanic and oceanic-continental convergence involve subduction and the formation of a deep-sea trench and a Benioff zone with earthquakes increasing in depth. However, in oceanic-oceanic convergence, the melting of the mantle above the subducting slab leads to the formation of a volcanic island arc on the overriding oceanic plate. In oceanic-continental convergence, the magma rises through continental crust, forming a continental volcanic arc.
93. The primary role of transform faults is to:
(a) Accommodate the movement of plates towards each other at convergent boundaries.
(b) Facilitate the creation of new oceanic crust at divergent boundaries.
(c) Allow plates to slide past each other horizontally, accommodating differences in spreading rates or directions.
(d) Drive the process of subduction at convergent boundaries.
Answer: (c)
Explanation: Transform faults are a type of strike-slip fault where plates move past each other horizontally. Along mid-oceanic ridges, they offset ridge segments and accommodate the different rates at which the seafloor is spreading along those segments. They also occur in continental settings, accommodating the relative motion between large crustal blocks.
94. Evidence for mantle plumes includes:
(a) The presence of magnetic stripes on the seafloor.
(b) The occurrence of deep-focus earthquakes at subduction zones.
(c) Chains of volcanic islands and seamounts that show a progression in age away from a hotspot.
(d) The jigsaw fit of continental coastlines.
Answer: (c)
Explanation: Hotspots are localized areas of volcanic activity thought to be caused by rising plumes of hot mantle material. As a tectonic plate moves over a stationary hotspot, it creates a chain of volcanoes that become progressively older with distance from the active hotspot, providing a track of the plate’s movement over the plume. The Hawaiian Islands are a classic example.
95. Paleomagnetic studies of ancient rocks have been crucial in understanding:
(a) The chemical composition of the Earth’s core.
(b) The rate of radioactive decay in the mantle.
(c) The apparent polar wander of continents, which provided early support for Continental Drift.
(d) The mechanisms responsible for earthquake generation.
Answer: (c)
Explanation: Paleomagnetism, the study of the Earth’s ancient magnetic field recorded in rocks, showed that the apparent position of the magnetic poles seemed to wander over time relative to individual continents. However, when the continents were placed in their proposed past positions (as suggested by Continental Drift), the apparent polar wander paths aligned, providing strong evidence that it was the continents that were moving, not the magnetic poles themselves.
96. Major mountain building events (orogenies) are primarily associated with:
(a) The erosion and weathering of continental landmasses.
(b) The movement of plates away from each other at divergent boundaries.
(c) The collision of tectonic plates at convergent boundaries.
(d) Volcanic activity at hotspots located beneath continents.
Answer: (c)
Explanation: Orogenesis, or mountain building, is a direct consequence of the intense compressional forces that occur when tectonic plates collide at convergent boundaries, particularly continental-continental or oceanic-continental collisions. These collisions lead to the folding, faulting, and uplift of the crust, creating large mountain ranges like the Himalayas and the Andes.
97. Normal faults are most commonly found at:
(a) Convergent plate boundaries where compressional forces dominate.
(b) Transform plate boundaries where shear stress is prevalent.
(c) Divergent plate boundaries where tensional forces cause the crust to stretch and thin.
(d) Areas affected by significant meteorite impacts.
Answer: (c)
Explanation: Normal faults occur when the hanging wall moves down relative to the footwall due to tensional forces pulling the crust apart. These are characteristic features of divergent plate boundaries, such as mid-oceanic ridges and continental rift valleys, where the lithosphere is being extended.
98. The Wilson Cycle describes the cyclical process of:
(a) Magnetic reversals recorded in oceanic crust.
(b) The formation and breakup of supercontinents and the opening and closing of ocean basins.
(c) The gradual cooling of the Earth’s interior over geological time.
(d) The periodic fluctuations in global sea level.
Answer: (b)
Explanation: The Wilson Cycle is a model that explains the long-term tectonic evolution of continents and ocean basins. It involves stages from continental rifting to the formation of a new ocean, its widening and eventual closure through subduction and continental collision, leading to the formation of a supercontinent, which then breaks apart to begin the cycle anew.
99. Slab pull, a significant driving force of plate tectonics, is generated by:
(a) The gravitational sliding of lithosphere off the elevated mid-oceanic ridges.
(b) The resistance of the asthenosphere to the movement of tectonic plates.
(c) The sinking of cold, dense oceanic lithosphere into the mantle at subduction zones.
(d) The upwelling of hot mantle plumes beneath tectonic plates.
Answer: (c)
Explanation: Slab pull is considered one of the most important forces driving plate motion. It occurs because old, cold oceanic lithosphere is denser than the underlying asthenosphere. As this dense slab subducts into the mantle, it “pulls” the rest of the plate along with it, similar to a heavy anchor pulling a chain.
100. A fundamental difference between oceanic and continental lithosphere is that oceanic lithosphere is typically:
(a) Thicker and less dense than continental lithosphere.
(b) Older and more rigid than continental lithosphere.
(c) Thinner and denser than continental lithosphere, and is continuously recycled at subduction zones.
(d) Composed primarily of granitic rocks, whereas continental lithosphere is basaltic.
Answer: (c)
Explanation: Oceanic lithosphere is generally thinner (around 50-100 km) and denser (composed mainly of basalt and gabbro) than continental lithosphere (which can be over 200 km thick and is composed of less dense granitic rocks). Oceanic lithosphere is also relatively young because it is continuously created at mid-oceanic ridges and recycled back into the mantle at subduction zones, whereas continental lithosphere is much older and more permanent.