Earth Science Quiz: Evidence For Plate Tectonics

What this quiz covers

This quiz focuses on Evidence For Plate Tectonics, giving you a quick way to practice the rules, question types, and explanations that matter most for Earth Science.

How to use this quiz

Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.

Question 1

1. The hotspot's magma source is chemically distinct from the magma at mid-ocean ridges.
2. The hotspot's volcanic output has remained constant over tens of millions of years.
3. The hotspot's source in the mantle is fixed or moves very slowly relative to the moving plate. (correct answer)
4. The hotspot is located precisely in the center of the Pacific Plate, far from any plate boundaries.

Explanation: The entire principle of using hotspot tracks to determine absolute plate motion rests on the assumption that the mantle plume feeding the hotspot is a fixed reference point. If the hotspot itself were moving significantly, the volcanic track would represent a combination of both plate motion and hotspot motion, making it impossible to isolate the plate's velocity. While the other statements may be true to some extent, the stationarity of the hotspot is the fundamental assumption required for the calculation of plate motion vectors.

Question 2

1. A new, unusually wide stripe of normally polarized crust would form at mid-ocean ridges. (correct answer)
2. The existing stripes would slowly lose their magnetic signatures and become uniform.
3. Seafloor spreading would cease because magnetic reversals are necessary to drive the process.
4. Alternating stripes of normal and reversed polarity would continue to form, but with a different frequency.

Explanation: The paleomagnetic stripes are like a tape recording of the Earth's magnetic field polarity over time, with the 'tape' being the newly formed oceanic crust. The width of each stripe is determined by how long the polarity remained in that state and the rate of spreading. If the field stays in a normal polarity state for a very long time (a 'superchron'), a very wide stripe of normally polarized crust would be created on both sides of the mid-ocean ridges.

Question 3

1. India has remained stationary near the equator for the last 100 million years.
2. India was located in the southern hemisphere and has since drifted north, crossing the equator.
3. India was located near the equator 100 million years ago and has since drifted north into the northern hemisphere. (correct answer)
4. India was located near the North Pole and has since drifted south towards the equator.

Explanation: Magnetic inclination is near zero at the magnetic equator. A low inclination in the 100 Ma rock indicates India was near the equator at that time. A moderate northerly inclination in the 10 Ma rock indicates it was in the northern hemisphere at that time (field lines pointing downwards). Therefore, the evidence strongly supports the conclusion that the Indian subcontinent drifted significantly northward from a near-equatorial position over the last 100 million years, consistent with its eventual collision with Asia.

Question 4

1. A north-south line of volcanoes. (correct answer)
2. An east-west line of volcanoes.
3. A line of volcanoes oriented northeast-southwest.
4. Two separate lines of volcanoes diverging from the ridge.

Explanation: The orientation of a hotspot track is determined by the motion of the plate relative to the fixed hotspot. The question states the plate is moving due north over the hotspot. Therefore, the chain of volcanoes will be imprinted on the plate as a north-south line, with the ages increasing to the south (in the direction opposite to plate motion). The motion of the mid-ocean ridge is extraneous information designed to confuse the test-taker; it affects where the crust is created but not the orientation of the track left by the hotspot on the overriding plate.

Question 5

1. Ridge A has a faster spreading rate than Ridge B. (correct answer)
2. Ridge B has a faster spreading rate than Ridge A.
3. Both ridges have the same spreading rate, but Ridge B is older.
4. The spreading rate cannot be determined without knowing the paleomagnetic history.

Explanation: The age of the crust at a certain distance from a ridge is inversely related to the spreading rate (Rate = Distance / Time). For Ridge A, the half-rate is 500 km / 10 Ma = 50 km/Ma. For Ridge B, the half-rate is 500 km / 20 Ma = 25 km/Ma. Therefore, Ridge A is spreading faster than Ridge B. The sediment thickness is a distractor, but consistent with the age data; the older crust at Ridge B has had more time to accumulate sediment.

Question 6

1. The age of the island relative to neighboring islands in the same chain.
2. The depth of earthquakes occurring in the crust beneath the island.
3. The primary chemical composition (e.g., basaltic vs. andesitic) of the lava.
4. The thickness of the lithosphere on which the island has formed. (correct answer)

Explanation: A) Hotspot islands show a clear age progression; island arcs may have coeval volcanism. B) Subduction zones have deep earthquakes (Wadati-Benioff zone); mid-plate hotspots do not. C) Hotspot volcanoes are typically basaltic; island arc volcanoes are typically andesitic or more silica-rich. D) Both mid-plate hotspots (by definition) and island arcs form on oceanic lithosphere. While the thickness might vary, it's a much less definitive and direct piece of evidence compared to the clear differences in age progression, seismicity, and geochemistry.

Question 7

1. Thermal subsidence of the lithosphere and erosion of the volcanic edifice by wave action. (correct answer)
2. Increased sediment loading on the older crust and a global rise in sea level over time.
3. Bending of the lithosphere due to volcanic loading and isostatic adjustment to the mantle.
4. Slower seafloor spreading rates in the past and increased density of older volcanic rocks.

Explanation: As the oceanic lithosphere moves away from the heat source (the hotspot, which is often near a ridge), it cools and contracts. This cooling increases its density, causing it to sink lower into the underlying asthenosphere in a process called thermal subsidence. Concurrently, as a volcanic island becomes inactive and moves off the hotspot, it is subject to erosion from waves, wind, and rain, which lowers its elevation. The combination of the plate sinking and the island itself eroding explains the increasing water depth of older volcanoes in the chain.

Question 8

1. It will be a 10-km-wide stripe of reversed polarity on each side of the ridge.
2. It will be a 20-km-wide stripe of reversed polarity on each side of the ridge. (correct answer)
3. It will be two 5-km-wide stripes of reversed polarity, one on each side of the central 10-km normal stripe.
4. It will be a single 20-km-wide stripe of reversed polarity centered on the ridge.

Explanation: Seafloor spreading occurs symmetrically on both sides of a mid-ocean ridge. The width of a magnetic stripe is proportional to the duration of the magnetic polarity period and the spreading rate. Stripe N is 10 km wide. Since this stripe exists on one side of the ridge, 10 km of crust was formed during that normal polarity period. If the next reversed polarity period lasts twice as long, twice as much crust will be created on that one side. Therefore, a 20-km-wide stripe of reversed polarity will form on each side of the ridge, adjacent to Stripe N.

Question 9

1. The Earth's magnetic pole split into two separate poles around 250 million years ago.
2. Continents X and Y were part of the same landmass before 250 million years ago and began to drift apart after. (correct answer)
3. The Earth's magnetic field was unstable before 250 million years ago, causing the poles to wander, but has been stable since.
4. Continents X and Y have always been stationary, and the diverging paths reflect complex movements of the true magnetic pole.

Explanation: Apparent polar wander paths (APWPs) track the apparent position of the magnetic pole relative to a continent over time. The key insight is that the pole itself is relatively stable, and the 'wandering' is due to the continent's movement. When two continents are joined, their rocks record the same magnetic pole position, so their APWPs are identical. When they split and move independently, their APWPs diverge. Thus, the data show the continents were together before 250 Ma and have been drifting apart since.

Question 10

1. The seamounts were formed by a stationary hotspot, and the plate is moving parallel to the ridge.
2. The seamounts were formed by volcanism along a transform fault, and are not related to a hotspot.
3. The seamounts were formed by a hotspot, but the plate has recently stopped moving.
4. The seamounts are part of the mid-ocean ridge system itself, possibly formed by eruptions along a fracture zone. (correct answer)

Explanation: A classic hotspot track should have a clear age progression and its orientation indicates the direction of plate motion. A lack of age progression and an orientation perpendicular to the ridge axis is inconsistent with a simple hotspot model. Plate motion is generally perpendicular, not parallel, to the ridge from which it forms. Volcanism can occur along fracture zones that are perpendicular to the main ridge axis. This would explain the orientation and potentially the lack of a simple age progression compared to a hotspot track. Transform fault volcanism is less common and would be parallel to plate motion, not perpendicular to the ridge.

Question 11

1. The Earth's magnetic field did not exist in its current dipolar form before the Jurassic period.
2. Older oceanic crust has lost its original magnetic signature due to heat and chemical alteration over time.
3. Seafloor spreading is a relatively recent geological process that began only around 200 million years ago.
4. Most oceanic crust older than 200 million years has been destroyed through subduction at convergent plate boundaries. (correct answer)

Explanation: The process of plate tectonics involves the creation of new oceanic crust at mid-ocean ridges and the destruction of old oceanic crust at subduction zones. Oceanic crust is dense and, as it cools and moves away from the ridge, it eventually gets recycled back into the mantle at deep-sea trenches. Because of this continuous recycling, very little oceanic crust is older than about 200 million years. This is the primary reason the seafloor paleomagnetic record has a limited age range.

Question 12

1. A volcano erupted on the deep ocean floor, forming a seamount that was then flattened by submarine landslides.
2. A volcano formed at the mid-ocean ridge, rose above sea level, was eroded flat by waves, and then subsided as the plate cooled and moved away from the ridge. (correct answer)
3. A block of continental crust was rifted, submerged, and flattened by deep-sea currents before being transported to its current location.
4. A volcano formed above sea level and was flattened by glaciers during an ice age, then submerged when sea levels rose.

Explanation: This question requires integrating multiple concepts. Guyots are formed as volcanic islands, typically near a hotspot or mid-ocean ridge. The volcano grows above sea level, where wave action erodes its top flat (creating a feature called an atoll or tablemount). The presence of shallow-water organisms confirms it was once at or near sea level. As the oceanic plate moves away from the spreading center, it cools, contracts, and becomes denser, causing it to subside deeper into the asthenosphere, carrying the eroded volcano with it into the deep ocean.

Question 13

1. Ridge A has a faster spreading rate than Ridge B. (correct answer)
2. Ridge B has a faster spreading rate than Ridge A.
3. Both ridges have the same spreading rate, but Ridge B is older.
4. The spreading rate cannot be determined without knowing the paleomagnetic history.

Explanation: The age of the crust at a certain distance from a ridge is inversely related to the spreading rate (Rate = Distance / Time). For Ridge A, the half-rate is 500 km / 10 Ma = 50 km/Ma. For Ridge B, the half-rate is 500 km / 20 Ma = 25 km/Ma. Therefore, Ridge A is spreading faster than Ridge B. The sediment thickness is a distractor, but consistent with the age data; the older crust at Ridge B has had more time to accumulate sediment.

Question 14

1. The Earth's magnetic pole split into two separate poles around 250 million years ago.
2. Continents X and Y were part of the same landmass before 250 million years ago and began to drift apart after. (correct answer)
3. The Earth's magnetic field was unstable before 250 million years ago, causing the poles to wander, but has been stable since.
4. Continents X and Y have always been stationary, and the diverging paths reflect complex movements of the true magnetic pole.

Explanation: Apparent polar wander paths (APWPs) track the apparent position of the magnetic pole relative to a continent over time. The key insight is that the pole itself is relatively stable, and the 'wandering' is due to the continent's movement. When two continents are joined, their rocks record the same magnetic pole position, so their APWPs are identical. When they split and move independently, their APWPs diverge. Thus, the data show the continents were together before 250 Ma and have been drifting apart since.

Question 15

1. The age of the island relative to neighboring islands in the same chain.
2. The depth of earthquakes occurring in the crust beneath the island.
3. The primary chemical composition (e.g., basaltic vs. andesitic) of the lava.
4. The thickness of the lithosphere on which the island has formed. (correct answer)

Explanation: A) Hotspot islands show a clear age progression; island arcs may have coeval volcanism. B) Subduction zones have deep earthquakes (Wadati-Benioff zone); mid-plate hotspots do not. C) Hotspot volcanoes are typically basaltic; island arc volcanoes are typically andesitic or more silica-rich. D) Both mid-plate hotspots (by definition) and island arcs form on oceanic lithosphere. While the thickness might vary, it's a much less definitive and direct piece of evidence compared to the clear differences in age progression, seismicity, and geochemistry.

Question 16

1. A volcano erupted on the deep ocean floor, forming a seamount that was then flattened by submarine landslides.
2. A volcano formed at the mid-ocean ridge, rose above sea level, was eroded flat by waves, and then subsided as the plate cooled and moved away from the ridge. (correct answer)
3. A block of continental crust was rifted, submerged, and flattened by deep-sea currents before being transported to its current location.
4. A volcano formed above sea level and was flattened by glaciers during an ice age, then submerged when sea levels rose.

Explanation: This question requires integrating multiple concepts. Guyots are formed as volcanic islands, typically near a hotspot or mid-ocean ridge. The volcano grows above sea level, where wave action erodes its top flat (creating a feature called an atoll or tablemount). The presence of shallow-water organisms confirms it was once at or near sea level. As the oceanic plate moves away from the spreading center, it cools, contracts, and becomes denser, causing it to subside deeper into the asthenosphere, carrying the eroded volcano with it into the deep ocean.

Question 17

1. A north-south line of volcanoes. (correct answer)
2. An east-west line of volcanoes.
3. A line of volcanoes oriented northeast-southwest.
4. Two separate lines of volcanoes diverging from the ridge.

Explanation: The orientation of a hotspot track is determined by the motion of the plate relative to the fixed hotspot. The question states the plate is moving due north over the hotspot. Therefore, the chain of volcanoes will be imprinted on the plate as a north-south line, with the ages increasing to the south (in the direction opposite to plate motion). The motion of the mid-ocean ridge is extraneous information designed to confuse the test-taker; it affects where the crust is created but not the orientation of the track left by the hotspot on the overriding plate.

Question 18

1. It formed at the North or South Pole.
2. It formed near the magnetic equator.
3. It formed at a mid-latitude location, approximately 15°-20° North or South of the equator. (correct answer)
4. It formed at a high-latitude location, approximately 60°-70° North or South of the equator.

Explanation: The magnetic inclination (the angle the magnetic field lines make with the horizontal) is related to latitude by the formula tan(I) = 2*tan(L), where I is inclination and L is latitude. A simple rule of thumb is that at the equator (0° latitude), the inclination is 0°; at the poles (90° latitude), the inclination is 90°. A shallow inclination of 30° indicates a low- to mid-latitude origin. Using the formula, L = arctan(tan(30°)/2) ≈ 16.1°. Therefore, the rock formed at a mid-latitude location, not near the equator (where inclination would be near 0°) or the poles (where it would be near 90°).

Question 19

1. The Earth's magnetic field did not exist in its current dipolar form before the Jurassic period.
2. Older oceanic crust has lost its original magnetic signature due to heat and chemical alteration over time.
3. Seafloor spreading is a relatively recent geological process that began only around 200 million years ago.
4. Most oceanic crust older than 200 million years has been destroyed through subduction at convergent plate boundaries. (correct answer)

Explanation: The process of plate tectonics involves the creation of new oceanic crust at mid-ocean ridges and the destruction of old oceanic crust at subduction zones. Oceanic crust is dense and, as it cools and moves away from the ridge, it eventually gets recycled back into the mantle at deep-sea trenches. Because of this continuous recycling, very little oceanic crust is older than about 200 million years. This is the primary reason the seafloor paleomagnetic record has a limited age range.

Question 20

1. Thermal subsidence of the lithosphere and erosion of the volcanic edifice by wave action. (correct answer)
2. Increased sediment loading on the older crust and a global rise in sea level over time.
3. Bending of the lithosphere due to volcanic loading and isostatic adjustment to the mantle.
4. Slower seafloor spreading rates in the past and increased density of older volcanic rocks.

Explanation: As the oceanic lithosphere moves away from the heat source (the hotspot, which is often near a ridge), it cools and contracts. This cooling increases its density, causing it to sink lower into the underlying asthenosphere in a process called thermal subsidence. Concurrently, as a volcanic island becomes inactive and moves off the hotspot, it is subject to erosion from waves, wind, and rain, which lowers its elevation. The combination of the plate sinking and the island itself eroding explains the increasing water depth of older volcanoes in the chain.