Evidence of Plate Motions

Evidence of Plate Motions

Middle School

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Learning Objectives

• Describe the continental drift hypothesis and the evidence that supports it.

• Explain how mid-ocean ridges and sea-floor spreading create new oceanic crust.

• Detail how subduction at ocean trenches recycles old oceanic crust.

• Connect sea-floor spreading and subduction to the movement of continents.

Key Vocabulary

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Continental Drift

The hypothesis that continents have slowly moved to their current locations over geological time.

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Pangaea

The name for the supercontinent that existed before breaking apart into today's continents.

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Mid-Ocean Ridge

An undersea mountain chain where new ocean floor is produced by volcanic activity at a boundary.

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Sea-Floor Spreading

The process where molten rock creates new oceanic crust at mid-ocean ridges, pushing older rock away.

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Subduction

The sinking movement of a denser tectonic plate back into the mantle at a plate boundary.

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Ocean Trench

A long, deep valley on the ocean floor where the process of subduction takes place.

Hypothesis of Continental Drift

• Alfred Wegener proposed the continental drift hypothesis in 1912.

• He proposed a supercontinent named Pangaea that broke apart over time.

• Scientists were initially skeptical as the 'how' of the drift was missing.

Multiple Choice

What is the main idea of the continental drift hypothesis?

• That the Earth's continents were once a single landmass that broke apart.

• That the oceans are slowly getting deeper over time.

• That new continents are constantly being formed by volcanoes.

• That the Earth's magnetic poles have reversed multiple times.

Multiple Choice

What was Alfred Wegener's primary contribution to the continental drift hypothesis described in the text?

• He explained how the continents were able to move.

• He proposed the idea of a supercontinent called Pangaea.

• He was the first to map the ocean floor.

• He proved that the Earth was round.

Multiple Choice

Based on the information, what is the most likely reason scientists were initially skeptical of the continental drift hypothesis?

• His idea of a supercontinent was considered too simple.

• He could not provide an explanation for how the continents moved.

• There was no evidence that the continents had ever been connected.

• The hypothesis was proposed too long ago to be considered valid.

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Evidence for Continental Drift

• Mountain ranges on different continents, such as South America and Africa, match up.

• ​Coal deposits from ancient tropical plants are found today in very cold climates.

• Fossils like Mesosaurus and Glossopteris were found on widely separated continents.

• Tropical plant fossils on the arctic island of Spitsbergen suggest a climate shift.

Multiple Choice

What main idea is supported by evidence like matching mountain ranges and similar fossils on different continents?

• The idea that continents have moved over time.

• The process of how mountains are formed.

• The study of different types of plant fossils.

• The reason why animals live in different habitats.

Multiple Choice

How does finding fossils of the same land animals, like Mesosaurus, on both South America and Africa support the theory of continental drift?

• It proves that the animals could swim across the ocean.

• It shows that the same types of animals evolved on every continent.

• It suggests these continents were once joined together.

• It means that the fossils were moved by ocean currents.

Multiple Choice

What is the best scientific explanation for finding fossils of tropical plants in a cold region like the Arctic?

• Tropical plants were able to survive in freezing temperatures.

• The landmass moved from a warmer climate zone to the Arctic.

• Ancient explorers transported the plant fossils to the Arctic.

• The fossils were carried there by wind and ocean currents.

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Mid-Ocean Ridges and Sea-Floor Spreading

• Scientists found long, undersea mountain chains called mid-ocean ridges.

• At these ridges, molten rock rises and hardens to form new rock.

• This process, sea-floor spreading, pushes older rock away from the ridge.

• Evidence includes pillow-shaped rocks and the age of the seafloor rocks.

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Multiple Choice

What is sea-floor spreading?

• The process where new rock forms at mid-ocean ridges and pushes older rock away.

• The process where underwater volcanoes erupt and form islands.

• The process where continents drift apart on land.

• The process where ocean currents move sediment across the seafloor.

Multiple Choice

Based on the process of sea-floor spreading, what is the relationship between the age of seafloor rock and its distance from a mid-ocean ridge?

• The farther the rock is from the ridge, the older it is.

• The closer the rock is to the ridge, the older it is.

• All rocks on the seafloor are the same age, regardless of location.

• The age of the rock depends on the ocean depth, not the ridge.

Multiple Choice

If scientists were gathering evidence to support the idea of sea-floor spreading, which finding would provide the strongest support?

• Rock samples taken farthest from the ridge are the oldest.

• Rock samples taken closest to the ridge are the oldest.

• All rock samples collected are the same age and have a pillow shape.

• The only rocks found on the seafloor are from hardened lava.

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Ocean Trenches and Subduction

• Subduction is when the ocean floor sinks back into the mantle.

• This occurs at ocean trenches, the deepest parts of the ocean.

• Denser oceanic crust sinks under a less-dense plate, which can create volcanoes.

• The Pacific Ocean is shrinking, while the Atlantic Ocean is getting wider.

Multiple Choice

What is the definition of subduction?

• The sinking of the ocean floor back into the mantle.

• The creation of new crust at the ocean's surface.

• The process of two continents moving apart.

• The widening of an ocean basin like the Atlantic.

Multiple Choice

What is the relationship between the process of subduction and the formation of volcanoes?

• It causes the Atlantic Ocean to shrink.

• It makes the sinking crust less dense.

• It recycles crust, which can lead to the formation of volcanoes.

• It only happens in the center of the oceans.

Multiple Choice

Given the information about subduction, what is the most likely reason the Pacific Ocean is shrinking?

• The Pacific Ocean must have many trenches where the ocean floor is sinking.

• The crust under the Pacific Ocean is much less dense than other plates.

• The Atlantic Ocean is pushing the Pacific plate, causing it to get smaller.

• There are no volcanoes forming in the Pacific Ocean.

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Common Misconceptions

Misconception	Correction
Continents float around on top of the oceans.	Continents are part of large tectonic plates that move.
Sea-floor spreading must be making the Earth expand in size.	The creation of new crust is balanced by the destruction of old crust through subduction.
Volcanoes only happen on land.	The largest and most extensive volcanic activity on Earth occurs underwater at mid-ocean ridges.

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Summary

• Alfred Wegener proposed that all land was once a supercontinent called Pangaea.

• Evidence includes matching fossils, landforms, and climate data across the oceans.

• Sea-floor spreading at mid-ocean ridges creates new crust, pushing plates apart.

• Subduction at ocean trenches recycles older crust back into the mantle.

Poll

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