Plate Tectonics

Plate Tectonics

Middle School

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Learning Objectives

• Use evidence from fossils and rocks to understand past plate movements.

• Explain how processes like sea-floor spreading change the Earth's surface over time.

• Describe the three types of plate boundaries and the landforms they create.

• Connect plate movement to events like earthquakes, volcanoes, faults, and folds.

Key Vocabulary

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Plate Tectonics

The theory explaining the movement of large plates that make up Earth's hard outer layer.

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Continental Drift

The hypothesis that the continents have moved slowly across Earth's surface over millions of years.

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Pangaea

The name for the ancient supercontinent that included all the world's landmasses before they broke apart.

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Sea-floor Spreading

The process where new oceanic crust is formed at mid-ocean ridges and spreads outwards.

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Subduction

The process where one tectonic plate moves under another and is forced to sink into the mantle.

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Mid-Ocean Ridge

An underwater mountain system formed by plate tectonics, where new ocean floor is created by magma.

Key Vocabulary

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Ocean Trench

An ocean trench is a deep, narrow valley on the ocean floor where old crust is pushed down.

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Divergent Boundary

A divergent boundary is a location where two of Earth's tectonic plates are moving apart from each other.

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Convergent Boundary

A convergent boundary is a place where two of Earth's tectonic plates collide or move towards one another.

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Transform Boundary

A transform boundary occurs where two of the Earth's tectonic plates slide horizontally past one another.

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Fault

A fault is a large crack or fracture in the Earth's crust where rock has moved.

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Earthquake

An earthquake is the shaking of the ground caused by the sudden movement of rock along a fault.

The Hypothesis of Continental Drift

• Alfred Wegener suggested all continents were once a single supercontinent called Pangaea.

• The coastlines of South America and Africa fit together like puzzle pieces.

• Similar fossils and rocks are found on continents that are widely separated today.

Multiple Choice

What is the central idea of the continental drift hypothesis?

• All continents were once joined together in a single supercontinent.

• The continents have always been in their current locations.

• A giant ocean once covered the entire surface of Earth.

• Continents are formed by underwater volcanic eruptions.

Multiple Choice

What is the significance of finding similar fossils and rock types on continents that are now far apart?

• They show that all rocks are the same age.

• They prove that animals can swim across oceans.

• They provide evidence that the continents were once connected.

• They indicate that the climate has never changed.

Multiple Choice

If the coastlines of South America and Africa fit together like puzzle pieces and have similar fossils, what is the most logical conclusion?

• The two continents were likely part of the same landmass in the past.

• A land bridge must have connected the two continents.

• The same types of animals evolved independently on each continent.

• Ocean currents must have carried the fossils from one continent to the other.

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Sea-Floor Spreading and Subduction

Sea-Floor Spreading

• At mid-ocean ridges, magma rises from the mantle to create new oceanic crust.

• This process pushes the older, existing rock further away from the ridge, toward the continents.

• Rock samples confirm that the crust is youngest at the ridge and gets older with distance.

Subduction

• The Earth does not get larger because old oceanic crust is destroyed through subduction.

• At deep ocean trenches, the older, denser crust sinks back into the mantle.

• This recycling process for the Earth's crust takes place over millions of years.

Multiple Choice

What happens during the process of sea-floor spreading at a mid-ocean ridge?

• New oceanic crust is created by rising magma.

• Old oceanic crust is destroyed in deep trenches.

• Continents are pushed closer to the ridge.

• The mantle cools down to form rock.

Multiple Choice

What is the relationship between sea-floor spreading and subduction?

• Spreading creates new crust, while subduction destroys old crust, keeping the Earth's size stable.

• Subduction creates new crust at trenches, while spreading destroys it at ridges.

• Both spreading and subduction are processes that create new sections of crust.

• Both spreading and subduction are processes that happen in the mantle, not the crust.

Multiple Choice

If a scientist collects one rock sample from a mid-ocean ridge and another near a continent, what can be concluded about their ages?

• The rock sample taken near the continent is older.

• The rock sample taken near the ridge is older.

• Both rock samples are the same age.

• It is impossible to know the age from the location.

The Theory of Plate Tectonics

• This theory combines continental drift and sea-floor spreading.

• Earth’s lithosphere is broken into large, slowly moving tectonic plates.

• Powerful convection currents in the mantle move these plates.

Multiple Choice

Which statement best describes the Theory of Plate Tectonics?

• Earth’s lithosphere is a single, solid shell.

• Earth’s lithosphere is made of large, slowly moving plates.

• The continents have always been in their current positions.

• The mantle of the Earth is a cool, solid layer.

Multiple Choice

What provides the power to move the tectonic plates?

• The spinning of the Earth's core

• The pull of the moon's gravity

• Convection currents in the mantle

• The process of sea-floor spreading alone

Multiple Choice

Based on the relationship between the mantle and the lithosphere, what would happen if the convection currents in the mantle stopped?

• The movement of the tectonic plates would stop.

• The tectonic plates would move faster.

• The Earth's lithosphere would become thicker.

• The continents would combine into one landmass.

Plate Boundary Interactions and Landforms

• Divergent boundaries: two plates pull apart, forming mid-ocean ridges and rift valleys.

• Convergent boundaries: two plates collide, creating mountains, volcanoes, and ocean trenches.

• Transform boundaries: two plates slide past each other, causing powerful earthquakes.

Multiple Choice

What happens at a divergent plate boundary?

• Two plates pull apart from each other.

• Two plates collide with each other.

• Two plates slide past each other.

• Two plates remain stationary.

Multiple Choice

What is the relationship between plate collisions and the formation of mountains?

• They are formed when two plates collide and push land upwards.

• They are created when two plates slide past each other horizontally.

• They appear when two plates pull apart, creating a valley.

• They result from the cooling of lava in a single location.

Multiple Choice

If a location experiences powerful earthquakes but has no volcanoes or major mountain ranges, what is the most likely type of plate boundary present?

• A transform boundary, where plates slide past each other.

• A divergent boundary, where plates pull apart.

• A convergent boundary, where plates collide.

• A stationary boundary, where no movement occurs.

The movement of tectonic plates puts stress on the rock of the lithosphere.

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Tension

• ​This stress pulls on the crust, stretching it out.

• ​​It usually happens at divergent plate boundaries.

• ​This type of stress creates normal faults in the rock.

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Compression

• ​This type of stress squeezes the Earth's crust together.

• ​​It is common at convergent plate boundaries.

• ​This can form reverse faults and create folded mountains.

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Shearing

• ​Shearing pushes rock in opposite horizontal directions.

• ​​This type of stress occurs along transform plate boundaries.

• ​The result of this stress is a strike-slip fault.

Multiple Choice

What is the general term for the force that the movement of tectonic plates puts on the rock of the lithosphere?

• Stress

• Faulting

• Boundaries

• Folding

Multiple Choice

Which type of stress is correctly matched with the type of fault it creates?

• Tension creates a reverse fault.

• Compression creates a normal fault.

• Shearing creates a strike-slip fault.

• Tension creates folded mountains.

Multiple Choice

A geologist observes a mountain range formed from folded rock layers with numerous reverse faults. What combination of stress and plate boundary most likely created this feature?

• Tension at a divergent boundary.

• Shearing at a transform boundary.

• Compression at a convergent boundary.

• Stretching at a normal fault.

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Common Misconceptions about Plate Tectonics

Misconception	Correction
Continents drift around on top of the oceans.	Continents are part of larger tectonic plates that move over the mantle.
Most earthquakes and volcanoes occur randomly.	They mostly happen along plate boundaries, where different plates interact.
A magnitude 8 earthquake is twice as strong as a magnitude 7.	Each whole number increase means a 32-fold increase in energy released.

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Summary

• Evidence for past plate motion includes matching fossils, rocks, and seafloor patterns.

• Earth’s lithosphere is divided into moving plates, driven by mantle convection currents.

• Plate boundaries are sites of earthquakes, volcanoes, and mountain formation.

• These processes continually reshape Earth’s surface over millions of years.

Poll

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