Plate Tectonics

Plate Tectonics

Middle School

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Learning Objectives

• Define the lithosphere and describe the tectonic plates that make up Earth's crust.

• Explain how convection currents in the Earth's mantle cause tectonic plates to move.

• Describe the three main types of tectonic plate boundaries: convergent, divergent, and transform.

• Describe subduction and seafloor spreading as key processes that change Earth’s surface.

Key Vocabulary

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Tectonic Plates

Massive, slowly moving pieces of Earth's outermost solid layer, which is called the lithosphere.

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Lithosphere

The lithosphere is the rigid outer part of the Earth, including the crust and solid upper mantle.

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Asthenosphere

A dense, weak layer of the mantle that slowly flows just below the rigid lithosphere.

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Convection Currents

The circular flow of rock in the mantle that is the main driving force for plate movement.

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Seafloor Spreading

The process where new oceanic crust forms at mid-ocean ridges as tectonic plates move apart.

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Subduction Zone

A location where one tectonic plate is forced to slide under another plate when they converge.

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Earth's Layers and Tectonic Plates

Continental Crust

• Earth's crust is its rocky, outermost layer, forming part of the lithosphere.

• The lithosphere, broken into tectonic plates, consists of the crust and the uppermost mantle.

• This type of crust is thicker and less dense than oceanic crust.

• It forms the large landmasses we know as continents.

Oceanic Crust

• The lithosphere is broken into large, moving sections called tectonic plates.

• Oceanic crust is thinner and denser than continental crust.

  It forms the ocean floors and is mostly made of dark, dense rocks like basalt.

• This type of crust is thinner and denser than continental crust.

Multiple Choice

The Earth's lithosphere, which is broken into tectonic plates, is composed of which two layers?

• The crust and the upper mantle

• The crust and the core

• The inner and outer core

• The mantle and the core

How Plates Move: Convection

• Convection currents in the mantle move the tectonic plates.

• Hot, less dense rock from the mantle rises toward the crust.

• Cooler, denser rock near the crust sinks back down.

Multiple Choice

What is the relationship between convection in the mantle and the movement of tectonic plates?

• They create currents that move the tectonic plates.

• They transfer heat directly to the Earth's surface.

• They cause the asthenosphere to become solid.

• They make the tectonic plates float higher on the asthenosphere.

Where Plates Meet: Three Boundary Types

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Convergent Boundaries

• ​Plates are moving directly toward each other.

• ​​One plate can be forced under the other in a process called subduction.

• ​The collision can also cause the Earth's crust to form mountain ranges.

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Divergent Boundaries

• ​Tectonic plates are moving away from each other at these boundaries.

• ​​Magma from the mantle rises up to fill the resulting gap.

• ​This process creates new crust on the ocean floor or on land.

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Transform Boundaries

• ​Two plates slide horizontally past one another in opposite directions.

• ​​Friction between the plates causes a large amount of pressure to build.

• ​This pressure is eventually released in the form of powerful earthquakes.

Multiple Choice

A region is experiencing powerful earthquakes along a fault line, but there is no mountain formation or new crust. What is the most likely plate boundary type?

• Transform boundary

• Convergent boundary

• Divergent boundary

• Subduction zone

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Creating and Destroying Crust

New Crust is Created

• New oceanic crust forms at divergent boundaries through a process called seafloor spreading.

• This occurs at mid-ocean ridges where tectonic plates move away from each other.

• Magma from the mantle rises, then cools and hardens to form the new crust.

Old Crust is Destroyed

• Old oceanic crust is recycled at convergent boundaries in areas called subduction zones.

• This happens when a dense oceanic plate collides with a less dense continental plate.

• The denser oceanic plate sinks under the other plate and melts back into the mantle.

Multiple Choice

What are the primary ways that Earth's crust is changed at tectonic plate boundaries?

• It is created at divergent boundaries and destroyed at convergent boundaries.

• It is folded by mountains and eroded by oceans.

• It is cooled by magma and heated by the mantle.

• It is formed by volcanoes and recycled by earthquakes.

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What the Theory Explains

• It explains why Earth’s geography has changed and continues to change.

• It clarifies why earthquakes and volcanoes occur in specific belts.

• The theory also explains the formation and location of mountain ranges.

• It helps understand the locations of minerals and distribution of life.

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Multiple Choice

Which of the following does the theory of plate tectonics help to explain?

• The location of mountain ranges, volcanoes, and earthquakes.

• The daily weather patterns and climate zones.

• The composition of Earth's core.

• The effects of weathering and erosion on rocks.

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Common Misconceptions

Misconception	Correction
Earth's surface is completely solid and still.	The surface has tectonic plates in constant, slow motion.
The layer under the plates is liquid magma.	It's a solid rock layer that can flow slowly.
Continents drift independently across the ocean floor.	Continents are part of larger tectonic plates and move with them.

Multiple Choice

A geologist discovers a long chain of underwater mountains in the middle of an ocean. What type of plate boundary is this and what process is occurring?

• A divergent boundary where seafloor spreading is creating new crust.

• A convergent boundary where two oceanic plates are colliding.

• A transform boundary where two plates are sliding past each other.

• A subduction zone where one plate is sinking under another.

Multiple Choice

Why are strong earthquakes common at transform boundaries but large volcanoes are not?

• The plates slide past each other, creating friction and pressure, but no magma is generated.

• The plates are moving apart, allowing magma to rise, but the movement is too smooth for earthquakes.

• The plates are colliding and pushing upwards, creating mountains instead of volcanoes.

• Volcanoes only form in the middle of tectonic plates, far from boundaries.

Multiple Choice

If the process of subduction at convergent boundaries were to stop completely, but seafloor spreading at divergent boundaries continued, what would be the most likely long-term consequence for Earth?

• The planet would slowly expand as new crust is continuously created but not destroyed.

• The continents would stop moving and remain fixed in their current positions.

• The oceans would shrink as old crust is destroyed faster than new crust is created.

• Mountain ranges would erode and disappear without any new ones being formed.

Multiple Choice

The Himalayan mountains are formed by the collision of two continental plates. Based on your understanding of plate boundaries, why are these mountains so tall and why are there no active volcanoes there?

• Because both plates have low density, neither subducts, causing the crust to crumple upwards instead of melting.

• Because one plate is subducting under the other, pushing the land up and creating volcanoes.

• Because the plates are sliding past each other, creating immense pressure that pushes the land upwards.

• Because the plates are moving apart, allowing magma to rise and form mountains instead of volcanoes.

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Summary

• Earth's lithosphere is broken into plates that move due to convection in the mantle.

• Earthquakes, volcanoes, and mountains form at convergent, divergent, and transform boundaries.

• New crust forms at divergent boundaries, while old crust is recycled at subduction zones.

• Plate tectonics continuously changes Earth's geography over vast periods of time.

Poll

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