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McDougal Littell Science

How did Earth's layers form?

Earth's layers formed as denser materials sank toward the center and the less dense materials rose toward the surface.

What are Earth's main layers?

The core, the mantle and the crust

The core is Earth's densest region and is made of up of two parts.

Inner core and the outer core

Inner Core

a ball of hot, solid metals. It remains solid because of enormous pressure.

Outer core

a layer of liquid metals that surround the inner core. the lower pressure allow's the metals to remain a liquid.

The mantle

Earth's thickest layer is made of hot rock that is less dense than the core

The crust

a thin layer of cool rock that surrounds Earth.

continental crust

includes all the continents and some major islands

oceanic crust

includes all the ocean floors. the crust is home to all life on Earth.

Earth's crust and top of mantle

Lithosphere and Asthenosphere - the lithosphere sits on top of the Asthenosphere


made up of the curst and the very top of the mantle and is the most rigid of all layers.


a layer of hotter, softer rock in the upper mantle, it is soft enough to flow slowly

Tectonic Plates

the lithosphere is broken into many small slabs of rock. Most large plates include BOTH continental crust and oceanic crust

Why do you see only the dry land areas of tectonic plates on a typical world map?

The oceanic crust of the plates is underwater.

Continental Drift

A hypothesis made by Alfred Wegener that Earth's continents were once joined together in a single landmass and gradually moved or drifted apart.

From what three sources did Wegener gather evidence for his continental drift hypothesis?

Studies of ancient climate
The geology (rocks) of continents

What is Pangaea?

Wegener's name for when the continents were once joined together in a huge supercontinent.
Comes from Greek word meaning "all lands"

Mid-Ocean ridges

Huge underwater mountain ranges

Evidence for The Theory of Plate Tectonics

Sea-Floor Spreading, Age of the sea floor, Ocean trenches

Sea-floor spreading

molten rock that rises through cracks in the crust, cools, and forms new oceanic crust.

Age of sea floor

rock is youngest near the ridge, and older as you get farther away, showing that older rock was carried away from the ridge some time ago.

Ocean trenches

Huge trenches like deep canyons, in the sea floor where the dense ocean crust is sinking into the asthenosphere.

Causes of Plate Movement

Convection and Convection currents


energy transferred by the movement of a material.

convection currents

The hot, soft rock rises, cool and sinks, then is heated and rises again.

Theory of Plate Tectonics

The theory that states that Earth's lithosphere is made up of huge plates that move over the surface of the Earth.
One plate cannot move without affecting the others nearby.
The plates can move apart, push together, or scrape past each other.
Plate movements cause most major earthquakes, volcanoes and mountain ranges.

Plate Boundary

Is where the edges of two plates meet.

What are the three types of boundaries?

Divergent boundary
Convergent boundary
Transform boundary

Divergent Boundary

A boundary along which two tectonic plates move apart, characterized by either a mid-ocean ridge or a continental rift valley.
Most divergent boundaries are found in the ocean.

Convergent Boundary

A boundary along which two tectonic plates push together, characterized either by subduction or a continental collision.

Transform Boundary

A boundary along which two tectonic plates scrap past each other and crust is neither formed nor destroyed.

What features are found at divergent boundaries in the ocean?

Mid-ocean ridges
Rift Valleys

Rift Valleys

A deep valley formed as tectonic plates move apart, such as along a mid-ocean ridge.

What are the longest chains of mountains on Earth?

Mid-ocean ridge

What happens to the old crust as new crust is formed?

The old crust is split and pushed aside, forming horizontal cracks. The old crust forms part of the ridge.

What happens in a magnetic reversal?

Earth's magnetic north and south poles switch places.

Magnetic reversal

A switch in the direction of Earth's magnetic field so that the magnetic north pole becomes the magnetic south pole and the magnetic south pole becomes the magnetic north pole.

Where is each magnetic reversal recorded?

In the sea-floor rock

Explain how records of magnetic reversals show that plates move apart

The farther away from the ridge, the older the record of reversals on both sides of the ridge. This shows that the plates move away from the center.

Continental divergent boundaries

Continents spread apart at a divergent boundary.
It forms when hot material rises from deep in the mantle.
This heat causes the crust to bulge upward and the crust crakes as it is stretched and a rift valley forms.

What happens when the floor of a rift valley sinks below sea level?

The valley can be flooded by waters from nearby oceans or rivers, eventually forming a lake or sea.

Hot Spots

An area where a column of hot material rises from deep within a planet's mantle and heats the lithosphere above it, often causing volcanic activity at the surface.

How were the Hawaiian Islands created?

Are being built as the Pacific Plate moves slowly over a hot spot.

How do you know which island or landform was formed first?

The island or landform farthest from the hot spot was formed first. As the plate moves, it carries the islands or landforms away from the hot spot; therefore, the ones first to form are the farthest away.

How does a hot-spot volcano form?

A plume of magma from the mantle melts the rock in the plate above it until a volcano forms.


The process by which an oceanic tectonic plate sinks under another plate into Earth's mantle

Continental-Continental Collision

A boundary along which two plates carrying continental crust push together
Both crusts are the same density; neither plate can sink beneath the other

When does folded crust form mountains?

When it is pushed up high

Explain how colliding plates form mountain ranges

Plates push together until rocks crumple and fold, rising higher and eventually forming mountain ranges

Oceanic-Oceanic Subduction

A boundary along which a plate carrying oceanic crust sinks beneath another plate with oceanic crust
Deep-ocean trenches - these trenches are like deep canyons that form in the ocean floor as a plate sinks
Island Arcs - chains of volcanic islands that form on the top plate, parallel to a deep-ocean trench

Oceanic-Continental Subduction

A boundary along which a plate carrying oceanic crust sinks beneath a plate with continental crust
Deep-ocean trenches - as the oceanic crust moves it causes underwater earthquakes
Coastal mountains - as oceanic crust sinks under a continent, the continent crust buckles to form a range of mountains

Why do deep-ocean trenches form at both types of subduction?

In both types of subduction, one plate sinks beneath another, forming a deep-ocean trench.

Where do two plates move past each other in opposite directions?

At transform boundaries

Where do transform boundaries occur?

Mostly on the sea floor near mid-ocean ridges
On land

Tectonic Plate Boundaries

Transform Boundaries
Divergent Boundaries
Convergent Boundaries

What future events can scientists predict using the theory of plate tectonics?

Sites of future volcanoes
Mountain building

Transform Boundaries

plates scrape horizontally past each other; crust is neither formed nor destroyed

Divergent Boundaries

as plates move apart, new crust is built, forming mid-ocean ridges and rift valleys

Convergent Boundaries

crust is destroyed where plates subduct; it is folded where plates collide

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