How Plate Movements

Create Landforms

By Moira Whitehouse PhD

Understanding how the plates of the Earth move help us understand how some landforms on Earth are built.

The forces of plate movements are constructive forces—forces that build new landforms on the surface of the Earth.

Remember how the crust of the Earth is broken into huge pieces of rock called plates. These plates fit together around the globe like a giant jigsaw puzzle.

The plates move as they float on the hot, melted rock found in the mantle.

Creative CommonsWikipedia Commons

The Earth’s crust consists of the oceanic and the continental crusts, both of which float on the magma (melted rock).

• The continental crust is the layer of rock which forms the continents and those areas of shallow seabed close to the shore. The continental crust is much thicker than the oceanic crust--about 19 miles thick.

• The oceanic crust is the layer of rock which forms the floor of an ocean. It is about 4-7 miles thick.

magma

The oceanic crust is made of mostly basalt, a very dense rock that is much heavier than the granite of the continental crust.

As a result, the oceanic crust sinks deeper into the magma (the molten rock) when the continental and oceanic crust meet.

The continental crust is mainly made of a rock called granite.

magma

This rock is mainly granite.

This rock is mainly basalt.

Wikipedia CommonsFirst of all, notice that most plates have both oceanic and continental crust and that few have only oceanic crust.

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Notice the arrows to see how the plates interact.

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2. apart

3. side by side

1. together

You may have noticed that plates can move in one of three ways:

Image for educational use http//serc.carleton.educ

This map of volcano and earthquake activity mirrors a map of plate boundaries.

Two continental plates colliding

When two continental plates collide, the rock where the two plates meet rises and folds forming jagged mountains.

USGS

These collisions produce Earth’s most spectacular mountain ranges and deepest valleys.

Mountain ranges that were formed in this way include the Himalayas where the Indian Plate is moving into the Eurasian Plate.

The Himalayas are the highest mountains in the world, towering as high as 29,000 feet.

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Wikipedia CommonsHimalayan Mountains from the air

Pictures of folded mountains.

The Himalayas, the Andes in South America, the Alps, the Rocky Mountains and the Appalachian Mountains are all folded mountains.

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When continental plates meet and push up “new” mountains, the land behind the mountain chain often is also up lifted. However, it doesn’t break or fold. As a result a high flat area is formed---a plateau.

The Tibetan Plateau was created when the Indian Plate and Eurasian Plate collided. The Himalayan Mountains formed along the edge of the collision, and the unbroken plateau behind them rose as a “flat table”.

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Continental and oceanic plates

colliding

When an oceanic plate moves into a continental plate, it slides under because it is denser and thus, heavier.

The resulting magma rises and gathers in pools under the continental crust.

The extreme heat and pressure causes the leading edge of the oceanic plate to melt. USGS

First, a deep ocean trench forms where the oceanic plate moves under the continental plate.

Second, when enough magma collects in the pools under the continental plate, and enough pressure develops, a volcano erupts.

As a result:

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Image courtesy of FEMA

An example of an oceanic plate is moving under a continental plate would be on the western coast of South America.

•The Nasca Plate (oceanic plate) is moving under the South American Plate. Result: the Andes Mountains.

Many volcanoes and earthquakes occur in this region.

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Image courtesy of National Geographic

Andes Mountains

Another place, closer to home, where an oceanic plate is moving under a continental plate is on the west coast of United States.

There a small oceanic plate called the Juan de Fuca Plate is subducting (moving under) under the North American Plate.

This subduction is occurring on the coast of Washington state, Oregon and northern California. The Juan de Fuca Plate, what is left of an old oceanic plate, is pushing under the North American Plate.

This subduction results in the building of the Cascade Mountain Range. Well-known volcanoes in this range are Mount St. Helens, Mount Adams and Mount Hood.

Mount St. Helens erupting in 1980Wikipedia commons

Now we will consider where two plates pull apart.

We will look at what happens when two oceanic plates move apart.

Oceanic plates moving apart

When two oceanic plates move apart, magma from the mantle flows upward filling the gap between the two plates. When the lava hits the cold water it solidifies as basalt rock. If this process occurs over a long, long time, a new mountain range is built. This type of mountain chain is called a midoceanic ridge.

Image courtesy of USGS

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Here we see magma building up to form a chain of mountains as two oceanic plates pull apart.

As a result of this process, new oceanic crust is continuously being built.

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This is what is happening in the middle of the Atlantic Ocean. The North American plate and the Eurasian plates are pulling apart in the North Atlantic and the South American plate and the African plate in the South Atlantic.

Magma oozing out of these “pull aparts” over millions and millions of years has built and is still building an underwater mountain range down the middle of the Atlantic Ocean called the Mid-Atlantic Ridge.

http://www.navmetoccom.

Iceland is an example of an island formed by magma that came from between diverging oceanic plates. It sits on top of the Mid-Atlantic Ridge.

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javier.losa's photostream

http://www.flickr.com

Photos from Iceland

Image from NASA

Here we see a chain of mountains being built under all Earth’s oceans where ocean plates move apart.

As the ocean plates move apart, the oceans are getting wider--a few centimeters each year.

As a result, oceans all over the world are increasing in size. However, at the same time, new continents are building at the faults (cracks) that run down the middle of these oceans.

Tectonic plates that move side

by side

Now we will consider where two plates slide side by side:

This type of movement commonly produces earthquakes.

Sometime the touching surfaces get stuck.As the movement of the plates continues, pressure builds up.

When pressure to move is greater than the force holding the surfaces still, a sudden violent thrust occurs.

This is an earthquake.

Earthquakes are common where plates slide by one another.

We have our own transform fault. Along the west coast of North American, the Pacific Plate is sliding past the North America Plate creating a fault called the San Andreas Fault.

In fact, the Pacific Plate is very gradually carrying the western-most part of California northward.

The city of Los Angeles rides on top of the oceanic Pacific plate.

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Let’s look at that image of the plates again.

Here you can see the Pacific Plate moving northeast and the North American Plate sliding southwest creating the San Andreas Fault.

In some parts of California, you can actually see the San Andreas Fault line where the two plates are sliding by one another.

Aerial view of the fault USGS

The land to the west of the San Andreas Fault is slowly moving north. The land to the east of the fault is moving south.

1906 San Francisco earthquake

1994 collapse of Los Angeles overpass

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http://wapedia.mobi/en/Northridge_earthquake

“Hot spot” volcanic activity

Most earthquakes and volcanic eruptions occur where plates move together, apart or side by side.

However, a few volcanoes erupt in the middle of plates.

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Red dots are some of the hotspots found around the world.

For example, the Hawaiian Islands, which are entirely of volcanic origin, have formed in the middle of the Pacific Ocean more than 3,200 km from the nearest plate boundary.

How do the Hawaiian Islands and other volcanoes that form in the interior of plates fit into the plate tectonics picture?

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So as the plate moves on the present volcano becomes extinct and a new one develops above the plume forming new land.

Scientists believe that below the crust in these areas, a hot plume of magma rises from deep within the Earth.

When the plumes breaking through the Earth’s surface a volcano erupts. These plumes are thought to be stationary relative to the plates that move over them.

Source: Maurice Krafft, Centre de Volcanologie, France)

Image courtesy of National Geographic

Mount Kilauea in Hawaii erupting

Now let us review.

Look at each landform shown.

Be ready to show by moving your hands and by telling us how each landform was formed.

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Himalayan Mountains from the air

Mount St. Helens erupting in 1980Wikipedia commons

Image courtesy of National Geographic

Mount Kīlauea volcano erupting in Hawaii.

http://www.arthursclipart.org/

Earthquake

Image from NASA

A chain of mountains being built under all Earth’s oceans.