Plate Tectonics

Plate Tectonics Causes Many Natural Disasters

Earth changes over many time periods.

An earthquake can be instantaneous,

a tornado, minutes,

a hurricane, days,

a flood, weeks,

a drought, years,

greenhouse warming, decades, centuries,

plate tectonics, millions of years.

A natural disaster is a natural event that causes considerable damage or loss of life. In the Natural Disasters & Health simulation you investigate injuries caused by six different natural disasters – some you may have already studied in school, seen on the news, or experienced.

This lesson reviews why and how these disasters happen.

Learning Objectives

To understand how plate tectonics cause natural disasters, you should be able to:

Describe three layers of the Earth.
Compare and contrast three movements at plate boundaries.
Describe what happens to earth’s crust at three plate boundaries.
Explain how natural disasters can be caused by plate tectonics.

Vocabulary

asthenosphere—the upper layer of the earth’s mantle that flows easily.

convection current cycle—circulating masses of molten material driven by temperature difference; hot material rises, cool material sinks.

convergent plate boundary—where two plates move toward each other; example-subduction zones.

core—the inner layer of Earth.

inner core—the inner, solid part of the core.

outer core—the outer, liquid part of the core.

crust—the outer rocky surface of the Earth; the plains, mountains, deserts, ice caps and the floor beneath the oceans.

divergent plate boundary—where two plates move apart from each other; example-sea-floor spreading zones, the Mid-Atlantic Ridge.

earthquake—a sudden, violent shaking of the ground caused by movements within the earth’s crust or by volcanic action.

fracture—break.

lithosphere—the crust and the upper part of the mantle.

magma—molten material inside the earth.

mantle—the layer of earth beneath the crust.

Mid-Atlantic Ridge—an underwater mountain system formed by magma erupting through a divergent plate boundary.

plate boundaries—the edges of plates where they meet each other.

plate tectonics—the theory that says the Earth’s crust is made up of plates moving across Earth’s surface.

San Andres fault—a transform plate boundary in California where the Pacific Plate sides northwest past the continent of North America.

sea-floor spreading—movement of crust and formation of new crust caused by the upwelling of magma at mid-ocean ridges.

subduction zone—a place where an oceanic plate and a continental plate collide causing the less dense plate to sink under the more dense plate into the mantle.

transform plate boundary—where two plates scrape past each other; example-San Andreas Fault.

volcano—an opening in the earth’s crust through which lava, steam and ash flow; a mountain formed by volcanic activity.

Plate Tectonics Causes Earthquakes & Eruptions

Let’s look at plate tectonics to understand how it causes eruptions and earthquakes. The diagram below of a sliced-open Earth shows our planet’s three main layers – crust, mantle and core - all involved in plate tectonics.

The crust is the rocky surface of the Earth with plains, mountains, deserts, ice caps and the floor beneath the oceans. The crust contains soils and plants, animals and every kind of life. The crust is made of hard rocks.

Beneath the crust is the 2,900 kilometer (km) thick mantle, which is hotter than the crust. Mantle rocks don’t break but slowly flow, which allows plates to move across Earth’s surface. This movement is plate tectonics.

Plates are made up of the crust and the uppermost part of the mantle (together called the lithosphere). The lithosphere is thin, only 10 kilometers (km) thick under oceans and up to 100 km thick or more under the continents.

Just under the lithosphere is a 100-250 km thick mantle layer that easily flows, the asthenosphere. As plates move, they fracture (break), creating earthquakes. Deep fractures open pathways for magma (melted rock) from the mantle to rise up to the surface, producing volcanic eruptions. Earthquakes can’t happen in most of the mantle because its rocks are too hot and soft to fracture; they just slowly flow like taffy being pulled apart.

Below the mantle is Earth’s core, which has two parts. The outer core is liquid – a liquid of melted rocks that flow around the inner core which is solid. Heat moves from the core into the mantle, adding enough energy to cause blobs of mantle rocks to rise towards the top of the asthenosphere. What a strange planet we live on!

We have talked about the thickness of the plates, but how wide are plates? Most volcanoes and earthquakes occur along plate boundaries so a map of volcanoes and earthquakes defines plate edges shown as dark lines on the map below. There are seven big plates and several small ones that rub against each other as they all move. On the map, you can see that most plates include both continents and ocean floors. Plates are amazing structures - they are slabs of rock hundreds to thousands of kilometers wide but only 10 to 100 km thick. Plates are more like DVDs than dinner plates. And the crust is so thin it is like the skin on an apple.

Image: Map from USGS

Plates move against each other in only three ways.

The red arrows on the world map show the direction each plate moves. There are just 3 main ways the plates interact: they break apart and new ocean floor is created (divergent), they collide and eliminate ocean crust (convergent), and they scrape past each other (transform fault).

Image: USGS

A Divergent Plate Boundary

On the world map, look at the middle of the Atlantic Ocean where the Eurasian and African plates are moving eastward. On the other side of the plate boundary (the dark line) the North American and South American plates move to the west. These Atlantic Ocean-centered plates move away from a long chain of underwater mountains – the Mid-Atlantic Ridge - that run from the Arctic to the Antarctic. This is the longest mountain range on Earth, and it is a chain of volcanoes, created where magma erupted onto the Earth’s surface.

Did you know? The whole Earth is involved in plate tectonics, which is how the entire planet loses heat. Over the next few billion years the Earth will cool down. Without adequate heat, plate tectonics will stop and so will earthquakes and volcanic eruptions.

As the magma flows up, it pushes or spreads the plates apart. This is called a divergent plate boundary. This is where new lithosphere is created, which shoves the older material out of its way. Plates move across the Earth at speeds of 2-4 cm/yr – the same rate that your fingernails grow!

A Convergent Plate Boundary

Now look on the map at the Pacific Plate. It forms at a long chain of underwater mountains called the East-Pacific Rise. Red arrows show that the newly created ocean floor spreads from the rise and moves westward. Along the western side of the Pacific plate, the red arrows point together, so the Pacific Plate and the plates next to it are colliding with each other! Where plates collide is a convergent plate boundary, for the plates come together or converge.

When two plates collide they become crumpled as they squeeze together building mountains and one slides below the other. The Pacific plate dives down under the edge of the Australian plate and into the mantle. This is a subduction zone. Just think – the new ocean floor crust created at the East Pacific Rise travels thousands of kilometers and then disappears. The Earth isn’t getting larger, it is recycling material! Crust is destroyed at a convergent boundary.

A Transform Fault Boundary

The third way plates interact is by sliding sideways. Look on the map where the Pacific plate scrapes against the California edge of the North American plate.

The arrows move in opposite directions; on the ocean side the arrow points north, and on the land side the arrow points south. The San Andreas fault is formed by scraping together of the two lates. That is called a transform plate boundary. Everyone who lives in California has felt the San Andreas fault move as an earthquake shakes everything. One of the most famous earthquakes in history occurred along this fault in 1906 when most San Francisco buildings were knocked down, burned in a huge fire, and 700 people died.

The Convection Current Cycle

What causes plates to move?

Two main forces cause plates to move.

One is heat energy and the other is gravity.

Scientists know that Earth’s core is much

hotter than the mantle and the crust. The

core provides heat energy to the lower Image: Wikipedia modified by C. Wood

mantle, causing large blobs of hot molten

mantle material to rise to the top. The material spreads sideways away from the spreading center. Tectonic plates are carried away from the spreading centers by riding on top of the flowing mantle. The cycle of rising blobs of hot mantle, its split into two at spreading centers and then flowing back down into the mantle at subduction zones is called a convection current cycle. When ocean plates collide with another plate they are pulled downward (subducted) because they are cold and dense.

Plates move because all three of these process act on them:

· Sea-Floor Spreading creates new crust that pushes ocean floor plates apart.

· Convection Currents carry the plates away from the spreading center.

· Subduction of the plate at convergent margins pulls the entire plate down into the mantle.

Gravity plays the most important role in plate movement. The new lava erupted at the spreading ridge builds 2 km mountains. The new plate is pulled downhill by gravity from the spreading ridge mountains. The new ocean floor plate cools and becomes heavier as it travels away from the ridge. At some point, the moving ocean floor collides with another plate and dives under it.

As gravity pulls the sinking plate down through the upper mantle, it pulls in the entire slab of ocean floor behind it. This slab pull constantly weakens the crust at the spreading ridge, making it easier for rising magma to break through to the surface as an eruption.

Plate tectonics – the moving of

lithosphere plates across Earth’s

surface – has been going on for at

least 600 million years, with plates

breaking apart, roaming across the

surface, and colliding to form new

continents and plates. Look at the

diagrams that show what the Earth

looked like at different times in the

past due to plate movement.

Today the continents are mostly

spread apart, but they are still