Seafloor Spreading

This diagram shows the Americas being separated from the Europe and Africa as the seafloor spreads. Continents are grey in color. Youngest seafloor is red and as seafloor gets older it becomes yellow, green and then blue. Click on image for full size version (40K GIF) Image from: NOAA/NESDIS/National Geophysical Data Center, Boulder, CO

This diagram provides evidence of seafloor spreading by showing the ages of ocean floor in the Atlantic and eastern Pacific Oceans. The red colors are the youngest parts of the seafloor, where fresh new crust is formed as lava seeps up from the deep interior of the Earth at spreading ridges. As new crust forms at these spreading ridges, older crust, colored green in the diagram, moves away from the ridge. The blue portions are the oldest regions of the seafloor. They are either next to continents, which also contain very old rocks, or are near areas on Earth where subduction is taking place.

The diagram shows the American continents being separated from the Eurasian and African continents. This is a very fast model of what has happened to the Earth’s plates over the past 250 million years.

Subduction

This is a drawing of a portion of the Earth's crust undergoing subduction. Click on image for full size version (40K GIF) Image copyright 1997 by the American Geophysical Union. Further electronic distribution is not allowed.

When two sections of the Earth's crust collide, one slab of crust can be forced back down into the deeper regions of the Earth, as shown in this diagram. This process is called subduction. The slab that is forced back into the Earth usually undergoes melting when the edges get to a depth which is hot enough. (A temperature hot enough to melt lithosphere is about a thousand degrees!). This process is called "subduction".

Melted crust rises back towards the surface where it helps make volcanoes and islands. Thus the formation of some volcanoes, mountains, and islands is connected to the process of subduction and continental drift.

The melted lithosphere also releases gases of the atmosphere which had become trapped in the ground. Thus subduction of the lithosphere contributes to recycling of the atmosphere!

How Do Plates Move?

This image is a cross section through the Earth showing the convection cells of the mantle. Ridge push happens at spreading centers where plates are moving apart. Slab pull happens at subduction zones where one plate is pulled down into the mantle. Click on image for full size (11K GIF) Windows Original after Northcott

Plates at our planet’s surface move because of the intense heat in the Earth’s core that causes molten rock in the mantle layer to move. It moves in a pattern called a convection cell that forms when warm material rises, cools, and eventually sink down. As the cooled material sinks down, it is warmed and rises again.

Scientists once thought that Earth’s plates just surfed on top of the mantle’s giant convection cells, but now scientists believe that plates help themselves move instead of just surfing along. Just like convection cells, plates have warmer, thinner parts that are more likely to rise, and colder, denser parts that are more likely to sink.

New parts of a plate rise because they are warm and the plate is thin. As hot magma rises to the surface at spreading ridges and forms new crust, the new crust pushes the rest of a plate out of its way. This is called ridge push.

Old parts of a plate are likely to sink down into the mantle at subduction zones because they are colder and thicker than the warm mantle material underneath them. This is called slab pull.

Clues to Plate Movements

This is a drawing of the Earth's lithosphere. Click on image for full size version (40K GIF)

Many kinds of surface features are clues that our lithosphere is sliding.

Two types of features can form when plates move apart. At mid ocean ridges, the bottom of the sea splits apart and new crust is formed from molten rock, or magma, rising from the mantle. Continental rifts form when a continent begins to split apart. If a continental rift continues to split a continent apart it can eventually form an ocean basin.

When two plates move towards each other, several features can form. Often, one of the plates is forced to go down into the hot asthenosphere at a subduction zone. Volcanoes may form when a subducted plate melts and the molten rock comes to the surface. If neither plate is subducted, the two crash into each other forming huge mountains.

If these features are found on a planet's surface, they provide evidence that the planet's surface is in motion. The sliding lithosphere makes Earth special because there are only a few other planets that have a surface in motion.

What causes a Planet's surface to change its appearance?

This is an image of the Martian dunefields. Click on image for full size version (40K GIF) Image from: NASA

Over the course of time there are many things which can cause the surface of a planet to change its appearance.

• winds, as shown in the example from the Martian surface
  • Monument Valley on Earth is an example
• weather & water, which cause erosion
• volcanism, which pours out a new surface
  • The Moon is an example.
• continental drift
• slow forces of deformation like those which cause mountains to form.
  • The crust of Ganymede is an example.
• slumping of craters, mountains and volcanoes.
  • The surface of Callisto is an example.

In their earliest histories, every planet & moon was bombarded with the remains of the material which formed them. If a planet's surface does not show many craters, it means that the surface is new, and the planet has been resurfaced, perhaps by one of the processes above. If the planet's surface still shows the many craters left over from it's formation, then that surface is very old, and has not been changed by any activity.

Plate Tectonics

Earth's tectonic plates. Plate boundaries are shown in red. Learn more about the geologic features related to Earth's tectonic plates at This Dynamic Planet Modified from USGS

The main force that shapes our planet’s surface over long amounts of time is the movement of Earth's outer layer by the process of plate tectonics.

This picture shows how the rigid outer layer of the Earth, called the lithosphere, is made of plates which fit together like a jigsaw puzzle. These plates are made of rock, but the rock is, in general, lightweight compared with the denser, fluid layer underneath. This allows the plates to "float" on top of the denser material.

Movements deep within the Earth, which carry heat from the hot interior to the cooler surface, cause the plates to move very slowly on the surface, about 2 inches per year. There are several different hypotheses to explain exactly how these motions allow plates to move.

Interesting things happen at the edges of plates. Subduction zones form when plates crash into each other, spreading ridges form when plates pull away from each other, and large faults form when plates slide past each other.

What Is an Earthquake?

This diagram shows an earthquake along a fault. The focus of the earthquake is where the energy is released underground. The epicenter is the spot on the Earth’s surface directly above the focus.
USGS

The expression “on solid ground” is often used to describe something as stable. Usually the solid ground underfoot seems very stable. But sometimes it is not.

"The ground seemed to twist under us like a top while it jerked this way and that, and up and down and every way," wrote a person describing the experience of being in the large 1906 earthquake in San Francisco, CA.

Earthquakes happen as large blocks of the Earth’s crust move suddenly past one another because of the force of plate tectonics. These blocks of the Earth’s crust meet at cracks called faults. Sometimes those pieces do not slide smoothly past one another. There can be friction along the fault – jagged edges that snag the blocks of rock. This makes it difficult for them to move past each other. Sometimes they get stuck together temporarily. When the pieces of rock overcome the snags, energy is released. The release of energy causes shaking at the ground surface.

The location inside the Earth where an earthquake begins is called the focus. The point at the Earth’s surface directly above the focus is called the epicenter. The strongest shaking happens at the epicenter.

Each year, more than a million earthquakes occur worldwide. Most of these are so small that people do not feel the shaking. But some are large enough that people feel them, and a few of those are so large that they cause significant damage.

Earthquakes can cause damage to things like buildings, bridges, and roads. Earthquakes can cause landslides and mudslides, too. If a large earthquake happens under the ocean it can cause a tsunami – a giant ocean wave or series of waves.

Scientists can figure out whether an earthquake is likely to happen in a place by studying plate tectonics, the faults underground, and the history of the area’s earthquakes. However, unlike weather events, earthquakes can not be forecast ahead of time.

Surface Features of the Earth

This image shows the topography, or shape, of the Earth's surface, on land and below the oceans. Mountain ranges, subduction trenches, tectonic plates, and mid-ocean ridges are all visible in the image. Click on image for full size version with these features identified.(637K GIF) Image from: U.S. Geological Survey

This image of the surface of the Earth shows the North and South American continents, as well as the floor of the Pacific Ocean. As can be seen in the image, the ocean floor shows evidence of

• mid-ocean spreading ridges
• individual volcanic island formation.
• subduction

The continental regions show evidence of

• volcanoes (a portion of the "Pacific Rim of Fire")
• non-volcanic mountain ranges
• island arcs
• faulting.

(Click on the image to see labeled examples of these features.)

These things provide evidence that, unlike other planets, the surface of the Earth is in motion. Motion of the Earth's surface is called plate tectonics.

Compare this active surface with that of Venus, Mars, or Europa.

The Earth's Crust, Lithosphere and Asthenosphere

This drawing shows the Earth's lithosphere (crust and upper mantle) on top of the asthenosphere. Click on image for full size version (40K GIF)

Crust, the upper layer of the Earth, is not always the same. Crust under the oceans is only about 5 km thick while continental crust can be up to 65 km thick. Also, ocean crust is made of denser minerals than continental crust.

The tectonic plates are made up of Earth’s crust and the upper part of the mantle layer underneath. Together the crust and upper mantle are called the lithosphere and they extend about 80 km deep. The lithosphere is broken into giant plates that fit around the globe like puzzle pieces. These puzzle pieces move a little bit each year as they slide on top of a somewhat fluid part of the mantle called the asthenosphere. All this moving rock can cause earthquakes.

The asthenosphere is ductile and can be pushed and deformed like silly putty in response to the warmth of the Earth. These rocks actually flow, moving in response to the stresses placed upon them by the churning motions of the deep interior of the Earth. The flowing asthenosphere carries the lithosphere of the Earth, including the continents, on its back.

Surface and Interior of Earth

Click on image for full size version (378K GIF) Image courtesy of NASA

Earth, the largest and densest rocky planet, was formed about 4.5 billion years ago. The Earth's interior is divided into four layers, which is typical of rocky planets. Each layer has different characteristics and is made of different elements and minerals.

There are many different types of features on Earth’s surface due to the complexity of our planet. The surface is unique from the other planets because it is the only one which has liquid water in such large quantities. Water forms some features of Earth's surface such as rivers, oceans, beaches and lakes. Other surface features, such as mountains, earthquakes and volcanoes, are formed when large pieces of the Earth’s outer layer move slowly by plate tectonics.

Structure of the Interior of Earth

Click on image for full size version (64K GIF)

Earth has a diameter of 12,756 km (7,972 mi). The Earth's interior consists of rock and metal. It is made up of four main layers:
1) the inner core: a solid metal core made up of nickel and iron (1200 km diameter)
2) the outer core: a liquid molten core of nickel and iron
3) the mantle: dense and mostly solid silicate rock
4) the crust: thin silicate rock material

The temperature in the core is hotter than the Sun's surface. This intense heat from the inner core causes material in the outer core and mantle to move around.

The movement of material deep within the Earth may cause large plates made of the crust and upper mantle to move slowly over the Earth’s surface. It is also possible that the movements generate the Earth's magnetic field, called the magnetosphere.

Surface of the Earth

Click on image for full size version (383K GIF) Image from: Rick Kohrs, Space Science and Engineering Center, University of Wisconsin, Madison

Most of the Earth's surface (70%) is covered with water, and the remaining 30% is taken up by the seven continental landmasses. However, underneath the water that fills the oceans, and the dirt and plants that cover the continents, the Earth’s surface layer is made of rock. This outer layer formed a hard, rocky crust as lava at the surface cooled 4.5 billion years ago.

The crust is broken into many large plates that move slowly relative to each other. Mountain ranges form when two plates collide and their edges are forced up. In addition, many other surface features are the result of the moving plates. The plates move about one inch per year, so millions of years ago the continents and the oceans were in different positions. About 250 million years ago, most of the land was connected together, and ov

This image shows the divisions of the Earth's interior. Click on the image to see labels. (Adapted from Beatty, 1990)

er time has separated into seven continents.