Tectonic Plates

According to the generally accepted plate-tectonics theory, scientists believe that Earth's surface is broken into a number of shifting slabs or plates, which average about 50 miles in thickness. These plates move relative to one another above a hotter, deeper, more mobile zone at average rates as great as a few inches per year.

Most of the world's active volcanoes are located along or near the boundaries between shifting plates and are called plate-boundary volcanoes. However, some active volcanoes are not associated with plate boundaries, and many of these so-called intra-plate volcanoes form roughly linear chains in the interior of some oceanic plates.

There are eight major tectonic plates and several more minor ones. These plates are known as Lithosphere Plates - continental and oceanic. The word lithosphere is the scientific word for the Earth's crust.

Each plate moves at a different speed and in different directions. This results in different types of boundaries between the plates.

When two oceanic plates collide one plate goes on top of - subducts - the other. The lower plate begins to melt. This melted oceanic crust will then rise to the surface as volcanic material. This material will eventualy build up to a height that extends above sea level. These volcanic peaks form a chain of islands parallel to the plate boundary. This island chain is known as an Island Arc. Aside from volcanic activity, the area around an island arc experiences a lot of seimic activity. This activity is the result of the movement of the one plate past the other and the related volcanic activity in the area. Island arcs can be found in the Carribean and the South Pacific.

Click here for more information and images.

Plate Tectonics, the Cause of Earthquakes - From NASA

The plates consist of an outer layer of the Earth, the lithosphere, which is cool enough to behave as a more or less rigid shell. Occasionally the hot asthenosphere of the Earth finds a weak place in the lithosphere to rise buoyantly as a plume, or hotspot. The satellite image below shows the volcanic islands of the Galapagos hotspot.

Only lithosphere has the strength and the brittle behavior to fracture in an earthquake.

The map below locates earthquakes around the globe. They are not evenly distributed; the boundaries between the plates grind against each other, producing most earthquakes. So the lines of earthquakes help define the plates:

In cross section, the Earth releases its internal heat by convecting, or boiling much like a pot of pudding on the stove. Hot asthenospheric mantle rises to the surface and spreads laterally, transporting oceans and continents as on a slow conveyor belt. The speed of this motion is a few centimeters per year, about as fast as your fingernails grow. The new lithosphere, created at the ocean spreading centers, cools as it ages and eventually becomes dense enough to sink back into the mantle. The subducted crust releases water to form volcanic island chains above, and after a few hundred million years will be heated and recycled back to the spreading centers.

The map below of Earth's solid surface shows many of the features caused by plate tectonics. The oceanic ridges are the asthenospheric spreading centers, creating new oceanic crust. Subduction zones appear as deep oceanic trenches. Most of the continental mountain belts occur where plates are pressing against one another. The white squares locate examples given here of the different tectonic and earthquake environments.

There are three main plate tectonic environments: extensional, transform, and compressional. Plate boundaries in different localities are subject to different inter-plate stresses, producing these three types of earthquakes. Each type has its own special hazards.

Sometimes continental sections of plates collide; both are too light for subduction to occur. This causes land areas to rise.

The satellite image above shows the bent and rippled rock layers of the Zagros Mountains in southern Iran, where the Arabian plate is impacting the Iranian plate.