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About Plate Tectonics

A starting point for exploring plate tectonics

By Andrew Alden, About.com Guide

See More About:

plate tectonics

lithosphere

crust

mantle

Geologists have an explanation—a scientific theory—of how the Earth's surface
behaves called plate tectonics. Tectonics means large-scale structure. So "plate
tectonics" says that the large-scale structure of the Earth's outer shell is a
set of plates. (see the
map)

Tectonic Plates

Tectonic plates don't quite match the continents and the oceans on the
Earth's surface. The North America plate, for instance, extends from the west
coast of the U.S. and Canada into the middle of the Atlantic Ocean. And the
Pacific plate includes a chunk of California as well as most of the Pacific
Ocean (see the list of
plates). This is because the continents and ocean basins are part of the Earth's
crust. But plates are made of relatively cold and hard rock, and that
extends deeper than the crust into the upper mantle. The part of the Earth that
makes up the plates is called the lithosphere. It averages about 100 kilometers
in thickness, but that varies greatly from place to place. (see About
the Lithosphere)

The lithosphere is solid rock, as rigid and stiff as steel. Beneath it is a
softer, hotter layer of solid rock called the asthenosphere ("es-THEEN-osphere")
that extends down to around 220 kilometers depth. Because it's at red-hot
temperatures the rock of the asthenosphere is weak ("astheno-" means weak in
scientific Greek). It cannot resist slow stress and it bends in a plastic way,
like a bar of Turkish taffy. In effect, the lithosphere floats on the
asthenosphere even though both are solid rock.

Plate Movements

The plates are constantly changing position, moving slowly over the
asthenosphere. "Slowly" means slower than fingernails grow, no more than a few
centimeters a year. We can measure their movements directly by GPS and other
long-distance measuring (geodetic) methods, and geologic evidence shows that
they have moved the same way in the past. Over many millions of years, the
continents have traveled everywhere on the globe. (see Measuring
Plate Motion)

Plates move with respect to each other in three ways: they move together
(converge), they move apart (diverge) or they move past each other. Therefore
plates are commonly said to have three types of edges or boundaries: convergent,
divergent and transform.

In convergence, when the leading edge of a plate meets another plate, one
of them turns downward. That downward motion is called subduction. Subducted
plates move down into and through the asthenosphere and gradually disappear.
(see About
Convergent Zones)

Plates diverge at volcanic zones in the ocean basins, the mid-ocean
ridges. These are long, huge cracks where lava rises from below and freezes
into new lithosphere. The two sides of the crack are continually pulled apart,
and thus the plates gain new material. The north Atlantic island of Iceland is
the foremost example of a divergent zone above sea level. (see About
Divergent Zones)

Where plates move past each other is called a transform boundary. These
are not as common as the other two boundaries. The San Andreas fault of
California is a well-known example. (see About
Transforms)

The basic
cartoon map of the plates uses only these three boundary types. However,
many plate boundaries are not sharp lines but, rather, diffuse zones. They
amount to about 15 percent of the world's total and appear in more
realistic plate maps. Diffuse boundaries in the United States include most
of Alaska and the Basin and Range province in the western states. Most of China
and all of Iran are diffuse boundary zones, too.

What Plate Tectonics Explains

Plate tectonics answers many basic geologic questions:

On the three different types of boundary, plate movement creates
distinctive kinds of earthquake faults. (see Fault
Types in a Nutshell)

Most large mountain ranges are associated with plate convergence,
answering a long-standing mystery. (see The
Mountain Problem)

Fossil evidence suggests that continents were once connected that are far
apart today; where once we explained this by the rise and fall
of land bridges, today we know that plate movements are responsible.

The world's seafloor is geologically young because old oceanic crust
disappears by subduction. (see About
Subduction)

Most of the world's volcanoes are related to subduction. (see About
Arc Volcanism)

Plate tectonics also lets us ask and answer new kinds of questions:

We can build maps of world geography in the geologic past—paleogeographic
maps—and model ancient climates.

We can study how mass extinctions are related to effects of plate
tectonics such as volcanism. (see Extinction: On
the Destiny of Species)

We can examine how plate interactions have affected the geologic history
of a specific region.

Plate Tectonic Questions

Geoscientists are studying several major questions about plate tectonics
itself:

What moves the plates?

What creates volcanoes in "hotspots" like Hawaii that are outside
subduction zones? (see A Hotspot
Alternative)

How rigid are the plates, and how precise are their boundaries?

When did plate tectonics begin, and how?

How is plate tectonics connected to the Earth's mantle below? (see About the
Mantle)

What happens to subducted plates? (see The Death
of Plates)

What kind of cycle do plate materials go through?

Plate tectonics is unique to Earth. But learning about it during the last 40
years has given scientists many theoretical tools to understand other planets,
even those that circle other stars. For the rest of us, plate tectonics is a
simple theory that helps make sense of the Earth's face.

Plate Tectonic Maps

The Basic Plate Map

Plates and Topography, Eastern Hemisphere

Plates and Topography, Western Hemisphere

Plate Tectonic Questions

How Many Plates Are There?

Why Not Dispose of Waste in Ocean Trenches?

The Plate Tectonics Geo-Quiz

Plate Tectonic Frontiers

The Hotspot Controversy

5 Myths of Plate Tectonics

True Polar Wander

Related Articles

Measuring Plate Motion - How We Track Plate Tectonic
Movements

Elementary Educators - Online Project Center - Geophysics
(Huh?) - from Abo...

About Divergent Zones - Plate Tectonic Divergent
Boundaries

Tectonic Plate Boundaries, Western Hemisphere

Tectonic Plate Boundaries, Eastern
Hemisphere

Andrew Alden
Geology Guide

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