Plate Tectonics in a Nutshell

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 don't quite match the continents and the oceans on the Earth's surface. The North American plate, for instance, extends from the west coast of the U.S. and Canada into the middle of the Atlantic Ocean. The Pacific plate includes a chunk of California as well as most of the Pacific Ocean (see the full list of plates). This is because the continents and ocean basins are part of the Earth's crust, and the plates go deeper than the crust. The part of the Earth that makes up the plates is called the lithosphere. It's about 100 kilometers thick, but that varies greatly from place to place. (more on 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 can bend slowly in a plastic way, like a bar of Turkish taffy. In effect, the lithosphere floats on the asthenosphere even though both are solid rock.

The plates are constantly changing position. The lithospheric plates move slowly over the asthenosphere. "Slowly" means slower than fingernails grow, no more than a few centimeters a year. The forces that move them are not fully clear, but the plates certainly move—we've measured their movements directly, 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. (more on 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 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. (more on convergent margins)

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 continually move apart, and thus the plates gain new material. (more on divergent margins)

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. (more on transforms)

Plate tectonics explains a lot of things:

• On the three different types of boundary, plate movement creates distinctive kinds of earthquake faults. (more on fault types)
• Most large mountain ranges are associated with plate convergence, answering a long-standing mystery. (more on mountains)
• Fossil evidence suggests that continents were once connected that are far apart today; plate movements are responsible.
• The world's seafloor is geologically young because old oceanic crust disappears by subduction. (more on subduction)
• Most of the world's volcanoes are related to subduction. (more on arc volcanism)

Plate tectonics also lets us answer new kinds of questions:

• We can build maps of world geography in the geologic past and model ancient climates.
• We can study how mass extinctions are related to effects of plate tectonics such as volcanism. (more on mass extinction)
• We can examine how plate interactions affected the geologic history of a region.

There are several unanswered questions about plate tectonics itself:

• What moves the plates?
• What creates volcanoes in "hotspots" that are outside subduction zones?
• 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? (more on the mantle)
• What happens to subducted plates? (more on 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.

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