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 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.
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 points where the edges of three plates meet are called triple junctions. They move across the Earth's surface in response to the different motions of the three plates. (see Triple Junctions)
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.