Today, and every day, somewhere on Earth, an earthquake or volcano is causing destruction. An average of 2,000 strong quakes and large eruptions occur every year all around the world. Throughout history, people have been all too familiar with the deadly power of earthquakes and eruptions such as these. The fear and danger these events inspire is heightened by their seemingly unpredictable and random nature.
But in the last few decades, scientists have been able to track these geologic activities, and they have discovered that their occurrence is, in fact, quite orderly. Both earthquakes and volcanoes are largely restricted to narrow, overlapping zones. This observation helped lead to the development of the grand unifying theory of earth science, the theory of plate tectonics.
Tectonics theory states that the surface of the earth is not fixed and eternal, but is in constant, ever-changing motion. Mountains rise and fall, oceans open and close, and life flourishes because of tectonic activity. Put simply, plate tectonics explains how the world works.
The tectonic forces that sculpt the surface of the earth originate deep within the planet. There are three internal layers—a central core, a thick mantle, and a very thin outer crust. Tectonic activity is concentrated in the upper 700 kilometers of the planet, in the uppermost mantle and the crust.
The outer mantle and the crust are fused together into a rigid, rocky layer, about 100 kilometers thick, called the lithosphere. The lithosphere encases the entire Earth, but it is broken into pieces, or plates, that butt up against one another.
Below the plates is the asthenosphere, a solid but soft layer of the upper mantle. The asthenosphere is squishy—it can deform and flow, though extremely slowly—so that the heavy plates literally float on top of it. Heat from the core of the earth rises into the asthenosphere and stirs it into sluggish motion. As it circulates, the plates above it begin to move.
Because there are no open spaces on the surface of the earth, as soon as the plates begin to move, they begin to interact with one another. Although plate motion is slow—averaging about the same speed that fingernails grow—it is relentless. Wherever plates touch, tremendous stresses are built up and released. Plate boundaries are home to the most violent and spectacular geologic activity on the planet.
There are three types of boundaries—plates can pull apart, they can collide, or they can slide past one another. Each type of boundary generates distinct geologic processes and landforms. Where plates pull apart, or diverge, a narrow rift valley forms as the crust splits open. Lava spews from long fissures and geysers spurt superheated water. Frequent earthquakes strike along the rift.
Beneath the rift, magma—molten rock—rises from the mantle. It oozes up into the gap and hardens into solid rock, forming new crust on the torn edges of the plates. Magma from the mantle solidifies into basalt, a dark, dense rock that underlies the ocean floor. Thus at divergent boundaries, oceanic crust, made of basalt, is created.
In contrast with divergent boundaries where plates pull apart, at convergent boundaries two plates collide with one another. The impact buckles the edge of one or both plates up into a rugged mountain range, and sometimes bends the other down into a deep seafloor trench. A chain of volcanoes often forms parallel to the boundary, to the mountain range, and to the trench. Powerful earthquakes shake a wide area on both sides of the boundary.
If one of the colliding plates is topped with oceanic crust, it is forced down into the mantle where it begins to melt. Magma rises into and through the other plate, solidifying into new crust. Magma formed from melting plates solidifies into granite, a light colored, low-density rock that makes up the continents. Thus at convergent boundaries, continental crust, made of granite, is created, and oceanic crust is destroyed.
The third type of plate boundary, where plates slide past one another, is called transform. Natural or manmade structures that cross the boundary are offset—split into pieces and carried in opposite directions. Rocks that line the boundary are pulverized as the plates grind along, creating a linear fault valley or undersea canyon. As the plates alternately jam and jump against each other, earthquakes rattle through a wide boundary zone.
In contrast to convergent and divergent boundaries, no magma is formed. Thus, crust is cracked and broken at transform margins, but is not created or destroyed.
The word tectonics is derived from the Greek word for carpenter, chosen because the action of plate tectonics over the long ages of the Earth has built and shaped the world around us. It is impossible to overstate the importance of plate tectonics. Earth’s atmosphere and surface water come from the volcanoes that line plate boundaries.
Tectonics creates the deep basins that hold the world’s oceans, and the dry land we live on. The eternal recycling of rocks from the mantle to the crust and back to the mantle replenishes the surface with the nutrients needed for life and the minerals and metals that support our civilization.
Nearly every feature and every natural event on Earth is influenced in some way by plate tectonics. To understand plate tectonics is to begin to understand the Earth.