Plate tectonics is the unifying theory of geology. This theory explains how crustal plates move around the surface of the earth, and it allows geologists to find explanations for geologic events such as earthquakes and volcanoes, as well as the many other processes that form, transform, and destroy rocks.
The crust of the earth is separated into ten major plates and a few smaller ones. These plates interact with each other along their edges as they shift position on the earth's surface. The motion of crustal plates is described as the relative motion between two plates where they touch; this motion fits into one of three categories:
Convergent: Where two plates are moving toward one another, they form a convergent plate boundary.
Divergent: Where two plates are moving away from each other, they form a divergent plate boundary.
Transform: Where two plates are moving alongside one another, they form a transform boundary.
Early in the twentieth century, a scientist named Alfred Wegener proposed that the continents had once been attached to one another, forming a single large land mass or supercontinent, and had then drifted apart.
While he had some good evidence to support his hypothesis, it wasn't until after World War I that scientists made progress in developing a solid theory of plate movement. The use of submarines in WWI prompted extensive mapping and study of the ocean floor. Through these studies, scientists discovered that the rocks on the floor of the Atlantic Ocean were of different ages — and the ages could be traced from oldest (nearest the continents) to youngest (along a ridge down the middle of the ocean). What they had discovered was that new ocean floor is created along a divergent plate boundary in the middle of the Atlantic Ocean.
This finding in the Atlantic Ocean provided new energy to supporters of Wegener's earlier hypothesis and led the way to decades of intensive undersea geological research. Only fairly recently — in the 1960s — did researchers have enough evidence to propose an explanation for how crustal plates move around the earth's surface and interact with one another: the theory of plate tectonics.
