The Earth’s Plates
Most earthquakes and volcanoes are caused by large scale movements of the Earth’s plates and occur at the boundaries between the plates. Earth scientists recognize seven to twelve major plates and a number of smaller ones. The plates take their names from continents (the North American Plate), from oceans (Pacific Plate), or from geographic areas (the Arabian Plate).
The plates are in very slow but constant motion, so if seen from above, the Earth’s surface might look like a slowly moving, spherical jigsaw puzzle. The plates move at a rate of 5 to 15 centimeters (or several inches) a year, or about as fast as our
fingernails grow. A faster moving plate will cause a greater geologic impact than a plate that moves at a lesser rate of speed. From a human point of view, this is a rate of movement only the most sophisticated instrumentation can detect. On the scale of geologic time, it is a dizzying speed. The three-billion-year-old rocks scientists have dated could have traveled all the way around the world at this rate.
The result of this slow, steady motion is the opening and closing of ocean basins. At the present time, the Atlantic ocean is opening, causing the North American continent to move slowly Westward, thereby closing the Pacific Ocean. Over the past several million years, the Mediterranean basin has opened and closed as the African and European plates moved toward each other. It is this motion that causes the numerous earthquakes and related volcanoes of the Mediterranean region.
Three Kinds of Plate Movements
The movement of plates relative to one another is generally a result of spreading, colliding, or sliding. When plates are spreading, or separating from each other, scientists call this movement divergent. When they are colliding or pushing each other, scientists call the movement convergent. Movement in which plates slide past each other is called lateral or transform plate movement. Earthquakes can accompany each of the three types of movement, but the most forceful quakes result from convergent and lateral movement. Volcanoes are normally associated with divergent and convergent types of movement.
Continental Drift - 1910 to 1960
The theory of plate tectonics originated early in this century, although it did not gain general acceptance until the late 1960s. The German meteorologist, geophysicist, and explorer, Alfred L. Wegener, is now given credit for the first step in understanding the movement of the lithosphere. Between 1910 and 1912, he formulated the theory called Continental Drift and collected evidence from the rocks, fossils, and climate of various continents that had once been joined together. Wegener had little data on the oceanic crust and thought the continents merely moved through that crust.
Plate Tectonics - 1960 to Present
In the early 1960s, oceanographers Fred Vine and Drummond Matthews, using magnetic data from the seafloor, demonstrated the ocean floor was spreading apart at the mid-ocean ridges. They named the process seafloor spreading. As study continued, it was soon realized the continents were also moving. By 1968 a new explanation for the dynamics of the Earth’s surface had been devised. It is now referred to as plate tectonics.
The discovery of plate tectonics has enabled scientists to understand earthquakes and volcanoes better and have greater respect for the forces that are shaping our Earth. Perhaps one day researchers will be able to predict earthquakes and volcanoes better, thus lessening the loss of life and property associated with these events.
