Plate Tectonics: The Mechanism
The main features of plate tectonics are:
Advances in sonic depth recording during World War II and the subsequent development of the nuclear resonance type magnometer (proton-precession magnometer) led to detailed mapping of the ocean floor and with it came many observation that led scientists like Howard Hess and R. Deitz to revive Holmes' convection theory. Hess and Deitz modified the theory considerably and called the new theory "Sea-floor Spreading". Among the seafloor features that supported the sea-floor spreading hypothesis were: mid-oceanic ridges, deep sea trenches, island arcs, geomagnetic patterns, and fault patterns.
The mid-oceanic ridges rise 3000 meters from the ocean floor and are more than 2000 kilometers wide surpassing the Himalayas in size. The mapping of the seafloor also revealed that these huge underwater mountain ranges have a deep trench which bisects the length of the ridges and in places is more than 2000 meters deep. Research into the heat flow from the ocean floor during the early 1960s revealed that the greatest heat flow was centered at the crests of these mid-oceanic ridges. Seismic studies show that the mid-oceanic ridges experience an elevated number of earthquakes. All these observations indicate intense geological activity at the mid-oceanic ridges.
Occasionally, at random intervals, the Earth's magnetic field reverses. New rock formed from magma records the orientation of Earth's magnetic field at the time the magma cools. Study of the sea floor with magnometers revealed "stripes" of alternating magnetization parallel to the mid-oceanic ridges. This is evidence for continuous formation of new rock at the ridges. As more rock forms, older rock is pushed farther away from the ridge, producing symmetrical stripes to either side of the ridge. In the diagram to the right, the dark stripes represent ocean floor generated during "reversed" polar orientation and the lighter stripes represent the polar orientation we have today. Notice that the patterns on either side of the line representing the mid-oceanic ridge are mirror images of one another. The shaded stripes also represent older and older rock as they move away from the mid-oceanic ridge. Geologists have determined that rocks found in different parts of the planet with similar ages have the same magnetic characteristics.
Deep Sea Trenches
The deepest waters are found in oceanic trenches, which plunge as deep as 35,000 feet below the ocean surface. These trenches are usually long and narrow, and run parallel to and near the oceans margins. They are often associated with and parallel to large continental mountain ranges. There is also an observed parallel association of trenches and island arcs. Like the mid-oceanic ridges, the trenches are seismically active, but unlike the ridges they have low levels of heat flow. Scientists also began to realize that the youngest regions of the ocean floor were along the mid-oceanic ridges, and that the age of the ocean floor increased as the distance from the ridges increased. In addition, it has been determined that the oldest seafloor often ends in the deep-sea trenches.
Chains of islands are found throughout the oceans and especially in the western Pacific margins; the Aleutians, Kuriles, Japan, Ryukus, Philippines, Marianas, Indonesia, Solomons, New Hebrides, and the Tongas, are some examples.. These "Island arcs" are usually situated along deep sea trenches and are situated on the continental side of the trench.
These observations, along with many other studies of our planet, support the theory that underneath the Earth's crust (the lithosphere: a solid array of plates) is a malleable layer of heated rock known as the asthenosphere which is heated by radioactive decay of elements such as Uranium, Thorium, and Potassium. Because the radioactive source of heat is deep within the mantle, the fluid asthenosphere circulates as convection currents underneath the solid lithosphere. This heated layer is the source of lava we see in volcanos, the source of heat that drives hot springs and geysers, and the source of raw material which pushes up the mid-oceanic ridges and forms new ocean floor. Magma continuously wells upwards at the mid-oceanic ridges (arrows) producing currents of magma flowing in opposite directions and thus generating the forces that pull the sea floor apart at the mid-oceanic ridges. As the ocean floor is spread apart cracks appear in the middle of the ridges allowing molten magma to surface through the cracks to form the newest ocean floor. As the ocean floor moves away from the mid-oceanic ridge it will eventually come into contact with a continental plate and will be subducted underneath the continent. Finally, the lithosphere will be driven back into the asthenosphere where it returns to a heated state.