Tectonic plates do not randomly drift or wander about the Earth's surface; definite, yet unseen, forces drive them. Scientists believe that the relatively shallow forces driving lithospheric plates are also working with forces that originate much deeper in the Earth.
From seismic and other geographical evidence and laboratory experiments, scientists generally agree with Harry Hess's theory that the plate-driving force is the slow movement of the hot, softened mantle that lies below the rigid plates. Scientists also accept that the circular motion of the mantle carries the continents along, much like a conveyor belt. As John Tuzo Wilson stated in 1968, "The Earth, instead of appearing as an inert statue, is a living mobile thing." Both the Earth's surface and interior are in motion.
Below the lithospheric plates, the mantle is partially molten and can slowly flow in response to steady forces applied for long periods of time. When solid rock in the Earth's mantle is subjected to heat and pressure in the Earth's interior over millions of years, it can be softened and molded to different shapes.
Within the mantle, the movement is as a convection cell in a circular motion, similar to heating a pot of thick soup to boiling. The heated molten mantle material rises toward the Earth's crust, spreads, and begins to cool, and then sinks back toward the Earth's core, where it is reheated, rises again, and repeats the process. Convection in the Earth is very slow. In order for convection to occur, there must be a source of heat. Heat within the Earth comes from two main sources: radioactive decay and residual heat.
Radioactive decay is a spontaneous process that involves the loss of particles from the nucleus of an isotope (the parent) to form an isotope of a new element (the daughter). An isotope is one variation of an element, different from other variations by its unique number of neutrons. Radioactive decay of naturally occurring chemical elements (such as uranium, potassium, and thorium) releases energy in the form of heat, which slowly migrates toward the Earth's surface. Residual heat is gravitational energy left over from the formation of the Earth 4.6 billion years ago.
How and why the escape of interior heat becomes concentrated in certain regions to form convection cells still remains largely a mystery. Scientists do believe that plate subduction plays a more important role than seafloor spreading in shaping the Earth's surface features and causing the plates to move. The gravity-controlled sinking of a cold, denser oceanic slab into the subduction zone, dragging the rest of the plate along with it, is considered to be the driving force of plate tectonics.
Forces deep within the Earth's interior drive plate motion. Because these powerful forces are buried so deeply, no mechanism can be tested directly and proven beyond a doubt. The fact that the tectonic plates have moved in the past and are still moving today is certain, but the details of why and how they move will continue to challenge scientists in the future.
As the plates move, they create characteristic landforms on the Earth's surface.
Because the Earth's plates have been in motion for millions of years, they have moved hundreds of miles. Seafloor spreading over the past 100 to 200 million years has caused the Atlantic Ocean to grow from a tiny inlet of water between the continents of Europe, Africa, and the Americas into the vast ocean of today.
Oceanic trenches are the deepest parts of the ocean floor. One of the most famous trenches is the Mariana Trench. The faster-moving Pacific plate converges against the slower-moving Philippine plate. The Challenger Deep, at the southern end of the Mariana Trench, is the deepest part of the ocean and plunges deeper into the Earth's interior—36,000 feet (11,000 m)—than Mount Everest, the world's tallest mountain, rises aboveground.
Oceanic-oceanic plate convergence also results in the formation of volcanoes. Over millions of years, the erupted lava builds up on the ocean floor until the submarine volcano rises above sea level to become an island volcano. Earthquakes are common in these areas as well. The Pacific Ring of Fire, a string of active volcanoes around the Pacific basin, is the world's most important example of oceanic-oceanic convergence.
Not all plate boundaries are as simple as the three types mentioned above. In some regions, the boundaries are not well defined because the plate movement deformation extends over a broad belt—called a plate boundary zone. These areas typically have larger plates with several smaller fragments of plates, called microplates, involved. One example of this is the Mediterranean-Alpine boundary, which involves two major plates and several microplates. Another microplate is the Juan Fernandez at the Pacific-Nazca-Antarctic junction.
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