||Plate Tectonics & Heat Flow
What causes the plates to move? It turns out to be a consequence of the high temperatures inside Earth. Common experience tells us that heat flows from hot to cold, so the heat in Earth's deep interior must be flowing somehow to the surface. Hot lavas and gases coming out of volcanoes are direct evidence of heat flowing out of Earth. Another indicator of heat flow is the increase in temperature with depth inside deep mines. These measurements of heat flow, however, are all made near the surface. The processes by which heat moves in Earth's deep interior are investigated by computer simulations, which can be compared with seismic and heat flow data that show temperature variations in Earth's interior.
Both the measurements and simulations show that the hottest part of Earth's interior is the iron core. Part of the heat down there is actually left over from the fiery formation of Earth; part is from latent heat released by the freezing of liquid iron in the outer core onto the solid inner core, and part is (possibly) from the slow decay of naturally radioactive elements like uranium and potassium mixed in the core. The core heats the bottom of the rocky mantle. The hottest rock near the bottom of the mantle becomes slightly less dense than the somewhat cooler rock above it, so buoyancy forces try to push the hottest rocks upward. Although the rock in the mantle is solid, the pressures and heat are so great that the rock can deform slowly, like hot wax. So the hot rock creeps upward through the cooler rock. As the hot rock rises, cooler rock flows downward to take its place next to the core, where it is heated and becomes buoyant enough to rise again later. The rising hot rock comes in contact with cold rocks near the surface of Earth where it gives off its heat, cools, and sinks again. Most of the rock in the mantle moves in this broad cyclic flow, indicated by the arrows in the figure. This zone, where rock is soft enough to flow, is called the asthenosphere.
(This means of heat transport--the cyclical movement of hot and cold material--is called convection. You can see an example of this in your kitchen by heating a pan of water to what is called a "rolling boil": hot water from the bottom of the pan rises up the sides, flows to the center, and sinks to the bottom again.)
Occasionally, however, masses of hotter-than-normal rock rise independently of the broad flow, like bubbles through a flowing stream. These masses of very hot rock form rising columns with rounded tops, called plumes.
Rock near the surface of Earth is so cold and at such low pressures that it cannot flow like mantle rock. So how does heat get through this rigid layer lithosphere, to the surface? One way is by conduction which describes heat flow in an iron pan held over a fire. The part of the pan over the flame gets hot first, followed by the handle, which is not over the flame. The heat in the handle came from the pan, but there was no movement of hot material from one part of the pan to another as in convection. (The metal in the pan and handle certainly didn't flow!) The heat, which is vibrations of atoms in the solid pan, moves as a result of fast moving (hot) atoms bouncing off slow moving (cool) atoms, causing the slow atoms to move faster (heat up). So at the top of the asthenosphere, the hot rock flows along the bottom of the lithosphere, transferring its heat to the cold rocks by conduction. The heat then flows through to the surface, again by conduction.
A second way of getting heat through the lithosphere is more exciting: melt some of the mantle rock and let it flow through cracks in the lithosphere to the surface! Sound familiar? Places where liquid rock (lava) flows onto Earth's surface are usually called volcanoes!
How does all this relate to the motion of the plates on Earth's surface? The movement of heat by convection in the asthenosphere causes the rock of the mantle to slowly move in huge streams. The solid (but brittle) rock of the lithosphere is resting directly on top of the solid (but soft) rock of the asthenosphere. As the rock of the asthenosphere moves in different directions, it carries parts of the lithosphere along with it. The lithospheric rock can't stretch, so it breaks into pieces--forming the plates. Interestingly, once the plates form, they begin to act somewhat independently of the convection flow because their cold edges tend to sink into the mantle. The detailed relation between of the motions of the plates and the underlying convective motions is still being studied.
This whole group of observations and ideas describing the motions of the plates and their associated geologic features is called plate tectonics. The word plate, of course, refers to the pieces of rigid lithosphere that comprise Earth's surface. Tectonics is derived from the Greek word for builder and is used in geology to describe structures like folds, faults, and mountains. Since one of the important results of plate collisions is rock fracture and mountain building, the use of this word should also be clear. Plate tectonics is the current "paradigm," or unifying philosophy, for understanding most of the geologic features on the surface of Earth. The development of plate tectonics in the 1960s and 1970s represented an enormous leap forward in understanding how Earth works.
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