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Heat Flow in Brief,
 Planets are great heat engines,
 Heat flow is primarily associated with the plate tectonic cycle whereby hot material
upwells at spreading centers and then cools.
 The cooling oceanic lithosphere moves away from the ridges, and eventually
reaches subduction zones where it descends in downgoing slabs (‫)لوح‬, reheating as
it goes
 Measuring the temperature at several depths and the thermal conductivity began in
1939 on land and in 1952 at sea.
 This is done at sea using a probe that penetrates into the soft sediment on the sea
floor, and on land using boreholes drilled for oil or other purposes.
 Comparison with depth, heat flow, and geoid (gravity) data shows that the plate
thermal model is a good, but not perfect, fit to the average data, because processes
other than this simple cooling also occur.
 Combining the oceanic and continental heat flow estimates gives a total global heat
loss of 44 TW (Tera Watt). Of this loss, 70% is in the oceans and 30% in the
continents, reflecting both the higher heat flow and larger area of the oceans.
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.
 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.
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 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.
 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.
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!
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.
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