Heat Flow (‫)التدفق الحراري‬
‫عند وضع جسمين عند درجات حرارة مختلفة بينهما اتصال حراري فإن الحرارة تنتقل من الجسم‬
،‫األعلى درجة حرارة إلى الجسم األقل درجة حرارة‬
.‫أي أن التدفق الحراري هو انتقال الطاقة الناتج عن اختالف درجات الحرارة‬
‫ودرجة الحرارة ما هي إال مقياس للطاقة الداخلية للمادة وكلما زادت درجة الحرارة زادت الطاقة‬
.‫الداخلية أي زادت الطاقة الحركية لجزيئاته‬
It is said that "heat is the geological lifeblood of planets". Planets are great
heat engines, whose nature and history govern their thermal, mechanical, and
chemical evolution. The most direct constraint on how the heat engine
operated at depth through time is the present outward heat flow at the surface.
On Earth, this heat flow is primarily associated with the plate tectonic cycle
whereby hot material upwells at spreading centers and then cools. Because the
strength of rock decreases with temperature, the cooling material forms the
strong plates of the lithosphere. 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. The other important component of
surface heat flow is that conducted through the continents, which are not
subducted.
Measuring the temperature at several depths and the thermal conductivity
began in 1939 on land and in 1952 at sea. By now, heat flow has been
measured at about 30,000 sites worldwide. 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.
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Figure 1 This map shows color-coded contours of the global distribution
of heat flow at the surface of the Earth's crust. Major plate boundaries
and continent outlines are also shown. The fundamental data embodied
in this map are the more than 24,000 field measurements in both
continental and oceanic terrains, supplemented by estimates of the heat
flow in the unsurveyed regions.
In general, oceanic heat flow data varies with the age of the lithosphere.
Values greater than 100 mW/m2 occur near the ridges, and decrease smoothly
to about 50 mW/m2 (milli watt per square meter) pin the oldest oceanic
lithosphere. Similarly, ocean depth is about 2500 m at the ridges, and
increases to about 5600 m for the oldest sea floor. These variations can be
described using a simple model for the formation of the lithosphere by hot
material at the ridge, which cools as the plate moves away. The modeled
depth to a given temperature increases as the square root of lithospheric age,
predicting that ocean depth should increase with the square root of age and
heat flow should similarly decrease. Because ocean depth seems to ``flatten''
at about 70 Myr, we often use a modification called a plate model, which
assumes that the lithosphere evolves toward a finite plate thickness with a
fixed basal temperature. The flattening reflects the fact that heat is being
added from below, so the predicted sea floor depth and heat flow behave for
young ages like in the halfspace model, but evolve asymptotically toward
constant values for old ages.
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. In particular, heat flow in
lithosphere younger than about 50 Ma is lower than the model's predictions.
This is generally assumed to reflect water flow in the crust transporting some
of the heat, as shown by the spectacular hot springs at midocean ridges. If so,
the observed heat flow is lower than the model's predictions, which assume
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that all heat is transferred by conduction. Unfortunately this hydothermal heat
transport is hard to quantify, so based on the model predictions global oceanic
heat flow is generally assumed to be 50% larger than directly measured.
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. 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.
Figure 2 show 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
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.
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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
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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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