|How does the mantle affect the movement of the crust?|
|Question Date: 2019-04-24|
Great question about the mantle!
You should think of most of the mantle as
consisting of a very thick liquid—like cold
honey in terms of its runniness, not its
temperature. On top of that honey-like mantle
floats the rigid crust and the brittle
uppermost part of the mantle (which
together form the plates of plate tectonics).
Well…the deeper in Earth we go, the hotter it
gets. So, Earth’s core, is its hottest
part. The core is like a flame on a stove,
heating the stuff above it, only the core heats
360 degrees in three dimensions, since “above” is
all directions away from the core.
Most substances expand (and become less dense)
when they get warmer, causing that warm material
to rise. (Water is weird. Cooling it forms ice,
which floats (!), because it’s LESS dense than
liquid water—but that’s a different story). The
base of the mantle, sitting near the hot core,
warms up, and begins to rise very slowly
(since the mantle’s very thick—viscous is the word
scientists use). As the mantle moves upward, it
encounters cooler and cooler temperatures, causing
it to become denser, and ultimately sink.
After sinking, it heats up again, and then (you
guessed it) rises again. This cycle of rising and
falling sets up big currents in the mantle (called
convection cells). These slow currents are
one of the things that push plates around,
causing, among other things, continental
Water convects in a boiling pot of water for the
same reason that the mantle convects, its density
changes as it alternately cools and warms (only
when liquid water turns to a solid, does water act
oddly—that is, it becomes less dense).
Although you probably don’t drink coffee (which is
the best for your health), it’s a wonderful way to
learn about convection. After pouring yourself a
hot cup (a clear cup, or measuring cup works
especially well), let it sit for a minute. Then
add a couple of tablespoons of milk from the
bottle or a pitcher. At first, the cold milk sinks
to the bottom of the cup, and most of the coffee
at the top of the cup remains black. The milk
sinks because it’s colder and denser than the
coffee. A second or two later, however, after the
milk is warmed by the coffee, it wells upward, and
soon all the coffee and milk in the cup become
mixed, even without stirring. Convection has
done all the work. A marshmallow floating on
that coffee would be pushed around by those
convection cells, the same way that the crust is
shoved around as it floats on the convecting
Imagine what happens when you
boil water on the stove. You heat the water from
below, and the hot water from below rises to the
top, whereas the cool water from above sinks to
the bottom. See image.
We call this convection. Why does
the water move? The reason is that hot water
is less “dense” than cold water, and
anything less dense than the surrounding will
The same thing happens with hot air that rises
above cold air. This convection is the reason
why a hot air balloon flies. On the
Earth, you have a similar situation than in a hot
stove. The center of the Earth is hot, and the
surface of the Earth is relatively cool.
Therefore, the mantle that is heated from below
will rise very slowly to the surface, and the
cool mantle at the surface will sink. This
convection in the mantle affects the rigid crust
of the Earth and drives the movement of the plates
on Earth’s surface.
Great question! Imagine taking something thick and
gooey like a bowl of pudding and placing something
light and solid like a graham cracker on top. If
you jiggle the cup of pudding, the graham cracker
on top is going shake back and forth. In this
example, the pudding is the mantle and the
graham cracker is the crust.
In the Earth, we can think of the crust as
"floating" on top of the mantle, like how the
cracker floats on the pudding. Because the
pudding (mantle) is less rigid it can
flow more easily, and when it flows the graham
cracker (crust) floating on top will move with it.
One important thing to remember though is that
the Earth's mantle is not liquid like
magma! It is made of hot, solid rock. Because
the rock is so hot, it can flow like the pudding.
The mantle moves because of something called
Convection is basically the way the Earth
cools itself off by moving heat from the inside of
the Earth to the outside. As hot rock rises
up from the deep Earth, it carries heat with
it. That heat comes through the crust through
volcanoes and rocks in the crust. Then when the
mantle rock cools off, it falls back down towards
This pattern of convection is what causes
the hot mantle rock to flow and carry the floating
crust on top!
The mantle allows the crust to move and also,
in a way, drives the motion of the tectonic
plates. The mantle is more or less solid rock,
but the high temperatures (>1000°C / 1832°F) and
forces present far below Earth's surface can
deform the mantle, albeit slowly. The mechanism
for motion of tectonic plates is still an open
question. Early in the development of the theory
of plate tectonics, convection currents in the
mantle were thought to push and carry plates. In
this process, hot material from near the core of
the earth would rise (due to lower density, like
of hot/cold air ). To make room, the
cooler mantle rock above it would be pushed
sideways. Because the cooler rock is in contact
with the plates, some of that sideways force would
be transferred to the plates, and they would also
move. However, modern research has found several
inconsistencies with this theory.
First, convection cells large enough to drive
motion of the plates have not been found, and the
material between the mantle and the plates is weak
enough that the forces transferred from the
convecting mantle to the plates is small.
Further, measurements indicate that there are
multiple convection cells beneath plates. This
is problematic because they would not push plates
in the same direction; their effects would
counteract each other and hinder plate motion.
The modern theory is that gravity drives plate
motion through two processes called "ridge
push" and "slab pull". Convection in the
mantle still plays an important role though. The
hot rock in the mantle rises and pushes up
mid-oceanic ridges at boundaries between plates.
At these ridges, the plates are essentially on top
of a hill.
Analogously to an avalanche, gravity pulls the
plates downward. As the plates slide down the
slope of the mid-ocean ridge, they push adjacent
plate material away from the ridge. This is the
"ridge push" process. At other plate boundaries
(namely, subduction zones), one plate is driven
below another. The sinking plate is more dense
than the surrounding rock, so the gravitational
force on it is stronger. As the plate sinks, it
pulls the material behind it farther down as well.
This is the "slab pull" process.
Another effect of the mantle in this theory is
to prevent motion, essentially through friction
against the moving plates.
While this second set of mechanisms is the current
theory, movement of the crust is still
undecided and a matter of much debate and current
Incorporated Research Institutions
for Seismology , including the video on the
right side of the page;
Geological Society; and
Columbia University .]
It's complicated, but interesting. The crust is
sitting on top of
the mantle, but is attached to the top of the
mantle because the mantle is an extremely
viscous fluid (it's so viscous that we would
normally call it a solid, not a liquid. It's far
more viscous than glacier ice, for example).
Ocean crust is dense and so sinks, and pulls
the mantle along with it. Continental crust is
less dense and so floats, but because it's
attached to the mantle underneath it, wherever the
mantle moves, the continents have to go with
it. Finally, hot plumes of mantle rock coming
up from the core or just deeper into the mantle
force the cooler mantle that they're moving
through away, which also causes continents to
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