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4.3 Read
Why Does the Mantle
Move the Way it Does?
In the demonstration, you observed warm water rising through cool water.
You also observed cool water sinking to replace the warm water.
The movement of a fluid material caused by density differences is called
convection. Convection carries heat energy from one location to another
location within a fluid. Often, convection is driven by either heating from
below or cooling from above the fluid. The warm water you observed
carried heat energy as it moved from an area of higher temperature to an
area of cooler temperature. You saw this happen in the demonstration as
warm water from the cup moved upward through the cooler water until it
reached the top.
cooling
rising
sinking
heating
convection: the
movement of a
fluid material
caused by density
differences. Often,
convection is
driven by either
heating from below
or cooling from
above the fluid.
Convection occurs in many places. Think of what happens when a pot of
water is heated on a stove. When the water reaches a full boil, the liquid will
bubble and roll. However, long before the bubbles start forming, the heated
water is already moving due to convection, even though it is hard to see.
That is because the heat from the stove adds energy to the water molecules
at the bottom of the pot.
Molecules are always moving and bumping into one another. In the pot, the
heat energy from the stove will cause the water molecules to move faster
and hit one another harder. This hitting causes them to spread apart.
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EVER-CHANGING EARTH
Learning Set 4 • What Causes Earth’s Plates to Move?
That spreading means that when water is hot, the same amount of space
will have fewer molecules in it than when the water is cool. Where there are
fewer water molecules in a space, the water is less dense. Like any liquid,
water that is less dense than the surrounding liquid will rise. The warmer
water on the bottom of the pot rises because it is less dense than the water
around it.
The floating of the heated, less dense water is not all that happens in a pot
of water on a hot stove. As the water molecules reach the top of the pot,
they release some of their heat energy to material in the surroundings, such
as air. As it releases its heat energy into other materials, the water cools.
It becomes more dense, and it begins to sink. The heating and cooling of
the water causes it to float and then sink in a process that can repeat many
times. That process is what is known as convection.
convection
current: the cycle
of movement when
warm, less-dense
fluids rise and
cool, and moredense fluids sink
due to differences
in temperature.
When a substance rises and sinks due to differences in heat energy, the
movement is called a convection current. A convection current is a
flowing pattern in which warmer, less-dense quantities of a substance rise
and are replaced by cooler quantities that sink because they are more dense.
plate
mantle
The heating and
cooling of mantle
material changes
the density of the
material. This sets
convection currents
in motion within
Earth.
outer core
inner core
The same process by which water in a heated pot moves in a convection
current happens in Earth’s mantle, but on a much larger scale and over a
longer period of time. Earth’s core is very hot, so it can provide a source
Project-Based Inquiry Science
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4.3 Read
of heat that causes convection in the mantle. Scientists have calculated that
temperatures at Earth’s core reach as much as 5000°C (over 9000°F).
The mantle material that is farthest down is heated by the core. Remember
that the mantle extends a long way from its upper to lower parts (its
thickness is about 2700 km, or 1800 mi). Because of its thickness,
temperatures in the upper mantle, nearest to the crust, are less hot (just
under 900°C, about 1600°F), than the temperatures in the lower mantle,
nearest the core (more like 2200°C, just under 4000°F). This temperature
difference is thought by scientists to cause convection in Earth’s mantle.
In Earth’s mantle, convection occurs when the core heats material in the
lower mantle. The molecules making up the mantle material begin to move
more quickly and spread apart. As they spread farther apart, the mantle
material nearest the core becomes less dense than the material above it,
and it rises.
This heated, less dense
mantle material eventually
plume
reaches the upper region
of the mantle. As it rises
up to just below the crust,
the mantle material spreads
outwards. As it spreads
under the crust, the material
loses heat energy. With less
heat energy, the molecules
move less quickly, and
begin to get closer together.
This loss of heat causes the
material in the upper mantle
to become more dense than
the material below it, so it sinks slowly back down toward Earth’s core.
This rising and sinking due to differences in heat in the mantle material is a
convection current.
plume: a hot,
rising column of
mantle material.
Some evidence indicates that convection currents in the Earth cause
material that is hotter than the material around it to rise as a column of
upward flowing magma in the mantle. This kind of hot, rising column of
magma is called a plume. Scientists are studying the possibility that a plume
may contribute to several kinds of geologic events.
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Learning Set 4 • What Causes Earth’s Plates to Move?
You have thought about three models that help you understand how Earth’s
mantle can move. The first was the model you built in Section 4.2 with the
cup of colored water. The second was in the video. The third was when
you thought about heating water in a pot as a model of convection in the
mantle. All of these can help you understand how convection might occur in
Earth’s mantle. Like any model in science, though, there are some parts of
the models you thought about that do not represent the real thing very well.
For example, both models used water to represent Earth’s mantle material.
However, Earth’s mantle material is not a thin liquid like water. In fact,
the mantle is much more like a solid than like a liquid, so the convection
currents occur very slowly, over millions of years. It can take millions of
years for material from deep in the mantle to reach the upper part of the
mantle.
Also, other than its temperature, the water in each model does not differ
from the bottom to the top. However, evidence from seismic waves and
studies of the composition of lava indicate that the mantle might have
layers, each made of different materials. It may be that convection currents
in the lower part of the mantle, very close to the core, are different from
convection currents in the upper part of the mantle. As well, it may be that
Earth’s core is not the only thing that heats mantle material and causes
convection currents. Some of the material in the mantle may be radioactive,
giving off its own heat and affecting convection currents. Scientists are still
studying evidence to learn more about the composition and structure of
Earth’s mantle.
hotspot on
magma from
the core
magma plume
convection
cell
crust
crust
upper mantle
upper mantle
lower
mantle
lower mantle
core
core
hotspot on
magma from
the core
magma plume
convection
cell
Scientists are still studying evidence to understand whether Earth’s mantle moves continuously from
bottom to top, or if the movement occurs in layers.
Project-Based Inquiry Science
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4.3 Read
Stop and Think
1. How do changes in heat energy affect the density of Earth’s mantle
material?
2. Write a short story detailing the journey of a cubic centimeter (cm3) of
mantle material through a convection current beginning at the core.
Be sure to include information about what happens to its molecules.
Support your story with sketches.
3. Draw a cross section of Earth, showing the core, mantle, and crust.
Draw arrows to represent a convection current in Earth’s mantle.
Sometimes called the Blue Planet, Earth’s blue waters and land areas look very
different from other planets in our solar system. However, beneath its cool and inviting
crust is a very different scene where temperatures are high enough to melt rock.
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Learning Set 4 • What Causes Earth’s Plates to Move?
What Else Drives Plate Motion?
mid-ocean ridge:
a continuous
mountain range
through the
oceans, which
is subject to
earthquakes.
lava: fluid rock
that comes from a
volcano; also, the
same material that
becomes solid by
cooling.
Scientists generally agree that circular motion created by convection in
the mantle is the strongest force driving plate movement. But there are
other forces, too, that cause Earth’s plates to move. When Earth’s plates
move apart from each other, mantle material rises to Earth’s surface. This
causes two more forces that drive plate movement: ridge push and slab pull.
There are places where two plates are moving away from each other
and mantle material reaches Earth’s surface. One such place is within a
large system of underwater mountains known as mid-ocean ridges. A
deep crack runs through their center. Mantle material rises up from this
crack, filling in the gap between the two plates. As this lava cools and
hardens, it forms new crust on the ocean floor. This process is called
ocean-floor spreading.
ocean-floor
spreading: a
process by which
new ocean floor is
created in the area
where two plates
are moving apart.
As the mantle
material fills in gaps
between two plates
at a mid-ocean ridge,
new crust is formed
on the ocean floor.
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4.3 Read
In places where new ocean floor is being added, a depressed valley fills the
gap between the plates. The valley is surrounded by the shoulders of the
plates that are moving apart. The gap is filled in, but the plates continue
to move. As the plates move farther apart, new gaps are created, and the
same thing happens again. As new gaps form and fill again and again, a
pattern forms on the seabed. At the mid-ocean ridge, where the plates
meet, the crust is composed of young rocks. On either side, the rocks
become older as you move away from the plate boundary. Successive bands
of rock form, with the youngest rock near the current plate boundary and
older rock on both sides of it. This evidence first led scientists to recognize
that the sea floor was spreading apart at the mid-ocean ridges. The push of
older crust away from the plate boundary, called ridge push, is a force that
drives plate motion.
ridge push: the
push of older crust
away from a plate
boundary.
mid-ocean ridge
younger crust
older crust
As new crust is created on the ocean floor along the mid-ocean ridge, older crust
is pushed outward. This means that rocks making up the crust nearest to the ridge
are the youngest. Moving away from the ridge, the rocks making up the crust are
increasingly older.
Earth’s crust stays the same size because the older crust that is pushed away
melts back into the mantle. As the older crust moves away from the midocean ridges, it cools and becomes more and more dense. Eventually, it
sinks deep down into Earth. The weight of the plunging crust pulls the rest
of the plate along with it. This is called slab pull, and it also drives plate
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slab pull: the
movement of
older crust down
toward the center
of Earth.
EVER-CHANGING EARTH
Learning Set 4 • What Causes Earth’s Plates to Move?
movement. When the rocks making up this crust move deep enough, they
are melted by the heat of the Earth’s core and, once more, their material
becomes part of the mantle. Eventually, this material may rise once again to
the surface in a mid-ocean ridge.
The Mid-Atlantic
Ridge, the part
of the mid-ocean
ridges beneath the
Atlantic Ocean, rises
above the surface
of the ocean in
Iceland. This allows
scientists to observe
phenomena that
more commonly
occur deep under
water.
Reflect
1. Look at your sketch of the convection current you drew earlier. Add
arrows and notes to show how convection currents in the mantle cause
the plates to move.
2. Think back to the example of the pot of water. If you take the water off
the heat, the bubbling stops after a few seconds. Why does this happen?
Why is this not happening on Earth?
3. Describe three factors that drive plate motion and discuss how they work
together to move Earth’s plates.
4. At the beginning of this Learning Set, you evaluated several ideas
about the forces that cause Earth’s plate to move. Reread and evaluate
these ideas again. Which ideas do you now have evidence to support?
Describe the evidence and how each force moves Earth’s plates.
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4.3 Read
What’s the Point?
Scientists have evidence that motion in Earth’s mantle is caused by
convection currents. Convection currents are formed when material
is heated while other material around it is cooler. The heating causes
molecules in the material to move faster and spread apart, making the
material less dense. The less-dense material rises, while cooler material
sinks. This rising and sinking due to temperature differences is a convection
current. In Earth’s mantle, material is heated by Earth’s very hot core.
This heating causes mantle material to rise, possibly in the form of plumes
of heated magma. While convection currents in water move quickly,
convection currents within Earth’s mantle move very slowly, taking many
millions of years to move material from the lower mantle up to Earth’s crust.
Hot magma can escape through cracks in Earth’s crust. As the magma
reaches the surface, it cools and forms new crust. This happens in the
ocean at mid-ocean ridges. As new crust forms at a mid-ocean ridge, older
crust is pushed aside exerting a force called ridge push. The formation
of new crust, along with the force of ridge push, means that usually the
crust material that is farther away from a mid-ocean ridge is older than the
material nearer to the center of the mid-ocean ridge. Eventually, that slab
of crust material is pushed to a place where it sinks deep into Earth. The
slab of crust material exerts a force as it sinks called slab pull, which helps
pull more crust material back down into Earth. As slabs of crust material
reach Earth’s mantle, the heat there melts them. It may be that as the old
crust material melts, it is recycled, becoming magma that can eventually
contribute to the formation of new crust.
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