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Density of the Earth
Background Information:
Imagine you could get into an elevator and ride down to the earth’s
center. What would you see? It’s impossible to do such a thing –
Earth’s interior is much too hot, but we can learn a lot about the
inside of Earth by studying earthquake waves. We also gather
clues about the inside of earth by looking at landforms such as
volcanoes, deep sea trenches and mountains.
Primary (P) waves and secondary (S) waves from earthquakes
travel at different speeds through Earth. Both kinds of waves slow
down in the molten (melted) rock layer of the mantle. When
earthquake waves are bent or slowed down, scientists know that
the waves have hit a layer within Earth that has different properties.
From this data, scientists are able to infer that Earth must have a
solid inner core surrounded by a liquid outer core. Scientists can
tell the outer core is liquid because S-waves are completely stopped at this layer. They used a
model to show that S-waves cannot travel through fluids. It has taken many years and many
different types of experiments and models to get a clear picture of the inside of Earth, and
scientists learn more every day.
We now think that as the earth formed, it layered according to density. We are usually familiar with
the solid crust that we walk around on every day and the oceans. The oceans are on top of a layer
of crust because water is less dense than the crust, which is made of rock. Under the crust is the
mantle. It is made of rock that has melted because it is so hot. (Think lava hot.) Under the mantle
is the outer core. It is made of dense minerals such as iron and nickel. It is a liquid because it is
so hot – even hotter than lava. The inner core is a solid and is very dense. It is hotter than the
outer core, which should make it a gas, but it is under so much pressure from all the layers of rock
above, that the molecules are pressed together to form a solid.
1. Why are iron and nickel found in earth’s core?
a. They have a higher density than most of Earth’s substances.
b. They have a lower density than most of Earth’s substances.
c. The core is hotter than the other layers of the earth.
d. The core is colder than the other layers of the earth.
Outer Core
Inner Core
Approximate Density
1.0 g/mL
2.7 – 3.0 g/mL
3.3 – 5.7 g/mL
9.0-12.0 g/mL
12.7 – 13.0 g/mL
2. If you found an object with a mass of 90 grams and a volume of 30 ml, which layer of the earth
would it have come from?
a. Crust
b. Mantle
c. Outer Core
d. Inner Core
A CAT Scan of Earth
If a doctor needs to see what's wrong inside you, he doesn't have to cut a big hole in you and look in. He uses
other methods such as CAT scans and X-rays. The earth's interior is too hot, and there is too much pressure for
humans to explore it directly. So, like doctors, scientists rely on other methods to find out what's inside the earth.
Rarely, volcanic and tectonic actions give us clues to the interior of the earth. Erupting lava sometimes brings
xenoliths. These are parts of the earth's mantle. Earthquakes sometimes expose huge pieces of mantle rock. Scientists
then study these materials to know what the mantle is made of.
Scientists know the earth has a magnetic field. It is what makes compasses work. It also helps animals on their
paths of migration. Scientists also know that in order to have this magnetic field, there must be a high concentration
of metal inside the earth. They also know that this metal must be able to be in a liquid state at high pressures. This
information gives clues as to what the core of the earth is made of.
Geophysicists are scientists who study the interior of the earth. They conduct laboratory experiments to help solve
these mysteries. They expose surface materials to high temperature and pressure that is similar to the earth's interior.
Then they study the changes in these materials. They compare these to changes that take place inside the earth.
Mathematical calculations also help us to know what the earth is like inside. We can't measure the temperature of
the core directly. But scientists know that temperature and pressure increase with depth. They also know the rate at
which they increase. By knowing the thickness of each layer, they can calculate the temperature of the core.
All these methods give us glimpses of what the interior of the earth is like. However, scientists have learned the
most by studying earthquake waves. Since many earthquakes occur every day, it is easy to study this data.
When an earthquake occurs, seismic waves travel throughout the earth. Sensitive seismographs can detect these
waves very far from the epicenter. These waves move faster through denser rock. Scientists measure the time it takes
for the waves to reach locations far from the epicenter. They can then determine the density of the material they have
moved through.
Scientists studied the speed of these waves as they moved through the earth. They found that at a depth of
about 60 kilometers, the waves increased their speed. The boundary between the crust and the mantle was
discovered. The waves moved faster through this denser layer.
Scientists use many methods to study the interior of the earth. Most of the information they have gathered has
come from studying earthquakes. The seismic waves travel to the depths of the earth and then return to its surface.
As they move, the speed and direction of the waves change. They allow us to "see" inside the earth, much like a CAT
scan allows a doctor to see inside you.
3. Name three methods scientist use to study the
interior of the earth.
4. Scientists figure out the temperature of the core by
CAT scans
Laboratory experiments
Mathematical calculations
5. What method has given scientists the most
information about the interior of the earth?
6. Which seismic waves can move through liquid?
Both P-waves and S-waves