Download Earth`s Structure Is Affected by Density

Earth’s Structure Is Affected
by Density
Utah OER DOE
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Printed: April 6, 2016
AUTHOR
Utah OER DOE
www.ck12.org
C HAPTER
Chapter 1. Earth’s Structure Is Affected by Density
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Earth’s Structure Is
Affected by Density
Do you remember our discussion on mixtures? If you do then you remember that a mixture is a combination of
different materials that are mixed together but not chemically combined. In some mixtures the different parts mix
together and move evenly throughout the container. This type of mixture is called a solution. A salt water solution
is a good example. As the salt dissolves in the water, the salt molecules diffuse (move around) until the salt particles
are evenly distributed throughout the solution.
Other mixtures are not like solutions and the particles tend to separate. The particles in mixtures separate according
to particle size and the density of the particles in the mixture. When particles separate according to density, the most
dense particles will always be on the bottom and the least dense particles will always be on top.
The materials that make up the earth follow all of the principles of density that we have discussed. The most dense
materials in the earth tend to be located closest to the center, or core, of the earth. The least dense materials are on
top of the earth. The air, which is the least dense earth material, is located above the earth.
The Core
The very center part of the earth is called the core. Earth’s core is divided into the inner core and the outer core.
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The inner core of the earth is a solid ball and is made up mostly of iron and nickel. These are very dense materials.
The inner core is about 1,230 km in diameter.
The outer core, like the inner core, is mostly iron and nickel, but the outer core is a liquid and is about 2,200 km
thick.
Temperature increases as you get closer to the center of the earth. If both the inner and outer core are made of similar
materials then why would the inner core be solid and the outer core liquid even though the inner core was hotter? In
order for a solid to turn to a liquid it must be able to expand. The weight of all of the upper materials on the inner
core is so intense that it cannot overcome the pressure and expand. Therefore, it must remain solid. If the pressure
were to ease up, the inner core would expand and liquefy.
The core of the earth is tightly compacted. The density of the core ranges between 9 and 13 g/cm3 . The core
contains about 33% of earth’s total mass.
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Chapter 1. Earth’s Structure Is Affected by Density
The Mantle
The part of the earth that lies between the core and the surface is the mantle. The mantle is about 2,900 km thick
and is composed of upper and lower parts. About 67% of earth’s mass is located in the mantle.
The mantle is located far enough below the crust that no one has been able to go there and study it. Scientists must
rely on interpreting data that they have in order to study the mantle. Volcanoes offer some help in studying the
mantle. Since magma from volcanoes come from the mantle scientists can use lava flows and magma domes to see
what materials are present in the mantle. From these sources we know that magnesium is one of the more common
materials found in the mantle.
The mantle is divided into the upper and lower mantle. The lower mantle is about 2,550 km thick and is made up
of solid rock. Most of earth’s mass is located here. The upper mantle is about 250 km thick and is made of rock
that flows very slowly. This rock can best be explained as a semi-solid. It is “plasticky”, having a consistency like
peanut butter. Heat in the lower mantle circulates in convection currents. Scientists believe that, since the earth’s
tectonic plates sit on top of the lower mantle, they ride these currents. It is these currents that are responsible for the
movement in plate tectonics.
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The mantle is a fairly dense region of earth’s interior but nowhere near as dense as the core. The density of the
mantle ranges between 3 and 9 g/cm3 .
The Crust
The upper part of the earth where most life exists is called the crust. The crust ranges in thickness from about
5 to 100 km. The thickest spots are on land and are called continental crust. The thinner parts of the crust are
under the ocean, called oceanic crust. The crust and the very top layer of the mantle compose a layer called the
lithosphere. The top layer of the mantle is included with the lithosphere because it is solid and does not flow like the
asthenosphere. The lithosphere contains the tectonic plates.
The most abundant elements in the crust are oxygen, silicon, and aluminum. Many other elements are present as
well, but in lower quantities.
The density of earth’s crust is between 2.7 and 3.0 g/cm3 . Since water has a density of 1.0 g/cm3 it sits on top of the
crust and fills in any seams, cracks, and empty areas that may be present.
The Atmosphere
The least dense layer of the earth is the atmosphere. The atmosphere is the layer of the earth that contains all
of the oxygen and other gases in the air around us. It extends several miles above earth’s surface. The reason our
atmosphere is above the earth is because it is far less dense than each of the other layers. The atmosphere has a
density of less than .0013 g/cm3 .
Remember, the densest materials in the earth tend to be the closest to the core. Likewise, the least dense materials
tend to be located on the surface and above the surface of the earth.
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Chapter 1. Earth’s Structure Is Affected by Density
How Do Scientists Know About the Inside of the Earth?
Seismic waves are another way that Scientists can study the earth’s interior without actually visiting there. Seismic
waves are the energy waves that are generated by earthquakes. To understand what these waves are like, fill a tub
with water and drop a rock into the water. Watch the waves that spread out from the point that the rock entered the
water. When an earthquake occurs, the energy waves move out from the point of the quake in much the same way.
There are two types of seismic waves, S waves and P waves. These waves act differently in solids and liquids. When
P waves pass through liquids they slow down. They pick up speed when they reach a solid on the other side of the
liquid. The S waves stop completely in liquids. Waves also travel faster in materials that are denser. Scientists look
at data that they collect from earthquakes and can determine the composition of earth’s interior by how the waves
slow down, speed up, and disappear.
Meteorites
Scientists study meteorites to learn about Earth’s interior. Meteorites formed in the early solar system. These objects
represent early solar system materials. Some meteorites are made of iron and nickel. They are thought to be very
similar to Earth’s core. An iron meteorite is the closest thing to a sample of the core that scientists can hold in their
hands!
Models of the Earth: Accurate or Inaccurate
Models are representations of a concept or process that help teach about that principle. They can be very effective
ways to study the earth and learn about the relationships of various parts of the earth. You can study a map in order
to learn more about how countries or landscapes are situated. A globe, which is another type of model, can help you
to have a better understanding of how the earth is organized. There are many types of maps and other earth models
that you can learn from. Models can have drawbacks as well. A flat map cannot correctly show exactly what a round
world looks like. Likewise, a round globe can never show the detail that a flat section of a map can show. Basically,
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models can show one or two things very well, but in order to really understand, a series of different types of models
would be best. It is always important to keep in mind the strengths and weaknesses of each type of model that you
are using.
Models are not always accurate. In fact, there have been times when an entire model that had been accepted was
thrown out with the discovery of new evidence or the development of new technology. Scientific theories are truly
dynamic. That means that they are constantly changing. The theories that we have today will be adjusted, or possibly
changed completely as technology adds new information to what we already know.
On Your Own
Task: Find a map of the world and a globe.
Guide: Look carefully at the two models. What differences do you see? What kind of study do you think each
would be good for? Find a country that is far North such as Greenland on both models. Do you see a difference in
how they country is presented on each model? What differences do you see?
Summary: If you used good models, you probably noticed several strengths and weaknesses of each model. The
map gave you better details. It probably listed many cities and rivers that the globe did not show. The Northern (or
Southern) Country that you picked out was probably greatly distorted in size on the flat map. It is also hard to see
the relationship between two countries on a map when they are on opposite sides of the paper.
Web link: http://www.usoe.k12.ut.us/curr/science/core/7thgrd/sciber7/EARTH/HTML/MODEL.HTM
Online Interactive Activity
• Plate Tectonics: http://tinyurl.com/UT7th2-2-b
MEDIA
Click image to the left or use the URL below.
URL: https://www.ck12.org/flx/render/embeddedobject/179352
Review
1. List the main layers of the earth.
2. What is the state of matter for each of the layers of the earth? What is unique about the mantle?
3. Based on what you have learned about expansion and contraction of particles when heated, why do you think
the inner and outer core are similar in composition and temperature, but the inner core is solid and the outer
core is liquid?
4. List the main materials found in each of earth’s layers.
5. Give the average density for each of earth’s layers.
6. From what you learned in objective 1, why do you think the layers of the earth are separated by density?
7. Explain how convection currents work.
8. What is the main gas that makes up earth’s atmosphere?
9. Explain how scientists use earthquakes and meteorites to learn about the interior of the earth.
10. How are models of the earth helpful to scientists?
11. Based on what you have learned about models, what do you think some of the limitations are for models of
the earth?
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