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The Geosphere
106
1.
The Earth: origin and composition
Before or after reading each section, listen to the
Talking book
.
1.1.
The origin of the Earth
Before starting this section, remind students about the concepts
of density and gravity. It will help them to understand clearly the
origin of our planet.
It is important that students differentiate the concepts of density
and mass. Otherwise, they will not be able to understand the
process of density differentiation. You can take a scale to the
classroom, a small pebble and a big cork. Make sure the cork
weighs more than the pebble. Put the cork and the pebble in a
bow with water and ask them to observe what happens. The cork
will float and the pebble will sink. This is a practical and visual way
to understand the difference between these two concepts.
Show the video.
Video:
THE ORIGIN OF THE EARTH
This video is about how the planets were formed. The presenta-
tion of the video and the answers to the questions will help stu-
dents to deepen their understanding of the following question:
After hearing about the Big Bang theory about the formation of
the universe, how do you think the Earth was formed?
1.2.
Studying the Earth’s interior
For students it is always a mystery to know the interior of the
Earth without being there. They should understand that the waves
produced by an earthquake are the same as the ones when you
throw a pebble into a pond. Those waves travel on the surface of the
water as well as in its interior and the same process happen to the
waves produced by the earthquakes. Light waves are deflected when
they pass from air to water and it makes us see a stick in the water
as if it was crooked. The same happens to the seismic waves when
they are deflected from one place to the other. Those deflections
affect the speed of the wave transmission through the interior of the
layer. This way, and also because of the seismographs, we are able to
know the internal structure of the planet.
On the planet there are lots of network seismographs to detect any
earthquakes that couldhappen in thecrust.At least three seismographs
are needed to precisely detect the epicenter of an earthquake.
1.3.
Layers of the geosphere
A practical way to explain the layers of the geosphere is to make
a chart where students can compare depth, density, state of the
materials and its composition. This chart will help students to easily
follow the explanations and understand the contents
Layers of the
geosphere
Depth Density Physical state Composition
Crust
Continental crust
Up to 70 km 2,7 g/cm
3
Solid
Granite,clay...
Oceanic crust
Up to 10 km 3 g/cm
3
Solid
Basalt
Mantle
Upper mantle
Up to 670 km 3,3 g/cm
3
Solid
Peridot
Lower mantle
Up to 2900 km 5,5 g/cm
3
Solid
Peridot
Core
Outer core
Up to 5120 km 10,6 g/cm
3
Liquid
Iron
Inner core
Up to 6370 km 13 g/cm
3
Solid
Iron
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3. Thegeosphere
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1.
THE EARTH: ORIGIN AND COMPOSITION
TheEarth is the thirdclosestplanet to theSun. It isa rockyplanetand theonlyplanet
that has water in all three states. As a result, it is the only planet with living things.
The Earth can be divided into four layers: the
geosphere
or solid layer; the
atmosphere
or gaseous layer; the
hydrosphere
which contains water in all three
states and the
biosphere
where life exists.
1.1.
The origin of the Earth
According to the latest studies, 4600
billion years ago the Sun formed from
chemical reactions in a giant cloud of
dust and gas, called a nebula. In the
cloud of matter that surrounded the
Sun, smaller dust particles collided and
grew in size. This process which formed
the planets is called
accretion of
planetesimals
.
For 1000 million years the Earth was
incredibly hot. Due to the immense
heat stored in the Earth’s interior, there
was a lot of volcanic activity during
this period. As the Earth’s temperature
decreased, gravity pushed denser
materials, such as iron, towards the
Earth’s interior. Less dense materials,
such as oxygen, moved towards the
Earth’s surface. This process is called
density differentiation
. As the Earth
cooled, it maintained this structure of
layers.
1.2.
Studying the Earth’s interior
The Earth’s is 6370 km at the Equator but we only have direct knowledge of the
most superficial layers under the Earth’s surface. Mines or drill holes
1
have allowed
us to reach depths of 8-12 km.
Tounderstand thecompositionof theEarth’s interior, scientistshave touse indirect
methods.Themostcommonmethod is thestudyofearthquakescalled the
seismic
method
. This method analyses the energy generated by earthquakes.
When you throw a pebble into water, the water moves across the surface in
all directions in waves. Similarly, when an earthquake occurs, the movement
generates waves of energy that travel to the interior of the Earth, called
seismic
waves
. These waves can be detected by an apparatus called a
seismograph
.
This shows us that the speed of the waves vary as they pass from one layer to
another.
The studyof thedataobtained from seismographshasallowed scientists todeduce
the composition of the Earth’s interior. This information has been used to create a
model of the Earth’s structure. It is divided into three layers: the crust, mantle and
core.
1.3.
Layers of the geosphere
The solid part of the Earth is divided into different layers separated by areas
known as
discontinuities
. In these areas the seismic waves change velocity. This
allows us to identify where each layer ends.
1.3.1.
The crust
This is a thin layer covering the Earth’s surface and it
is the least dense layer. There are two types of crust:
❚
The
continental crust
is more than 1000 million
years old and it is between 10 and 70 km thick.
The continental shelf, continents and islands are all
partof thecontinentalcrust. It iscomposedof rocks
such as granite, clay and slate.
❚
The
oceanic crust
is 200 million years old and it is
between 6 and 10 km thick. It forms the seafloor
and is composed mainly of basaltic rock.
1.3.2.
The mantle
The density of this layer varies. It is composed
mainly of a type of rock called peridot. The mantle
has two parts.
❚
The
upper mantle
has a higher density than
the crust and it is solid. However, scientists have
discovered some areas of liquid or molten rock.
❚
The
lower mantle
is the densest layer of the
mantle and contains materials in a solid state.
1.3.3.
The core
This is the most internal and densest layer of the
geosphere. It is composed mostly of iron, although
other metals such as nickel can be found. It is divided
into two layers.
❚
The
outer core
is not as dense as other layers. It is
composed of molten materials and it is constantly
moving.
❚
The
inner core
has the densest materials. It is the
hottest layer. Even though the temperatures are so
hot, thematerials in this layerare ina solid statedue
to the immense pressure found in it.
Key concepts
❚
The Earth formed by the
process of accretion of
planetesimals. The materials
that make up the Earth
were distributed in layers
according to their density.
❚
There are three layers in
the geosphere: the crust
(continental and oceanic),
mantle (upper and lower)
and thecore (outerand inner).
1
drill hole:
a perforation in the
ground inorder tostudy the rocks
under the Earth’s surface
Seismograph
Formationof the Solar System
Layersof thegeosphereanddiscontinuities that separate them
Understand
1.
Explain in your own words the meaning of
accretion of planetesimals
.
2.
Listen and find the parts of the geosphere on the diagram.
Create
3.
Findoutabout thedensityof the layersof thegeosphere.Makea table
with the information in the order of least dense to densest materials.
