47
3. The geosphere
www
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 1billion years
old and it is between 10 and 70 km thick. The
continental shelf, continents and islands are all part
of the continental crust. It is composed of 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, the materials in this layer are in a solid state due
to the immense pressure found in it.
Key concepts
❚
The Earth was formed by the
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 the core (outer and inner).
Layers of the geosphere and discontinuities 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.
Find out about the density of the layers of the geosphere. Make a table
with the information in the order of least dense to densest materials.
