The Lithosphere…
The Earth’s crust is not a single layer
like the shell of an egg; rather, it is
broken into (at least) 23 pieces similar
to a cracked eggshell or a jigsaw puzzle.
The pieces are moving and are termed
PLATES.
Plates are composed of lithosphere. The
lithosphere is a rigid, brittle outer
layer of the solid Earth. Above the
lithosphere is the atmosphere, below
the asthenosphere.
The asthenosphere means region
without strength and is the uppermost
region of the mantle. The Lithospheric
plates are thought to ride on top of the
asthenosphere. This region is made up
of partially molten rock and is said to
be plastic.
1. Why do you think the asthenosphere is described as being plastic in nature?
The lithosphere is divided into two types; oceanic or continental, depending on the nature of
crustal material.
Using the diagram of the Lithospheric plates give an example of;
2. A plate containing sea-floor and continent.
3. A plate containing sea-floor only.
OCEANIC LITHOSPHERE
Oceanic crust is produced at MOR’s. Molten magma rises to the surface and solidifies to form
dark mafic rocks on contact with ocean water. Mafic rocks are rich in magnesium and iron. The
upper layers of the mantle are fined grained basalt, further down larger grained gabbro can
be found. The thickness of the oceanic crust is almost always around 6 to 9km. This type of
crust is similar in density to the underlying asthenosphere and mantle. The oldest rocks ever
discovered on the ocean floor are around 175 million years old.
4. Why is the oldest oceanic lithosphere only 175 million years old?
5. What does the term mafic mean?
6. What is the difference between basalt and gabbro?
7. Explain the term density.
CONTINENTAL LITHOSPHERE
Continental crust is thicker and more complex than oceanic crust and is mainly composed of
felsic rocks, such as granite and the minerals quartz and orthoclase. These plates are also
sialic in composition. That is the crust is rich in the elements silicon and aluminium. This type
of crust is lower in density than the asthenosphere. Beneath the light coloured grantic rocks
is a layer of mafic rocks, mostly gabbro. Continental crust is about 40km thick, but can get up
to 65km thick under mountain ranges.
8. What does felsic and sialic mean?
9. Create a table in your book with the following headings;
Crust type
Rock type
Rock examples
Elements present
Thickness
Density