Module 5: Theme4 Lithosphere – Key Idea 1
The Lithosphere
Knowledge:
Why the Earth has a lithosphere. Temperature changes with depth and surface heat
flow. Variation of rock strength with depth. Partial melting.
Evidence for a lithosphere comes from global seismology. Asthenosphere, low velocity
zone.
Base of the lithosphere is 1300 C isotherm. Lithospheric thickness varies.
Skills:
Comparing geotherms for continental and oceanic.
Use earthquake refraction to show the presence of a low velocity zone and layers in the
crust.
Interpret P and S wave velocity – depth curves.
Evidence for the lithosphere
Using earthquakes
The internal structure of the Earth is composed of a series of concentric layers. The
evidence for this comes from earthquake waves. There are three main types of seismic
waves: P, S and L.
Exercise 1
Compare and contrast P, S and L waves.
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Module 5: Theme4 Lithosphere – Key Idea 1
The P and S waves can be used to determine the structure of the Earth because as they
pass from one layer to another, they become refracted.
Exercise 2
Using diagrams explain what “refracted” means and show how a seismic wave becomes
refracted as it passes from (i) a fast layer to a slow layer and (ii) from a slow layer to a
fast layer.
Exercise 3
(a) Sketch and label a diagram to show the typical pattern on a seismogram.
(b) Sketch and label a similar diagram for an earthquake further away from the recording
station.
If the recording station is only a few hundred kilometres from the focus and the
earthquake originated within the crust, then the seismogram records P and S waves that
have only travelled through the crust. These are very useful for determining the structure
of the crust and how its thickness varies.
The behaviour of P and S waves
Near the top of the mantle, the P wave speed is approx 7.8 – 8.1 km/s to about 13.6 km/s
at the base. S waves increase in speed from 4.35 km/s to 7.25 km/s over the same depths.
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Module 5: Theme4 Lithosphere – Key Idea 1
Exercise 4
Write a sentence describing the relationship between speed and depth.
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Seismometers all around the world are used to measure and record earthquake events.
The seismic waves pass through the earth and are recorded at the seismograph station, but
at a certain distance from the epicentre, the seismic waves are not recorded. These
regions are called shadow zones. The P wave shadow zone begins at 105 from the
epicentre and then reappear at 143 . The S wave shadow zones begins at the same point
but they do not reappear.
Exercise 5
Sketch two diagrams which show the shadow zones and the epicentral angles.
Diagrams showing the changes in speed of P and S waves with depth are used to identify
the changes from one layer to another. These are called discontinuities. In some regions,
the change from one layer to another is not sharp and sudden, but more gradual. These
regions are called transition zones.
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Module 5: Theme4 Lithosphere – Key Idea 1
Exercise 6
Compare the compositional and mechanical layering models.
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Layers in the Earth
Layer
Depth (km)
Velocity of P waves
(km/s)
Density g/cm
CRUST
A
Oceanic 4.8
3.3
Continental 33
UPPER MANTLE
33
B
7.9 - 8.1
50
250
L.V.Z.
2.7 – 2.9
3.32
7.8
8.1
400
400
8.9
9.13
3.54
3.72
670
10.27
3.99
C
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Module 5: Theme4 Lithosphere – Key Idea 1
LOWER MANTLE
D
OUTER CORE
E/F
INNER CORE
G
670
10.75
4.38
2891
2891
13.71
8.06
5.57
9.90
5149
5149
10.36
11.03
12.2
12.8
6371
13.1
Why does the Earth have a lithosphere?
What is it?
The velocity-depth curves show that there is a Low Velocity Zone (LVZ) within the
mantle. This corresponds to mantle material at or near its melting point (1300 C). This
shows that the mantle material at this depth behaves differently compared with mantle
above and below. The low velocity zone is ductile, while the mantle around it is rigid.
So it makes sense to group the rigid mantle above the LVZ with the crust because they
behave similarly; together they are called the lithosphere. The thickness varies between
80 and 125 km, the lower boundary of the lithosphere is defined by the 1300 C isotherm.
The LVZ is also called the asthenosphere.
Exercise 7
Explain why the deepest earthquake foci are recorded at a maximum depth of 720 km.
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Temperature changes
So far we’ve said that the lithosphere is defined on the basis of the temperature changes
within the mantle. You should remember that this can be described by graphs showing
the geotherm.
Exercise 8
Define the geotherm.
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Define the geothermal gradient
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Module 5: Theme4 Lithosphere – Key Idea 1
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When the Earth formed, the outer part cooled quicker than the rest of the planet and so
formed a solid outer shell. This part is rigid and is what we now call the lithosphere.
Because it is rigid it loses heat mainly by conduction but the asthenosphere below is
ductile, so it flows, losing its heat by convection.
Exercise 9
Sketch and label a geotherm through the outer part of the Earth (not near a plate
boundary).
The material of the asthenosphere is near to its melting conditions, so changes to the
physical conditions may lead to partial melting.
Exercise 10
Define partial melting.
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Exercise 11
Draw a diagram similar to your answer in ex. 8 to show how partial melting can occur.
ESTA GEOTREX The Geology Teachers Resource Exchange Contributor: Owain Thomas
School Date: 22:04:05
Establishment: Amman Valley
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