Download - P1 L4
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Lesson 5
Structure of the Earth
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Learning Outcomes:
You will be successful if you can:
• Recall the physical structure of the Earth.
• Discuss how multiple sources of data are required in order to formulate a scientific theory.
• Explain how the use of seismic data led to the ‘discovery’ of Earth’s structure.
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• Some seismometers measure vertical ground movement
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• Some seismometers measure vertical ground movement.
• When combined we can use this information to calculate the location of an earthquake (the epicenter).
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Seismogram
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Seismic wavesEarthquakes travel as waves through the Earth – we call them SEISMIC WAVES. There are two types:
P waves:1) They are longitudinal so they cause the
ground to move up and down
2) They can pass through solids and liquids
3) They go faster through more dense material
S waves:1) They are transverse so they cause the
ground to move from right to left
2) They ONLY pass through solids
3) They are slower than P waves
4) They go faster through more dense material
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Seismic WavesSeismic WavesSeismic WavesSeismic Waves
Seismic waves
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SeismogramP Wave
S Wave
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The Structure of the EarthA thin crust - 10-100km thick
A mantle – has the properties of a solid but it can also flow
A core – made of molten nickel and iron. Outer part is liquid and inner part is solidHow do we know this? These facts have all been
discovered by examining seismic waves (earthquakes)
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Seismic waves
P waves travel through the Earth and are refracted when they pass through a medium
S waves will only travel through a solid
The paths of these waves are all curved because density is gradually changing
These observations tell us 3 things about the Earth: 1) It has a thin crust, 2) it has a semi-fluid mantle where density increases with depth, 3) a core with a liquid outer part and a solid inner part.
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Shadow zone
• Area of the earth from angular distances of 104o to 140o degrees that, for a given earthquake, does not receive any direct P waves. The shadow zone results from S waves being stopped entirely by the liquid core and P waves being bent (refracted) by the liquid core.
• Through measuring how P and S waves travel through the earth and out the other side, a seismic wave shadow zone was discovered in about 1910. From the lack of S waves and a great slowing of the P wave velocity (by about 40%) it was deduced that the outer core is made of liquid. The shadow zone also defined the diameter of the core.
140o
- 143o
103o
- 104o