Lesson 5

Structure of the Earth

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.

• Some seismometers measure vertical ground movement

• Some seismometers measure vertical ground movement.

• When combined we can use this information to calculate the location of an earthquake (the epicenter).

Seismogram

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

Seismic WavesSeismic WavesSeismic WavesSeismic Waves

Seismic waves

SeismogramP Wave

S Wave

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)

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.

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