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# P1 L4

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### P1 L4

1. 1. Lesson 5 Structure of the Earth
2. 2. 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.
3. 3. • Some seismometers measure vertical ground movement
4. 4. • Some seismometers measure vertical ground movement. • When combined we can use this information to calculate the location of an earthquake (the epicenter).
5. 5. Seismogram
6. 6. Seismic waves Earthquakes travel as waves through the Earth – we call them SEISMIC WAVES. There are two types: P waves: 2) They are longitudinal so they cause the ground to move up and down 3) They can pass through solids and liquids 4) They go faster through more dense material S waves: 2) They are transverse so they cause the ground to move from right to left 3) They ONLY pass through solids 4) They are slower than P waves 5) They go faster through more dense material
7. 7. Seismic Waves
8. 8. Seismogram P Wave S Wave
9. 9. The Structure of the Earth A 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 solid How do we know this? These facts have all been discovered by examining seismic waves (earthquakes)
10. 10. Seismic waves S waves will only travel through a solid P waves travel through the Earth and are refracted when they pass through a medium 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.
11. 11. 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 103o S waves travel through the - 104o 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 140o made of liquid. The shadow - 143o zone also defined the diameter of the core.