P1 L4
- 1. Lesson 5 Structure of the Earth
- 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. Some seismometers measure vertical ground movement
- 4. Some seismometers measure vertical ground movement. When combined we can use this information to calculate the location of an earthquake (the epicenter).
- 5. Seismogram
- 6. Seismic waves Earthquakes travel as waves through the Earth – we call them SEISMIC WAVES. There are two types: P waves: They are longitudinal so they cause the ground to move up and down They can pass through solids and liquids They go faster through more dense material S waves: They are transverse so they cause the ground to move from right to left They ONLY pass through solids They are slower than P waves They go faster through more dense material
- 7. Seismic waves Seismic Waves
- 8. Seismogram P Wave S Wave
- 9. The Structure of the Earth How do we know this? These facts have all been discovered by examining seismic waves (earthquakes) 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
- 11. Seismic waves 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. P waves travel through the Earth and are refracted when they pass through a medium S waves will only travel through a solid
- 12. Shadow zone Area of the earth from angular distances of 104 o to 140 o 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. 140 o - 143 o 103 o - 104 o