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How Do Earthquakes Tell Us About the Earth’s Interior?

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Presentation on theme: "How Do Earthquakes Tell Us About the Earth’s Interior?"— Presentation transcript:

1 How Do Earthquakes Tell Us About the Earth’s Interior?

2 Wave Refraction When a wave (light, sound, H 2 O) passes from one material to another, it refracts (bends) or reflects (bounces back) – Waves change speed between mediums – In water, changes occur with depth – In Earth, w/density

3 An Earth Sized Sonogram… A. Mohorovičić (1909) – Waves from distant earthquakes traveled at a faster velocity than nearby quakes Increase in density Traffic example – Identified boundary between crust and mantle

4 Different Types of Crust Earthquakes travel faster through oceanic rock than continental rocks – Must be difference in density OCEAN CRUST Basalt (Gabbro) High iron (Fe) content, dense ~7 km thick on average CONTINENTAL CRUST “Granites” Higher silicon (Si), low density 20-70 km thick (~40 km average)

5 Further Subdivisions Depth (km)

6 Crust and lithosphere “Low Temp Shell” – The crust and the top part of the mantle down to an average of 100 km constitutes the rigid lithosphere

7 Below the rigid lithosphere, is the asthenosphere – Hot and weak (plastic) – Though solid, over 1000s to millions of yrs flows like hot wax due to pressure of crust – Solid Mantle below this When the Temperature Rises…

8 The Core? Between 105° and 140° no seismic signature – Due to refraction – No S-waves beyond 105° Liquid outer core at ~2900 km Solid inner core at ~5150 km

9 The Core Crust and mantle occupy 85% of Earth by volume but are below average density – Core must have density of 10-12 g/cm 3 – Iron + nickel Remember Wiechert and his meteorites

10 War + Science? WWII and the Nazis: MORs – A seismically active ridge the length of Atlantic – And in most of the other ocean basins


12 Earthquakes of the World Mid-ocean ridges

13 Holmes’ Radical Idea Oceanic crust separated along the mid-ocean ridge – Hot magma would well up out of the crack, cool, and the oceanic crust would grow!

14 The Outer Core as a Magnet Why: Why: – Electricity passed through a conducting material generates a magnetic field Outer core is hotter at base than top Outer core is hotter at base than top – Like pot of water, it convects – Convecting hot metal produces magnetic field – Acts like a bar magnet, causes compass needles to point ~N

15 Rocks: Magnetism and Time

16 The Irony of Science WWII: sensitive magnetic devices used to detect steel hulls of German submarines Identified magnetic bands on oceanic crust perfectly symmetrical about mid-ocean ridge

17 Mid-ocean ridges

18 Support for “Seafloor Spreading” Hypothesis – Molten material rises, cools against both sides of spreading axis, and magnetic band is formed – Ocean crust is growing!

19 Wasn’t Mantle ~ Solid? Mantle is HOT, solid under HIGH pressure Crust splits, pressure drops Temp wins

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