Presentation on theme: "How Do Earthquakes Tell Us About the Earth’s Interior?"— Presentation transcript:
How Do Earthquakes Tell Us About the Earth’s Interior?
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
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
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 km thick (~40 km average)
Further Subdivisions Depth (km)
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
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…
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
The Core Crust and mantle occupy 85% of Earth by volume but are below average density – Core must have density of g/cm 3 – Iron + nickel Remember Wiechert and his meteorites
War + Science? WWII and the Nazis: MORs – A seismically active ridge the length of Atlantic – And in most of the other ocean basins
Earthquakes of the World Mid-ocean ridges
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!
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
Rocks: Magnetism and Time
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
Support for “Seafloor Spreading” Hypothesis – Molten material rises, cools against both sides of spreading axis, and magnetic band is formed – Ocean crust is growing!
Wasn’t Mantle ~ Solid? Mantle is HOT, solid under HIGH pressure Crust splits, pressure drops Temp wins