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geology
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8/25/13
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The Earth’s Interior As defined by its
chemical composi>on….
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What are the Crust & Mantle made of?
• Silicate Rock: – Rock made with the silicon-‐oxygen tetrahedron.
• Silicate rocks will also have varying amounts of iron:
– Felsic -‐ Less than 31% Fe/Mg content. – Intermediate – 32-‐47% Fe/Mg content. – Mafic – 48-‐54% Fe/Mg content. – Ultramafic – greater than 55% Fe/Mg content
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USGS
Ocean Crust – Juan de Fuca Ridge (Pacific) Con>nental Crust – Yosemite, CA (Half Dome)
Bri4le Mantle
NOAA USGS
Mohorovičić Discon>nuity Note: Mojo for short
Boundary between the Earth’s crust and the mantle.
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Crustal Thickness and Depth to the Mojo
Note: Rocks behave In a brifle manner Above and below the Mojo.
• Con7nental Crust • Thickness: 30-‐70 km
– Thickest at mountains
• Lightest of all layers – Density: 2.7 g/cm3
(Felsic)
• Oldest of all layers – Up to 3.8 billion years old
• Best understanding – It’s right underneath our feet!
• Oceanic Crust • Thickness is 3-‐10 km • Denser
– 3.0 g/cm3 (Mafic)
• Very young – Less than 200 million years old.
• Primarily Igneous – Mid-‐ocean Ridge Volcanism
– ~75% of all volcanism happens here.
5 6 South Pacific, 2009 (NOAA)
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Mantle
• Composi>on – Ultramafic
• Very thick – 2,883 km – or ~1791 miles
• Density – 3.3 g/cm3
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Core
• Composi>on – Mostly iron and nickel. (90%)
• Very thick! – 3,486 km – or ~2,160 miles
• Density – 11 g/cm3
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The Earth’s Interior As defined by its physical proper>es...
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Physical Proper>es: How rocks respond to stress….
• Liquids – The state of mafer in which molecules flow freely by one another and lack crystal structure.
• Solids – Brifle
• Breaks under stress – Duc>le
• Flows under stress, but not freely
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Flow in the solid state…….
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Before Aper
Flow in the solid state for Earth’s layers is MUCH slower than clay!
Lithosphere
• Earth’s cool, rigid brifle outer layer
• Includes both: – Con>nental and oceanic crust
– the uppermost cool and brifle part of the mantle
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(also brifle)
Duc>le
Reminder: The difference between the crust and the uppermost mantle is composi>onal only. They behave the same when stressed (brifle).
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Asthenosphere
• Hot, duc>le and slowly flowing solid.
• Moves independently of the lithosphere.
• Moves very very slowly compared to the clay example.
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(also brifle)
Duc>le
Mesosphere (aka Lower Mantle)
• Chemical Composi>on is similar to the asthenosphere – Why?
• Temperature is hofer, but it does not melt – Why?
• Flows even more slowly than the asthenosphere. 14
Outer Core
• Dense viscous liquid • Why is the outer core liquid even at these extreme pressures?
• Generates the Earth’s magne>c field due to the movement of metallic iron.
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Inner Core
• Behaves as a solid • Immense pressure strengthens metallic minerals at this depth.
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Some important notes from lecture: • The lithosphere does not equal the crust!
– These two layers are defined differently and occupy different (but overlapping) spaces in Earth’s interior.
• The base of the lithosphere, the asthenosphere and mesosphere are all ultramafic (and therefore are all a part of the mantle).
• Plates are made of the en>re lithosphere, not just the crust.
• The asthenosphere and mesosphere are considered solid because atoms are s>ll bonded together in a crystalline larce.
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Evidence for Earth’s Layers
By Physical Proper>es By Composi>on
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Evidence for Layering by Physical Proper>es
• Seismicity – Body Waves
• Primary Waves • Secondary Waves
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Primary Waves (P-‐waves)
• Compressional waves • Can travel through both liquid and solid • Move faster than S-‐waves • Travels at different speeds through different mediums
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Secondary Waves (S-‐waves)
• Transverse waves • Only travels through solids • Travels at different speeds through different mediums
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What waves can do?
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The Evidence for Layering by Composi>on
• Crust – Field observa>ons – Ocean floor drilling
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