EARTH… What lies beneath. WHAT LIES BENEATH… CRUST: Top layer of Earth’s internal structure...
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Transcript of EARTH… What lies beneath. WHAT LIES BENEATH… CRUST: Top layer of Earth’s internal structure...
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EARTH…What lies beneath
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WHAT LIES BENEATH…
CRUST: Top layer of Earth’s internal structure that has two parts…
1.Basalt-rich oceanic crust
2.Granite-rich continental crust (much thicker than oceanic)
• Relatively cold in temperature• Rocky and brittle so it can
fracture during earthquakes
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WHAT LIES BENEATH…
MANTLE: Most of Earth’s mass comes from the mantle
•Comprised of iron, magnesium, aluminum, and silicon-oxygen compounds
• Upper 1/3 is known as the ASTHENOSPHERE• Asthenosphere is more plastic
in nature than rest of the mantle
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WHAT LIES BENEATH…
CORE: Center of Earth that has two parts…
1. Inner Core2. Outer Core
-Comprised of mostly iron
Inner Core – Under EXTREMELY high pressure so it remains solid
Outer Core – Temperature is so hot that it remains molten
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What is Plate Tectonics?
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If you look at a map of the world, you may notice that some of the continents could fit together like pieces of a puzzle.
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Plate Tectonic Theory
•Scientific theory which describes the large scale motions of Earth’s lithosphere
•Arose out of two separate observations:
•Continental Drift
•Seafloor Spreading
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Seafloor Spreading
Continental Drift
Plate Tectonics
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The Continental Drift Hypothesis
Proposed by Alfred Wegener in 1915.
Supercontinent Pangaea started to break up about 200 million years ago.
Continents "drifted" to their present positions.
Continents "plowed" through the ocean crust.
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Continental Drift: Evidence
Geographic fit of South America and Africa
Fossils match across oceans
Rock types and structures match across oceans
Ancient glacial features
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Tight fit ofthe
continents, especially
usingcontinental
shelves.
Continental
Drift:Evidence
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Continental Drift: Evidence
Fossil critters and plants
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Continental Drift:
Evidence
Correlation of
mountains with nearly
identical rocks and structures
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Continental
Drift:Evidence
Glacial features
of the same age
restore to atight polar
distribution.
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Seafloor SpreadingU.S. Navy mapped seafloor with echo sounding (sonar) to find and hide submarines. Generalized maps showed:
oceanic ridges—submerged mountain ranges
fracture zones—cracks perpendicular to ridges
trenches—narrow, deep gashes
abyssal plains—vast flat areas
seamounts—drowned undersea islands
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How magnetic reversals form at a spreading center
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Bands of seismicity—chiefly at trenches and oceanic ridges
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Plate Tectonics•The Earth’s crust is divided into 12
major plates which are moved in various directions.
•This plate motion causes them to collide, pull apart, or scrape against each other.
•Each type of interaction causes a characteristic set of Earth structures or “tectonic” features.
•The word, tectonic, refers to the deformation of the crust as a consequence of plate interaction.
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Tectonic Plates on Modern Earth
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What are tectonic plates made of?
•Plates are made of rigid lithosphere.
The lithosphere is made up of
the crust and the upper part of the
mantle.
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What lies beneath the tectonic plates?
•Below the lithosphere (which makes up the tectonic plates) is the asthenosphere.
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Plate Movement•“Plates” of lithosphere are moved
around by the underlying hot mantle convection cells
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What happens at tectonic plate boundaries?
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The Theory of Plate Tectonics
Earth’s outer shell is broken into thin, curved plates that move laterally atop the asthenosphere
Most earthquakes and volcanic eruptions happen at plate boundaries.
Three types of relative motions between plates:
“group authorship” in 1965-1970
divergent convergent transform
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•Divergent
•Convergent
•Transform
Three types of plate boundary
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•Spreading ridges
•As plates move apart new material is erupted to fill the gap
Divergent Boundaries
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Divergent boundaries: Chiefly at oceanic ridges
(aka spreading centers)
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Age of Oceanic Crust
Courtesy of www.ngdc.noaa.gov
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Divergent boundaries
also can rip apart (“rift”)
continents
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How rifting of acontinent could
lead to formation of
oceanic lithosphere.
e.g., Red Sea
e.g., Atlantic Ocean
e.g., East Africa Rift
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• Iceland has a divergent plate boundary running through its middle
Iceland: An example of continental rifting
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Presumably,Pangea was ripped apart
by such continental
rifting & drifting.
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•There are three styles of convergent plate boundaries
•Continent-continent collision
•Continent-oceanic crust collision
•Ocean-ocean collision
Convergent Boundaries
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•Forms mountains, e.g. European Alps, Himalayas
Continent-Continent Collision
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Himalayas
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•Called SUBDUCTION
Continent-Oceanic Crust Collision
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• Oceanic lithosphere subducts underneath the continental lithosphere
• Oceanic lithosphere heats and dehydrates as it subsides
• The melt rises forming volcanism
• E.g. The Andes
Subduction
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• When two oceanic plates collide, one runs over the other which causes it to sink into the mantle forming a subduction zone.
• The subducting plate is bent downward to form a very deep depression in the ocean floor called a trench.
• The worlds deepest parts of the ocean are found along trenches.
• E.g. The Mariana Trench is 11 km deep!
Ocean-Ocean Plate Collision
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•Where plates slide past each other
Transform Boundaries
Above: View of the San Andreas transform fault
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Most transform boundariesare in the oceans.
Some, like the one in California, cut continents.
The PAC-NA plate boundary is MUCH more complex than this diagram
shows.
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