the world as it turns
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The World as it Turns. Changes in the Earth’s Crust…Plate Tectonics. Parts of the Earth…. There are 5 main layers, from surface to center: Lithosphere (crust) : solid outermost layer, about 5-50km deep; - PowerPoint PPT PresentationTRANSCRIPT
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The World as it TurnsChanges in the Earth’s Crust…Plate Tectonics
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Parts of the Earth…• There are 5 main layers, from surface
to center:– Lithosphere (crust): solid outermost
layer, about 5-50km deep; – Asthenosphere: the lower layer of the
earth's crust; less rigid than the lithosphere—more slushy/flexible
– Mantle: region below crust extending down to Earth's core; some molten rock; less flexible
– Liquid Outer Core: made of molten iron/nickel; flows & creates Earth’s magnetic field
– Solid Inner Core: solid iron/nickel center of the Earth; high temperatures & pressure; like a solid
How do we know?
Earthquakes
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The Theory of Continental Drift
Alfred Wegener, 1912
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Continental Drift…• proposed – continents slowly drift in
relation to one another; continents were once joined, single super-continent (Pangaea)
Interactive Map of Pangaea
Alfred Wegener
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Continental Drift: The Evidence…• shape of the continents - puzzle-like fit• same animal fossils – fresh water fossils-opposite
shores of Atlantic Ocean • same sequence of rock layers - found on opposite
shores of Atlantic Ocean along coastlines• fossils of tropical plants - found in polar areas• glacial deposits - found in tropical areas
More on the evidence
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Continental Drift: Lack of Support…• Wegener’s theory did not go over well in the scientific
community…– no explanation how the continents “drift” – continents plow through sea floor , really???– What strong force moves huge mass-large distance???
• To read an excellent historical account of Continental Drift with informative graphics, visit the U.S. Geological Survey.
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Crustal History Confirmation
A.Describe the evidence Wegener used to support his theory of Continental Drift.
B.Why did the scientific community not believe in Wegener’s theory?
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Sea Floor Spreading
More Evidence is Found
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Sea Floor Spreading• 1940’s, use sonar to map
ocean floor• Discovery of ridges
(underwater mountains), middle Atlantic Ocean– extended entire length (N -
S) of ocean– More ridges found in Indian
and Pacific Oceans
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Sea Floor Spreading: The Evidence…• 1968, scientists drill cores of rock from ocean floor
– youngest rock at the mid-ocean rift; oldest at continental coasts
– Earth’s magnetic field constantly shifting, • On average, Atlantic Ocean spreads ~ 1.25 cm/yr, similar to
fingernail growth
Polarity reversals same
distance from rift on either
side
N N N NN N N S SS SS SS S
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Sea Floor Spreading: the How…• Occurs at MID-OCEAN RIDGES • CONVECTION – Hot, less dense magma rises, flows sideways ,
cools & sinks• Magma/crust friction pulls crust; rift fills with magma/lava, cools -
creating NEW crust• Ocean lithosphere (crust) is youngest NEAR mid-ocean ridge
Mid Ocean Ridge
Convection Currents
Oceanic Crust (lithosphere)Rising
Magma
youngest
oldest
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Sea Floor Spreading: The How…• Sea Floor Spreading animation• Remember: convection currents CAUSE sea floor spreading
—gravity causes convection currents• Convection Currents and Plate Movement animation
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Crustal History Confirmation
A.Explain sea-floor spreading.B.When examining rock samples from
the ocean floor, identify 2 things that can be found that support the hypothesis of sea floor spreading.
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Where are we today?
The Plate Tectonic Theory
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The Plate Tectonics Theory• Plate Tectonics Theory combines continental drift & seafloor
spreading• Lithosphere floats on denser, liquid rock in mantle called
asthenosphere.• Lithosphere broken into tectonic plates• Tectonic plates move about 1-8 cm/yr
• This movement is the “continental drift” referred to by Wegener
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The Plate Tectonics Theory• New plate added by rising magma at divergent boundaries (think
mid-ocean ridges)• Plates are destroyed in subduction zones at convergent
boundaries
• Transform boundaries slide past each other
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Plate Boundary Movements…• 3 types of plate boundary movements
– Convergent: Boundary between two plates that are pushing together.
– Divergent: Boundary between two plates that are moving apart.
– Transform: Boundary between two plates that are sliding past one another.
• ANIMATIONS
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When Boundaries Collide: Convergence…• Continental vs. oceanic
– More dense oceanic plate dives under less dense; called subduction
– dense, leading edge of the oceanic plate actually pulls the continental plate down forming a trench
– Crust heats and melts as it is forced downward below the continental crust
– Hot gases and magma forced upward creating a volcanic mountain range on the continent
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When Boundaries Collide: Convergence…• Oceanic vs. Oceanic
– the farther a plate gets from the mid-ocean ridge that created it, the colder and denser it gets
– when two oceanic plates collide, the plate that is older, therefore colder and denser, is the one that will sink (subduct)
– this subduction zone forms a curved volcanic mountain chain
– Aleutian Peninsula of Alaska is an example of a very volcanically-active island arc
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When Boundaries Collide: Convergence…• Continental vs.
Continental– two continental plates
meet head-on, neither can sink; both plates are too buoyant
– solid rock is folded and faulted
– huge chunks of rock many kilometers wide are thrust on top of one another, forming a towering mountain range
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Himalayan Mtns. Taken by Satellite, 2004…
India(Indian Continental Plate)
Nepal
Tibet (\Eurasian Continental Plate)
Himalayan Mountain Range
* Himalayan mountain range is the perfect example of how the highest mountains in the world continues to grow
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Plate Tectonics: Boundaries• They Move! – Convergence
• Turn to your neighbor and explain converging tectonic boundaries
• Use handy supplies to help your explanations
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When Boundaries Collide: Divergence…• hot magma moves slowly upward, cooler magma near surface
moves slowly downward forming convection currents within the asthenosphere
• Rising convection currents diverge where they approach the surface pulling on the plate above it creating a divergent plate boundary
• two sides move away in opposite directions, cracks between the diverging plates fill with molten rock which cools and quickly solidifies, forming new oceanic crust
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When Boundaries Collide: Transform…• At transform plate boundaries, plates slide/grind past each other.• separates the North American plate from the Pacific plate along the
San Andreas fault, a transform plate boundary responsible for many California’s earthquakes
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The World as it TurnsIn Closing…
• Turn to your neighbor and explain converging, diverging and transform boundaries tectonic boundaries
• Use handy supplies to help your explanations• Be prepared to share your “demonstrations”
with the class
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Review…• As plates move, the structure of the earth changes, creating
volcanoes and building mountains