Plate Boundaries Chapter 3. 1.Divergent 2.Convergent 3.Transform.
Divergent Plate Boundaries
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Transcript of Divergent Plate Boundaries
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Divergent Plate Boundaries
Finz 2012
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Plates move about 2 inches/year - about the same rate as your fingernails grow!
Plate Tectonics
There are 3 types of plate boundaries:1. divergent2. convergent3. transform
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Tectonic Plate Boundaries
Divergent boundariesPlates move apartMagma rises, cools and forms new lithosphereTypically expressed as mid-oceanic ridges
Transform boundariesPlates slide past one another Fault zones, earthquakes mark boundarySan Andreas fault in California
Convergent boundariesPlates move toward each otherMountain belts and volcanoes commonOceanic plates may sink into mantle along a subduction zone, typically marked by a deep ocean trench
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Divergent Plate Boundaries:Divergent plate boundaries are where seafloor spreading occurs, producing new oceanic crust. Material from mantle intruded into fractures as plates are move apart.
Ocean basins form when continents split apart!
The crust of the ocean is basaltic rock on top and gabbro on the bottom.
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Early evidence of seafloor spreading
Old mountain belts show us where continents used to be connected
Old mountains belts (Appalacians and Caledonides) now separated but if continents are fit together, mountain chains form a continuous belt
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More recent evidence of seafloor spreading
1. Symmetry of magnetic stripes (defined by polarity of magnetic minerals in basaltic rock of seafloor)
The pattern of normal and reverse polarities on either side of a divergent boundary can only be explained if new crust was being formed and repeatedly split apart as magnetic field reversed
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But how does seafloor spreading (divergence) start?
Hot plume in mantle upwarps lithosphere of continent
Cracks develop forming rift valleys
Rift zones allow further spreading to produce an ocean because the water moves into the low area created from the divergence
•http://www.iris.edu/hq/files/programs/education_and_outreach/aotm/11/AOTM_09_01_Divergent_480.mov
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Red Sea-Gulf of Aden: An ocean basin in the making
Future ocean basin
East AfricanRift will probably stop spreading and become a “failed arm”
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Divergent boundaries• Perhaps the best known of
the divergent boundaries is the Mid-Atlantic Ridge.
• The rate of spreading along the Mid-Atlantic Ridge averages about 2.5 centimeters per year (cm/yr), or 25 km in a million years.
• The mechanism that drives seafloor spreading was thermal convection cells in the mantle
• hot magma rises from mantle to form new crust
• cold crust subducts into the mantle at oceanic trenches, where it is heated and recycled
Mid-Atlantic Ridge
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Oceanic Crust Is Young
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• Ridges also have – high heat flow– and basaltic flows or pillow lavas
Divergent Boundaries
• Pillow lavas have– a distinctive
bulbous shape resulting from underwater eruptions
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• Divergent boundaries
Divergent Boundaries
• Beneath a continent, – magma
wells up, and
– the crust is initially • elevated, • stretched • and thinned
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• The stretching produces fractures and rift valleys.
Rift Valley
• Example: East African Rift Valley
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Narrow Sea• As spreading proceeds, some rift valleys
– will continue to lengthen and deepen until
a narrow linear sea is formed,
– Examples: •Red Sea •Gulf of
California
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Modern Divergence– View looking down the
Great Rift Valley of Africa.• Little Magadi
soda lake
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Oceanic Divergent BoundaryExample: Mid-Atlantic Ridge
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Continental Divergent BoundaryExample: Red Sea / E. African Rift
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Continental Divergent BoundaryExample: Baja California
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• Spreading ridges– As plates move apart new material is
erupted to fill the gap
Divergent Boundaries
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• Iceland has a divergent plate boundary running through its middle
Iceland: An example of continental rifting
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Divergent Boundary
• http://www.youtube.com/watch?v=FK1s1-OJ5BE
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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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Presumably,Pangea was ripped apart
by such continental
rifting & drifting.
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Divergent Cross Section View