Lithosphere Environmental Science pp' 124 127

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LithosphereEnvironmental Science pp. 124 - 127

• The solid part of the Earth
• Internally the Earth is divided into 3 zones
• Core - makes up a significant proportion of
  Earths mass despite its small volume due to its
  high density
• Composed primarily of iron with some nickel
• 2 distinct regions the liquid outer core (2,200
  km thick) and solid inner core (1,250 km thick)

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• Mantle - thick shell of rock that separates the
  core from the crust (2,900 km thick)
• Makes up most of Earths volume
• Much richer in iron and magnesium than the crust
  which makes it more dense
• Crust - outer layer of rock that forms a thin
  skin over the Earths surface (up to 100 km
  thick)
• Less dense than mantle
• 2 of Earths volume and less than 1 of Earths
  mass

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2 types of crust

• Continental crust - 30-40 km thick
• Rich in aluminum and silicate
• Lighter than oceanic crust so it floats higher
• Oceanic crust - 5-7 km thick
• Rich in iron and magnesium which make it very
  dense
• Forms through volcanoes at fissures called
  mid-ocean ridges - heat escapes the interior as
  matter emerges from below it gradually cools and
  moves away from the ridge in a process called
  seafloor spreading

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Lithospherecrustupper mantle (100km)Asthenosphe
remobile zone in mantle

• The asthenosphere is solid but convection occurs
  through the deformation of the rock
• Convection cells and mantle plumes
• Mantle material is heated at core-mantle boundary
• This material rises then eventually cools and
  thickens becoming more dense than the mantle
  below causing it to sink ? convection!
• This slow circulation is the dominant force in
  shaping the Earths surface structure

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History of plate tectonics

• 1915 - Alfred Wegener proposed theory of
  continental drift which says that continents
  migrate around the planet like ice floes
• Based on fit of South America and Africa, similar
  plant and animal fossils and similar geological
  structures
• His ideas were ignored because he could not offer
  a mechanism ?What forces could be strong enough
  to move such large masses of solid rock over such
  great distances?

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• 1940s and 50s - development of echo-sounders
  led to discovery of mid-ocean ridges (underwater
  mountain chain)
• 1960s - Glomar Challenger expedition
• Drilling into mid ocean ridges
• Found that crust gets thicker moving away from
  the ridge magnetic orientation reversals
• Concluded that there is growth of ocean floor at
  mid-ocean ridges where new crust is formed over
  convective currents in the asthenosphere

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Plate tectonics

• The theory states that plates of rigid
  lithosphere move above and upon a less viscous,
  fluid asthenosphere
• Tectonic plate massive, irregularly shaped slab
  of solid rock, often composed of both continental
  and oceanic crust
• 7 large plates African, North American, South
  American, Eurasian, Australian, Antarctic,
  Pacific
• http//geology.er.usgs.gov/eastern/plates.html

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• There are several minor plates including the Juan
  de Fuca, Arabian, Nazca and Phillipines plates
• The plates are all moving in different directions
  and at different speeds (2-15 cm per year - about
  the rate of fingernail growth!)
• Movement probably due to convection cells in
  mantle which carry plates along

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3 types of plate boundaries

• Constructive margins (divergent) - new crust is
  generated as the plates pull away from each other
  (youngest crust at ridges)
• Destructive margins (convergent or subduction
  zones) - crust is destroyed as one plate dives
  under another
• Transform boundaries - crust is neither produced
  nor destroyed but the plates slide horizontally
  past each other
• http//pubs.usgs.gov/publications/text/understandi
  ng.html

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Movement of continents

• Continents have been coming together and
  separating for hundreds of millions of years
• http//www.ucmp.berkeley.edu/geology/tecall1_4.mov
  
• Pangaea - supercontinent formed approximately 345
  mya (million years ago)
• About 190 mya Pangaea broke up into 2 large
  continents Laurasia (N. America and Eurasia) and
  Gondwanaland (Africa, Arabia, Australia,
  Antarctica, India, South America)
• http//www.exploratorium.edu/origins/antarctica/id
  eas/gondwana2.html

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Effect of plate tectonics on biodiversity

• Pangaea provided a stable environment for
  millions of years
• Upon the break up of Pangaea evolution through
  natural selection occurred in isolation on the
  two continents - ? biodiversity
• Generation of new and diverse habitats
• Greater habitat diversity means
• Speciation through geographical isolation
• Formation of physical barriers

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Physical barriers separate populations

• Mountain ranges
• e.g. Himalayas
• Canyons
• e.g. the Grand Canyon
• Oceans
• Land bridges - allow for migration of species
  which may lead to extinctions of some groups or
  colonization of new habitats