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Plate Tectonics
Structure of the Earth | Plate
Tectonics | Links
- Structure of the Earth
- Earth’s lithosphere is broken up into
plates
- Hot, weak asthenosphere allows for plates
to move
- Plates are in motion and continually
changing in shape and size
- Move very slowly – 5 cm/yr
- Internal Composition
- Layers – by composition
- Layers – by physical properties
- Lithosphere
- Asthenosphere
- Mesosphere
- Outer Core
- Inner Core
- Crust
- Thinnest of Earth's divisions
- Varies in thickness (exceeds 70 km
under some mountainous regions while oceanic crust
ranges from 3 to 15 km thick)
- Two parts
- Continental crust
- Average rock density about
2.7 g/cm3
- Composition comparable to
the felsic igneous rock granodiorite
- Oceanic crust
- Density about 3.0 g/cm3
- Composed mainly of the igneous
rock basalt
- Mantle
- Contains 82 percent of Earth's volume
- Solid, rocky layer
- Upper portion has the composition
of the ultramafic rock peridotite
- Two parts
- Mesosphere (lower mantle)
- Asthenosphere or upper mantle
- Core
- Larger than the planet Mars
- Mostly iron with some nickel
- Average density is nearly 11
g/cm3
- Two parts
- Outer core - liquid outer layer
about 2270 kilometers thick
- Inner core - solid inner sphere
with a radius of 1216 kilometers
- Responsible for Earth’s magnetic
field
- Made of material that conducts
electricity
- Core is mobile
- Seismic waves
- P waves
- Travels through liquids as well as
solids
- In all materials, P waves travel
faster than do S waves
- S waves
- Cannot travel through liquids
- Seismic waves refract as they pass from
one material to another
- Isostacy
- The balancing of pressures exerted by
mass of continents and ocean crust on mantle
- Continents float on mantle like icebergs
in water
- Plate Tectonics
- Plate boundaries
- All major interactions among individual
plates occur along their boundaries
- Types of plate boundaries
- Divergent plate boundaries
- Convergent plate boundaries
- Transform fault boundaries
- Divergent Plate boundaries
- Most are located along the crests
of oceanic ridges
- Also continental rifts
- One presently is on land
- Quicktime
Movie
- Convergent plate boundaries
- Where two plates collide
- Types
- Oceanic-continental
convergence
- Denser oceanic slab sinks
into the asthenosphere
- As the plate descends, partial
melting of mantle rock generates magmas having
a basaltic or, occasionally andesitic composition
- Mountains produced in part
by volcanic activity associated with subduction
of oceanic lithosphere are called continental
volcanic arcs (Andes and Cascades)
- Oceanic-oceanic convergence
- When two oceanic slabs converge,
one descends beneath the other
- Often forms volcanoes on
the ocean floor
- If the volcanoes emerge as
islands, a volcanic island arc is formed (Japan,
Aleutian islands, Tonga islands)
- Continental-continental convergence
- Continued subduction can
bring two continents together
- Less dense, buoyant continental
lithosphere does not subduct
- Result is a collision between
two continental blocks
- Process produces mountains
(Himalayas, Alps, Appalachians)
- Transform fault boundaries
- Plates slide past one another
- No new lithosphere is created or
destroyed
- Transform faults
- Most join two segments of a mid-ocean
ridge as parts of prominent linear breaks in the
oceanic crust known as fracture zones
- A few (the San Andreas fault
and the Alpine fault of New Zealand) cut through
continental crust
- Hot spots
- Caused by rising plumes of mantle
material
- Volcanoes can form over them (Hawaiian
Island chain)
- Most mantle plumes are long-lived
structures and at least some originate at great depth,
perhaps at the mantle-core boundary
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