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Plate Tectonics
Continental drift | Sea
Floor Spreading Hypothesis | Structure of the
Earth | Plate Tectonics | Links
- Continental drift
- Background
- Researchers noted geographic fit of continents,
e.g. Africa and S. America
- Atlantic formed by separation of Africa
from S. America
- Seuss, 1885, proposed super continent
by studying fossils, rocks, mountains
- Alfred Wegener
- First proposed his continental drift
hypothesis in 1915
- Published The Origin of Continents
and Oceans
- Continental drift hypothesis
- Evidence used in support of continental drift
hypothesis
- Fit of the continents
- Fossil evidence
- Rock type and structural similarities
- Paleoclimatic evidence
- Objections to the continental drift hypothesis
- Inability to provide a mechanism capable
of moving continents across the globe
- Wegner suggested that continents broke
through the ocean crust, much like ice breakers
cut through ice
- Sea Floor Spreading Hypothesis
- Proposed by Harry Hess in the late 1950s
/ early 1960s
- New crust forms at ocean ridges
- Old crust consumed at trenches
- Developed theory of sea floor spreading
driven by convection cells
- Evidence
- Earthquakes at ridges and subduction
zones
- Heat flow highest at ridges, decreases
away from ridges
- Radiometric dating shows oldest rocks
on sea floor of 200 m.y.
- Sediments older away from ridges
- Earth’s magnetic field reverses,
this change recorded by iron minerals in rocks
- Earth's magnetic field resembles
that produced by a large bar magnet.
- Magnetic north is not true north
- Paleomagnetism
- Magnetic field of the Earth
is imprinted upon iron-bearing rocks when
they form
- Provides a record of magnetic
reversals and original latitude
- Seafloor mapping in 1950’s
and 1960’s revealed magnetic striping
on the ocean floor.
- Putting it all together: The Theory of Plate
Tectonics
- Continental Drift
- + Sea-Floor Spreading
- + Paleomagnetism
- = Plate Tectonics
- 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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