Transcript No Slide Title - physicalallen

Continental Drift
Alfred Wegener proposed theory in
1912 that the continents had moved.
All the continents were once one big
continent and moved apart.
Evidence included jigsaw fit of
continents, fossils, rocks and
continuation of geologic features.
Alfred
Wegener
Meteorologist
Mechanism of movement of continents wrong.
Unexplained Observations
Volcanoes occur on edges of
continents and rim of Pacific Ocean
Mountain range extending around
the world (40,000 miles) capped by
a grand canyon-like valley.
Deep ocean trenches along volcanic
mountain chains.
• Why mountain ranges are located
where they are and why there are
different types of mountain ranges.
• Seismologists observe earthquakes
occur in a line and deep
earthquakes occur along inclined
planes.
20th Century Technology
Breakthroughs
• Radiometric Dating
• Fathometer (echo sounder) WWII
Hess measured m-Ar and guyots,
led to conclusion that ocean crust
pulling apart and new sea floor was
created by volcanic eruptions.
• Magnetometer and drilling ships
Sea Floor Spreading
• Dietz-new crust formation SFS
• Paleomagnetism~ magnetic bands
occur in pairs on both sides of plate
boundaries (Vine and Matthews)
• Younger rocks found closer to
spreading centers (s.c.), age
increases as distance increases
away from s.c. (drillships)
<---------Older
Older ------->
Plate Tectonics
“construction”
• Study of the formation and
movement of plates
Crust is divided into a dozen major
plates & many other smaller plates
Each plate is moving relative to the
other plates creating Earth’s
surface features ~ mountains,
trenches & volcanoes
Thickness of Plates
• Crust and upper mantle make up
the lithosphere ~~ 100 Km thick
• Most of lithosphere is made of
3
basalt~~ oceanic , thin, 2.9 g / cm
• Continental crust made of granite
material ~~ thicker, 2.7 g / cm 3
Plate Boundaries
• Diverging Boundaries- spreading
centers where plates are moving
apart.
• Location of mid-ocean ridges, rift
valleys and fracture zones
• Examples ~~ mid-Atlantic Ridge
and East Pacific Rise, East African
Rift
Converging Boundaries
2 plates move toward each
other
Three Types
Subduction 1)Ocean-continent
2)Ocean- Ocean
Collision
3)Continent-Cont
Subduction Boundary
• oceanic plate collides with
continental plate or two ocean
plates collide with each other
• Oceanic plate denser/ subducts
under continental plate
• Trenches bordered by volcanoes
and mountain chains, earthquakes
deeper, lithosphere destroyed
Collision BoundaryCollision of two continents.
Examples:
Himalayas - India and Asia
Ural- North America and Europe
Appalachians - NA and Africa
Sliding Boundaries
Two plates are moving past one
another
• Example ~~ San Andreas Fault
Pacific Plate moving NW relative
to the North American Plate
Plate Movement
• Lithosphere rests on the
asthenosphere ~ a layer in the
mantle
• Rocks in the asthenosphere are
partially melted and flow slowly
• Convection currents rising and
sinking push the plates apart and
pull them together
Craton
• Continent cores that are the oldest
and most deformed rocks on cont.
• North American craton where
exposed is called the Canadian
Shield, rest is below surface
• The N.A. craton shows the shape of
continent approximately 2.5 bya
• Continent has grown since
Sources of Growth Material
• Deep-sea sediments - scraped off
during subduction of oceanic plate
• Volcanic Rock- volcanism
associated with subduction
• Deposition of sediments by rivers
• Thin-Skinned Thrusting - pushing
of thin, horizontal sheets of rock
onto continental margins
Terranes - a block of lithospheric
plate that has been moved a great
distance and attached to continent
Identification of Terranes
• bounded on all sides by faults
• rocks and fossils do not match with
neighboring terranes
• magnetic polarity does not match
Ex. Cache Creek terrane BC Canada