Transcript Tectonics 1 - Montville.net

Plate
Tectonics
Part 1
Whose Idea Was This
In The First Place?
The idea of continents in motion is
hundreds of years old
• In 1596, Dutch mapmaker Abraham
Ortelius stated that “the Americas were
torn away from Africa And Europe by
earthquakes and floods.”
Alfred Wegener• The idea was revived in the early
1900’s by German meteorologist,
Alfred Wegener
• He was inspired by the near
perfect fit of Africa and South
America.
• He would spend years researching all areas of
earth science to help support his theory.
Continental Drift
• Wegener searched for evidence that
supported his hypothesis that continents
were once joined.
• Evidence: Coastlines match with near
perfection
• Evidence: matching plant and animal
fossils found on both continents
• Impossible for the plants or animals to
swim or be transported across ocean, the
land must have been joined.
Matching Coastlines
Matching Mountain Ranges
Glacier Evidence
Fossil Evidence-Mesosaurus
More Evidence
• Drifting continents explained dramatic
climate changes on some continents.
• Tropical plant fossils (coal) in frozen
Antarctica
• Tropical fern fossils (Glossopteris) in now
colder climates.
• Glacial deposits in South Africa.
• Polar dinosaurs found in Australia
What has happened since
Pangaea broke up?
Theory Rejected
• Geologists at the time rejected Wegener’s
theory.
• Wegener suggested the continents plowed
through the ocean floor, or gravitational
pull from the moon could tug the
continents slowly into new positions
• Other scientists argued correctly that this
was physically impossible
• Despite so much evidence, without a
mechanism for moving the continents,
the theory was not accepted
Developing the theory
• Until new evidence was revealed, Wegener’s
continental drift theory lay dormant.
• Discoveries of ocean floor topography-trenches
and mid-ocean ridges and rift valleys
• Discovery of Earth’s changing magnetic field
• Development of Seafloor Spreading model
• Concentration of earthquakes and volcanoes at
plate boundaries
Ocean Floor Mapping Timeline
• Before WW1-believed ocean floor was flat
and featureless
• WW1-primitive sonar revealed a mid
Atlantic ridge
• 1947-seismic survey showed thin
sediments on ocean floor-expected to be
thicker because ocean floor was believed
to be very old
• 1950’s-mid ocean ridges discovered in all
oceans
Paleomagnetism
• 1950’s-scientists discovered magnetism in
ocean floor basaltic rocks
• The magnetic pattern created a striped
appearance when mapped
• The pattern was a mirror image on either
side of a mid-ocean ridge
Changing magnetic field
recorded in iron bearing rocks
Seafloor spreading
• Idea came from the magnetic striping
• Ages of rocks near mid-ocean ridges was
youngest
• Age increased away from ridge
• Age of rocks with same magnetism is
same
• Therefore must have been formed at mid
ocean ridge and been forced away by
formation of new crust
Seafloor spreading cont’d
• If new crust is formed at mid-ocean ridges,
why isn’t earth getting larger?
• Answer: it is being subducted and
recycled into the mantle at ocean trenches
at convergent boundaries
• Idea was proposed by Harry Hess, a
Princeton geologist and Navy Admiral
Ridge Push and slab pull
• Gravity causes raised mid ocean ridges to
push apart, causing seafloor spreading
• Gravity pulls at subducting ocean slabs at
convergent boundaries, dragging oceanic
crust into mantle
• The oldest ocean crust is 180 mya
• The oldest continental crust is almost 4
bya (that’s 4000 mya)
Ridge push and slab pull driven
by mantle convection
Earthquakes
• Earthquakes are concentrated at plate
boundaries