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Plate Tectonics
Definition of ‘Plate Tectonics’
Plate(s) – “flat” pieces of the lithosphere
that cover the dynamic earth
Tecton – Greek word for ‘movement’
Plate Tectonics – The study of
movement, creation, and destruction of
lithospheric (crustal) plates
Dynamic – constantly changing
Continental Drift Theory
Alfred Wegener (meteorologist), 1912
• Claimed that the continents
were once connected into one
huge continent called Pangaea
(means all lands)
• Scientists rejected Wegener’s
theory because he could not
prove WHY or HOW the
continents moved.
Alfred Wegener (cont.)
First clue:
• Continents
seem to ‘fit
together’
(South
America &
Africa)
Alfred Wegener (cont.)
original drawings from Wegener’s book
“Origin of the Continents and Oceans”
Evidence I – Fossil Clues
• Fossils found on continents 1000’s of miles
apart: Mesosaurus and Glossopteris
Evidence II - Glaciers
• Glacial
deposits
and
striations
in tropical
areas
Evidence III – Mountain Ranges
• Mountain
ranges of same
geology (rocks)
separated by
oceans.
For example: the
Appalachian &
the Caledonian
Mountains
Evidence IV
Mineral and Rock Deposits
Other Paleoclimate indicators:
• Coal deposits in North America
and Europe (and artic areas)
• Coral reefs in Northern Canada
• Vast sand dunes (desert
indicators) not in latitudes
associated with deserts.
Alfred Wegener (cont.)
**Scientists rejected Wegener’s theory
because he could not prove WHY or
HOW the continents moved.
Wegener died on a meteorological
expedition in Greenland in 1930;
his theory still unaccepted.
Sea Floor Spreading
Harry Hess (1960)
• Professor of Geology at Princeton University
• World War II broke out, while on active
duty as a navy sea captain, he took
“soundings” of the ocean floor topography
and found the Mid-Atlantic Ridge system.
• Continuing his research after the war, Hess
speculated that the sea floor was spreading
out from vents in the cracks where hot
magma oozed up to the surface.
Sea Floor Spreading
• Theory developed that magma from the
Earth’s mantle rises to the surface at the
mid-ocean ridge and cools to form new
seafloor – like a conveyor belt
Evidence
•
•
Rock Age – rocks are youngest at
the ridge, and they get older as you
move away from the ridge / toward
the continents
Magnetic Clues – iron minerals in
rocks, like basalt, align themselves
with the magnetic north pole at the
time they crystallize.
Sea Floor Spreading
• ridges had high
heat flow and
universal volcanism
throughout
• new magma slowly
pushes away from
the ridge, causing
separation of the
ocean floor.
(not in notes)
Sea Floor Spreading
Sea Floor Spreading
• the Mid-Ocean Ridge
is the longest
mountain chain in
the world:
 nearly 46,000 miles
long in total
 undersea mountains
more than 12,000
feet high & 1,200
miles wide
Sea Floor Spreading
Paleomagnetism
• Study of the
intensity and
orientation of the
earth's magnetic
field
• Magnetic
orientation is
‘frozen’ in metal
bearing igneous
rocks
Sea Floor Spreading
(In 1963, Fred Vine & Drummond Matthews)
• Discovered that throughout geologic time, the
polarity of the Earths Magnetic field has been
reversed
• This reversal has been recorded in the rocks on
either side of the mid-ocean ridges
Magnetic Inclination
• The Earth's magnetic
field is curved, and the
inclination of the
magnetic grains gives an
estimate of the
“paleolatitude”:
– Equator: horizontal
– Mid-latitude: high angle
– Pole: straight up
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