Transcript Developing a Theory of Plate Tectonics

Developing a
Theory of Plate Tectonics (1960)
What we believe today:
• The Theory of Plate Tectonics states that the
Earth’s Lithosphere (crust and uppermost mantle) is
broken into pieces called tectonic plates.
• The plates of the earth slowly move due to
convection currents in the Asthenosphere.
The Lithosphere is broken into
approximately 20 pieces called
Tectonic Plates
Where did this theory come from?
•
The theory of plate tectonics is the sum
of two earlier theories:
1. The Theory of Continental
Drift
2. The Theory of Seafloor
Spreading
The Theory of
Continental Drift (1912)
• Alfred Wegener said that
continents were together
in one large landmass
called Pangea, which
means “all lands.”
• Pangea drifted apart
horizontally between 250 300 million years ago
(mya).
Alfred Wegener (1880-1930)
German Meteorologist
Geologic Time Line
Pangea – “All Land
(Early Permian (290 mya to Early Jurassic (200 mya))”
225 Million Years Ago (mya)
• During the Triassic
Period (248209mya) the
continents were still
together in a large
landmass known as
Pangea and began
to drift apart about
200 (mya).
• The climate was
warm and dry for
Ginko- “living fossil”
Tree Fern
Horsetail
Conifer Forest
Ferns
Evidence
Supporting the
Continental Drift Theory
1. Jig Saw Puzzle Fit
• The apparent fit of
continental coast
lines was the first
clue Wegener used
to support his
theory.
Observe a World Map. Which
continents look like they could
fit together?
2. Fossil Clues
Glossopteris Fossil (Tree Fern)
• Fern fossil found on 5
different land masses:
Africa, South America,
Antarctica, Australia and
India.
What is the significance of this discovery?
The fern contained seeds that were far
too large to be carried by wind
across the oceans, so Wegener
concluded that the landmasses had to
be together when they grew on th
planet.
Mesosaurus Fossil
• Small swimming reptile
that lived in freshwater
and land about 280 mya.
Fossil remains are found
in South America and
Africa.
How could these fossils be found so
far apart?
They couldn’t swim in salt water
and could not swim the distance
between the continents, so Wegener
hypothesized that they lived on both
continents when they were together.
3. Climate Clues
Glacial Deposits and Glacial
Grooving (scratches in rocks)
• Glacial deposits of the
same age and type &
grooved bedrock are found
in southern areas of South
America, Africa, India, and
Australia.
Warm Weathered Plants
• Fossils of warm
weathered plants
found on island of
Spitzbergen in the
Arctic.
What does this fossil find suggest
about the past climate of
Spitzbergen?
Wegener believed that the island drifted
from warm weathered areas.
Coal Found in Cold Climates
• Coal forms
from
compressed
plant matter
from swamps
in Tropical
areas!
4. Rock Clues
Mountain Ranges Match
• The North American
Appalachian Mountain
Range has parts found in
Greenland and Western
Europe.
What is the significance of this
discovery?
•Mountains are created as a massive mountain range
when landmasses collide and their edges fold upward.
•These landmasses had to be connected to create the
Appalachian chain because they line up when pieced
together.
Similar Rock Structures
• Similar rock
structures were
found on different
continents.
How do these clues support
the idea that the continents
were once together?
Weakness of Theory
• Wegener could not
explain How or
Why the continents
drifted apart, so his
theory was
rejected at the
time.
The Initial Break Up
• About 180 -200 million
years ago the
Supercontinent Pangea
began to break up.
• Laurasia was the
Northernmost continent
and consisted of present day
North America, Asia and
Europe
• Gondwanaland was the
Southernmost continent
and consisted of South
America, Africa,
Antarctica, Australia and
India
• Panthalassa (“all
Ocean”)was the name of
Watch the Break up of
Pangea.
Watch the Break up of
Pangea.
http://emvc.geol.ucsb.edu/forteachers/flashmovies/Pangea.swf
"Pangea Ultima"