Transcript Continental Drift

Continental Drift
Plate Tectonics
 The lithospheric plates float on top of
the liquid asthenosphere.
 This causes the plates to move around
slowly.
 Over a long period of time, they can
move large distances.
Discovery
 Alfred Wegener was the first person to
come up with the concept of
Continental Drift.
 He suggested that the continents
formed from a super-continent called
Pangaea, breaking apart about 200
million years ago
We are now going to look at the
pieces of Evidence Wegener used
to prove his theory…
1. Jigsaw Puzzle
 Wegener’s first clue was the perfect fit
of the African/South American
Coastlines
2. Geological Structure
 Analysis of rocks suggested similarities
between continents (similar age/type)
 Some mountain ranges end at one
coastline and begin at another
3. Fossil Evidence
 This is the best support for Wegener’s
theory
 Similar fossils have been found in
different locations across the world
 Eg. Mesosaurus
 The only explanation:
all the land was
once connected
4. Glacier Evidence
 (Paleoglaciation)
 When glaciers move, they
leave distinctive patterns
 (large U shaped valleys, sediments,
deep scratches)
 All of these were found in tropical areas
5. Coal Beds
 Coal beds form from the decomposition
of tropical swamps
 So how can there be coal beds in
Antarctica?
The best answer for the
coal/glacier evidence is that the
continents must have been in
completely different locations,
which gave them completely
different climates!
The only question Wegener
couldn’t answer was…
 How can continents move if they are
large and made out of solid rock?
 Plate tectonics and the soft
asthenosphere were unknown at the
time so he had no answer…
Plate Tectonics
Proof – Magnetism
The discovery of the mid-atlantic ridge
(which is a mountain range under
water)
 It has bands of opposite polarity called
magnetic striping
 See p512
Explanation of Striping
 Earth is like a bar magnet. It flips
polarity every 500,000 years.
 As the ridge opens, it makes new rock.
As the rock hardens, its domains line up
with the earth’s magnetic field.
 As the earth’s
magnetic polarity
flips, so does that of
the rock.
 This creates
magnetic stripes.
Sea Floor Spreading
 Harry Hess
 Magma is less dense so it rises through
breaks in the sea floor
 We call this spot a spreading ridge
 It cools and hardens, making new sea
floor
This process continues, pushing
older rock aside. We call this
process sea floor spreading
Plate Tectonic Theory
 J. Tuzo Wilson
 Island arcs form when
plates move over
stationary hot spots
 This is an area where
molten rock rises.
 He helped form the
Plate Tectonic Theory