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.  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  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  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  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  (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?  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…  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 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.  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.  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  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  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