12.1 Continental Drift Evidence

People thought the world was flat, but it wasn’t. People also thought the continents didn’t move, but they do.   The first main theory about this was proposed in the early 1900’s by a scientist named Alfred Wegener

There were three main clues used to piece this idea together.   1. Looks like a puzzle: Many people realized that the “map of the earth” looked like puzzle pieces. (ex: Africa and S. America). If you used the outline for the “pieces” as the continental shelf, they fit even better.

2. Matching Geological structures and rocks 2. Matching Geological structures and rocks. Quite often it looks like a mountain range that begins on one continent will continue on the other side of the ocean. Also, types of rock and rock formation

3. Matching fossils. You sometimes find fossils for things (like a fern) in areas that seem strange. The best explanation for this describes how the continents may have similar fossils in similar areas but designed for a climate very different from today.

This was a great idea but needed evidence to support it This was a great idea but needed evidence to support it. One place they found it was in paleoglaciation: the study of ancient glaciers and the rock markings they left behind.

There is evidence on some “tropical” continents of glaciation There is evidence on some “tropical” continents of glaciation. The way that best describes this (in association with all the other evidence) is that the continents must have been arranged differently in the past.

Coal deposits were another clue Coal deposits were another clue. Coal is formed by decaying plant material (tropical swamp stuff). The South Pole has never had a tropical climate, yet there are coal deposits there. The best explanation again is that the continents must be moving.

It is originally believed that all of the continents were once all together in one big supercontinent called Pangaea.   So how do things as large as continents move?

The surface of the earth is not one big piece The surface of the earth is not one big piece. It is broken up into large movable slabs of rock called tectonic plates. Continental plates are made up of mostly granite while oceanic plates are mainly basalt.

Volcanic eruptions allow us to see the movement of these plates as do earthquakes. Both are examples of these plates interacting with one another. By plotting the locations of these events you can see the outline of each of the tectonic plates.

The discovery of the Mid-Atlantic ridge strangely gave more evidence The discovery of the Mid-Atlantic ridge strangely gave more evidence. Scientists wanted to discover how there could be a long mountain chain right along the middle of the Atlantic Ocean.

WWII developed a lot of technology WWII developed a lot of technology. Mapping of the Atlantic Ocean increased a lot for military purposes. Further research into this mid Atlantic ridge discovered that there was more sediment and build up away from the ridge.

The planet earth has a N and S pole because earth is like a giant magnet. Interestingly enough, the magnetic north/south switch about every 250000 years.

So how does this tie into moving tectonic plates?   -The heat from within the earth drives this process. Molten rock (magma) rises because it is less dense than cooler rock. -The magma cools and hardens when it breaks through at an area called a spreading ridge. -The Mid Atlantic ridge is an area this happens

-Proof for this is found in the magnetic striping around the ridge   -The fact that the seafloor is getting bigger means it has to move somewhere.

One last bit of evidence includes island chains (like the Hawaiian Islands). They are formed when a tectonic plate passes over a stationary hot spot (area where molten rock rises to Earth’s surface