Evidence for Continental Drift and The Theory of Plate Tectonics

Did you know that the coal that is mined in Pennsylvania was actually formed from tropical plant life near the Equator? How did it travel northward to Pennsylvania? Scientists believe that 200 million years ago, when the dinosaurs dined upon tropical ferns and tall tropical vegetation, what is now Pennsylvania was at a different location, namely the equatorial region.!

1. Alfred Wegener was the first scientist to suggest that the fit of the continents of South America and Africa may not be a coincidence. He thought that the continents may have been together at some point in the past. He called this supercontinent Pangaea (this means all land ).

2. In 1912, he proposed the theory of continental drift, which means that the continents have moved to their current locations.

3. Besides the puzzle-like fit of some of the continents, there is substantial rock & fossil evidence to support Wegener s theory:

a. Like animal fossils have been found oceans apart.

b. Fossils of the same green plants have been found on almost all continents, suggesting that they all once had a similar climates.

c. Additionally, fossils of tropical plants have been found on islands in the Arctic Ocean, suggesting that area may have been much warmer in the past.

d. Glacial deposits and weathered rock (scientists think the is from moving ice) have been found on South America, Africa, India, and Australia. This could mean that these lands were once cold enough for glaciers to form and eventually move around on the land.

e. Similar rock structures are found on different continents. For example, mountains in the United States are similar to ones found on Greenland and in Europe. Also, rocks in South America and Africa are very similar.

4. The discovery of sea-floor spreading on the ocean s floors has given Wegener s theory of continental drift more support.

a. In the 1950 s, it was found that the ocean floors have mountains, valleys, and ridges, just like the land on earth.

b. It was suggested (and later agreed upon) that these formations have been caused by magma from the mantle being pushed up through the crust. Then, it flows to each side as the process repeats. http://www.wwnorton.com/earth/egeo/animations/ch2.htm

c. This idea was supported by the dating of rock found along the ocean s ridges. The youngest rocks are found right along the ocean s ridges. The ages of the rocks becomes increasingly older as you move away from the ridges. http://www.wwnorton.com/college/geo/earth2/home.asp#animations

This map shows the ages of the rocks that make up the floor of the Atlantic Ocean. Red represents the youngest rocks; the deepest red marks the Mid-Oceanic Ridge, where continental plates are pulling apart and new crust is being formed. Older rocks are yellow, green, and blue.

5. Also supporting the idea of seafloor spreading are clues from the earth s magnetic poles.

a. Think of the earth having a giant magnet in its core.

b. This creates a north and a south poles on earth. We evidence of these poles when we use a compass.

c. Anyhow, scientists theorize that these poles have reversed several times over the lifespan of the earth. This is supported by the discovery of patterns in iron containing minerals and rocks (remember, iron is magnetic). Magnetic rocks are attracted to the north pole, so they are pulled towards it, and away from the south pole.

Interesting Our planet's magnetic field reverses about once every 200,000 years on average. However, the time between reversals is highly variable. The last time Earth's magnetic field flipped was 780,000 years ago, according to the geologic record of Earth's polarity.

d. When the iron-containing material on earth is studied, it shows that this pattern has changed over time. This is especially apparent on the oceans floor.

e. Did you know that the magnetic north on earth is still wandering? The magnetic pole is currently 966 km (600 miles) from the geographic one.

6. All of this scientific work has led scientists to the theory of plate tectonics, which states that the earth s crust is broken in to sections which move around (or float) on the mantle below. It is this theory that helps to explain how the earth s surface has changed and will continue to change over time.

So what will the earth look like in the future? We can only guess