Earth's Outermost Layer

The Earth has not always looked the same as it does today. Over the last 100 years, scientists have gathered much evidence to show that the Earth has changed.

In the early 1900's Alfred Wegener proposed that the continents were once joined together as a supercontinent named Pangaea. He had several pieces of evidence to help prove his theory...the same fossils can be found on different continents, mountain ranges line up when the continents are moved together, the shape of the continents match like puzzle pieces, and climate evidence suggests that Africa was once cold enough to have glaciers and Antactica was warm enough for tropical plants. Unfortunately, Wegener could not explain HOW the continents moved, so his Theory of Continental Drift was not accepted.

In the 1950's, new technology allowed scientists to study the ocean floor. There, they discovered a huge mountain range called the Mid-Ocean ridge. This is the longest mountain range in the world. Upon further investigation, scientists realized that magma was squeezing through an opening in the ridge and was pushing older rock to the sides. As rock gets pushed to the side, continents also get pushed along. This process is referred to seafloor spreading and it helps explain Wegener's theory.

Today, scientists use the Theory of Plate Tectonics to explain how the continents have moved. This theory states that the lithosphere is broken into several pieces, called tectonic plates. These plates float on the asthenosphere and are in constant motion. Scientists believe that convection currents in the mantle, as well as slab pull, cause the plates to move. There are 3 different types of plate boundaries...convergent (coming together), divergent (moving apart), and transform (sliding past each other).

California is very tectonically active. Down in Southern California, the San Andreas Fault separates the North American Plate from the Pacific Plate. Movement along this transform plate boundary creates earthquakes. In northern California, subduction of the Gorda and Juan de Fuca Plates is responsible for Mt. Shasta and other Cascade Range volcanoes.

The Earth's surface has changed alot over the past 4.6 billion years. Even though the plates are only moving a few centimeters per year, they will continue to change the face of our planet.