Plate Tectonics

The seeds of Plate Tectonics were planted when a German astronomer, Alfred Wegener, noticed that the continents would fit together like puzzle pieces when cut from a map. He also noticed that fossil distribution on the different continents seemed to correspond with each other, as did lithology (strata).

Wegener called his theory Continental Drift. Unfortunately, he was almost universally ignored by his scientific colleagues.

Paleomagnetism also seemed to support Wegener’s theory. In igneous lava or unlithified sediments (those sediments that have not turned into rock), magnetic particles will align themselves with Earth’s magnetic field. This record is found in the igneous rocks when they cool and the sediments when they harden and become rock. These magnetic records show that the continents have moved.

In the 1960’s, Drummond Matthews and Fred Vine discovered magnetic anomalies on both sides of the Mid-Atlantic Ridge. Strips of oceanic crust showed alternating changes of magnetic alignment that indicated that the seafloor has been spreading. This led to the Plate Tectonics Theory, which states that all of Earth’s continents started as one super-continent, called Pangea, which broke up into many smaller landmasses. These masses have slowly moved from their original positions over millions of years and will continue to do so.

Continental plates are moving at different rates. The African plate moves about 25 mm per year and the Australian plate moves at the rate of 60 mm per year. The rate of movement can be determined by the pattern of magnetic anomalies on either side of a spreading oceanic ridge. The Earth’s polarities have reversed themselves at regular intervals throughout its history. Thus, crust formed during these reversed polarity periods show this through their magnet orientation.

Where two plates meet, one will slide under the other at an approximate 45-degree angle and will become part of the Earth’s mantle. This is called a subduction zone. This zone can be seen as a deep ocean trench, and the path of the descending plate can be seen through deep earthquakes. When two oceanic plates collide, a volcanic island arc may form. The descending plate melts at a depth of 90-150 km and the magma rises to form volcanic island chains.

The HartRAO website gives a very good, basic explanation of Plate Tectonics.

Another good site is The Life in Universe Organization website, which states “If plate tectonics were to cease, then so would this mountain building activity. Erosion would then, eventually, abrade down the mountains. Sediments removed by erosion, carried into the oceans by rivers and streams, would eventually lead to a rise in sea level. If the process continued, it is calculated that eventually a global ocean would cover the earth completely, resulting in catastrophic extinction of all land-based species.

You can view a 3D Paleographic map of the Cretaceous