The Theory of Plate Tectonics Tom and Lawrence

Contents How the theory came about Evidence to support the theory Types of plate boundary Volcanoes Earthquakes

How the theory came about In the late 19 th and 20 th centuries, geologists believed the plates were fixed except for varying degrees of vertical movement. In 1912 German scientist, Alfred Wegener put forward the idea that the large land masses of Earth in fact fitted together. For example, America’s coastline fits in to that of West Africa’s. From then on Wegener publicly advocated his theory, however there was not significant evidence to prove it either way. It wasn’t until 33 years after his death that the Royal Society for science in London adopted his theory, the theory of continental drift.

Evidence for the theory. Large land masses appear to fit together. Samples taken proved India had once been in the Southern hemisphere as stated by Wegener. Evidence for continental drift is now extensive. Similar plant and animal fossils are found around different continent shores, suggesting that they were once joined. The fossils of Mesosaurus, a freshwater reptile rather like a small crocodile, found both in Brazil and South Africa, are one example; another is the discovery of fossils of the land reptile Lystrosaurus from rocks of the same age from locations in South America, Africa, and Antarctica. There is also living evidence — the same animals being found on two continents. An example of this is a particular earthworm found in South America and South Africa. An example of how the plates once fitted together can be found on the next page.

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The various types of plate boundary Three types of plate boundaries exist, characterized by the way the plates move relative to each other. They are associated with different types of surface phenomena. The different types of plate boundaries are: 1.Passive boundaries occur where plates slide or, perhaps more accurately, grind past each other along transform faults. The relative motion of the two plates is either sinistral (left side toward the observer) or dextral (right side toward the observer). The San Andreas Fault in California is an example of a transform boundary exhibiting dextral motion. 2.Divergent boundaries occur where two plates slide apart from each other. Mid- ocean ridges (e.g., Mid-Atlantic Ridge) and active zones of rifting (such as Africa's Great Rift Valley) are both examples of divergent boundaries. 3.Convergent boundaries (or active margins) occur where two plates slide towards each other commonly forming either a subduction zone (if one plate moves underneath the other) or a continental collision (if the two plates contain continental crust). Deep marine trenches are typically associated with subduction zones. The subducting slab contains many hydrous minerals, which release their water on heating; this water then causes the mantle to melt, producing volcanism. Examples of this are the Andes mountain range in South America and the Japanese island arc.

Volcanoes Volcanoes are generally found where tectonic plates are diverging or converging. A mid-oceanic ridge, for example the Mid-Atlantic Ridge, has examples of volcanoes caused by divergent tectonic plates pulling apart; the Pacific Ring of Fire has examples of volcanoes caused by convergent tectonic plates coming together. Volcanoes are usually not created where two tectonic plates slide past one another. Volcanoes can also form where there is stretching and thinning of the Earth's crust such as in the African Rift Valley, the Wells Gray-Clearwater volcanic field and the Rio Grande Rift in North America.

Earthquakes An earthquake is the result of a sudden release of energy in the Earth's crust that creates seismic waves. Earthquakes are recorded with a seismometer, also known as a seismograph. The Richter magnitude of an earthquake is normally reported, with magnitude 3 or lower earthquakes being mostly unnoticeable and magnitude 7 causing serious damage over large areas.