Plate tectonics

Plate tectonics in geology is a scientific theory which explains many features of our planet, as the existence of continents and oceans and their movements though geologic time. It also provides an explanation for mountain ranges, deep-sea trenches, and mid-ocean ridges. As such it is a theory broad in scope and rooted in fundamental physical principles, explaining many geological features previously only understood piecemeal using ad hoc hypotheses.

Conventionally, the earth has three major layers: crust, mantle, and core. Continents and oceans are both features of the crust. Until the early 1960s, with the exception of a very few people such as Alfred Wegener and Arthur Holmes, most scientists believed the continents and seas to be largely static. It was believed "local" crustal movement formed mountain ranges, understood by the geosyncline concept which had been elaborated in the latter 19th century.

Arguments for an early "super-continent" which split apart over eons were put forward by Alfred Wegener, a German physicist born in 1880, in his book Die Entstehung der Kontinente und Ozeane (The Origin of Continents and Oceans - first edition 1915). His theory was based on several significant coincidences which, taken together, convinced him that the continents had split apart from an enormous land mass he called Pangaea (translated: whole earth), which he believed existed millions of years in the past. Among the coincidences he was trying to explain was the way the shapes of continents seemed to fit together, for example, Africa to South America.

Wegener elaborated the theory of continental drift, which assumed that the continents 'plowed' through the oceans, and can be considered a precursor to modern plate tectonics.

In brief, the current theory of plate tectonics states that:

• The external layer of the earth, constituted by the crust and the outermost mantle and called lithosphere, lies on a plastic horizon. This horizon is visible in seismic prospections and is called low velocity channel.
• The lithosphere is subdivided into discrete parts, called plates.
• Plates can move with respect to the others, and cover all of the earth's surface. Thus, plate margins may show different behaviours.
• Divergent plate margins are rifting zones and mid-ocean ridges. In divergent margins, new oceanic crust is formed through the continuous eruption of basalts which derive from partial melting of the mantle underneath.
• Convergent plate margins are ocean trenches. Crust is consumed here, being subducted and melted into the mantle. When two continents face at a convergent margin, a mountain range arises, subduction stops, and eventually the two plate merge into one.
• A third kind of margin forms where plates move laterally one each other.

Refs

• Intro to Plate Tectonic Theory
• Plate Tectonics
• Harry Hammond Hess: Spreading the seafloor