TUKUL 2: Geology and Volcanology

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Plate Tectonics and Volcanoes

Volcanoes are not casually distributed on the Earth surface. Many of them are located near the boundaries of plates (volcanism of oceanic ridge and of subduction zone), but also inside the plates, as for example the volcanism of hot spots.

The volcanism of oceanic ridge

There are submarine volcanoes along the oceanic ridge, where new oceanic lithosphere  is produced. We know this kind of volcanism thanks to the exploration of oceanic backdrop, but also by two particular examples located in the Northern Atlantic ridge of Iceland, and in Afar (Eastern Africa).

In the last one the ridge was elevated above the sea level and formed lands which are natural laboratories for studying the volcanism of divergent borders.

Volcanism of oceanic ridge
Deep fractures of the lithosphere are equally able to create continental rifts. The lithosphere of the continents, in contact with the upper mantle, results affected by a divergent movement separating the two edges of the rift. This is the origin of the Ethiopian Rift. The enlargement of this opening can create a ridge with production of oceanic lithosphere. The  volcanism of Afar illustrates this situation.

The volcanism of subduction zones

Volcanism of subduction zone

The volcanism related to the plunging of one plate below an other creates ranges of volcanoes. The so called Ceinture du Feu of the Pacific is the expression of this converging volcanism.  It is generally caused by a collision between a portion of the oceanic lithosphere and a portion of the continental lithosphere. In this case, the volcanoes are located on the margin of a continent and form a continental arc. A good example of this situation is the Cascades Range, to the West of North American continent, as is explained by the following scheme, showing the relations between the three lithosphere plates of Pacific, Juan de Fuca and North America.

The volcanism of hot spots

The volcanism of hot spots is an intraplate type, typical but not exclusive of the oceanic lithosphere. Volcanic ranges of hot spots confirm the theory of drift of oceanic plates. The melted material at the level of hot spots is less dense than the neighbouring one. For this reason it goes towards the surface and perforates the lithosphere in order to create a volcano. These volcanoes of hot spots are numerous. The oceanic backdrops of Pacific represent a good example. The hot spots do not move and are  active for many millions of years, until a maximum of 100 Myr. If a lithosphere plate moves below a hot spot, it gives origin to a volcanic range. The most ancient volcanoes are located at the extremity of the range more distant from the hot spot, while the younger ones are built below the hot spot. A good example is the range from the Hawaii Islands to the Aleutians-Kurili (Range Hawaii-Emperor) in the Northern Pacific. 

Volcanism of hot spots


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