Plate tectonics and volcanism

The Earth's outer crust is composed of different tectonic plates. These drift apart, collide or slide over each other and can cause earthquakes.

Mid-ocean ridges:

They are the longest volcanic mountain ranges in the world  – zones at which two tectonic plates drift apart and form a so-called divergent plate boundary. Large amounts of magma are transported from the Earth's mantle to its surface, which is largely submerged below water. A basaltic oceanic crust forms, thus creating a new lithosphere (solid rock cover). The Mid-Atlantic Ridge, with a length of more than 20,000 kilometres, is the longest of these submarine volcanic mountain ranges. It is connected to the Pacific Ridge.

In addition to this, in many plates a subduction process taking place at a far-distant opposite plate boundary pulls the plate away from the spreading centre.Two different mechanisms cause the drift of plates: The rising magma below the plate boundaries drives the plates apart. However, a distant force often exerts an influence. In addition to this, in many plates a subduction process taking place at a far-distant opposite plate boundary pulls the plate away from the spreading centre.

Origin of new spreading centres:

Below large continents hot magma can accumulate and thin out the crust by partially melting it. The result is a graben, such as the East African Rift system with its many volcanoes, among them Mount Kilimanjaro and Ol Doinyo Lengai. Eventually this rift tears apart, the middle section sinks and a small sea forms. Today, for example, the Arabian Peninsula is already separated from Africa by the Red Sea.

Also the Upper Rhine Valley is part of a large rift zone which extends from the Norwegian North Sea through the European mainland to the Mediterranean. Here, the crust expands and thus thins out. As part of this faulting activity, the Kaiserstuhl, Hegau and Eifel volcanoes emerged along with Europe's largest volcano, the Vogelsberg. Even today, the Upper Rhine Valley is sinking by up to one millimetre per year, which repeatedly leads to weak earthquakes in the region.

Such a plate expansion is the initial stage of a new divergent plate boundary. If hot, viscous mantle material continues to rise, this results in a spreading zone.

The rock in the mantle is not equally hot and dense in all places. In some areas mantle plumes exist. These are areas in the mantle where molten rock ascends and towards the top spreads out in the shape of a mushroom. The source of this plume can reach down as far as the border between the core and the mantle.  Hot spots form along the boundary between asthenosphere and crust and are quite stable over geological time periods.  When an oceanic plate, such as the Pacific plate, moves over such a hot spot, this usually results in a chain of volcanic islands, as for example the Hawaiian Islands. Hot spots below a continental crust can lead to the formation of huge volcanoes such as the volcanic Yellowstone complex in the United States. Over time, the volcanoes move in unison with the moving plate.

In some regions there are also hot spots next to a mid-ocean ridge. Below the South Atlantic for example, there is a very old mantle plume. Its ancient volcanic remnants can today be found both in Brazil and in Namibia, Africa. In contrast to this, the young and still active volcanoes fed by this plume are found on the island of Tristan da Cunha in the middle of the South Atlantic. Iceland is another example where a mantle plume coincides with a spreading centre. On land it is even possible to observe that the Atlantic is splitting along its centre and that North America and Eurasia are slowly drifting apart.

Plate convergence:
In places where two plates move towards each other, elongated collision zones build up. Depending on the composition and age of the two converging plates, these zones have dif¬ferent characteristics and thus lead to different types of collision.

Where an oceanic plate drifts towards a continental plate, subduction zones form, such as around the Pacific Ring of Fire. The oceanic crust is drawn beneath the continental crust. Two forces are involved in this process across wide areas:

On the one hand, the mid-ocean ridges push the oceanic crust away from the spreading centre and therefore towards another plate. At the same time, caused by its high density and the convection currents, the oceanic crust is pulled downwards at the contact area of the two plates.
The oceanic plate sinks together with the sea water and the sediment by which it has been covered and penetrated. Heat and pressure cause mineral transformations which leads to the release of fluids such as water and gases at a depth of about 90-120 km. If the water released from the subducting plate penetrates the wedge-shaped overlying mantle, it reduces the melting point of the mantle rock. The resulting magmas ascend and create elongated chains of volcanoes on the continental plate as, for example, along the Andes or in New Zealand.

Ocean – ocean convergence:
When two oceanic plates move towards each other, the older, denser plate sinks beneath the younger, less dense plate. In the ocean, volcanic island arcs form, such as the Philip-pines, the Aleutian Islands or the Izu-Bonin-Mariana Arc.

Continent – continent convergence:

When two continental plates collide with each other, high mountains form. This is due to their similar density. They therefore give way to the enormous pressure by moving upwards. This is how the highest mountain ranges of the world formed, such as the European Alps through the collision of Africa with Europe, or the Himalayas through the collision of India with Eurasia. However, as such collisions are very complex processes, smaller local subduction zones occur which can give rise to volcanoes such as Mount Vesuvius in Italy.

Text: Christina Bonanati,GEOMAR Helmholtz Centre for Ocean Research Kiel

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