What happens at a destructive plate boundary
People used to think that the surface of the earth was a rigid and continuous solid shell that enveloped our planet. The only movements would be caused by shrinkage cracks, which were caused by an alleged loss of mass and the associated volume reduction through volcanic eruptions. But you were wrong! Despite volcanic eruptions and degassing of the earth's interior, the earth does not shrink and our earth's surface is anything but rigid.
The solid earth crust is wafer-thin compared to the earth's diameter. It has an average thickness of 35 km across the continents; under the ocean it is only 6 - 7 km thick.
The crust is broken into 7 large and numerous smaller plates that float on the plastic material of the earth's mantle. They move like icebergs in the ocean, only much more slowly. They only cover a few centimeters a year.
Despite the relatively low speed, gigantic forces act on the tectonic plates and a number of different movements occur. Especially in the zones where two or more plates collide, something dramatic happens: deep-sea trenches arise, mountains unfold, volcanoes erupt and the earth shakes. It is also possible that forces act on a continent whose vectors point in different directions, with the result that the continent breaks up.
The German climatologist and polar researcher Alfred Wegener came up with the idea of continental drift as early as 1915 and coined the term plate tectonics. But his theory about continents that move was ridiculed at the time. It was not until the 1960s that, thanks to modern measurement methods, it came back into the focus of scientists and was confirmed. Today one researches exactly what happens at the plate boundaries and defines 4 different boundary types:
At divergent plate boundaries two plates move away from each other. As a result, lava constantly escapes at the seam and also pushes the plates apart. Divergent plate boundaries are found primarily in the mid-ocean ridges (Mid-Atlantic Ridge, Iceland), but also in continental rifts (rifts). The East African Rift Valley is an example of continental divergence.
At convergent plate boundaries two plates butt against each other. The heavier oceanic plate dips under the lighter and thicker continental plate. This usually happens in the deep-sea trenches off the coast; the volcanologist speaks of subduction zones here. The subducting (subducting) plate pulls the rest of the plate behind it, resulting in a further opening at the divergent plate boundaries. The submerged plate partially melts in the earth's mantle. Magma then rises behind the subduction zone and emerges at volcanoes. Convergence of two oceanic plates is less common and mostly leads to island arc volcanism. When two continental plates collide, mountains form (orogeny).
At conservative plate boundaries the plates move sideways past each other. Earthquakes do occur here too, but volcanic eruptions and mountain formation are less common. An example of conservative plate boundaries is the San Andreas Trench shear zone in California. These fault zones are also called blade shifts.
Passive plate boundaries are only mentioned here in passing. There are currently no movements on these continental margins.
In relation to volcanism, the convergent and divergent plate boundaries are of particular importance. A good 90% of all active volcanoes on earth lie along these borders.
The engine behind the plate tectonics can be found deep inside the earth. In the outer core of the earth and in the earth's mantle there are convection currents made of plastic material. These currents form vertically rotating cells several thousand kilometers in diameter. The direction of rotation between 2 neighboring cells is opposite.
In the earth's mantle it is plastic rocks that form these convection cells. In the outer core of the earth, it is a molten nickel-iron alloy that rotates slowly. Thereby not only relative movements to the earth's mantle take place, but also to the solid inner core of the earth. That has the effect of a dynamo and the earth's magnetic field is created. This protects us from the bombardment of cosmic rays.
Image source: USGS
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