We can compare the structure of the Earth to that of a peach with a large nucleus.
The very thin skin is the surface on which we live.
We call it the lithosphere, it consists of solid, compact, and difficult to deform rocks.
This skin is all cracked, it is even split into several pieces, like a puzzle.
Each piece of the puzzle is what is called a tectonic plate. The pieces of the puzzle are not very well fitted together, they can move around a bit, and sometimes even overlap.
This is what leads in particular to continental drift. Below the lithosphere is the asthenosphere,
also made of rocks. But this layer, always solid, is more deformable than the lithosphere.
The temperature here reaches 1300 degrees, and would be enough to melt the rock if we were on the surface.
But the pressure exerted by the lithosphere keeps the rock-solid. However, it takes little for it to found.
If that’s the case, the molten rock, which is called magma, undergoes pressure from surrounding rocks
and will, therefore, tend to move towards areas of lower pressure, ie the surface.
The magma will squeeze into existing cracks in the lithosphere. If it encounters an obstacle, i.e. an area without cracks, it will accumulate to form a kind of bubble, which is called a magma chamber.
Pressures or earthquakes will eventually fracture the rock. When the magma finally comes to the surface, we have a volcano.
Why the rocks of the asthenosphere do they start to melt at certain points?
Geologists are far from knowing everything about what’s going on under our feet, but they identified at least three mechanisms that lead to the fusion of these rocks, therefore the possible formation of a volcano.
The first appears where a plate dives under another plate. it sinks into the asthenosphere, and if it’s an oceanic plate, therefore covered by an ocean, it also brings water and carbon dioxide, which is enough to modify the properties of the rock and to make it melt. This magma is very viscous, and when it comes to the surface,
it will not sink on the side but will stay at the top of the crack, gradually forming a plug.
The magma under the cap, pushed by the material which continues to rise from the overlapping area of the plates, sees his pressure increase. Finally, the cork will jump in a gigantic explosion,
sending rocks and ashes in all directions at a speed of up to several hundred km / h. It’s during a rash like this that in less than a minute, Mount Pelée destroyed the town of Saint Pierre, in Martinique.
Another famous eruption of this type is that of Vesuvius, in the year 79 which resulted in the destruction of Pompeii. The second mechanism appears in places where, unlike the previous case, the tectonic plates move away from each other. In the space thus created, the lithosphere is finer and more fragile,
and below, the pressure decreases and the mantle rocks melt. The magma formed is very viscous, but more fluid than in the previous case. Upon reaching the surface, it is still sufficiently liquid
to be pushed on either side of the crack, and flow on the sides of the volcano, in the form of lava.
Most of these plate separation zones are at the bottom of the oceans, This type of volcano is rare on the surface,
with a few exceptions, such as Iceland. The third mechanism is linked to the existence of hot spots in the asthenosphere where the temperature is higher, without anyone really knowing why.
The rocks melt to form a relatively fluid magma. So we have on the surface a volcano with lava flowing.
These hot spots are scattered across the surface of the globe, on continents and at the foot of the oceans.
As the lava builds up, underwater volcanoes may eventually emerge, thus forming a volcanic island, like Reunion Island or the Hawaiian Archipelago. These three mechanisms give a very simplified image of volcanism.
A given volcano can alternate very well explosive events and lava flows, as in the case of Vesuvius for example.
And there are probably still other mechanisms to discover …