There are a variety of volcanoes on the surface of the earth, some active, some dormant. While each has its own specificities, meaning no two eruptions are alike, the volcanic machinery obeys certain general principles.
Regardless of whether the volcano is explosive or effusive, an eruption is based on the rise of magma, ie molten rock, through the thickness of the lithosphere. This ascent takes place along fissures that act as channels, transporting the magma from its source to the surface or to temporary storage areas commonly referred to as magma chambers.
No volcano without magma
Magma normally forms in the mantle, at the base of the lithosphere, where temperatures exceed 1,200°C, about 100 to 110 kilometers deep. Under the action of heat or rapid rise, the rocks of the mantle begin to melt locally and partially, creating a liquid called “magma”. The liquid droplets created, which are less dense than the surrounding rocks, will then begin to rise through the numerous interstices of the lithosphere. This ascension will then benefit from the presence of pre-existing cracks and faults affecting the Earth’s crust. But these lines are rarely continuous to the surface.
As a result, as it rises, the magma can be found temporarily stored in the crust at very different depths, generally between 20 and 50 kilometers. These are the deep magma chambers. Today, people often prefer to use the term “reservoir” instead, because the word “chamber” indicates an empty space, a kind of pocket or cavity that would be filled with magma. Today, however, we know that magma storage is quite diffuse and extensive, within a network of more or less connected fissures that form magmatic lenses trapped in a sort of crystalline mixture (the mash) where liquid and solid elements are intimately mixed are mixed up.
From this deep reservoir, magma can continue to rise to the surface until it reaches a shallow reservoir just a few kilometers from the surface. This storage level results from a balance of density and pressure. In other words, the Archimedean thrust that pushed the magma up until then is now zero.
A storage of several thousand years in magmatic deposits
During its ascent, the magma undergoes an important chemical modification. In fact, numerous exchanges and contaminations take place in contact with the surrounding rocks. During its stay in the magmatic reservoirs (which lasts several hundred or thousand years), the magma slowly begins to cool, which leads to the beginning of crystallization, but only of certain minerals.
This is a process called fractional crystallization. The composition of the residual liquid changes significantly due to the depletion of certain chemical elements that form new minerals. This is magmatic differentiation. Many parameters are then changed compared to the original magmatic liquid, in particular the silica content, density, temperature, viscosity and the ability of the liquid to absorb gases (especially water vapor and CO2).
Within the surface reservoir, the gases thus gradually become supersaturated and create an overpressure until they reach a critical value that leads to the opening of the previously closed system. The overpressure in the reservoir then exceeds the mechanical strength of the rock. New channels then open, allowing the magma to quickly escape to the surface. These volcanic vents will then release the pressure contained in the reservoir: this is the eruption.
Eruption and partial emptying of the magma reservoir
The pre-eruption stage, when the reservoir becomes unstable, is accompanied by several warning signs. Many small earthquakes are recorded under the volcano, whose flanks will also “swell”. The characteristics of the eruption itself will depend on several factors, most notably the viscosity of the magma. If it is liquid we will witness an effusive eruption with many lava flows. If it is very viscous, its rise in volcanic chimneys will be slow and complicated, creating overpressure that leads to an explosion.
In any case, the emptying of the magma reservoir does not usually reach 10%. In addition, the reservoir becomes too unstable and collapses, creating a caldera.
Once the pressures within the reservoir equalize again, the eruption stops. The volcano then enters a dormant phase, but this does not mean that the deep magmatic processes have stopped. Reinjection of magma from the deep reservoir into the surface reservoir is often the cause of an awakening and a new eruption.
What types of volcanoes are there?