What is magma

The word magma comes from a word of Greek origin that could be translated as ‘pasta’. The volcanic magma is a combination of molten rock and solids and volatile compounds that form on the inside of the Earth . This substance is very unstable and can also contain gas bubbles and suspended crystals. Magma can easily be found in the chambers of a volcano, but also in nearby rocks. Once the magma cools and crystallizes it gives rise to igneous rocks.

The compounds that make up magma range from 700ºC to 1,300ºC . These very high temperatures can only be reached in the subduction zones of the planet, that is to say, in oceanic ridges, continental areas and in other hot spots on Earth. The magma formation process is quite complex, as we will explain later.

You can define quite a few types of magma. However, the most common are three:

Magmas basálticos

Basaltic magmas arise from the fusion of ultrabasic rocks , although their composition changes depending on the area in which they are formed. If they occur in oceanic ridges they have a low silica content (-50%) and if they occur inside tectonic plates they are more alkaline and rich in sodium and potassium. They are the most common.

Magmas andesíticos

Andesitic magmas are formed in subduction zones, both in the continental and oceanic crust, and have up to 60% content of silica and hydrated minerals , such as amphiboles or biotites. Andesitic magma is the richest in water, but when it erupts it evaporates as vapor. When this magma crystallizes in depth, it forms diorite and the water becomes part of amphiboles.

Granitic magma

This magma has the lowest melting point and can crystallize into large plutonic rocks . They are formed in orogenic areas like the andesitic ones, but from andesitic or basaltic magmas that manage to cross and melt sedimentary rocks or igneous rocks of the crust. These rocks alter the composition of magma when they are incorporated into it.

Magma originates in those areas of the earth’s crust and upper mantle where the temperature reaches a point where the minerals that make up the rocks begin to melt. However, the melting temperature also depends on other factors such as pressure or the presence / absence of water.

Thus, an increase in pressure accompanied by the absence of water makes fusion difficult, as occurs for example in the great depths of the earth. On the contrary, the presence of water lowers the melting point of the rocks. Therefore, magma only forms and remains (except magma leaks) where favorable conditions for its formation exist, such as in the crust and upper mantle.

When volcanic eruptions occur , magma manages to escape to the outside in the form of lava . As lava crystallizes rapidly, pieces of volcanic glass, such as obsidian or pumice, form instead of large crystals. 

The formation of magma occurs gradually when the rocky substance that forms our planet melts. The rocks of our planet are made up of minerals with different melting points and physicochemical properties, which determine the formation of different types of magmas . The high pressure of the Earth’s interior determines that the solid components soften.

Within the liquid complex of magma, temperatures are very high, as we have already indicated before, being in ranges between 700ºC and 1,300ºC. Usually, the composition of the different types of magma is associated with the environment where they are formed, which are usually subduction zones such as ocean ridges, continental areas and other hot spots on Earth .

Magma ceases to exist for two reasons, one is the evolution towards crystallization and the other as an exit to the outside in the form of lava in a volcanic eruption. In both cases, when it solidifies, it gives rise to igneous rocks , such as diorite, basalt or granite.