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The igneous rocks are primarily made up of minerals silicates, like pyroxenes, amphiboles, feldspars and quartz (among others). The proportions of these minerals are different among the various known igneous rocks. In deep igneous rocks, the crystals are often centimetric because they crystallized from a magma during million of years. In volcanic igneous rocks, the crystals are generally much smaller, hardly visible by eye.
The Earth’s Mantle is the source of many magmas. It is not a gigantic underground lake of magma as one could believe by looking at a volcanic eruption. In contrast, the Earth Mantle is composed of quite solid but plastic rocks (a little like chewing-gum). These rocks are exposed to very high pressures and temperatures (above 50 kbars and 1500°C).
If faults occur, the pressure, is locally released brutally, whereas the temperature remains constant with this depth. This led to the fusion of the minerals that are most fusible. Nets of magma are formed, go up towards surface (because the density of a magma is always lower than that of its surrounding rocks), then merge within magmatic chambers, which can easily reach a few tens of kilometers in diameter.
These rooms are localized in the Earth's crust, therefore much closer to surface. Hence, their temperature decrease even more quickly, thus facilitating the growth of the crystals starting from the magma. Thus, the proportion of magma decreases and that of the crystals increases gradually.
Faults at the top of this magmatic room will creates a volcanic eruption on the Earth’s surface that ejects what remains of magma, with possibly some unattached crystals. But the main part of the crystals remains confined in the chamber during million years, until its crystallization is complete. The erosion of the rock layers located above the chamber then reveals the rocks of the former magmatic room to the surface, as observed in Brittany, in the Limousin or the Vosges mountains.
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