school energy&environment

Magmatic rocks

2/3 the Earth’s Crust are made up of magmatic rocks that derive from the cooling of a mineral mass in melted state, mixed with gaseous substances. The mixture is called magma: it consists of different minerals that belong to the group of silicates. The gradual cooling of the magmatic mass leads to the crystallization of minerals and rock formation. Crystal structures form more easily when the magma contains gaseous substances, which are retained more easily when the rock originates inside the Earth. In this case the cooling process is slow and gradual and leads to the creation of intrusive magmatic rocks.
An example of it is granite, a rock whose crystals (belonging to different minerals) can be seen very clearly. Another example is diorite.
When the cooling process occurs on the surface, it is characterized by a sudden temperature drop. The gases contained in the magma disperse into the atmosphere; their change into solid state takes place all of a sudden and the rock is made up of very small crystals. These rocks are called magmatic effusive rocks. Examples are basalt, rhyolite, andesite, which are used in the building sector for road flooring and railway ballasts as they have a uniform colour. The colour depends on their structure where the mineral crystals are not visible. The rock that derives from the most rapid cooling of magma is called obsidian as it has a “vitreous structure”, while another very particular rock is pumice. This rock derives from a magma that has lots of gases and that hardens before the gases can disperse. Its structure is porous and the various pores, due to gas bubbles, make the rock so light that it floats on the water.
The first magma mineral that hardens at very high temperatures is olivine, that is present in large quantities in the internal layers of the Earth. Closer to the Earth’s surface, magma temperature diminishes and minerals with lots of silica are formed: amphiboles, biotite, feldspars, muscovite and quartz: these minerals form magmatic rocks that are located close to the Earth’s surface.
Can we see intrusive magmatic rocks?
Deposits of intrusive rocks originated deep underground and are surrounded by other types of rocks that can be altered by erosive phenomena occurring on the Earth’s surface. As a consequence, intrusive magmatic rocks are located on the Earth’s surface. The same thing happens when a mountain chain is formed, and tectonic movements lift deposits of intrusive rocks. Those deposits of intrusive magmatic rocks that are visible on the Earth’s surface are called plutons or batholithes.
Where does the magma form?
The magma originates on the mantle, at a depth of 100 kilometres, at a particular temperature and pressure conditions. After it is formed, the magmatic deposit can keep still for long periods of time, and only with temperature rises or pressure reductions it moves towards the Earth’s surface. This deposit feeds the volcanoes. During eruptions the magmatic material is expelled outside and acquires the name of lava. Lava
When it is very fluid, lava is expelled from the volcano without any explosive phenomena. It reaches a temperature of 1200°C and moves on the ground with a speed of up to 100 kilometres an hour. The presence of gas and silicon favours the volcano explosive activity: the magma is broken into pieces and the surrounding rocks are fragmented and violently launched into the air. The fragments are called pyroclasts and according to their size, it is possible to distinguish between: dusts (very fine), ashes, lapilli, bombs and blocks (big dimensions). Lava blocks are launched even at 10 km of distance, where they accumulate and form pyroclastic deposits, which are sedimentary rocks.