To date, igneous rocks are considered the most common rocks, which make up about 65% of the total crust. These substances are formed from liquid magma and are the result of its crystallization.
Igneous rocks and the mechanism of their formation
It is no secret to anyone that magma is located in the bowels of the earth - a viscous substance that contains complex silicate complexes enriched in gases and water vapor. The temperature of molten magma in places can be 1200 degrees Celsius. As a result of various geological processes (for example, the divergence of tectonic plates, volcanic eruptions), molten magma moves to the earth's surface.
As the temperature rises, the temperature of the igneous substance decreases - this is how crystallization processes and the formation of new minerals occur. First of all, substances with a high melting point crystallize - these are amphiboles and pyroxenes. This happens until the entire mass of magma turns into a persistent rock.
Igneous rocks and their structure
Rocks differ in their structure, which primarily depends on the speed with which magma solidifies. If the substance cools slowly, rising from the bowels of the earth, this leads to the formation of large crystals. But quick cooling gives the opposite effect - small crystals form.
But most often the rise and cooling of magma is mixed. Depending on the size of the crystals, igneous rocks are divided into:
- having an even-grained structure - they consist of crystals of relatively equal size (for example, plutonic);
- having a different-grained structure - they include crystals of different sizes.
Classification of igneous rocks by the method of eruption and cooling of magma:
1. Massive - are formed if magma is erupted in a continuous stream. Among these breeds, the following types are distinguished:
- deep - magma cools at a depth. These include diorite and granite;
- spilled - magma cools on the surface of the Earth (porphyrite, basalt).
2. Volcanic clastic rocks - the name itself indicates how exactly magma emerges from the bowels of the earth. Molten rocks erupt in the form of splashes and splashes, cooling in the air. It:
- loose rocks (volcanic sand and ashes);
- cemented (for example, volcanic tuff).
Igneous rocks and their mineralogical composition
To date, there are four main types of minerals. Almost any igneous rock consists of them. It:
- Quartz is a mineral composed of crystalline silica. These are opaque crystals with a characteristic glass luster. The substance tolerates the influence of the external environment and is not susceptible to acids (except hydrofluoric).
- Feldspars are a large group of minerals that are characterized by less (compared to quartz) hardness and perfect cleavage. They are painted in light shades of different colors. Feldspars are usually divided into plagioclases and orthoclases. These minerals easily erode, turning into clay.
- Mica is a mineral with perfect cleavage and a low degree of hardness. The most common representatives of this group are muscovite (transparent mica) and biotite (black). If the rock consists of a large amount of mica, then its mechanical properties are significantly deteriorated.
- Pyroxenes and amphiboles are rock-forming minerals that have a similar chemical composition. As a rule, these are magnesium silicates or double salts of calcium and magnesium.
Igneous rocks differ significantly from sedimentary rocks primarily in their chemical composition: they consist of silicates or aluminosilicates.