Igneous or magmatic rocks

In deep areas of the interior of the Earth, the temperature is so high that the rocks are melted, mixed with water and gases, forming magma. When conditions change and magma cools and solidifies, the resulting rocks will be magmatic  or igneous rocks.

Magma is a mass of molten rocks, formed by silicates with water and gases dissolved in its interior due to the effect of pressure.

The magma rises towards the surface through areas where the pressure is lower until it reaches the earth's crust or even comes to the surface. When it cools and solidifies, it forms igneous or magmatic rocks.

Remember that for magmatism to occur, there must be fusion of materials. If it does not melt, the resulting rock is a metamorphic rock.

Classification of magmatic rocks

Magmatic rocks are, therefore, those resulting from the solidification of magma, which can be produced in two ways:

• If they solidify inside the Earth: Plutonic rocks.
• If they solidify on the earth's surface: Volcanic rocks.

Plutonic rocks

Plutonic (or intrusive) rocks are formed when magma rises slowly towards the earth's crust and its temperature also decreases so slowly that it allows crystals of the minerals that compose them to form. Plutonic rocks have a crystalline appearance, with large crystals that can be seen with the simple help of a magnifying glass.

Some examples of plutonic rocks are:

• Granite. It is the most characteristic plutonic rock, abundant in the continental crust.
• Gabbro. Common on oceanic crust.
• Peridotite. Major rock of the Earth's mantle, formed by olivine, pyroxenes and amphiboles.
• Syenite. Continental crustal rock.

Volcanic rocks

Volcanic (or extrusive) rocks are formed when magma finds an outlet to the outside, leading to a volcanic eruption. The gases contained in the magma escape into the atmosphere and the lava solidifies so quickly that its chemical elements do not have time to organize themselves to form crystals or they are very small, forming volcanic glass. Volcanic rock is a vitreous mass in which all the mineralogical components are mixed.

Examples of volcanic rocks:

• Basalt. Volcanic rock prevalent in oceanic crust.
• Pumice or pumice stone. Very characteristic rock due to its low density, since it has many small holes produced by the gases that the lava contained before it solidified.
• Obsidian. Very shiny black rock.

Composition of magmatic rocks

The chemical composition of magmatic or igneous rocks depends on the composition of the magma from which they come and the evolution that magma has undergone along its path, such as, for example, if new materials have been added from rocks that it has been melting. That is, from magmas of the same chemical composition, we can obtain different magmatic rocks depending on how the cooling has been. You can find a plutonic rock, in which the minerals are distinguished, and another volcanic rock, in which they are not appreciated, with the same chemical composition.

Magmatic rocks are classified according to their silica (SiO₂) content. If it has a high silica content, it is acid rocks, and if it has less silica content, it is intermediate, basic and ultrabasic, respectively, depending on its content decreases.

As we have seen, each plutonic rock corresponds to another volcanic rock with the same chemical composition, although its structure is different because it has solidified in a different way.

Interactive activity: Characteristics of plutonic and volcanic rocks.

Crossword: Identify 5 magmatic and 5 metamorphic rocks.

Interactive Activity: Classify these 10 rocks as magmatic or metamorphic.

Interactive Activity: Classify rocks as sedimentary, metamorphic or magmatic.

Interactive Activity: Classify Some Rocks.