IGNEOUS ROCKS: CLASSIFICATION AND MAIN CHARACTERISTICS - GEOGRAPHY - 2024

The igneous rocks are those formed when hot molten rock crystallizes and solidifies. The melt originates very deep within the Earth, near the boundaries of hot plates or hot spots, and then rises to the surface.

Therefore, igneous rocks form when magma or lava cools. These types of rocks make up the majority of the planet's continental crust and almost all of the oceanic crust.

Although their formation can be made of different minerals, they have one thing in common: they were all formed by the cooling and crystallization of a melt.

The key concept of all igneous rocks is that they were once hot enough to melt.

The main classification of these rocks depends on where they solidify, so they can be intrusive or extrusive.

Additionally, they can be classified according to their texture, color, chemical composition and mineral composition.

These rocks are relatively strong and are made of primary minerals that are often black, white, or gray. Its texture is similar to that of something that was cooked in the oven.

Main features

Of all the major rock types (igneous, sedimentary, and metamorphic), igneous rocks are considered primary rocks because they crystallize from a liquid (molten rock).

These rocks can be divided into two categories: intrusive and extrusive. Intrusive or plutonic rocks crystallize from magma below Earth's surface. Extrusive or volcanic rocks crystallize from lava on the Earth's surface.

The texture of an igneous rock depends on how often it cools after melting: when it cools slowly, large crystals form and when it cools quickly, small crystals form.

The magma and its resulting plutonic rock bodies cool and gently crystallize; They are characterized by their coarse grain texture in which mineral crystals are visible to the human eye.

In contrast, rapidly cooling lava is characterized by its fine-grained texture in which the crystals are very small.

Quick-cooling lavas, typically those that are soaked in water, have a glassy texture. They cool very quickly to form crystals. Volcanic glass is called obsidian.

In addition to texture, igneous rocks can be classified according to their chemical composition.

The most general classification is based on the relative abundance of felsic and mafic minerals in the rock. Felsic minerals are light in color while mafic minerals are dark.

The 4 peculiarities of igneous rocks

1- They are the most abundant rocks on the earth's surface.

2- They are formed when magma cools and solidifies.

3- Its chemical composition has a limited range of silicates and oxides such as calcium, iron and magnesium.

4- They are classified according to their typical textures, so there are two main types: extrusive and intrusive. Intrusive rocks can be divided into plutonic (crystallization at great depth) and hypabysal (crystallization near the earth's surface).

Training

These rocks are formed from materials such as lava that rose from the surface of the Earth, or magma that is found deep in the crust a few kilometers deep.

These rocks are found in four main places:

- At divergent boundaries, such as ocean edges, where plates separate and form cracks that are filled by magma.

- The subduction zones, which occur when a dense oceanic plate moves under another oceanic or continental one. The water that descends from the oceanic crust lowers the boiling point of the upper blanket, and forms magma that rises to the surface and forms volcanoes.

- At convergent continental borders where large land masses collide, causing the crust to harden and heat until it melts.

- In places like Hawaii, where a hot spot forms when the crust moves over a thermal column that rises from the depths of the Earth. These hot spots form extrusive igneous rocks.

The 2 main classifications of igneous rocks

1- According to their training

Extrusive rocks

These rocks cool quickly (in periods from seconds to months) and have invisible or microscopic grains. Some of these rocks have distinctive textures:

- Obsidian: is formed when lava cools quickly and has a glassy texture.

- Pumice: they have a volcanic foam which causes them to be inflamed by millions of gas bubbles that give them a vesicular texture.

- Toba: a rock made entirely of volcanic ash that falls from the air.

- Cushioned lava: it is a lava rock that forms under water. It has lumps that are created by excluding lava.

Intrusive rocks

They form more slowly (over thousands of years) and have visible small or medium-sized grains. They form when magma becomes trapped deep within the Earth.

This means that individual mineral grains take a long time to grow, so they grow quite large and have a rough texture. Some examples include:

- Granite: it is a light colored rock that contains quartz and mica minerals.

- Peridot: a rock composed almost entirely of olivine.

- Pegmatite: it is a light-colored rock with extremely rough grains that forms near the margins of the magma chamber during the final stages of crystallization.

2- According to its minerals

Igneous rocks are also classified according to the minerals they contain. The main minerals in these types of rocks are the hard and primary ones, such as quartz, feldspar, amphiboles and olivine, along with a softer mineral called mica.

The two best known types of igneous rocks are basalt and granite, which have different compositions and textures.

Basalt is a dark, fine-grained rock rich in iron and magnesium, which is why it is considered a mafic rock (dark in color) that can be intrusive or extrusive.

Granite is a light-colored, coarse-grained rock that is rich in feldspar and silicates, which is why it is considered a felsic (light-colored) rock.

Most igneous rocks are basalt or granite; In-depth laboratory analysis is needed to determine the exact rock type based on general classifications.

References

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6. General classification of igneous rocks (2011). Recovered from tulane.edu