Igneous rock refers to one of the major types of rocks alongside metamorphic and sedimentary rocks. This rock is formed when lava or magma cools and solidifies. It can form with crystallization or even without it. Magma or lava is a molten rock. It can originate from hot spots or plate boundaries from deep down the earth. This molten rock rises to the surface of the earth where it cools and then solidifies to form igneous rocks.
Igneous rocks can be classified into two categories. These are the extrusive igneous rocks and the intrusive igneous rocks. This classification is based on the place where the molten magma solidifies from.
Intrusive igneous rocks are also called the plutonic igneous rocks. These are formed when molten rock is trapped in the earth. This is because great globes made of molten rock can rise to the surface of the earth. Some of them can feed the volcanoes on the surface of the earth while some are trapped below the surface from where they cool slowly over the years and then solidify. This cooling and solidification can take millions of years.
When these rocks cool slowly it implies that individual grains of the minerals in them may take some time to grow. This makes such intrusive rocks to have relatively large particles. The texture of most intrusive igneous rocks is course grained.
Extrusive igneous rocks are also called volcanic igneous rocks. These rocks are formed when molten rock or magma exits on the earth surface from where it cools and solidifies. They can also form when magma cools and solidifies near the surface of the earth. They are commonly formed when fissures ooze or when volcanoes erupt.
Lava or magma produced through such processes cools and solidifies on the surface of the earth almost instantly after being exposed to relatively low or cool temperature of the earth’s atmosphere. This implies that mineral crystals do not have sufficient time for growing. This makes extrusive igneous rocks to have fine grains and glassy texture. Bubbles of hot gas are trapped inside the cooling and solidifying lava which forms a vesicular, bubbly structure. Examples of extrusive igneous rocks include obsidian, basalt and pumice.
By volume, igneous rocks and metamorphic rocks comprises up to 90 to 95 percent of the top layer of 16 kilometers of the crust of the earth. Geologically, igneous rocks are important due to their minerals, ages, and features which help in various contemporary studies.
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