Igneous rocks form when magma or lava cools and solidifies beneath or above Earth’s surface. The term igneous is derived from a Latin word, ignis, which means fire. The type of igneous rock that forms is based on the composition of the magma. Magma is a mixture of molten rock, dissolved gases, and mineral crystals. The minerals found in magma are also present in the Earth’s crust. Magma is usually formed deep within the mantle. Temperature and pressure increase with depth. Water also affects the composition of magma because its presence can lower the melting point of specific minerals in rocks. Therefore the three factors that affect the formation of magma are heat, temperature, and water. Igneous rocks can be classified into two categories: intrusive igneous rocks and extrusive igneous rocks.
Intrusive igneous rocks are formed when magma cools, crystallizes, and solidifies below Earth’s surface. Granite and Gabbro are two types of intrusive igneous rocks that have different mineral compositions. Gabbro is a darker colored rock compared to granite. Gabbro also has a lower silica concentration than granite. Granite makes up the majority of the Earth’s continental crust. Most intrusive igneous rocks have a coarse grained texture. When the magma cools slowly below the Earth’s surface, large mineral crystals form inside the rock. Granite and gabbro can have crystal sizes larger than 1 cm. Igneous rocks that have coarse grained texture consist of large mineral grains or crystals. Granite and gabbro are used in kitchen countertops, cemetery markers, and tiles. After they are polished, they have a very appealing appearance. Both rocks also have the ability to resist weathering and abrasion.
Magma is referred to as lava once it reaches the Earth’s surface. After the lava cools and hardens it becomes extrusive igneous rock. Obsidian and pumice are examples of extrusive igneous rocks with different textures. When the lava escapes from a volcanic opening, the gases that were once in the magma are released. If the lava cools very rapidly and contains a small amount of dissolved gases, the rock will have a glassy texture such as obsidian. If the lava cools very rapidly and contains a large amount of dissolved gases, the rock will have holes called vesicles. The dense lava prevents the gases from escaping and they are trapped as bubbles as the rock forms. Pumice is an example of a rock with a vesicular texture. Many extrusive igneous rocks have a fine grained texture because they consist of small mineral grains when the lava cools very rapidly. Please watch “The Rock Cycle: Igneous Rocks” video located on the sidebar to learn more about formation of igneous rocks.