Igneous Rock Formation

Texture

Geologists determine whether an igneous rock is extrusive or intrusive by studying the rock's texture. If the crystals are small or impossible to see without a magnifying lens, the Rockies extrusive. If all the crystals are large enough to see and have an interlocking texture, the rock is intrusive.

Composition

Geologists study a rocks composition to help identify it. Igneous rocks are classified, in part,based on their silica content. If minerals are difficult to identify, you can sometimes estimate the composition by observing how dark in color the rock is. Magma composition, the location where the lava or magma cools and crystallizes, and the cooling rate determine the type of igneous rock that forms.

Intrusive Rocks

Because magma within Earth is insulated by sold rock, it cools more slowly than lava in Earth's surface. When magma cools slowly, large well-defined crystals form. The arrangement of minerals in intrusive rocks are random.

Intrusive Rocks

Igneous rocks that form as magma cools underground.

Igneous Rock Formation

Magma is molten rock below Earth's surface, and lava is molten rock that has erupted onto Earth's surface. When lava cools and crystallizes, it becomes igneous rock. It cools quickly after coming in contact with cooler air around it. The rock that results from subsurface cooling is different from the rock that results from lava cooling on Earth's surface.

Extrusive Rocks

Materials, such as lava and ash, solidify and form extrusive igneous rocks. Lava can cool rapidly in Earth's surface. This means that there might not be enough time for any crystals to grow. Extrusive igneous rocks, therefore, have fine-grained texture. Magma stored underground can contain dissolves gases. As magma moves toward the surface, pressure decreases, and the gases deprecate from the molten mixture.

Volcanic Glass

Rock that forms when lava cools too quickly to form crystals, such as obsidian.

Igneous Rock Identification

Texture and composition can help identify all rocks. Geologists identify an igneous rock using the arrangement and size of mineral crystals in the rocks. Chemical composition a also can be used for igneous rock identification.

Extrusive Rocks

When Volcanic material erupts and cools and crystallizes on Earth's surface, it forms igneous rock.