Igneous Rock Formation, Textures, and Composition

Igneous Rocks: Formation and Classification

Igneous rocks are formed when molten rock (magma or lava) solidifies. Those that solidify on the Earth's surface are classified as extrusive or volcanic rocks. Rocks that solidify at depth within the Earth's crust are called intrusive or plutonic rocks.

Igneous Textures: Describing Rock Appearance

Igneous textures describe the general appearance of a rock based on the size, shape, and arrangement of its constituent crystals.

Factors Influencing Crystal Size

Several factors affect the size of crystals within igneous rocks:

• The speed at which the magma cools.
• The amount of silica content in the magma.
• The amount of gases dissolved in the magma.

Common Igneous Textures

• Aphanitic Texture: Characterized by rapid cooling, resulting in a very fine-grained structure where individual crystals are too small to be seen with the naked eye.
• Phaneritic Texture: Forms when large masses of magma solidify slowly at depth, allowing for the growth of large, visible crystals.
• Porphyritic Texture: Originates in a large mass of magma that cools in two distinct stages. Initially, some crystals grow quite large during slow cooling, followed by faster cooling where smaller crystals begin to form around the larger ones.
• Glassy Texture: Occurs when ions are frozen out of order before they can join into an orderly crystalline structure, resulting in a non-crystalline, glass-like appearance.
• Pyroclastic Texture: Formed by the consolidation of individual rock fragments, often from explosive volcanic eruptions.
• Pegmatitic Texture: Refers to very coarse-grained igneous rocks, typically with crystals larger than one centimeter.

Igneous Rock Compositions

Igneous rocks are composed mainly of silicates. Oxygen and silicon are the most abundant elements, making up more than 98% by weight of many magmas. Igneous rocks can be categorized according to their proportions of light and dark minerals.

• Rocks formed predominantly by light-colored silicates, such as quartz and feldspar, are called granitic (or felsic).
• Rocks that contain abundant dark minerals and less silica are said to have a basaltic (or mafic) composition.

Classification Principles for Igneous Rocks

Igneous rocks are classified or grouped primarily according to their texture and mineral composition.

Major Igneous Rock Types by Composition

The first minerals to crystallize from magma are typically high in iron, magnesium, and calcium, and low in silica. These are referred to as mafic or basaltic rocks. Among the last minerals to crystallize are potassium feldspar and quartz, which are characteristic of felsic rocks. Intermediate igneous rocks contain a mix of minerals common in both mafic and felsic rocks; rocks with this composition are often called andesite.

Examples by Compositional Group

• Felsic Rocks: Rhyolite
• Intermediate Rocks: Andesite
• Mafic Rocks: Basalt
• Ultramafic Rocks: Komatiite