Igneous Rocks Formation

Igneous Rocks Formation

• Igneous rocks are formed from the cooling and crystallization of magma or lava.
• Magma is molten rock under Earth's surface, and it can cool and solidify above or below the surface.
• The cooling process determines the type of crystals formed, which in turn affects the appearance of the igneous rock.

Types of Igneous Rocks

• There are two main types of igneous rocks: intrusive and extrusive.
• Intrusive rocks form below Earth's surface, where magma cools slowly, allowing for large crystal formation.
• Extrusive rocks form above ground, where lava cools quickly, resulting in fine-grained or glassy textures.

Intrusive Igneous Rocks

• Also known as plutonic rocks.
• Formed when magma cools slowly inside Earth's surface.
• Coarse-grained and rough-textured due to slow cooling.
• Examples: granite.

Extrusive Igneous Rocks

• Also known as volcanic rocks.
• Formed when lava cools quickly above ground.
• Fine-grained or glassy textures due to quick cooling.
• Examples: obsidian, pumice.

Igneous Rock Textures

• Texture depends on cooling rate.
• Porphyritic texture: two distinct sizes of minerals, with larger minerals mixed with smaller groundmass.
• Phaneritic texture: slow cooling, coarse-grained, with large interlocking crystals.
• Aphanitic texture: fast cooling, very fine-grained, with microscopic crystals.
• Glassy texture: fast cooling, smooth like glass.

Classification of Igneous Rocks

• Igneous rocks can be classified according to mineral content, particularly silica dioxide.
• Felsic rocks: light in color, contain >65% silica dioxide, with abundant quartz and feldspar.
• Mafic rocks: darker and denser, contain 45-55% silica dioxide, with abundant magnesium and iron.
• Ultramafic rocks: very low silica content (<45%), with abundant magnesium and iron.
• Examples: rhyolite (felsic), basalt (mafic), peridotite (ultramafic).

Learn about the formation of igneous rocks, one of the three types of rocks in the rock cycle, and how they are created through extreme heat and pressure deep under Earth's surface.