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).