3.2 Igneous rock classification and textures

Igneous Rock Classification and Textures

Igneous rocks form from cooling magma or lava, and geologists classify them using two main criteria: texture (grain size) and mineral composition (what minerals are present). These two properties together let you identify almost any igneous rock you'll encounter, and they also reveal how and where the rock formed.

The key principle behind texture is simple: cooling rate controls crystal size. Slow cooling gives crystals time to grow large, while fast cooling produces tiny crystals or no crystals at all.

Classification of Igneous Rocks

Classification comes down to two questions: What does the rock look like? (texture) and What is it made of? (composition).

Texture refers to the grain size and arrangement of minerals in the rock. There are four main textures to know:

• Phaneritic (coarse-grained): Crystals are larger than 1 mm and visible to the naked eye. Granite is the classic example.
• Aphanitic (fine-grained): Crystals are smaller than 1 mm and too small to see without magnification. Basalt is the most common aphanitic rock.
• Porphyritic (mixed grain sizes): Large crystals called phenocrysts sit within a fine-grained background called the groundmass. Porphyritic andesite shows this well.
• Glassy: No visible crystals at all because cooling happened too fast for any crystal structure to form. Obsidian is the go-to example.

Mineral composition is determined primarily by silica ($SiO_2$) content, which also affects the rock's color:

• Felsic: High silica content (roughly 65%+), light-colored minerals like quartz, potassium feldspar, plagioclase feldspar, and muscovite.
• Intermediate: Moderate silica content (roughly 52–65%), a mix of light and dark minerals. Andesite and diorite fall here.
• Mafic: Low silica content (roughly 45–52%), dark-colored minerals like pyroxene, amphibole, olivine, and calcium-rich plagioclase.
• Ultramafic: Very low silica content (below ~45%), composed almost entirely of olivine and pyroxene. These rocks are very dark and very dense.

Textures in Igneous Rocks

Each texture tells a story about the rock's cooling history.

Phaneritic texture forms when magma cools slowly deep underground (intrusive/plutonic setting). The slow cooling gives ions in the melt plenty of time to migrate and attach to growing crystal faces, producing large, interlocking crystals you can see and identify by eye. Granite, diorite, and gabbro all have phaneritic texture.

Aphanitic texture results from rapid cooling at or near Earth's surface (extrusive/volcanic setting). Because the magma loses heat quickly, crystals don't have time to grow large. You can tell the rock is crystalline under a microscope, but the grains are too small to distinguish with the naked eye. Basalt, rhyolite, and andesite are common aphanitic rocks.

Porphyritic texture records a two-stage cooling history:

1. Magma first cools slowly at depth, and large crystals (phenocrysts) begin to grow.
2. The magma then moves to a shallower level or erupts at the surface, where the remaining liquid cools rapidly and forms a fine-grained groundmass around those early phenocrysts.

This texture can occur in both intrusive and extrusive rocks. When you spot it, you know the magma experienced a change in its cooling environment.

Glassy texture forms when lava cools so rapidly that atoms can't arrange themselves into any crystal structure at all. The result is volcanic glass. Obsidian forms this way, often at the edges of lava flows where the melt contacts air or water.

Two additional textures worth knowing:

• Vesicular: Contains holes (vesicles) left by gas bubbles escaping from the lava. Pumice and scoria show this texture.
• Pegmatitic: Exceptionally large crystals (often several centimeters or more), formed from water-rich magmas that allow rapid crystal growth in the final stages of cooling.

Common Igneous Rock Types

Granite

• Felsic composition, high silica content
• Phaneritic texture from slow cooling at depth
• Contains quartz, potassium feldspar, plagioclase feldspar, and biotite (sometimes muscovite or hornblende)
• One of the most abundant rocks in continental crust

Basalt

• Mafic composition, low silica content
• Aphanitic texture from rapid cooling at the surface
• Composed of pyroxene, calcium-rich plagioclase, and often olivine
• The most common extrusive igneous rock, and the rock that makes up the ocean floor

Obsidian

• Typically felsic in composition (similar chemistry to granite)
• Glassy texture from extremely rapid cooling
• Lacks crystal structure, giving it a smooth, conchoidal fracture and glass-like appearance

Cooling Rate vs. Crystal Size

The relationship between cooling rate and crystal size is one of the most testable concepts in this unit. Here's the pattern:

• Slow cooling → more time for crystal growth → larger crystals → phaneritic texture (granite, diorite, gabbro)
• Moderate/rapid cooling → limited crystal growth → small crystals → aphanitic texture (basalt, rhyolite, andesite)
• Extremely rapid cooling → no crystal growth → glassy texture (obsidian, pumice)

This is why intrusive rocks (which cool underground, insulated by surrounding rock) are coarse-grained, while extrusive rocks (which cool in air or water) are fine-grained or glassy.

Bowen's Reaction Series describes the order in which minerals crystallize as magma cools. It's not about texture, but about which minerals appear at which temperatures:

1. High-temperature minerals crystallize first: olivine and calcium-rich plagioclase.
2. As the magma continues to cool, intermediate minerals form: pyroxene, amphibole, and sodium-rich plagioclase.
3. Low-temperature minerals crystallize last: biotite, potassium feldspar, muscovite, and quartz.

This sequence explains why mafic rocks contain olivine and pyroxene (early crystallizers) while felsic rocks contain quartz and potassium feldspar (late crystallizers). If magma cools completely, the remaining melt becomes increasingly silica-rich, which is why the last minerals to form are the felsic ones.