Chapter – Textures of Igneous Rocks
Geography / Geology — Study Notes
Textures of Igneous Rocks
A complete, exam-ready chapter with definitions, diagrams, examples & key points | virtualstudycircle.com
1. Introduction to Texture
Definition
Texture of an igneous rock refers to the overall physical appearance and arrangement of its mineral grains/crystals — including their size, shape, degree of crystallization, and the relationship of one crystal to another within the rock.
Texture is one of the most important features used to identify and classify igneous rocks in the field, because it directly tells us the cooling history of the magma or lava that formed the rock — i.e., how fast and where it cooled.
Why texture matters
Two rocks can have the exact same chemical/mineral composition but look completely different because of texture. For example, Granite and Rhyolite have almost identical composition, but Granite is coarse-grained (cooled slowly, deep underground) while Rhyolite is fine-grained (cooled quickly, on the surface).
virtualstudycircle.com
2. Factors Controlling Texture
The texture of an igneous rock is controlled mainly by four factors:
- Rate of Cooling — the single most important factor. Slow cooling allows large, well-formed crystals to grow; fast cooling produces tiny crystals or no crystals at all.
- Viscosity of Magma — thick/viscous (silica-rich) magma resists the free movement of ions, restricting crystal growth; thin/fluid (silica-poor) magma allows ions to move and form larger crystals more easily.
- Gas / Volatile Content — dissolved gases (like water vapour, CO₂) escaping from lava as it nears the surface create holes/cavities, producing vesicular textures.
- Mode of Occurrence (Depth of Solidification) — magma solidifying deep underground cools slowly (coarse texture); lava erupted on the surface cools rapidly (fine/glassy texture).
Fig. 1 — As the rate of cooling increases, crystal/grain size decreases (inverse relationship).
Virtual Study Circle
3. Classification of Textures
Textures of igneous rocks are classified under several different heads. This chapter covers the four major classification bases used in geology and geography textbooks.
A. Degree of Crystallization
B. Grain Size
C. Grain Shape
D. Fabric / Mutual Relationship
A. On the Basis of Degree of Crystallization
This tells us how much of the rock is made of actual crystals versus glass (non-crystalline material).
- Holocrystalline — the entire rock is made up of crystals; no glass present at all. Typical of slow-cooled plutonic rocks. Example: Granite, Gabbro.
- Hypocrystalline (Merocrystalline) — the rock is a mixture of crystals and glass. Typical of rocks that cooled at an intermediate rate. Example: Andesite.
- Holohyaline (Vitreous) — the entire rock is glass, with no crystals at all, because cooling was too rapid for any crystals to form. Example: Obsidian.
B. On the Basis of Grain Size
- Phaneritic (Coarse-grained) — crystals are large enough to be seen and identified with the naked eye (>5mm). Formed by slow cooling deep underground. Example: Granite, Gabbro, Diorite.
- Aphanitic (Fine-grained) — crystals are too small to be seen without a microscope (<1mm). Formed by rapid cooling at/near the surface. Example: Basalt, Rhyolite.
- Pegmatitic (Very coarse-grained) — exceptionally large crystals (sometimes several cm to metres long), formed from water-rich residual magma that cooled extremely slowly in the final stage of crystallization. Example: Pegmatite.
C. On the Basis of Grain Shape (Crystal Form)
This describes how well-formed (well-bounded by flat crystal faces) each individual crystal is.
Fig. 2 — Crystal shape spectrum: Euhedral (perfect) → Subhedral (partial) → Anhedral (no faces).
- Euhedral — crystal is completely bound by its own well-developed flat faces. Forms when a crystal grows freely without obstruction.
- Subhedral — crystal is partly bound by its own faces and partly by irregular surfaces, because growth was slightly obstructed by neighbouring crystals.
- Anhedral — crystal has no original faces at all; its shape is entirely irregular, because it was the last to crystallize and had to fill in the leftover space between earlier-formed crystals.
virtualstudycircle.com
D. On the Basis of Fabric / Mutual Relationship Between Grains
This is the most detailed classification, describing the special arrangement/relationship between different minerals in a rock.
Granular
Crystals of roughly similar (equal) size fit tightly together, like a mosaic. Example: Granite.
Porphyritic
Large crystals (phenocrysts) embedded in a fine-grained groundmass, showing two-stage cooling. Example: Porphyry.
Ophitic
Small lath-shaped plagioclase crystals are fully enclosed within larger pyroxene crystals. Example: Dolerite.
Poikilitic
Numerous small crystals of one mineral are randomly enclosed within a single large crystal of another mineral (host).
Graphic
Quartz crystals arranged within feldspar in a pattern resembling cuneiform/ancient script. Example: Graphic granite.
Vesicular
Rock is full of small rounded holes/cavities left by escaping gas bubbles. Example: Pumice, Scoria.
Amygdaloidal
Same as vesicular, but the holes are later filled with secondary minerals like zeolite, calcite or quartz.
Spherulitic
Radiating, needle-like crystals arranged outward from a common centre, forming spherical clusters. Example: some Rhyolites.
Pyroclastic
Made of fragmented volcanic material (ash, cinders, blocks) explosively ejected and later compacted. Example: Tuff, Volcanic breccia.
Virtual Study Circle — www.virtualstudycircle.com
4. Master Comparison Table of Textures
| Texture | Basis | Key Feature | Example Rock |
|---|---|---|---|
| Holocrystalline | Crystallization degree | 100% crystals, no glass | Granite |
| Hypocrystalline | Crystallization degree | Mix of crystals + glass | Andesite |
| Holohyaline | Crystallization degree | 100% glass, no crystals | Obsidian |
| Phaneritic | Grain size | Coarse, visible crystals | Granite, Gabbro |
| Aphanitic | Grain size | Fine, invisible crystals | Basalt, Rhyolite |
| Pegmatitic | Grain size | Extremely large crystals | Pegmatite |
| Euhedral / Subhedral / Anhedral | Crystal shape | Degree of well-formed faces | Varies by mineral |
| Porphyritic | Fabric | Big phenocrysts in fine groundmass | Porphyry |
| Ophitic | Fabric | Small crystals enclosed in larger ones | Dolerite |
| Vesicular | Fabric | Gas-bubble holes | Pumice, Scoria |
| Amygdaloidal | Fabric | Holes filled with secondary minerals | Amygdaloidal basalt |
| Pyroclastic | Fabric | Fragmented, explosively ejected material | Tuff |
5. Key Points & Quick Revision
Points to remember
- Texture = size + shape + arrangement of crystals; it reflects the cooling history of the rock.
- Slower cooling → larger, better-formed crystals (Phaneritic, Euhedral); faster cooling → smaller/no crystals (Aphanitic, Glassy).
- Porphyritic texture indicates two distinct stages of cooling — slow at first (forming phenocrysts), then fast (forming the fine groundmass).
- Vesicular texture forms due to trapped gas; when those holes get filled later with secondary minerals, it becomes Amygdaloidal.
- Order of crystal formation: earliest-formed crystals tend to be Euhedral; last-formed crystals (filling leftover gaps) tend to be Anhedral.
Slow cooling = Coarse + Euhedral
Fast cooling = Fine/Glassy
Gas escape = Vesicular
Two-stage cooling = Porphyritic
virtualstudycircle.com
6. Practice / Revision Questions
- Define texture of an igneous rock. Name the four factors that control it.
- Differentiate between Holocrystalline, Hypocrystalline, and Holohyaline textures with examples.
- What is Porphyritic texture? Explain how it forms, using a labelled sketch.
- Differentiate between Euhedral, Subhedral, and Anhedral crystal forms.
- What is the difference between Vesicular and Amygdaloidal texture?
- Why does Granite have a coarse-grained texture while Basalt has a fine-grained texture, despite both being igneous rocks?
- Explain Ophitic and Poikilitic textures with suitable examples.
Virtual Study Circle — Offering Brighter Future
© Notes prepared for www.virtualstudycircle.com | Please do not reproduce without credit.