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4.2 Igneous Texture

Igneous Rock Texture

Texture is the key to understanding the cooling history of most igneous rocks that are solidified underground or on the surface. When molten rock starts to cool, it crystallizes. The rate that the rock cools and the presence of gases in the molten rock affect igneous textures. Why is this useful? Because texture is a helpful method to identify some igneous rocks! Below are some common examples of igneous texture [1]:

Phaneritic/Coarse-grained texture, where individual crystals and minerals are easily spotted by the naked eye within the rock.
Figure 4.2.1 Phaneritic/Coarse-grained texture, where individual crystals and minerals are easily spotted by the naked eye within the rock.
Aphanitic/Fine-grained texture, where individual crystals and minerals cannot be spotted by the naked eye within the rock.
Figure 4.2.2 Aphanitic/Fine-grained texture, where individual crystals and minerals cannot be spotted by the naked eye within the rock.
Porphyritic texture illustrated by large, visible crystals found within a fine-grained matrix in igneous rock.
Figure 4.2.3 Porphyritic texture, illustrated by large, visible crystals found within a fine-grained matrix in igneous rock.
Glassy texture, which is very smooth without the presence of crystals or minerals
Figure 4.2.4 Glassy texture, which is very smooth without the presence of crystals or minerals
Pyroclastic texture shown by angular rock fragments mixed with fine ash and volcanic glass in igneous rock.
Figure 4.2.5 Pyroclastic texture, shown by angular rock fragments mixed with fine ash and volcanic glass in igneous rock.
Vesicular texture, in which the rock contains many pores where gases escaped during formation.
Figure 4.2.6 Vesicular texture, in which the rock contains many pores where gases escaped during formation.

Let’s Review!

From the previous section, we briefly learned about cooling history and which rocks are extrusive and intrusive. Can you determine what type of rock these textures are?

**Note that Poryphryitic rocks vary by sample. In general, they cool slowly within the Earth where the crystals grow large, then cool rapidly near or at the surface.

In the exercises below, you will review samples that represent the main types of igneous rocks. You will not need to name them–yet. However, you will need to assign them a texture and determine their cooling history.

Sample 1: Click on the image below to view the sample as an interactive, 3D model!

Peridotite Interactive Model. Peridotite is a bright green, coarse-grained rock made mostly of olivine.
Figure 4.2.7 3D interactive model by Sara Carena CC BY-NC.

Sample 2: Click on the image below to view the sample as an interactive, 3D model!

Rhyolite Interactive Model. Rhyolite is a fine-grained rock with light-tan to pink coloring.
Figure 4.2.8 3D interactive model by Dr. Parvinder Sethi CC BY

Sample 3: Click on the image below to view the sample as an interactive, 3D model!

Scoria interactive model. Scoria is a dark-colored rock with holes where gases have previously escaped.
Figure 4.2.9 3D interactive model by Sara Carena CC BY-NC.

Sample 4: Click on the image below to view the sample as an interactive, 3D model!

Tuff interactive model. Tuff is usually a light-colored rock with angular volcanic fragments and volcanic gas.
Figure 4.2.10 3D interactive model by rocksandminerals CC BY.

Sample 5: Click on the image below to view the sample as an interactive, 3D model!

Obsidian Interactive Model. Obsidian is an extremely smooth, glassy rock with no visible minerals and conchoidal fracture at its broken edges.
Figure 4.2.11 3D interactive model by jonathan.davidson CC BY.

Sample 6: Click on the image below to view the sample as an interactive, 3D model!

Andesite Interactive Model. Andesite is light gray and porphyritic, meaning that it is mostly fine-grained with larger dark minerals in its matrix.
Figure 4.2.12 3D interactive model by Sara Carena CC BY-NC.

Remember that intrusive rocks cool slowly and extrusive rocks cool quickly from the previous part’s exercise.

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