Engineering geology

Textures of Igneous rocks

A body of mostly molten (heated until liquefied) rock below Earth’s surface is called magma. In addition to its liquid molten rock portion, or melt, magma contains dissolved gases (e.g., water, carbon dioxide, sulfur dioxide) and solid particles. The solid particles may be pieces of rock that have not yet melted and/or mineral crystals that may grow in size or abundance as the magma cools. The textures of an Igneous rocks is a description of its constituent parts and their sizes, shapes, and arrangement.

Textures of Igneous Rocks

You must be able to identify the common textures of igneous rocks (highlighted in bold text below) and understand how they form (Figures 1 and 2). This will help you to classify and infer the origin of igneous rocks.
The size of mineral crystals in an igneous rock generally indicates the rate at which the lava or magma cooled to form a rock and the availability of the chemicals required to form the crystals.

Large crystals require a long time to grow, so their presence generally means that a body of molten rock cooled slowly and contained ample atoms of the chemicals required to form the crystals. Tiny crystals generally indicate that the magma cooled more rapidly (there was not enough time for large crystals to form). Volcanic glass (no crystals) can indicate that magma was quenched (cooled immediately), but most volcanic glass is the result of poor nucleation as described below.

The crystallization process depends on the ability of atoms in lava or magma to nucleate. Nucleation is the initial formation of a microscopic crystal, to which other atoms progressively bond. This is how a crystal grows. Atoms are mobile in a fluid magma, so they are free to nucleate.

If such a fluid magma cools slowly, then crystals have time to grow—sometimes to many centimeters in length. However, if magma is very viscous (thick and resistant to flow), then atoms cannot easily move to nucleation sites. Crystals may not form even by slow cooling. Rapid cooling of very viscous magma (with poor nucleation) can produce igneous rocks with a glassy texture (see Figure 1).

Several common terms are used to describe igneous rock texture on the basis of crystal size (Figure 1). Igneous rocks made of crystals that are too small to identify with the naked eye or a hand lens (generally less than 1mm) have a very fine-grained aphanitic texture (from the Greek word for invisible).
Those made of visible crystals that can be identified are said to have a phaneritic texture (coarse-grained; crystals 1–10 mm) or pegmatitic texture (very coarse-grained; more than 1mm).

Textures,Igneous rock analysis and classification
Figure 1. Igneous rock analysis and classification.
Step 1—Estimate the rock’s mafic color index (MCI).
Step 2—Identify the main rock-forming minerals if the mineral crystals are large enough to do so, and estimate the relative abundance of each mineral.
Step 3—Identify the texture(s) of the rock.
Step 4—Use the Igneous Rock Classification Flowchart to name the rock. Start on the left side of the flowchart, and work toward the right side to the rock name.

Some igneous rocks have two distinct sizes of crystals. This is called porphyritic texture (see Figure 1). The large crystals are called phenocrysts, and the smaller, more numerous crystals form the groundmass or matrix. Porphyritic textures may generally indicate that a body of magma cooled slowly at first (to form the large crystals) and more rapidly later (to form the small crystals). However, recall from above that crystal size can also be influenced by changes in magma composition or viscosity.

Combinations of igneous-rock textures also occur.
For example, a porphyritic-aphanitic texture signifies that phenocrysts occur within an aphanitic matrix. A porphyritic-phaneritic texture signifies that phenocrysts occur within a phaneritic matrix.

When gas bubbles get trapped in cooling lava they are called vesicles, and the rock is said to have a vesicular texture. Scoria is a textural name for a rock having so many vesicles that it resembles a sponge.
Pumice has a glassy texture and so many tiny vesicles (like frothy meringue on a pie) that it floats in water.

Pyroclasts (from Greek meaning “fire broken”) are rocky materials that have been fragmented and/or ejected by explosive volcanic eruptions. They include volcanic ash fragments (pyroclasts less than 2 mm), lapilli or cinders (pyroclasts 2–64 mm), and volcanic bombs or blocks (pyroclasts more than 64 mm).

Igneous rocks composed of pyroclasts have a pyroclastic texture (see Figure 1). They include tuff (made of volcanic ash) and volcanic breccia (made chiefly of cinders and volcanic bombs).

Figure 2. Igneous rock classification chart.
Obtain data about the rock in Steps 1–3, then use that data to select the name of the rock (Step 4).

Adapted from R.M. Busch, and D.Tassa

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