Question

The statements that follow relate to terms used to describe igneous rock textures. For each statement, identify the appropriate term. a. Openings produced by escaping gases b. The texture of obsidian c. A matrix of fine crystals surrounding phenocrysts d. Consists of crystals that are too small to be seen without a microscope e. A texture characterized by two distinctly different crystal sizes f. Coarse grained, with crystals of roughly equal size g. Exceptionally large crystals, most exceeding 1 centimeter in diameter

Short answer

a. Vesicular, b. Glassy, c. Porphyritic, d. Aphanitic, e. Porphyritic, f. Phaneritic, g. Pegmatitic.

Step-by-step solution

Identify Openings in Rocks

The statement refers to 'openings produced by escaping gases' which are found in igneous rocks. The term used to describe this texture is 'vesicular texture'. This occurs when gas bubbles are trapped in cooling lava and form holes in the rock.

Determine the Texture of Obsidian

Obsidian is a natural volcanic glass. The texture associated with obsidian is 'glassy'. This texture forms because the lava cooled too quickly for crystals to form, leading to a smooth, glass-like appearance.

Recognize Fine Matrix with Phenocrysts

The texture described as a 'matrix of fine crystals surrounding phenocrysts' is known as 'porphyritic texture'. In this texture, larger crystals, or phenocrysts, are embedded in a fine-grained groundmass.

Small Crystal Texture

The description 'consists of crystals that are too small to be seen without a microscope' is referred to as 'aphanitic texture'. This texture indicates rapid cooling, typically in volcanic rocks.

Texture with Different Crystal Sizes

A 'texture characterized by two distinctly different crystal sizes' identifies a 'porphyritic texture'. This texture features large crystals amidst a fine-grained or glassy background.

Coarse-Grained Texture

This statement describes 'coarse grained, with crystals of roughly equal size'. The term for this texture is 'phaneritic'. In phaneritic texture, the crystals are large enough to be discerned without a microscope.

Exceptionally Large Crystals

The description 'exceptionally large crystals, most exceeding 1 centimeter in diameter' refers to 'pegmatitic texture'. Pegmatites have such large crystals due to slower crystallization from a melt.

Key concepts

vesicular texture

Vesicular texture in igneous rocks is characterized by numerous openings or holes left behind by escaping gases. These holes form when gas bubbles are trapped inside the lava as it cools quickly and solidifies. When the pressure decreases, the gases expand and escape, creating voids where the gases were once present.
This texture is often found in rocks such as pumice or scoria, where the rapid cooling of volcanic eruptions is a common occurrence. Not only does this texture give the rock a unique appearance, but it also affects its buoyancy, often leading to rocks that are lightweight enough to float on water.

• The holes vary in size, depending on the amount and type of gas originally contained.
• Vesicular textures indicate past volcanic activity.
• These textures can also tell geologists about the conditions under which the lava cooled.

glassy texture

When we refer to a glassy texture in igneous rocks, we're talking about a smooth, glass-like appearance most commonly seen in obsidian. This occurs because the lava cools so quickly that crystals do not have time to form. Instead, the rock's surface becomes solid with an amorphous, or non-crystalline, structure.
Obsidian, known for its smooth, glassy sheen, is a classic example of a rock with this texture. This rapid cooling typically happens when lava enters water or meets the cooler atmosphere rapidly, freezing into a glass without distinct crystals.

• Glassy textures are smooth and can be very sharp if fractured, resembling broken glass.
• The absence of crystals distinguishes this texture from other igneous textures.
• These textures can be indicative of certain explosive volcanic environments.

porphyritic texture

Porphyritic texture in igneous rocks is identified by the presence of large, well-formed crystals known as phenocrysts embedded within a finer-grained groundmass. This texture reveals a two-stage cooling process, where the larger crystals form during slow cooling deep underground, and the finer matrix forms when the lava erupts and cools more rapidly at the surface.
The striking combination of different sizes tells us a lot about the cooling history of the magma. These distinctly sized crystals often give the rock a speckled appearance.

• The large crystals, or phenocrysts, are usually composed of minerals like feldspar and quartz.
• This texture can be commonly found in both volcanic and plutonic rocks.
• It indicates a complex cooling and solidification history involving varying temperatures and depths.

aphanitic texture

An aphanitic texture is defined by crystals that are so tiny they can only be seen with the aid of a microscope. These fine-grained crystals form when lava cools very quickly, typically at or near the Earth's surface, resulting in rocks like basalt and rhyolite.
This texture suggests rapid volcanic activity, where cooling occurs too fast for large crystals to grow. The minute size of the crystals requires careful microscopic examination to identify the minerals present.

• Aphanitic rocks often have a uniform appearance.
• This texture is most common in extrusive igneous rocks, which cool at the surface.
• Rocks with this texture are usually quite dense and compact.

phaneritic texture

Phaneritic texture in igneous rocks means the crystals are large enough to be seen without a microscope. This coarse-grained texture forms when magma cools slowly beneath the earth's surface, allowing ample time for large crystals to develop. Granite is a perfect example of a rock with a phaneritic texture.
The slow cooling of magma allows the transformation from liquid to solid to occur at a leisurely pace, enabling the growth of visible mineral crystals. Such rocks are often robust and visually striking due to the interplay of different colored minerals.

• The texture provides knowledge about the slow cooling history of the rock.
• Phaneritic textures are typically found in intrusive igneous rocks.
• This texture is characterized by crystals of uniform size and can include minerals like quartz, feldspar, and mica.

pegmatitic texture

Pegmatitic texture is associated with exceptionally large crystals, often exceeding 1 centimeter in diameter, and sometimes even larger. This texture forms under unusual conditions where magma retains its liquid state for a long time, allowing crystal growth of colossal sizes. Pegmatites are usually formed in the final stages of magma crystallization.
Pegmatites are noteworthy not just for their impressive crystal sizes but also for the diversity of rare minerals they can contain, such as tourmaline and beryl. These rocks provide valuable insights into the last stages of crystallization in a mineral-rich environment.

• Pegmatitic rocks are frequently mined for gemstones and rare earth elements.
• The large crystal sizes suggest extremely slow cooling and a high water content in the magma.
• Pegmatitic textures offer clues to the conditions within magma chambers.