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

Identifying Openings Produced by Escaping Gases

The openings in igneous rocks that are produced by escaping gases are called *vesicles*. These are small cavities that form as the gases are trapped within the lava or magma while it cools and solidifies.

Identifying the Texture of Obsidian

Obsidian is a type of volcanic glass and is known for its *glassy texture*. This texture occurs when the lava cools too quickly for crystals to form.

Identifying a Matrix of Fine Crystals Surrounding Phenocrysts

This texture is known as *porphyritic texture*. It consists of larger crystals known as phenocrysts embedded in a fine-grained matrix (groundmass) of smaller crystals.

Identifying Crystals Too Small to be Seen Without a Microscope

When the crystals in an igneous rock are too small to be seen without a microscope, the texture is referred to as *aphanitic*. This texture results from rapid cooling, typically at or near the Earth’s surface.

Identifying a Texture with Two Distinctly Different Crystal Sizes

An igneous rock texture characterized by two distinctly different crystal sizes is called *porphyritic*. The larger crystals are known as phenocrysts and the smaller ones make up the groundmass.

Identifying Coarse-Grained Texture with Crystals of Roughly Equal Size

A coarse-grained texture with crystals of roughly equal size is described as *phaneritic*. This texture forms when magma cools slowly, usually deep underground, allowing large crystals to grow.

Identifying Exceptionally Large Crystals

When crystals exceed 1 centimeter in diameter, they are referred to as *pegmatitic*. Pegmatitic textures occur in igneous rocks that crystallize from water-rich magma.

Key concepts

Vesicles

Vesicles are fascinating features found in many igneous rocks. They are essentially small cavities or holes that form during the cooling of lava or magma.
These openings are created by gases escaping from the molten rock as it begins to solidify. Think of it like bubbles in a loaf of bread.
As the bread bakes and rises, gases are released, creating pockets in the dough. Similarly, as lava cools, these vesicles remain as evidence of trapped gases.

• Vesicles are indicators of volcanic activity and can vary greatly in size.
• They often give the rock a Swiss cheese-like appearance.
• Rocks with abundant vesicles are called vesicular rocks.

Vesicles can be filled with minerals over time, resulting in formations known as amygdales.

Obsidian Glassy Texture

Obsidian is an intriguing volcanic rock known for its glassy texture. This glossy appearance is due to the rapid cooling of lava, which prevents crystals from forming.
Essentially, obsidian is like nature's glass. It has a smooth, often shiny surface and can appear in various colors, although black is the most common.
Obsidian's glassy texture results in sharp edges when broken, which historically made it a favored material for creating cutting tools and weapons.

• Obsidian forms with a high silica content, similar to pumice but without the vesicles.
• It serves as an excellent example of extrusive igneous rock.
• Despite its glassy appearance, it is not composed of mineral crystals.

These properties make obsidian a unique and valuable rock both historically and in modern times.

Porphyritic Texture

Porphyritic texture in igneous rocks is characterized by a combination of large and small crystals. This unique pattern occurs when magma experiences different cooling rates.
Initially, the magma begins to cool slowly, forming large crystals known as phenocrysts. Then, if the remaining magma cools rapidly, a fine-grained matrix called the groundmass surrounds these phenocrysts.
This texture offers clues about the rock's history and environmental conditions during formation.

• Porphyritic texture indicates a complex cooling process, often linked to volcanic eruptions.
• The phenocrysts can sometimes be seen with the naked eye.
• This texture is useful for identifying and studying the history of volcanic rocks.

By examining the sizes of these crystals, geologists can learn about the magmatic conditions that formed the rock.

Aphanitic Texture

Aphanitic texture occurs in igneous rocks when the crystals are too small to be seen with the naked eye. This fine-grained texture results from rapid cooling, commonly observed in volcanic erruptions.
Aphanitic rocks form quickly at the Earth's surface, with lava cooling rapidly to create a uniform appearance.
Often, one needs a microscope to distinguish the individual crystals within such rocks.

• Aphanitic textures are typical of extrusive igneous rocks, like basalt.
• Rapid cooling doesn’t give crystals time to grow large.
• Commonly associated with volcanic activity.

These rocks are significant in geological studies for understanding past volcanic activity.

Phaneritic Texture

Phaneritic texture is defined by large, visible crystals of roughly equal size throughout the rock. This coarse-grained texture forms when magma cools slowly deep underground.
The slow cooling process allows crystals to grow extensively, resulting in rocks like granite that can be identified by their large, visible components.
These textures are generally associated with intrusive igneous rocks.

• Phaneritic rocks are often associated with plutonic environments.
• The texture provides insights into the geological processes of deep Earth.
• Main minerals are typically quartz, feldspar, and mica.

Such textures are crucial for studying Earth's crust formation.

Pegmatitic Texture

Pegmatitic texture refers to igneous rocks that contain exceptionally large crystals, often exceeding 1 centimeter in diameter.
These massive crystals form from water-rich magma, which facilitates the growth of such large crystal structures due to enhanced mineral mobility.
Pegmatites can be a treasure trove of rare minerals and gemstones.

• Pegmatitic textures are often found in granite and similar compositions.
• The presence of water in magma aids the creation of large crystals.
• Pegmatite bodies can sometimes host valuable minerals like beryl and tourmaline.

Understanding pegmatitic textures helps geologists explore the conditions of deep magmatic processes.