Question

Identify the igneous rock textures described by each of the following statements. 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 rock fragments welded together 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. Pyroclastic; f. Phaneritic; g. Pegmatitic.

Step-by-step solution

Identify Vesicular Texture

The openings produced by escaping gases are characteristics of a vesicular texture. This texture forms when gas bubbles are trapped in the solidifying lava.

Identify Glassy Texture

Obsidian is known for its glassy texture. This texture is produced when lava cools very rapidly, preventing crystals from forming.

Identify Porphyritic Texture

A matrix of fine crystals surrounding phenocrysts indicates a porphyritic texture. This signifies two stages of cooling, where large crystals, or phenocrysts, are embedded in a finer-grained groundmass.

Identify Aphanitic Texture

Crystals that are too small to be seen without a microscope describe an aphanitic texture. This texture results from rapid cooling at or near the Earth's surface.

Identify Pyroclastic Texture

A texture characterized by rock fragments welded together is known as a pyroclastic texture. It forms from explosive volcanic eruptions that eject and deposit volcanic fragments.

Identify Phaneritic Texture

A coarse-grained texture with crystals of roughly equal size is referred to as phaneritic. This texture develops when magma cools slowly beneath the Earth's surface, allowing large crystals to form.

Identify Pegmatitic Texture

Exceptionally large crystals, most exceeding 1 centimeter in diameter, are indicative of a pegmatitic texture. This texture arises from the slow cooling of magma, usually in the presence of water, which facilitates crystal growth.

Key concepts

Vesicular Texture

Vesicular texture in igneous rocks is characterized by numerous small cavities, known as vesicles. These form because of gas bubbles trapped in the lava or magma, which escape as the rock solidifies. When volcanoes erupt, the magma reaches the surface, leading to a sudden decrease in pressure. This allows dissolved gases, such as water vapor, carbon dioxide, and sulfur dioxide, to expand and escape. The rapid cooling that follows preserves the bubbles as vesicles.

Examples of rocks with vesicular textures include pumice and scoria. These rocks are often lightweight due to the numerous voids within their structure. This texture provides insight into the eruptive history of volcanoes, indicating a volatile environment where gases played a crucial role.

Glassy Texture

Glassy texture is observed in igneous rocks like obsidian, which form when lava cools extremely rapidly. The rapid cooling prevents the formation of crystals, resulting in a smooth, glass-like surface. Obsidian is often black but can be found in other colors depending on the mineral impurities present.

The absence of crystal structure gives glassy rocks a conchoidal fracture, meaning they break along smooth, curved surfaces. These rocks are typically dense and quite brittle. Because of their fast formation, glassy textures are indicative of volcanic environments where lava cools quickly, such as when it meets water or air.

Porphyritic Texture

Porphyritic texture is an interesting feature where larger crystals, known as phenocrysts, are embedded in a finer-grained matrix or groundmass. This indicates a two-stage cooling process. Initially, the magma cools slowly, allowing large crystals to form. Later, the remaining melt cools more rapidly, producing a fine-grained matrix.

This texture can occur in both volcanic and plutonic environments. In volcanic settings, porphyritic textures develop when magma rises to the surface quickly after forming phenocrysts below the surface. For geologists, this texture is a valuable indicator of a complex cooling history.

Aphanitic Texture

Rocks with an aphanitic texture consist of very small crystals that are often too tiny to be seen with the naked eye. This fine-grained texture results from the rapid cooling of lava at or near the Earth's surface, which doesn't allow enough time for large crystals to form.

Aphanitic textures are common in volcanic igneous rocks like basalt and andesite. Such fast cooling is typical of lava flows and volcanic eruptions. These fine-grained rocks are an indicator of surface eruptions and give evidence regarding the speed and style of the lava cooling process.

Pyroclastic Texture

Pyroclastic texture is a result of explosive volcanic eruptions. It is characterized by a rock composed of various fragments welded together. These fragments include volcanic ash, glass shards, and other debris that are ejected during explosive activity, where they accumulate and eventually consolidate into a rock.

Tuff is a common example of a rock with pyroclastic texture. This texture reflects a highly explosive eruption, where rapidly cooling fragments get compressed and cemented together. Studying these textures can help scientists understand past eruptive events and predict future activities.

Phaneritic Texture

Phaneritic texture describes rocks with visible crystals of roughly equal size. This coarse-grained texture forms when magma cools slowly inside the Earth's crust, allowing atoms time to arrange into visible crystals.

Granite and diorite are classic examples of phaneritic rocks. Such textures are common in intrusive igneous rocks, hinting at a slow crystallization process perhaps miles beneath the Earth's surface. This texture is particularly useful to geologists for determining the cooling history and figure out the depth at which the rock solidified.

Pegmatitic Texture

The pegmatitic texture contains exceptionally large crystals, typically exceeding 1 centimeter in diameter. These textures form in the late stages of magma crystallization, when fluid-rich environments allow minerals to grow incredibly large.

Pegmatite is a rock with such a texture and often contains rare minerals that are not found in smaller-grained rocks. This texture is often associated with volatile-enriched, water-rich magmas that facilitate rapid ion migration, allowing crystals to grow large. Pegmatitic textures are valuable in mining, as they can contain concentrations of gems or rare elements.