Volcanic rock is a type of igneous rock formed from the cooling and solidification of molten material on or very near the Earth’s surface. It is a major component of the Earth’s crust, particularly where tectonic plates meet or hotspots exist. The properties of volcanic rock, including its composition and texture, are a direct result of the speed at which this molten material cooled. Geologists study these formations to understand past eruptions, the chemistry of the deep Earth, and how surface features have been sculpted over geologic time.
Formation: The Journey from Magma to Solid Rock
The creation of volcanic rock begins deep within the Earth, where molten material is known as magma. This magma, a complex mixture of liquid rock, dissolved gases, and solid crystals, rises toward the surface through conduits and fissures. Once magma exits the crust, it is referred to as lava.
Volcanic rocks are classified as extrusive igneous rocks because they form when lava is extruded onto the surface. In contrast, intrusive igneous rocks, also called plutonic rocks, form when magma cools slowly, insulated beneath the surface.
The rapid cooling that occurs when lava meets the atmosphere or water is the defining characteristic of volcanic rock formation. Because the molten material loses heat quickly, mineral crystals do not have enough time to grow to a visible size. This results in a rock with a fine-grained structure, or sometimes no crystalline structure at all.
This rapid heat exchange separates volcanic formations from their intrusive counterparts, which can take thousands or even millions of years to solidify. Volcanic activity forces the molten material to quench quickly, and the resulting fine-grained texture is the physical evidence of this rapid cooling history.
Textural Characteristics of Volcanic Rock
The speed of cooling imparts specific physical features to volcanic rock, offering clues about its formation. The most common texture is aphanitic, which describes a fine-grained structure where individual mineral crystals are too small to be seen without magnification. This texture occurs because the lava cooled so quickly that only microscopic crystals could form.
Another distinctive trait is vesicular texture, characterized by small, bubble-like holes called vesicles. These features are created by gases, such as water vapor and carbon dioxide, dissolved in the magma under high pressure. As the lava erupted, the pressure dropped, causing the gases to rapidly escape. The lava solidifies around these escaping gas bubbles, trapping them as open cavities within the rock.
In cases of extremely rapid cooling, such as when lava is instantly quenched by water, a glassy texture forms. This texture lacks any crystalline structure because the atoms did not have time to organize into a repeating lattice. The resulting material is an amorphous, natural glass that fractures with a distinctive conchoidal pattern.
Common Examples and Their Distinguishing Features
Many familiar rocks are examples of solidified lava, each with a unique combination of texture and chemical composition.
Basalt is the most widespread volcanic rock and is typically dark-colored due to its mafic composition, meaning it is rich in iron and magnesium. It exhibits an aphanitic texture, resulting from the rapid cooling of low-viscosity lava flows.
Rhyolite is the chemical equivalent of the intrusive rock granite, but it is volcanic. This rock is typically light-colored, ranging from pink to gray, because of its felsic composition, which is high in silica. Rhyolite often displays an aphanitic texture.
Pumice is a highly distinct example, a light-colored rock with an extremely vesicular texture. It is formed during explosive eruptions when gas-rich, silica-heavy lava is rapidly cooled and fragmented. The sheer number of trapped gas bubbles gives pumice such a low density that it can float on water.
Obsidian is a form of natural volcanic glass, usually dark in color, often black, despite typically having a felsic, silica-rich composition similar to rhyolite. Its color is usually due to minute inclusions of iron oxides. The extreme speed of its cooling prevents any crystal growth, yielding the characteristic glassy texture.