The question of whether granite is a volcanic rock is common when people first encounter geology. Igneous rocks, which include both granite and true volcanic rock, form from the cooling and solidification of molten material. This molten material is known as magma beneath the Earth’s surface and lava once it erupts. The location where this cooling takes place is the fundamental difference that separates granite from volcanic rocks.
How Granite Forms
Granite is formed from magma that cools very slowly deep beneath the planet’s surface, often miles down within the Earth’s crust. Because the surrounding rock acts as an insulator, the magma remains molten for thousands to millions of years. This extended cooling period allows the individual mineral components to organize themselves. The resulting structure is characterized by a coarse-grained texture, which geologists describe as phaneritic.
The slow cooling permits crystals of quartz, feldspar, and mica to grow large enough to be easily seen with the naked eye. These interlocking crystals are a defining feature of granite and a direct consequence of its deep-seated formation. When granite is observed at the surface today, it is because prolonged geological processes, such as uplift and erosion, have stripped away the overlying rock layers.
The Characteristics of Volcanic Rocks
In contrast to granite, a rock is considered volcanic when the molten material cools rapidly on or very near the Earth’s surface. This rapid cooling occurs because lava is exposed directly to the cooler atmosphere or water. The quick loss of heat restricts the time available for mineral crystals to form and grow. The resulting rock will possess a fine-grained texture, known as aphanitic, where the crystals are too small to be individually distinguished without a microscope.
Basalt is a widespread example of a true volcanic rock, forming the bulk of the oceanic crust and many lava flows on land. If the cooling is extremely fast, such as when lava is quenched in water, virtually no crystals have time to form at all. This instant solidification produces a glassy texture, or hyaline, seen in rocks like obsidian. The presence of vesicles, small holes left by escaping gas bubbles, also frequently marks a rock as having formed from an erupting lava flow.
Categorizing Igneous Rocks
The distinction between granite and volcanic rock is formally recognized by the geological classification system for igneous rocks. This system primarily divides them based on their formation location and texture. Rocks that solidify beneath the surface, like granite, are termed intrusive or plutonic rocks. Conversely, rocks that cool on or near the surface, such as basalt, are termed extrusive or volcanic rocks.
The size of the crystals in the rock acts as a reliable indicator of its cooling history and classification. The classification system also accounts for the rock’s chemical makeup. Granite is a felsic rock, meaning it is rich in silica, aluminum, potassium, and sodium. The extrusive equivalent of granite, possessing the same chemical and mineral composition, is called rhyolite.
The difference between granite and rhyolite is solely in their texture, which reflects their cooling environment. A body of felsic magma that cools slowly underground becomes coarse-grained granite, while the same magma erupted onto the surface cools quickly to become fine-grained rhyolite. This concept of chemical equivalents extends to other pairings, such as the mafic combination of intrusive gabbro and extrusive basalt.