Volcanic rock is defined as an igneous rock that forms when molten material solidifies at or very near the Earth’s surface. It is also known as “extrusive igneous rock” because the material is extruded, or erupted, onto the surface. This formation process above ground gives these rocks unique physical characteristics.
The Formation Process of Extrusive Igneous Rocks
The formation of volcanic rock begins with magma, the molten material found beneath the Earth’s surface. When magma travels up and erupts onto the surface, it becomes lava. The key difference in their formation lies in the cooling environment, which fundamentally changes the resulting rock structure.
Magma that remains underground cools extremely slowly over thousands or even millions of years, forming intrusive rocks like granite. In contrast, lava exposed to the atmosphere or ocean water cools very rapidly, often taking only hours to months. This quick cooling prevents mineral components from organizing into large, visible crystals.
The rapid loss of heat causes the mineral structure to “freeze” quickly, which is the defining characteristic of all extrusive igneous rocks. This rapid solidification can occur when lava flows out across the landscape or when fragmented material, known as pyroclasts, is violently ejected.
Categorizing Volcanic Rocks by Mineral Composition
Volcanic rocks are categorized primarily based on their chemical makeup, specifically the amount of silica they contain. This chemical classification is generally divided into four major groups: felsic, intermediate, mafic, and ultramafic. The silica content dictates the rock’s color, density, and the types of minerals that form within it.
Felsic rocks contain the highest concentration of silica, typically over 63% by weight, and are therefore relatively light in color and density. Rhyolite is the most common volcanic rock in this group. At the other end of the spectrum are mafic rocks, which have a lower silica content, generally ranging from 45% to 52%, but are rich in magnesium and iron.
Mafic rocks, such as basalt, are consequently darker in color and have a higher density compared to felsic rocks. Intermediate rocks, like andesite and dacite, fall between the felsic and mafic extremes, containing moderate amounts of silica, between 52% and 69%.
Understanding Rock Texture and Cooling Speed
The texture of a volcanic rock refers to the size, shape, and arrangement of its mineral grains and is a direct consequence of the cooling speed. Due to rapid cooling, the most common texture is aphanitic, meaning the individual crystals are too small to be seen without a microscope. This fine-grained texture is characteristic of rocks like basalt and andesite.
When cooling is extremely fast, such as when lava is quenched by water or air, it forms a glassy texture where no crystals develop. Obsidian is the prime example of this natural volcanic glass.
A third common texture is vesicular, characterized by small holes or cavities called vesicles. These vesicles form when gases dissolved within the magma come out of solution as pressure drops during eruption. As the lava solidifies around these expanding gas bubbles, the resulting rock becomes porous and lightweight. Pumice and scoria are examples of vesicular rocks.
Major Examples and Global Occurrence
Basalt is the most abundant volcanic rock on Earth, characterized by its mafic composition and aphanitic texture. It forms the majority of the oceanic crust and is found at mid-ocean ridges and volcanic hotspots like Hawaii. Its low-silica lava is fluid and forms broad, shield-shaped volcanoes.
Rhyolite, the felsic counterpart, is less common and found mainly in continental settings, such as the Yellowstone Caldera. Its high-silica composition makes the lava very viscous, often leading to explosive eruptions and the formation of lava domes. Andesite, intermediate in composition, is frequently associated with subduction zones and forms the large stratovolcanoes along the Pacific Ring of Fire.
Pumice and Obsidian represent textural extremes and are often found alongside rhyolite due to their silica-rich chemistry. Pumice is the highly vesicular form resulting from gas-rich, explosive eruptions, while obsidian is the glassy product of rapid cooling.