Igneous rocks originate from the solidification and cooling of molten material. This molten rock is known as magma when it exists deep beneath the Earth’s surface. Extrusive igneous rocks, also called volcanic rocks, form when this molten material is extruded onto or very near the Earth’s surface. The location where the cooling and solidification occur determines the final characteristics of the rock, dictating its resulting texture and composition.
From Magma to Lava
The formation of extrusive igneous rocks begins with magma, the high-temperature, liquid rock mixture found within the Earth’s crust or upper mantle. Because magma is less dense than the surrounding solid rock, it rises toward the surface. As it ascends, the material remains in a molten state under immense pressure.
The transition to an extrusive rock occurs when this molten material breaches the surface and is renamed lava. Eruptions happen through a central volcanic vent or ooze out from long cracks in the Earth’s crust known as fissures. Once exposed to the atmosphere or water, the material encounters a significantly cooler and lower-pressure environment.
The type of eruption impacts the form the lava takes upon reaching the surface. Non-explosive (effusive) eruptions typically produce lava flows that spread across the landscape. Highly explosive eruptions shatter the molten rock into fragments, such as volcanic ash and volcanic bombs, which are collectively called pyroclasts. Whether flowing or fragmented, the material is positioned for the rapid cooling phase that defines all extrusive rocks.
The Impact of Rapid Cooling
The defining characteristic of extrusive rock formation is the speed at which the molten material solidifies. When lava is exposed to the relatively cool temperatures of the atmosphere or ocean water, it can cool and solidify extremely quickly, sometimes almost instantly. This rapid cooling is the direct opposite of the process that forms intrusive rocks, which solidify slowly over thousands to millions of years deep within the crust.
This dramatic difference in cooling time has a profound effect on the internal structure of the rock. Mineral crystallization is a process where atoms must migrate and bond together to form an ordered, growing structure. With rapid cooling, the atoms do not have sufficient time to move into position and form large, well-defined crystal lattices.
The result is that the mineral grains remain microscopic, or in some cases, fail to form crystals at all. To illustrate this effect, consider making rock candy: cooling the sugar solution very slowly yields large crystals, while cooling it quickly produces many tiny crystals or a sugary sludge. Similarly, the fast cooling of lava forces the melt to “freeze” in place, locking in a texture composed of extremely fine grains.
Key Characteristics and Examples
The rapid cooling environment yields a specific set of physical properties that characterize extrusive igneous rocks. The most common texture is aphanitic, meaning the rock is fine-grained, with individual mineral crystals too small to be seen without a microscope. Basalt, a dark-colored rock that makes up much of the ocean floor and large continental lava flows, is the most common example of a rock with this fine texture.
If cooling is nearly instantaneous, the atoms cannot organize at all, resulting in a glassy texture. Obsidian, often dark and shiny, is a familiar example of volcanic glass that solidifies without forming any crystals.
Another distinctive feature is vesicular texture, characterized by numerous small holes or cavities called vesicles. These vesicles are preserved gas bubbles trapped as the lava solidified around them. Pumice, a very light and porous rock, is an example of a highly vesicular extrusive rock that often has a low enough density to float on water. Rhyolite is also an extrusive rock, chemically similar to the intrusive rock granite but displaying a fine-grained texture due to its rapid cooling on the surface.