Igneous rocks form when molten material cools and solidifies. Extrusive igneous rocks, also known as volcanic rocks, are created when this molten rock reaches the Earth’s surface and crystallizes. This unique formation environment, on or very near the planet’s exterior, is responsible for the distinct appearance and composition of these rocks.
The Journey from Magma to Lava
The source of the molten rock lies deep within the Earth, primarily in the upper mantle or the lower crust, where it is known as magma. This silicate melt forms under immense heat and pressure, often initiated by processes like decompression melting at mid-ocean ridges or the addition of volatiles at subduction zones. Magma, being less dense than the surrounding solid rock, begins an upward journey toward the surface through buoyancy.
As the magma rises, it forces its way through existing rock fractures and conduits, accumulating volatile gases like water vapor and carbon dioxide. This upward movement is driven by the pressure of the dissolved gases. The transformation from magma to lava occurs when the molten material exits a volcanic vent or fissure and is exposed to the atmosphere or ocean water.
Rapid Cooling and Solidification
The rapid cooling that follows the material’s release onto the surface is the defining characteristic of extrusive rocks. When the superheated lava, which can be over 1,200 degrees Celsius, encounters the air or water, heat dissipates almost instantly. This process is drastically faster than the cooling of magma trapped deep underground.
Rapid heat loss dictates the rock’s final structure. This speed means that constituent atoms and ions, such as silicon, oxygen, iron, and magnesium, do not have sufficient time to migrate and link up into an organized, repeating lattice structure. Since the building blocks of minerals cannot fully organize themselves, the resulting crystals are either microscopic or entirely absent. This quenching effect locks the material into its final, fine-grained state quickly.
The environment of extrusion also affects the final rock. Lava flows erupted on land cool quickly to form rock sheets. Lava extruded underwater, such as at mid-ocean ridges, forms characteristic pillow-shaped structures due to the rapid quenching by seawater. In explosive eruptions, airborne fragments of lava, known as pyroclastic material, cool almost instantaneously before falling back to the ground as ash or pumice.
Resulting Textures and Classification
The rapid cooling inherent to extrusive rock formation results in a characteristic fine-grained texture called aphanitic, where individual crystals are too small to be seen without the aid of a microscope. Basalt, a dark, dense rock that forms the bedrock of the ocean floor and many lava plains, is the most common example of an aphanitic extrusive rock.
In some cases, the lava cools so fast that no crystals form at all, yielding a rock with a glassy texture. Obsidian, a smooth, dark volcanic glass, is formed when lava is quenched so quickly that the atoms remain in a completely disorganized, amorphous state. Furthermore, the decompression of lava upon eruption allows dissolved gases to escape, creating bubbles that become trapped as the lava solidifies.
When gas bubbles are abundant, the resulting rocks have a vesicular texture. Pumice and Scoria are examples of these highly porous rocks. Pumice is a light-colored, frothy rock that often floats on water because its abundant vesicles make it less dense than water. Scoria is a darker, denser vesicular rock.