Magma is molten rock material found beneath the Earth’s surface, typically originating in the crust or upper mantle. This material is a hot mixture of liquid rock, suspended crystals, and dissolved gases, existing at temperatures between 600 and 1,300 degrees Celsius. When this molten material is forced out onto the Earth’s surface, it is known as lava. The cooling and solidification of magma and lava create igneous rock. Igneous rocks are categorized into two groups: intrusive rocks, which cool underground, and extrusive (or volcanic) rocks, which cool on the surface.
The Journey to the Surface
Magma’s journey upward is primarily driven by buoyancy, as it is significantly less dense than the surrounding solid rock of the mantle and crust. This density difference creates a powerful upward push. The magma exploits pre-existing weaknesses, cracks, and fractures in the lithosphere to move toward areas of lower pressure. This movement often involves collecting in large underground reservoirs known as magma chambers.
As the magma rises, the pressure on it decreases, which allows dissolved gases, such as water vapor and carbon dioxide, to expand. This expansion creates additional internal pressure, helping to propel the magma through narrow conduits and vents toward the surface. The final transition from magma to lava occurs the moment the molten material exits the Earth’s crust. This eruption is the prerequisite for the formation of extrusive igneous rock.
Rapid Cooling and Crystal Formation
The formation of extrusive rock is defined by the drastic and rapid change in temperature upon eruption. When lava is exposed to the atmosphere or water, the temperature contrast, often exceeding 1,000 degrees Celsius, causes near-instantaneous cooling. This rapid heat loss significantly limits the time available for the atoms within the liquid melt to organize themselves into crystalline structures.
The lack of time for atomic organization results in the characteristic fine-grained texture of extrusive rocks, which geologists call aphanitic. In an aphanitic rock, the individual mineral crystals are too small to be seen without a microscope. This contrasts sharply with intrusive rocks, which cool slowly deep underground, allowing large, visible crystals to form. In some cases, the cooling is so rapid that no crystals can form, resulting in a natural, amorphous volcanic glass.
Identifying Extrusive Rocks
The fast cooling process creates distinctive textures that aid in the identification of extrusive rocks. A common feature is the vesicular texture, characterized by small holes or cavities called vesicles. These form when trapped gas bubbles escape from the lava as it solidifies.
Pumice and scoria are highly vesicular rocks, and pumice is so light that it can float on water. Extrusive rocks that cool instantaneously form a glassy texture, with obsidian being the most widely known example of volcanic glass. The most common type of extrusive rock globally is basalt, a dark, fine-grained rock that makes up the majority of the ocean floor. Other examples include rhyolite and andesite, which are classified based on their specific chemical compositions and mineral content.