Lava rock, also known as volcanic rock, is a type of extrusive igneous rock. It forms from molten material expelled onto the Earth’s surface. This material begins deep within the planet as magma, a superheated liquid mixture of rock, dissolved gases, and mineral crystals. The formation of this rock is tied to volcanic activity, representing the final, solidified stage after extreme changes in temperature and pressure.
The Internal Source: Magma Beneath the Crust
The material that eventually becomes lava rock starts as magma, molten rock found primarily in the Earth’s upper mantle or lower crust. This melting process occurs under specific geological conditions, not solely due to the Earth’s internal heat. Rocks melt through three main mechanisms: decompression melting, flux melting, and heat-induced melting.
Decompression melting occurs when hot rock rises and the pressure decreases, lowering the rock’s melting point even if its temperature remains high. Flux melting involves volatile substances, such as water and carbon dioxide, which lower the melting temperature of the surrounding rock. This is common in subduction zones where oceanic plates carry water-rich minerals into the mantle.
Magma is less dense than the surrounding solid rock, giving it buoyancy that drives it upward through the crust. As it rises, it often collects in large underground reservoirs known as magma chambers, located several kilometers below the surface. The composition is rich in elements like silicon and oxygen, along with iron, aluminum, and magnesium.
Eruption and Transition: Magma Becomes Lava
The ascent of magma through the crust is driven by buoyancy and the pressure from dissolved gases. These gases—mainly water vapor, carbon dioxide, and sulfur dioxide—are held in solution within the molten rock under immense subsurface pressure. As the magma moves closer to the surface, the confining pressure decreases, causing the dissolved gases to exsolve and form bubbles.
This buildup of gas bubbles increases the internal pressure, forcing the molten material through conduits, fissures, and volcanic vents. The moment the molten rock is expelled onto the Earth’s surface, whether on land or underwater, it is officially designated as lava. The temperature of this extruded material is extremely high, typically ranging from 800 to 1,200 degrees Celsius.
The viscosity of the lava, which determines its flow characteristics and eruption style, is influenced by its silica content. Low-viscosity, fluid lava (often basaltic) flows easily and travels great distances before cooling. In contrast, high-viscosity, silica-rich lava flows slowly and tends to form steep domes or result in more explosive eruptions.
Formation of Lava Rock: Rapid Cooling and Porosity
The process that gives lava rock its lightweight and porous structure begins immediately upon exposure to the atmosphere or water. This rapid cooling, known as extrusive cooling, is the defining factor in the rock’s texture. When the hot lava, which can be over 1,000 degrees Celsius, meets the colder environment, it loses heat quickly.
This swift heat loss prevents mineral components from having enough time to organize into large, visible crystals. This results in a fine-grained or glassy texture, such as in basalt or obsidian. The rapid drop in pressure causes the gas bubbles formed during the ascent to expand and become trapped within the solidifying rock. These trapped gas bubbles leave behind small holes called vesicles, creating the high porosity common in many lava rocks.
For instance, scoria is a type of lava rock that is dense with vesicles, making it light and spongy. The cooling rate affects the final density and permeability of the rock, with the fastest cooling occurring when lava flows into water. This rapid process completes the transformation from deep Earth magma to the porous, solid lava rock found on the surface.