Lava is molten rock, or magma, that has been expelled from a volcano or fissure onto the Earth’s surface. This material begins in a liquid state, typically at temperatures between 700°C and 1,200°C (1,300°F and 2,190°F). The fate of erupted lava is determined by its initial temperature, chemical makeup, and the environment it encounters. The transformation from a flowing liquid to a hard, solid igneous rock is a fundamental geological process that shapes the Earth’s surface.
The Physics of Lava Flow
The movement of lava across the landscape is governed by its viscosity, which measures its resistance to flow. This property is controlled by the concentration of silica (silicon dioxide) within the molten rock. Low-silica lavas, such as basalt, have low viscosity, allowing them to flow rapidly and travel long distances.
Conversely, high-silica lavas, like rhyolite and andesite, are highly viscous, causing them to move slowly and pile up near the vent, often forming a thick dome. Temperature also plays a part, as hotter lava is less viscous than cooler lava of the same composition. As lava loses heat during flow, it becomes progressively thicker, slowing its advance.
Degassing occurs when dissolved gases, such as water vapor and carbon dioxide, escape from the lava as it rises and pressure decreases. These escaping gases form bubbles within the molten rock, which are preserved as small voids called vesicles when the lava solidifies. In fluid lava, these bubbles are typically spherical, but movement in thicker flows can stretch the vesicles into irregular or elongated shapes.
How Lava Solidifies
Lava solidifies through crystallization, a phase change where mineral components form ordered solid structures as the temperature drops. The speed of cooling dictates the final texture of the rock, a principle central to volcanic rock formation. Rapid cooling, which happens when lava is exposed to the atmosphere or water, does not allow crystals enough time to grow.
This quick quenching results in a fine-grained texture where individual crystals are microscopic, or sometimes a glassy texture, such as obsidian, where no crystal structure forms. If the lava flow is thick or insulated, it cools very slowly, allowing mineral crystals to grow larger and become visible to the unaided eye. This differential cooling rate can lead to a mix of crystal sizes within the same flow.
A crust forms almost immediately on the surface of an exposed lava flow, acting as an effective insulator that traps heat within the interior. This effect allows the liquid lava inside to remain molten and continue flowing for extended periods, sometimes for months or even years in very thick flows or lava lakes. When lava encounters a body of water, the intense thermal shock causes the surface to fracture and rapidly cool into a glassy rind.
The Shapes Lava Creates
The final form a lava flow takes is a direct consequence of its viscosity and how its surface solidifies. One common morphology is Pahoehoe, a Hawaiian term for flows characterized by smooth, billowy, or ropy surfaces. This texture forms from hot, fluid lava that cools with a thin, flexible skin, which is dragged and wrinkled by the liquid flowing beneath it.
The rough, jagged, and clinkery surface known as A’a results from cooler, more viscous lava that moves quickly enough for its thick, brittle crust to be continuously broken up and tumbled. While a smooth Pahoehoe flow can transform into a chaotic A’a flow as it loses heat and gas, the reverse transition is rarely observed. A lava tube forms when the outer crust of a fluid flow solidifies completely, creating a rigid, insulated tunnel.
Once the eruption ends and the internal liquid drains away, the empty conduit remains as a long, cylindrical cave. Another distinctive structure is pillow lava, which forms when molten rock erupts underwater. The rapid quenching creates bulbous, interconnected lobes that resemble stacked sacks, a morphology common on the ocean floor. Finally, when thick lava flows or pools cool slowly and uniformly, the resulting contraction stress creates a network of cracks, forming regular, multi-sided columns known as columnar jointing.