Lava
Lava is molten or partly molten rock that has erupted onto a planetary surface. On Earth, lava flows, fountains, domes, tubes, and volcanic fragments reveal magma composition, eruption style, cooling rate, gas release, terrain, and volcanic hazards.
What lava is
Lava is molten rock at the surface. The same material is called magma while it remains underground. Once erupted, lava cools, loses gases, crystallizes, and hardens into volcanic igneous rock. A lava flow may look simple from a distance, but it records changing temperature, chemistry, gas content, surface crust, and terrain.
From magma to lava
Magma becomes lava when it reaches the surface through a vent, crater, or fissure. Pressure drops during ascent, so dissolved gases can separate from the melt. Some eruptions release lava quietly in flows and fountains. Others involve lava mixed with ash, bombs, blocks, and explosive gas release.
Pahoehoe and aa
Two famous basaltic flow forms use Hawaiian names. Pahoehoe has smooth, billowy, or ropy surfaces because a thin flexible crust moves over still-molten lava. Aa has a rough, rubbly, clinkery surface made of broken crustal blocks. A single flow can change from pahoehoe to aa as it cools, loses gas, thickens, or moves over steeper ground.
Composition and viscosity
Basaltic lava is usually hotter, lower in silica, and more fluid than rhyolitic or dacitic lava. More silica generally increases viscosity, making lava thicker and harder to move. Viscous lava may form short flows or domes, while low-viscosity lava can travel farther, especially if insulated inside lava tubes.
Cooling and rock textures
As lava cools, minerals crystallize or volcanic glass forms. Rapid cooling can create fine-grained basalt, glassy obsidian, frothy pumice, or vesicular scoria. Slow cooling within a thick flow can make larger crystals than the chilled outer crust. Gas bubbles may leave holes called vesicles.
Lava tubes and landscapes
A crust can form over flowing lava while the interior continues moving. If the molten interior drains away, it may leave a lava tube. Lava also builds shields, cones, plateaus, deltas, benches, and fields of volcanic rock. Older lava flows can become cliffs, soils, cave systems, habitats, and landmarks after weathering and erosion.
Hazards
Lava flows usually move slower than many other volcanic hazards, but they can destroy roads, homes, farms, forests, utilities, and cultural sites. Heat, fire, toxic gases, unstable crust, ocean-entry explosions, collapsed benches, and difficult terrain can make active or recently cooled lava dangerous.
Why it matters
Lava matters because it is one of the clearest ways deep Earth material reaches the surface. It builds land, renews volcanic landscapes, creates igneous rocks, threatens communities, and records magma chemistry and eruption conditions. Studying lava helps scientists understand volcano behavior and the rock cycle.