Lava: molten rock that erupts and flows on Earth's surface

How lava forms and reaches the surface

Molten rock forms in the mantle or lower crust where heat, decompression, or addition of volatiles melts solid rock. When pressure forces this material upward it travels along conduits and fractures; these pathways can be long tubes or open cracks. The molten material flows through openings in the crust (vents and fissures) and emerges onto the exterior (the surface), where it is then called lava. Movement and behavior depend on temperature, chemical composition and gas content.

Common types and characteristics

Lava ranges from very fluid to very viscous. Scientists classify flows by texture and form:

• Pahoehoe — smooth, ropy surfaces formed by low-viscosity, hot lava.
• Aa — rough, clinkery blocks produced by faster surface cooling and higher viscosity.
• Blocky lava — very thick, slow-moving fragments that pile up near vents.
• Pillow lava — bulbous shapes that form when lava erupts underwater.

Viscosity, the resistance to flow, is a key property: more viscous lava resists movement and builds steep features, while fluid lava spreads widely (viscosity).

Hazards, uses, and scientific importance

Lava can ignite vegetation, destroy buildings and infrastructure, and reroute rivers. Despite its danger, cooled lava provides fertile soils and new land. Volcanologists study lava chemistry, temperature and flow patterns to assess eruption style and risk, and to interpret Earth’s geological history. Remote sensing and field measurements of flow behaviour (flow dynamics) help forecast hazards and guide evacuations.

Notable distinctions

Unlike the word magma, which refers to molten rock still underground, lava specifically denotes material on or above the surface. Its forms record eruption conditions: glassy textures indicate rapid cooling, while crystalline structures suggest slower cooling at depth. Understanding these differences helps geologists reconstruct past eruptions and anticipate future activity.