How Do Magma and Lava Differ

Both magma and lava are molten rock, but they differ in one key way: location. Magma is molten rock beneath Earth's surface. Once it erupts and reaches the surface, scientists call it lava. This name change marks an important shift in how the molten rock behaves and what happens next.

What matters most for volcanoes is not just the name change but what the magma is made of. Two properties shape every eruption. The first property is viscosity, which describes how thick and sticky the magma is. The second property is how much dissolved gas the magma holds. These two factors work together to determine how a volcano erupts.

Silica content controls viscosity. Magma low in silica is thin and runny, like syrup. It flows easily and allows gas bubbles to escape gently. In contrast, magma high in silica is thick and sticky, like peanut butter. This thick magma traps gas bubbles under pressure. When pressure builds too high, the gas explodes violently, throwing rock and ash into the air.

Scientists observe that viscosity and gas content determine eruption style. Evidence shows that thin, low-silica magma produces gentle eruptions with flowing lava. Mount Kilauea in Hawaii demonstrates this pattern with its steady lava flows. Thick, high-silica magma produces explosive eruptions. Mount St. Helens in Washington State erupted explosively in 1980 because its magma was thick and gas-rich.

These properties also shape the volcano itself. Thin lava spreads widely and builds broad, flat volcanoes called shield volcanoes. Thick lava piles up steeply near the vent and creates tall, cone-shaped volcanoes. Understanding magma composition helps scientists predict eruption behavior and warn communities at risk.

Interesting Fact: Some magma can remain underground for thousands of years, slowly cooling to form large underground rock formations called plutons without ever erupting.