Magma and lava are fundamentally the same molten rock material, distinguished by a single change in state that shifts the material’s name. The difference between the two names marks the boundary between the Earth’s interior and its surface, signifying a dramatic shift in the material’s physical and chemical properties. Both are composed of a complex mixture of liquid melt, suspended solid crystals, and dissolved gases, but the environment dictates their behavior and the resulting rock type.
The Primary Distinction: Location and Pressure
The most straightforward distinction between the two is location: magma is molten rock material found beneath the Earth’s surface, while lava is the name given to that material once it has extruded onto the surface. Magma exists in reservoirs and conduits within the Earth’s crust or mantle, often residing in large underground spaces known as magma chambers.
Magma chambers impose immense lithostatic pressure from the overlying rock layers. This confining pressure governs the difference between magma and lava, forcing gases to remain dissolved within the molten rock. The transition occurs only when the material escapes this confinement, typically through a volcanic vent or fissure during an eruption. As the material rises and the confining pressure is released, the dissolved gases begin to separate from the melt.
Impact of Volatiles and Chemical Composition
A defining chemical difference between magma and lava is the concentration of volatiles, which are dissolved gases like water vapor, carbon dioxide, and sulfur dioxide. Magma, under high subsurface pressure, holds these volatiles in solution, similar to how carbon dioxide is dissolved in a sealed bottle of soda. When the pressure drops drastically as magma ascends toward the surface, these dissolved gases rapidly exsolve, or bubble out, transforming the melt into a foamy mixture.
The release of volatiles is the driving force behind most volcanic eruptions, especially explosive ones, as the expanding gas bubbles rapidly increase the volume of the magma. Lava, which has reached the surface, is essentially degassed magma, having lost a significant portion of its volatile content to the atmosphere. The behavior of both magma and lava is also influenced by their chemical composition, particularly the concentration of silica (\(\text{SiO}_2\)).
Silica content controls the viscosity, or resistance to flow, of the molten rock. Magmas with a high silica content, like rhyolite, have a high viscosity because the silica tetrahedra link together to form complex structures within the melt. High-viscosity magmas trap the exsolving gas bubbles, leading to a buildup of pressure and violent, explosive eruptions when they finally reach the surface. Conversely, low-silica magmas, like basalt, are more fluid and have a low viscosity, allowing gases to escape easily, which often results in gentle eruptions and fast-moving lava flows.
Cooling Rates and Resulting Rock Types
The difference in environment between the subsurface and the surface leads to drastically different cooling rates, which in turn determines the texture of the solidified rock. Magma is insulated by the surrounding rock deep underground, which causes it to cool very slowly over thousands to millions of years. This prolonged cooling time allows mineral crystals to grow large enough to be easily visible to the naked eye.
When magma crystallizes slowly deep within the Earth, it forms coarse-grained intrusive, or plutonic, igneous rocks, such as granite and gabbro. This large crystal size is known as a phaneritic texture, resulting directly from the extended time available for crystal growth. The contrast is stark when the same material reaches the surface and becomes lava.
Lava is exposed to the much colder atmosphere or ocean water, causing it to cool rapidly. This rapid cooling prevents the mineral crystals from growing large, resulting in fine-grained extrusive, or volcanic, igneous rocks. Rocks like basalt and rhyolite, formed from lava, have an aphanitic texture, where the crystals are microscopic. In cases of extremely rapid cooling, such as when lava is quickly quenched by water, the molten material may solidify into volcanic glass, like obsidian, without forming any crystals at all.