What Is Lava?

Lava is molten or partially molten rock that has erupted onto the surface of a planetary body, along with the solidified rock formed when that melt cools. The distinction from magma is purely one of location: the same material is called magma while it remains within the crust and lava once it reaches the surface, where confining pressure drops and dissolved gases escape. Erupted temperatures generally fall between about 700 and 1,200 degrees Celsius, with the hottest values belonging to silica-poor basaltic melts and the coolest to silica-rich rhyolitic ones.

Because loss of volatiles and rapid cooling both change the material as it flows, lava is a system in transition rather than a fixed substance. Crystals nucleate and grow, gas bubbles expand and burst, and a chilled crust forms over a still-mobile interior. Nearly every feature that distinguishes one flow from another follows from how these processes compete against the rate at which the flow advances.

Composition and Viscosity

Silica content is the primary control on behavior. Basaltic lava contains roughly 45 to 52 percent silica by weight and has a viscosity low enough to run for kilometers in open channels, while rhyolitic lava exceeding about 69 percent silica may be many orders of magnitude more viscous and often barely moves, piling into steep-sided domes instead. Silicon and oxygen form polymerized chains in the melt, so more silica means more linkage and more resistance to shear. Temperature, dissolved water, and suspended crystal fraction modify the result further: cooling raises viscosity sharply, dissolved water lowers it, and a crystal cargo above roughly 30 percent by volume causes the melt to behave as a yield-strength fluid rather than a simple liquid. The Volcano Hazards Program glossary maintained by the US Geological Survey defines the standard compositional and morphological terms used across this literature.

Flow Morphology

Basaltic flows adopt one of two classic surface forms, both named from Hawaiian usage. Pahoehoe has a smooth, billowy, or ropy skin and advances by inflating small toes that break out from a flexible crust, which favors hotter, more fluid, slower-moving lava. The rough, spiny, clinkery surface of aa forms when higher flow rates or greater viscosity tear a partly solidified crust into rubble that is carried forward and overridden at the flow front. Explanations of basaltic flow types published by the National Park Service describe how a single eruption can produce both, with pahoehoe converting to aa downslope as the melt cools and degasses. More viscous andesitic and dacitic lavas produce blocky flows with angular fragments, and eruption underwater generates pillow lava, a stack of chilled lobes with glassy rinds. Roofed-over channels evolve into lava tubes, whose insulation lets flows travel far from the vent with little temperature loss, as comparisons of basaltic flow types compiled at Oregon State University explain.

Monitoring and Remote Sensing

Active lava is one of the strongest thermal targets visible from orbit, which makes satellite instruments central to modern volcano surveillance. Middle- and thermal-infrared radiometers detect hot spots and convert measured radiance into estimates of effusion rate, while shortwave sensors track flow extent between passes. Synthetic aperture radar penetrates the eruption plume and, through interferometry, measures the ground deformation that precedes and accompanies an eruption. Ground-based thermal cameras, tilt meters, and gas spectrometers supply the higher time resolution needed for hazard decisions, and flow path models driven by digital elevation data forecast which drainages a flow will follow.

Applications

The study of lava has applications in a range of fields, including:

• Volcanic hazard assessment and evacuation planning
• Satellite thermal and radar monitoring of active eruptions
• Geothermal energy resource evaluation
• Planetary geology of the Moon, Mars, Venus, and Io
• Igneous petrology and crustal evolution research
• Aggregate, pumice, and mineral resource assessment