The distinction between magma and lava is purely a matter of location, representing two phases of the same molten rock material. Magma is the term used for the molten rock mixture found beneath the Earth’s surface. The moment this material breaches the surface, whether through a volcanic vent or a fissure, it is instantly renamed lava. This transition is a physical process driven by changes in pressure, which alters the material’s internal chemistry and behavior.
Magma: Characteristics Below the Surface
Magma is a complex, high-temperature mixture of liquid rock, suspended mineral crystals, and dissolved gases, often stored in large underground reservoirs known as magma chambers. These chambers exist deep within the Earth’s crust or upper mantle, where temperatures typically range from 650°C to 1200°C, depending on the composition. The defining characteristic of magma at this depth is the immense lithostatic pressure exerted by the overlying rock layers.
This extreme pressure forces and holds volatile substances, such as water vapor (\(\text{H}_2\text{O}\)) and carbon dioxide (\(\text{CO}_2\)), in a dissolved state within the silicate melt. These dissolved volatiles significantly influence the magma’s physical properties, including its density and crystallization. Magma remains buoyant due to its lower density compared to the surrounding solid rock, causing it to slowly migrate upward through the crust.
The Critical Role of Volatiles and Pressure
The transformation from magma to lava occurs when the molten rock reaches a point of vapor saturation, linked directly to pressure release. As magma ascends toward the surface, the confining pressure from the surrounding rock decreases rapidly. This drop in pressure causes the dissolved gases to exsolve and form bubbles, much like opening a pressurized bottle of soda.
The depth at which this exsolution begins can be a few kilometers below the surface, driving the eruption. As the bubbles of water vapor and carbon dioxide nucleate and grow, they cause the magma to expand tremendously, sometimes to hundreds of times its original volume. This expansion forces the material upward through the volcanic conduit. The molten rock is termed lava the moment it passes the Earth’s surface.
Lava: Characteristics Above the Surface
Once extruded onto the Earth’s surface, lava exists in an environment of dramatically lower pressure, having already degassed most of its volatile content. This loss of dissolved gases changes the material’s bulk properties, often increasing its viscosity or resistance to flow. Lava flows exhibit temperatures between 700°C and 1200°C, but they cool much more rapidly than magma does underground.
The exposed surface quickly forms a solid crust, though the interior remains molten and continues to flow, sometimes traveling great distances. The behavior of the lava, whether it forms fast-moving, fluid sheets or thick, slow-moving mounds, is primarily determined by its chemical composition and remaining gas content.
The Resulting Extrusive Igneous Rocks
The final stage occurs when the lava completely solidifies, forming extrusive, or volcanic, igneous rocks. Because lava cools quickly upon exposure to the atmosphere or water, the mineral crystals within the rock have very little time to grow. This rapid cooling results in rocks with a fine-grained, or aphanitic, texture, where individual crystals are too small to be seen without magnification.
If the lava cools extremely fast, such as when it meets water, it can form a glassy texture with no crystalline structure at all. Common examples of these rocks include basalt, which is dark and fine-grained, and obsidian, which is a volcanic glass. The presence of small holes, called vesicles, in rocks like pumice and scoria is direct evidence of the gas bubbles that exsolved during the transition to lava.