The Earth’s deep interior, characterized by immense heat and pressure, causes solid rock to transform into a scorching liquid. This molten material drives volcanic activity, constantly seeking a path to the surface from deep within the crust and upper mantle. The intense thermal energy causes rock minerals to melt, creating a complex substance that shapes the planet’s geology. Its designation changes the moment it emerges from the subterranean environment.
Lava Versus Magma: The Key Distinction
The distinction between the two primary terms for molten rock depends entirely on its location relative to the Earth’s surface. Molten rock confined beneath the crust is known as magma. This material collects in large, underground reservoirs called magma chambers. Magma transitions to lava only when it is expelled through a volcanic vent or fissure and contacts the atmosphere.
The moment this liquid rock exits the surface, it is classified as lava. Lava is simply magma that has been extruded onto the Earth’s exterior. Although the chemical makeup remains largely the same upon eruption, the new designation reflects a significant change in the rock’s environment, pressure, and cooling rate.
What Molten Rock is Made Of
The primary constituent of all molten rock is the liquid portion called the melt, composed mainly of silicate minerals. Silicon and oxygen are the most abundant elements, forming a structural backbone with other elements like aluminum, iron, magnesium, calcium, sodium, and potassium. The percentage of silicon dioxide (\(\text{SiO}_2\)) determines its classification, ranging from approximately 45% to over 75% by weight.
Molten rock also contains a solid component of mineral crystals that have not fully melted or have begun to crystallize. Another element is a gaseous phase known as volatiles, which vaporize easily at surface pressures. The most common volatiles are water vapor (\(\text{H}_2\text{O}\)), carbon dioxide (\(\text{CO}_2\)), and sulfur dioxide (\(\text{SO}_2\)), dissolved into the melt under high pressure. The material’s temperature is tied to its composition; low-silica magmas are hotter (up to \(1,200^{\circ}\text{C}\)), while high-silica magmas are cooler (as low as \(650^{\circ}\text{C}\)).
How Lava Behaves on the Surface
Once molten rock becomes lava on the surface, its physical behavior is governed by its viscosity, or resistance to flow. The primary factor controlling viscosity is the percentage of silica in the lava. High-silica lavas (felsic lavas) have high viscosity because silica molecules form complex, interlocking chains. This high viscosity causes the lava to move slowly, pile up near the vent, and often results in thick, blocky flows.
Conversely, basaltic lavas have a lower silica content and are less viscous, allowing them to flow more easily and rapidly over great distances. These runny flows can form smooth, ropey surfaces known as pahoehoe, or rough, jagged surfaces called ‘a’ā. Hotter lava also generally has a lower viscosity and flows more freely than cooler lava of the same composition. High-viscosity lavas tend to trap dissolved gases, leading to more explosive eruptions, while low-viscosity lavas allow gases to escape easily, resulting in calmer, effusive flows.
The Final State: Extrusive Igneous Rock
When lava reaches the Earth’s surface, exposure to the atmosphere or water causes it to solidify rapidly. This quick cooling prevents mineral crystals from growing to a large size. The resulting solid material is classified as extrusive igneous rock, also referred to as volcanic rock.
The rapid solidification leads to a very fine-grained texture, where individual crystals are often microscopic. If cooling is immediate, no crystals form, producing a glassy texture, such as obsidian. Gas bubbles trapped during cooling can also create a porous, lightweight texture known as vesicular. Examples of this final state include basalt, andesite, and rhyolite, defined by their composition and the textures formed by rapid cooling.