The molten rock that emerges from a volcano is a dynamic, superheated fluid whose temperature varies widely. To understand the heat of a volcanic eruption, it is important to distinguish between magma and lava. Magma is the molten rock beneath the Earth’s surface, typically pooling in subterranean chambers. Once this material is expelled from a vent or fissure and flows across the land or ocean floor, it is called lava. The temperature of lava changes constantly based on its chemical makeup and the environment it flows into.
The Core Answer: Temperature Ranges
The temperature of most silicate lavas when they first erupt falls within a range of approximately 700°C to 1,200°C (about 1,300°F to 2,200°F). This represents a large spectrum, with the most common lavas usually erupting near the top of this range, often exceeding 1,100°C. Objects or materials with a melting point below 1,200°C, which includes most common metals and organic substances, cannot withstand direct contact with a fresh lava flow.
The radiant heat produced by a large body of lava is immense, making it dangerous to approach even from several meters away. The sheer volume and insulating properties of a lava flow mean it contains vast thermal energy. This massive heat reservoir allows the lava to remain molten for extended periods, even as its surface begins to cool and solidify.
Why Temperature Varies: The Role of Composition
The primary factor determining a lava’s temperature and behavior is its silica ($\text{SiO}_2$) content. Silica acts as a polymerizing agent, making the molten rock stickier and more resistant to flow, a property known as viscosity. Low-silica lava, known as basaltic or mafic lava, is the hottest and most fluid, often erupting at temperatures above 1,000°C. This runny material flows easily and spreads out over great distances, which is why it forms the broad, gently sloping mountains called shield volcanoes, such as those found in Hawaii.
In contrast, high-silica lava, such as andesitic or rhyolitic lava, is significantly cooler, sometimes erupting at temperatures as low as 700°C. The high silica content makes this lava highly viscous, causing it to pile up steeply around the vent rather than flowing away. This results in the formation of steep-sided, conical structures known as stratovolcanoes, which can be seen in places like Mount Fuji or Mount Vesuvius. The cooler, viscous lava often traps volcanic gases, leading to more explosive eruptions compared to the gentler, effusive flows of the hotter basaltic type.
Measuring the Heat
Volcanologists employ two primary methods to determine the temperature of a lava flow. For the most accurate measurement of the lava’s internal temperature, a specialized device called a thermocouple is used. This instrument must be physically inserted directly into the molten material, a process that is often challenging and hazardous in an active flow field. The direct contact method provides a precise reading of the actual temperature.
Because of the danger involved with direct contact, scientists also rely on remote sensing techniques, primarily using infrared pyrometers or thermal cameras. These instruments measure the thermal radiation emitted by the lava, providing a temperature reading from a safe distance. However, these remote measurements only capture the temperature of the flow’s surface, which can be considerably cooler than the interior due to rapid heat loss to the atmosphere. Comparing the surface reading to the more accurate thermocouple data helps researchers understand the cooling dynamics of the flow.
Cooling and Solidification
Once lava is exposed to the atmosphere, it immediately begins to cool and solidify, a process that dictates the final texture of the resulting igneous rock. The speed of cooling is the main factor controlling the size of the crystals that form within the rock structure. When lava cools extremely rapidly, like when it hits water or air, there is insufficient time for atoms to organize into mineral crystals, resulting in a glassy texture, such as that seen in obsidian. Conversely, a slightly slower, but still fast, cooling rate produces fine-grained rock, such as basalt, where the crystals are too small to be seen without magnification.
The physical appearance of a solidified flow is also strongly influenced by its cooling and flow dynamics, resulting in distinct surface textures. The smooth, sometimes ropy or billowy surface known as Pahoehoe forms from highly fluid lava that cools slowly and develops an insulating crust. In contrast, the rough, jagged, and broken surface called A’a forms from cooler, more viscous lava that tears itself apart as it moves, creating a flow made of sharp, clinkery fragments. The transition from the hotter, smoother Pahoehoe to the cooler, rougher A’a is a common observation as a single lava flow loses heat and viscosity over distance.