When Earth breathes fire: the science of volcanoes

The ongoing activity at Hawai‘i’s Kīlauea volcano has garnered intense media attention, with images and live footage circulating worldwide. During the most recent eruptive episode, lava jets soared as high as 1,200 feet (about 370 meters), lighting up the night sky and painting the crater in fiery orange (Fig. 1). Watching these towering fountains in video footage, one can easily imagine the sheer terror volcanoes must have inspired in ancient people as they were trying to flee the deadly flames. Today, we have a much better understanding of this natural phenomenon. So, what ignites the flame and causes the Earth to open its crust and propel molten rock high into the air?

Fig. 1 Lava jets erupt from the Hawaii Kilauea volcano, during the latest episode of activity, producing bright, high-temperature fountains and feeding fresh lava flows across the caldera floor. Image credit: U.S. Geological Survey (USGS), Hawaiian Volcano Observatory.

At the most fundamental level, volcanic eruptions are driven by pressure, heat, and gas within Earth’s interior. Our planet was formed 4.5 billion years ago from a nebula containing remnants of ancient stars. During its formation, intense heat from debris collisions, gravitational compression, and radioactive decay melted a planet into a jellylike state. The elements moved and separated according to their density, much like ingredients settling in a liquid mixture. Heavier elements, such as iron and nickel, sank to the core, while intermediate rock-forming elements settled in the mantle. Lighter minerals and gases formed the crust and atmosphere.

Fig. 2 Internal structure of the Earth showing the crust, mantle, outer core, and inner core.

As time passed, Earth cooled. Today, the Earth's crust and most of the mantle consist of solid rock (Fig. 2). Still, the mantle remains very hot, particularly near the core, where the rock becomes softer and moves slightly over geological timescales. This gradual movement can cause the crust above to thin or fracture. In these regions, the mantle can rise closer to the surface, where it encounters lower pressure with profound consequences.

Within the Earth, pressure increases significantly as you move toward the center due to the weight of the layers above. As a result, the pressure at Earth’s core is enormous compared with anything we experience in life, being 3.6 million times higher than the air pressure at the surface. As we move in the opposite direction, away from the center, the pressure decreases because the weight of the layers above us diminishes.

Lower pressure makes materials melt and boil at lower temperatures. As a familiar example, Water boils at a lower temperature on a mountain than at sea level. With less atmospheric pressure pushing down on it, water molecules escape into vapor more easily. The same principle applies inside Earth, though at vastly higher pressures and temperatures. Mantle rock that is completely solid at great depth can begin to partially melt when it rises to the surface, not because it gets hotter, but because the melting point drops in the lower-pressure environment.

When rock melts to form magma, its volume increases, similar to how most liquids take up more space than their solid forms. Being less dense than the surrounding rock, magma begins to rise toward the surface. However, this ascent doesn’t always go smoothly. Magma gets trapped in underground crevices and chambers, where it accumulates, building intense internal pressure. Gases within magma bubble out in the upward flow, further raising pressure in the pockets.

Eventually, the surrounding rock can no longer hold back this growing, destructive force. Magma breaks out through cracks and conduits. When it finally reaches open air, the trapped gases burst free, propelling molten rock skyward in spectacular fountains, similar to those recently observed at Kīlauea. The Earth releases built-up pressure, heat, and gases in a dramatic showcase of internal energy, reminding us of the power of nature.

Fig. 3. Video footage of the Hawaii Kilauea volcano eruption.

The vivid orange glow of erupting lava is more than just a striking visual; it is a direct clue to its temperature. Based on Wien’s displacement law, lava that glows a bright orange radiates most strongly at wavelengths corresponding to temperatures of about 1,000–1,200 °C (about 1,832–2,192 °F).  At these temperatures, the thermal radiation curve shifts sufficiently into the visible spectrum, creating the distinctive fiery hue observed in lava fountains and flows. Thus, the color of lava acts as a natural thermometer, enabling us to gauge its heat directly from the glow it casts into the night.