Lava is molten rock that has erupted onto the Earth’s surface. While its intense heat causes dramatic effects on many common objects, the capacity of lava to melt a substance is governed by the laws of thermodynamics. Melting depends on the relative difference between the lava’s temperature and the material’s melting point. This means not everything succumbs to the heat; some materials remain structurally intact, while others undergo chemical transformation entirely.
The Temperature Range of Lava
The temperature of lava varies significantly based on its chemical composition, primarily its silica content. Basaltic lavas, which are low in silica and flow easily, are the hottest, erupting at temperatures between \(1000^\circ\text{C}\) and \(1200^\circ\text{C}\). These fluid lavas represent the maximum thermal limit for most eruptions.
In contrast, andesitic lavas have a higher silica content, making them more viscous and cooler, generally ranging from \(800^\circ\text{C}\) to \(1000^\circ\text{C}\). Felsic lavas, the most silica-rich, can be even cooler, sometimes erupting as low as \(650^\circ\text{C}\).
For a material to melt, it must absorb enough thermal energy to reach its specific melting point. The typical range of \(800^\circ\text{C}\) to \(1200^\circ\text{C}\) is the boundary that determines what remains solid and what undergoes a phase change. A material with a melting point above \(1200^\circ\text{C}\) will resist liquefaction when exposed to Earth’s hottest lavas.
High Melting Point Materials
Materials that cannot be melted by lava are those whose melting points substantially exceed \(1200^\circ\text{C}\). This category includes refractory ceramics, specialized alloys, and certain natural minerals.
Pure aluminum oxide, or alumina (\(\text{Al}_2\text{O}_3\)), is a common refractory ceramic used in industrial applications, boasting a melting point of approximately \(2072^\circ\text{C}\). This temperature is nearly \(900^\circ\text{C}\) higher than the hottest basaltic lava.
Zirconia (\(\text{ZrO}_2\)), another robust ceramic, exhibits an even higher melting point, typically around \(2715^\circ\text{C}\). These materials resist the thermal energy that would melt most other substances.
Certain metals also possess melting points high enough to withstand lava’s heat. The refractory metal tungsten, for example, has a melting point of \(3422^\circ\text{C}\), making it one of the most heat-resistant pure elements.
Even more extreme are specialized composite materials. Hafnium carbide (\(\text{HfC}\)) has a melting point of up to \(3958^\circ\text{C}\). An alloy of hafnium and tantalum carbide (\(\text{Ta}_4\text{HfC}_5\)) melts at about \(3905^\circ\text{C}\), placing it among the highest melting point compounds ever synthesized.
Naturally occurring minerals like diamond also resist melting, requiring temperatures around \(3827^\circ\text{C}\) to melt at high pressure. The survival of these substances is a consequence of their strong atomic bonds, which require immense energy to break.
Non-Melting Interactions with Lava
Not all materials that avoid melting emerge unaffected; some undergo dramatic non-melting interactions, where they are consumed or transformed by chemical change rather than a simple phase transition. Water, in the form of ice or liquid, provides a clear example of this phenomenon.
When water comes into contact with lava, it rapidly vaporizes into steam. This process is often governed by the Leidenfrost effect, where the intense heat causes a layer of vapor to form instantly around the water. This insulating vapor layer temporarily prevents direct contact between the liquid and the molten rock, slowing the rate of heat transfer.
The water is converted directly to an explosive volume of steam, which can sometimes cause phreatic explosions, but the water molecule itself never melts into a liquid phase.
Organic materials, such as wood, plants, and plastics, similarly do not melt, but instead burn or decompose. Lava’s heat causes combustion, a chemical reaction with oxygen, or pyrolysis, which is thermal decomposition without oxygen.
Wood and other plant matter are consumed by combustion, turning into ash and gas, or they are “cooked” by the heat, leading to pyrolysis. The pyrolysis process breaks down the complex organic compounds into simpler flammable gases like methane, hydrogen, and carbon monoxide, which can then ignite.
Plastics, such as polyethylene, undergo thermal degradation where the polymer chains break down into smaller hydrocarbon fragments at temperatures far below their theoretical melting points. Polyvinyl chloride (PVC) is particularly susceptible to degradation at relatively low temperatures.
In all these cases, the material transitions to a gaseous state or a solid residue like char and ash, rather than transitioning from a solid to a liquid phase. The lack of a liquid-to-liquid phase change means it technically does not melt.