Granite is a common intrusive igneous rock composed primarily of quartz, feldspar, and mica. Granite does not burn because this natural stone lacks the organic compounds required to sustain combustion.
The Chemical Reason Granite Does Not Burn
Combustion relies on three elements: fuel, oxygen, and heat, often called the fire triangle. Granite fails to qualify as a fuel source because its chemical structure is entirely inorganic. It is a felsic rock composed largely of mineral oxides, with silica (SiO2) making up 70% to 77% of its weight, alongside significant amounts of alumina (Al2O3) and various metal oxides.
These components, such as silicon dioxide, are already in a highly oxidized state due to their formation deep within the Earth under immense heat and pressure. For a material to burn, it must undergo rapid oxidation, but granite’s minerals are essentially “pre-burned” chemically. Unlike organic substances, which contain carbon and hydrogen, the tightly bonded, non-combustible oxides in granite cannot react further to release heat and light. This chemical stability ensures that granite remains non-flammable, even when directly exposed to a flame.
Physical Damage: Spalling and Thermal Shock
While granite will not catch fire, it is susceptible to physical degradation when exposed to intense, rapid heating, such as in a structure fire. This damage is primarily caused by thermal shock, which occurs due to sudden or extreme temperature changes. Granite is a composite material made up of different minerals, each possessing a unique coefficient of thermal expansion, meaning they expand at different rates when heated.
As heat penetrates the granite, the uneven expansion of minerals like quartz and feldspar generates significant internal stress. Quartz, for example, undergoes a volume change when it transitions from its alpha to beta state around 573°C, which dramatically increases this internal strain. This stress causes the formation of microscopic cracks, weakening the material’s overall integrity.
The most visible sign of this damage is spalling, where layers or fragments of the rock surface pop off violently. This effect is exacerbated by moisture trapped within the granite’s porous structure. When internal water turns to steam under intense heat, the rapid volumetric expansion creates internal pressure that forces pieces of the stone to detach. Spalling can begin to occur at temperatures as low as 300°C, leading to structural failure and exposing fresh layers of rock to the damaging heat.
Granite’s Extreme Heat Tolerance
Despite the risk of physical damage from thermal shock and spalling, the temperature required for granite to melt is high. The melting of dry granite under normal atmospheric pressure typically occurs in the range of 1215°C to 1260°C (2219°F to 2300°F).
This melting point is far above the temperatures reached in most standard building fires, which rarely sustain temperatures above 1000°C for long periods. Although granite can be compromised by thermal shock and micro-cracking starting around 300°C to 500°C, its melting resistance reinforces its classification as a non-combustible material. The high threshold for material failure is a direct result of its formation from molten magma, granting it a high degree of intrinsic thermal stability.