Granite, known for its strength and durability, is a fundamental component of the continental crust. While it appears permanent, this rock can be melted. The process requires a combination of extreme heat and pressure found only in deep geological environments. Melting granite is a dynamic process central to the formation and evolution of continents over geologic time. The transformation to molten material, known as magma, is complex because granite is not a substance with a single melting point, but rather a mixture whose components melt sequentially.
Defining the Building Blocks of Granite
Granite is classified as a multi-mineral rock, meaning it is an aggregate of several distinct mineral types. Its composition is predominantly felsic, indicating a high concentration of lighter elements like silicon and oxygen. The primary minerals present in granite are quartz and two types of feldspar (potassium and plagioclase), which together account for over 90% of the rock’s volume.
The remaining portion consists of darker, iron and magnesium-rich minerals, such as micas. Because each component possesses a different inherent melting temperature, granite does not melt all at once when subjected to heat. Instead, the minerals with the lowest melting points begin to turn to liquid first. This process is called partial melting, which means the composition of the resulting magma is not identical to the original solid rock.
The Baseline: Dry Melting Point and Extreme Heat
To establish a baseline for granite’s melting temperature, scientists look at the “dry solidus.” This is the temperature at which the first melt appears in the absence of water or other volatile compounds. Under atmospheric pressure, this theoretical temperature is exceptionally high, typically falling in the range of 1215°C to 1260°C. This immense heat requirement is a reason why granite is so durable and common in the Earth’s upper crust.
The dry melting point of granite is comparable to the hottest temperatures recorded for basaltic lava. In the Earth’s crust, temperatures this high are rarely reached without the influence of other factors. Since the melting point of dry rock generally increases with depth and pressure, simply burying granite and heating it up is not the primary way granite magma forms in nature.
The Critical Factors: Pressure and Volatiles
Granite melting in the Earth’s crust involves two modifying factors: high pressure and the presence of volatile compounds, most significantly water. While pressure generally increases the melting temperature of dry materials, the effect of water completely changes the dynamic for granite. Water acts as a powerful flux, a substance that lowers the melting point of the surrounding rock by interfering with the crystal structure.
Dissolved water breaks the strong silicon-oxygen bonds within the aluminosilicate crystal lattice, chemically weakening the structure. This process is known as depolymerization, and it drastically reduces the energy required to transform the solid rock into a liquid state. With sufficient water content, the temperature at which melting begins, known as the “wet solidus,” can drop dramatically to between 650°C and 700°C at crustal depths where pressure is high. This reduction of several hundred degrees makes granite melting a common phenomenon in specific geological environments.
This melting under wet conditions is always partial, with the resulting felsic magma being rich in quartz and feldspar. The remaining solid rock, known as the restite, is left behind, enriched in minerals with higher melting temperatures, such as pyroxene and amphibole. A higher water content means the melt can form at a lower temperature, a principle that governs the creation of large granite bodies.
Where Granite Melts: Geological Context
The conditions necessary for granite melting—moderate heat, high pressure, and water—converge in two main tectonic settings on Earth. The most common location is within subduction zones, where one tectonic plate slides beneath another. As the oceanic plate descends, it carries water deep into the mantle, locked within the crystal structures of its hydrous minerals.
When the descending oceanic slab reaches a certain depth, the increasing temperature and pressure cause these hydrous minerals to break down, releasing large volumes of water vapor into the overlying continental crust. This water lowers the wet solidus of the continental rock, triggering widespread partial melting to form granitic magma. This magma, which is less dense than the surrounding solid rock, then rises and cools slowly beneath the surface to form massive intrusions known as batholiths.
A second environment where granite melts is within continental collision zones, such as the formation of the Himalayas. When two continental plates collide, the crust thickens dramatically, pushing vast quantities of rock deep underground. The insulating effect of this thickened crust, combined with heat generated by radioactive decay, raises the temperature of the lower crust. The intense pressure and heat eventually cause deep crustal melting, or anatexis, which generates granite-forming magma. The resulting molten material cools and crystallizes to form new igneous rocks, completing the cycle of continental crust formation.