Granite is a common igneous rock found extensively within the Earth’s continental crust. It forms from magma that cools and solidifies beneath the surface, characterized by its coarse-grained texture. Like all rocks, granite can melt under specific, extreme conditions, a process that occurs deep within the Earth. Understanding these conditions helps explain the rock’s behavior in geological processes.
The Melting Point of Granite
Granite does not possess a single, precise melting point like a pure substance. Instead, it melts over a range of temperatures because it is a mixture of various minerals, each with its own melting characteristics. At ambient pressure, dry granite typically begins to melt at temperatures ranging from approximately 1215°C to 1260°C (2219°F to 2300°F). This temperature range is significantly influenced by several external factors and the rock’s specific mineral makeup.
Factors Influencing Granite’s Melting
The temperature at which granite melts varies based on several geological factors. The presence of water, the surrounding pressure, and the specific mineral composition of the granite all play significant roles in determining its melting behavior. These variables collectively dictate when and where granite can transform into molten rock.
The presence of water lowers the melting point of granite. Water acts as a flux, reducing the temperature required for melting to occur. Even a small amount of water, such as 1 to 2 weight percent, can decrease the solidus temperature, which is the temperature at which partial melting begins. This effect is particularly pronounced at higher pressures, where water can dissolve more readily into the rock.
Pressure also influences granite’s melting temperature. Generally, increasing pressure tends to raise a rock’s melting point. However, in geological settings, the presence of volatiles can counteract this effect, allowing melting to occur at lower temperatures even under high pressure. A decrease in pressure, known as decompression melting, can also cause already hot rocks to melt without any additional heat input.
The specific minerals within a granite sample contribute to its varied melting behavior. Granite is primarily composed of quartz, feldspar, and mica minerals, which melt at different temperatures. This leads to a process called partial melting, where some minerals in the rock begin to melt while others remain solid. For instance, felsic minerals, which are abundant in granite, generally melt at lower temperatures compared to mafic minerals.
Why Granite is So Resistant to Melting
Granite exhibits resistance to melting under typical surface conditions due to the high temperatures and pressures required for its liquefaction. The high melting temperatures, often coupled with the need for water and immense pressure, mean that granite does not melt near the Earth’s surface. This characteristic contributes to granite’s stability and prevalence in the continental crust.
Melting of granite occurs deep within the Earth’s crust, typically at depths where temperatures and pressures are high. Such conditions are found in geological environments like subduction zones, where oceanic crust descends beneath continental crust, or in areas of crustal thickening. In these deep crustal settings, heat from the mantle or from the intrusion of other magmas can raise temperatures enough to initiate the melting process, especially when water is present.