Lowering the Melting Temperature:
* Hydrostatic Pressure: Water, when present in the pores of rocks, exerts pressure. This pressure opposes the pressure required for melting. Because the melting temperature of rock increases with pressure, water's pressure effectively lowers the melting temperature. This is particularly important in areas like subduction zones, where water-rich sediments are dragged down into the Earth's mantle.
* Flux Melting: Water acts as a "flux" by lowering the melting temperature of rock. This is because water molecules break down the chemical bonds within the rock's minerals, making it easier for them to melt. The presence of water allows rocks to melt at lower temperatures than they would otherwise. This process is essential for the formation of many magmas.
* Hydrous Minerals: Some minerals, like amphibole and mica, contain water molecules within their crystal structures. When these minerals are heated, they release water, which further lowers the melting temperature of the surrounding rock.
Other Considerations:
* Composition: The composition of the rock also plays a role. Some rocks, like those rich in silica, are more sensitive to water's influence on melting temperature than others.
* Depth: The depth at which melting occurs influences how water affects melting. At shallower depths, water is less effective at lowering the melting temperature.
* Pressure: The overall pressure within the Earth also influences the melting temperature. At higher pressures, water's effect on lowering melting temperature is less pronounced.
In summary:
Water acts as a catalyst for melting by:
1. Lowering the melting temperature: Water pressure and the breakdown of chemical bonds help rocks melt at lower temperatures.
2. Facilitating flux melting: Water helps to initiate melting by weakening the bonds within minerals.
This influence is crucial for many geological processes, such as the formation of volcanoes, the creation of metamorphic rocks, and the movement of tectonic plates.
