1. Decompression Melting:
* As the plume rises, the pressure surrounding it decreases significantly.
* This pressure reduction lowers the melting point of the silicate minerals within the plume.
* Imagine a bottle of soda: when you open it, the pressure is released, and the gas dissolved in the liquid forms bubbles and escapes. Similarly, when the pressure on the plume decreases, the minerals start to melt.
2. Adiabatic Expansion and Heating:
* The rising plume experiences adiabatic expansion. This means that the plume expands as it rises, and this expansion results in a slight decrease in temperature.
* However, the decrease in temperature is much smaller than the decrease in pressure, which lowers the melting point significantly.
* This is because the plume is rising slowly, allowing it to exchange heat with its surroundings.
3. Presence of Water:
* The mantle is not completely dry. It contains small amounts of water, often bound within minerals.
* As the plume rises and melts, the water is released and can further lower the melting point of the surrounding rock.
* The presence of water acts as a "flux" that facilitates melting.
4. Composition of the Plume:
* The composition of the plume itself can influence its melting point.
* Some minerals in the mantle, like those found in the lower mantle, are less stable at shallower depths and more susceptible to melting.
In summary:
The combination of decreasing pressure, adiabatic expansion, the presence of water, and the composition of the plume itself creates a scenario where the rising plume, despite slightly cooling, will experience significant melting as it approaches the base of the lithosphere. This melting leads to the formation of magma, which can rise further and potentially erupt at the surface, contributing to volcanic activity.
