1. Decompression Melting:
* Lower pressure: As tectonic plates move apart at divergent boundaries, the overlying pressure on the mantle rocks decreases.
* Reduced melting point: The reduced pressure lowers the melting point of the mantle rocks. This is because the pressure suppresses the melting process. When the pressure is released, the rocks can melt at a lower temperature.
2. Adiabatic Expansion:
* Rising mantle: As the plates move apart, the mantle material rises to fill the gap.
* Cooling: The rising mantle material cools as it expands, but not enough to compensate for the reduced pressure.
* Melting: This cooling, combined with the reduced pressure, leads to decompression melting.
3. Water Content:
* Subducted water: In some cases, water from subducted oceanic plates can be incorporated into the mantle.
* Lowering melting point: The presence of water significantly lowers the melting point of the mantle rocks.
* Enhanced melting: This leads to more extensive melting and the formation of larger volumes of magma.
In summary: The combination of decompression melting, adiabatic expansion, and water content contributes to the melting of rocks in the mantle at divergent boundaries. This process is responsible for the formation of new oceanic crust at mid-ocean ridges and volcanic activity along these boundaries.
