Plate subduction causes melting because the subducting oceanic plate carries water into the mantle, which lowers the melting point of the surrounding rocks.
The Role of Water in Subduction Zone Melting
When a tectonic plate from under the ocean subducts, or moves beneath another plate, it doesn't just disappear; it carries water into the mantle. This water, trapped within the minerals of the oceanic crust and sediments, is released under the high pressures and temperatures of the mantle wedge above the subducting plate.
The presence of this water is key. This water lowers the melting point of the mantle material. Normally, mantle rock (peridotite) requires very high temperatures to melt under the immense pressure found deep within the Earth. However, the introduction of water acts like a flux, disrupting the bonds in the rock minerals and allowing them to melt at lower temperatures than they otherwise would at that pressure.
As the mantle material melts due to this reduced melting point, it forms magma. This newly formed magma rises to form magma chambers, which are a significant part of the mechanism for volcanic activity often seen at convergent plate boundaries, such as along volcanic arcs.
Here's a simple breakdown of the process:
- Subduction: An oceanic plate goes beneath another plate.
- Water Transport: The oceanic plate carries water-rich minerals into the mantle.
- Water Release: Water is released from minerals due to heat and pressure.
- Melting Point Lowering: Released water lowers the melting point of the overlying mantle rock.
- Magma Formation: Mantle rock melts at the reduced temperature, forming magma.
- Magma Ascent: Buoyant magma rises towards the surface, potentially leading to volcanism.
This process explains why many of the world's most active volcanoes are located in regions where subduction is occurring, forming chains of volcanoes parallel to the plate boundary.
