Divergent Boundaries:
* Source Material: The mantle beneath divergent boundaries is mostly peridotite, an ultramafic rock rich in minerals like olivine and pyroxene.
* Melting Process: As plates move apart, decompression melting occurs. This means the pressure on the mantle rock decreases, allowing it to melt.
* Mafic Composition: Olivine and pyroxene have low melting points compared to other minerals, so they melt first, creating a magma that is rich in iron (Fe) and magnesium (Mg) - the defining characteristics of mafic magma.
Convergent Boundaries:
* Subduction: At convergent boundaries, one plate dives (subducts) beneath another.
* Water Release: The subducting plate releases water into the overlying mantle.
* Melting Point Reduction: Water lowers the melting point of mantle rock, causing it to melt.
* Intermediate to Felsic Magma: Melting at these depths involves more of the mantle and the crust, leading to a more complex mix of minerals.
* The presence of water also contributes to the assimilation of continental crust, which is rich in silica (SiO2), sodium (Na), and potassium (K).
* This process creates magma that is intermediate to felsic in composition, with higher silica content and lower iron and magnesium content.
Summary:
* Divergent boundaries: Decompression melting of mafic mantle rock produces mafic magma.
* Convergent boundaries: Water-induced melting of mantle and crustal material leads to intermediate to felsic magma.
This is a simplified explanation, and the actual process is far more complex. There are many factors influencing magma composition, including the specific mineralogy of the source rock, the depth of melting, and the presence of other elements. However, this breakdown helps explain the general trend of mafic magma at divergent boundaries and intermediate to felsic magma at convergent boundaries.
