When melting forms magma
Multiple Choice
None of these choices are correct.
rocks in the source region generally are completely melted.
partial melting produces a magma that is more felsic than the source.
melting of the mantle generally produces felsic magma.
melting of continental crust generally produces mafic magma.
The correct answer and explanation is :
The correct answer is:
Partial melting produces a magma that is more felsic than the source.
Explanation:
When rocks in the Earth’s mantle or crust are heated to the point of melting, the resulting liquid is referred to as magma. The process of forming magma is generally not a complete melting of the source rock but rather a partial melting. This partial melting occurs because different minerals within the rock melt at different temperatures. As a result, the composition of the magma will not be the same as the source rock, and the resulting magma will have a different chemical composition.
- Partial Melting and Felsic Magma:
When partial melting occurs, the minerals that melt first tend to be less dense, lighter-colored minerals, such as feldspar, quartz, and muscovite, which are common in felsic rocks. Felsic rocks are silica-rich and typically light in color, with higher amounts of silicon and oxygen. This means that the resulting magma is more felsic than the original source rock. If the source rock is mafic (like basalt), partial melting will produce magma that is more felsic, having higher silicon dioxide (SiO2) content than the original material. - Melting of the Mantle Produces Mafic Magma:
In contrast, the melting of the mantle typically produces mafic magma, which is rich in iron and magnesium and lower in silica. This type of magma is typically darker in color. The mantle is primarily composed of peridotite, which is mafic, so when it melts, it forms a magma with characteristics of basalt or gabbro, which are mafic rocks. - Melting of Continental Crust:
Melting of continental crust often produces more felsic magma (like granite or rhyolite) because continental crust is primarily composed of light-colored, silica-rich rocks such as granite. However, this process is more complicated, and some parts of the crust can melt to produce intermediate or mafic magmas.
Thus, partial melting results in magma that is generally more felsic than the source rock, particularly when dealing with more mafic source materials. The key point is that the chemical composition of the magma is not identical to the source material due to partial melting.