What is unique about the origin of granitic magma?
Basically, magma formed in the mantle rises up into continental crust. That causes part of the crust to melt. Since the continental crust is more felsic, the resultant magma is more felsic (more silica). Figure 4.5 shows this. • Important!! So let’s think about ocean crust and continental crust. Ocean crust is mostly gabbro and basalt. Ocean crust mostly forms at divergent plate boundaries where pressure-release melting creates gabbroic magma. Continental crust is formed when gabbroic magma from below melts some continental crust. If the resulting magma is more felsic, the resulting rocks that form will be granite and rhyolite. If the resulting magma is less felsic, the resulting rocks that form will be diorite and andesite. Look at Table 4.2. Section 5.7 does a very good job describing the types of magma at different types of crust. • Why and how does magma rise? Magma is partial liquid. It is less dense than the surrounding rock (country rock). You should know what assimilation is, b
