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4.10 Why don't all magmas erupt?
- The many intrusive rock outcrops tell us that not all magma erupts. Why not?
- To understand, let us look at why magma rises.
- Magma is less dense than surrounding rock of same composition.

But, as mafic material is denser than felsic, it generally doesn't rise through continental rock. Heat melts lower felsic rock, which rises instead.
Fig 4.30