Introduction to Physical Geology
Igneous Rocks
Igneous rocks form from magmas, and most magmas are associated with plate tectonics. Mafic (basaltic) and ultramafic magmas form along the divergent midoceanic ridges and are major components of new oceanic crust. More felsic magmas, such as andesites and rhyolites, are associated with the edges of continental crust at subduction zones along converging plate boundaries. Whether a magma is intermediate or felsic may depend on the relative amounts of oceanic crust and continental crust in the subduction zone that melt to form the magma. The great abundance of granitic intrusions in continental crust is thought to be related to the partial melting of the lower continental crust.
Intraplate igneous activity occurs in the interior of a single continental plate and is thought to be related to mantle plumes (such as the eruptions at Yellowstone National Park) or flood basalts. Intraplate activity is not associated with moving plate boundaries such as subduction zones.
