Where (and why) do igneous rocks form?
Rocks melt when the temperatures, pressures, and/or water content are sufficient to bring the rock to the melting tempertare (duh). This most occurs 1) when rocks have been brought up from deeper in the mantle for example at a RIDGE, 2) rocks with water in them are brought from the surface down into the mantle, for example at a SUBDUCTION ZONE, or there is some “HOT SPOT” arising from deep in the mantle unrelated to a plate boundary. About 90% of all melting occurs at plate boundaries (spreading or subducting). The rest is called “intra-plate” and is typified by Hawaii. For a summary of where melts form, see Figure 4.7 in your book. When a partial melt forms, it rises and collects in a magma chamber (see Figure 3.2 in your book). In the magma chamber, the melt continues to crystallize thus changing its chemistry. This is a process known as magmatic differentiation. As magmas cool, different minerals will crystallize out of the melt. By studying the crystallization of melts in the labor
