Igneous rocks form from the cooling of molten materials.  In the past, geologists believed that the occurrence of molten materials is random and that igneous activity could occur anywhere. With plate tectonics, we have come to see that igneous activity is far from random, and is associated with plate interactions. 

The ultimate source of all molten materials is the asthenosphere. From pieces of this layers called ophiolites, incorporated in suture zones, we have learned its composition and conditions that exist in this layer. 

One of the intriguing questions which arises when considering igneous rocks is: why are there so many different igneous rocks, especially when we consider that they all started as the same molten materials from the asthenosphere at divergent boundaries? One possible answer is that there are as many different rocks as there are types of magmas (molten material beneath the earth’s surface). Although this explanation may hold in those cases where there is localized melting, it does not explain how a material rich in iron, calcium, and magnesium silicates can eventually give rise to lavas and magmas rich in potassium and aluminum. It also begs the question as to why magmas would be different in the first place.