Basalt is thought to form from partial melting of peridotite and eclogite. Eclogite is a metamorphic rock with a chemical bulk composition similar to basalt. Unlike continental crust, oceanic crust can be subducted into the mantle. Partial melting initiates when the subducted slab reaches about 150 km depth in the mantle. The position of island arc ridges relative to slab position agrees agrees well with partial melting initiated at 150 km depth.
Oceanic crust has insignificant amounts of radiogenic elements.
Divergent plate motion. Convergent plate motion . For example, the collision of India with Eurasia is deforming and squeezing China towards the east along successions of strike-slip faults. Earthquakes occur in the brittle zone of the crust. In most regions, this is in the upper 12-15 km of the crust.