Plate Boundaries The action in plate tectonics is almost always at the plate boundaries. Two plates moving towards each other form a convergent boundary 1.Ocean crust meets ocean crust 2.Ocean crust meets continental crust 3.Continental Crust meets continental crust Two plates moving away from each other form a divergent boundary
Divergent Plate Boundaries in the oceans Extensional Stresses stretching ocean crust Creates New Seafloor Volcanoes Eruptions of basalt Low in Si, high in Fe Earthquakes Shallow foci Low to modest magnitudes
Partial melting of peridotite in the mantle produces basalt. Why doesn’t this process just produce more peridotite? Different mineral components in the peridotite melt at different T’s What will happen if only a small fraction of the rock melts? Will the melted part have the same composition as the original rock?
The rift occurring under the Red Sea is more advanced
Why do we get both high & low Si rocks at continental rifts? Basalt is dense and ponds at base of crust Heat from basalt melts some of the Si-rich crust producing granite/rhyolite magma Both types of magma may eventually erupt at the surface
Convergent Plate Boundaries Compressional Stresses Old Seafloor Destroyed (Subduction) Volcanoes Explosive and Very Dangerous (lots of gas) Earthquakes Shallow to Deep foci Large magnitudes Mountain Belts/High Deformation Accretion – Building of the Continents