Partial melting occurs when some minerals melt Continental Rifting Trench Spreading while others remain solid because their melting point has not been reached The Crust Center continental Subduction Basalt Volcanic rifting Granite Arc Accretionary process Oceanic Prism Crust a Granite q The role of partial melting Basalt Kk 3 Types of Plate Boundaries MODERN CONTINENTS EVOLVED FROM PANGAEA DIVERGENT CONVERGENT New lithosphere forms as plates pull apart. One plate dives beneath another (subduction) or two Convergent Divergent TRANSFORM plates collide without either Main Types of Plate Motion Plates grind past each other. subducting. Transform No change in Lithosphere Divergent Boundary Mid-Ocean Ridge Seafloor spreading leads to Spreading Center the formation of new crust that, compared to continental crust, is relatively enriched in iron and magnesium and depleted in silica (SiO2) (because it reflects the chemistry of the mantle). As two plates continue to move apart, the rock in the seafloor grows older as its distance from the rift zone increases, and as it ages, it cools and becomes denser and is buried under marine sediments that are deposited on the seafloor. 1 TRANSFORM Transform Boundaries Connect OCEANIC FRACTURE ZONE BOUNDARY Two Spreading Transform Boundary with INACTIVE (FRACTURE ZONE) (side-to-side plate Centers and ACTIVE (TRANSFORM FAULT) portions. movement) Occur where two plates slide past each other. Motion called shearing. Connect two spreading centers (less commonly, two subduction zones). Probably the most FAULT famous transform “a place where the crust is broken and the broken edges boundary in the are offset relative to each other world is the San Andreas Fault. (either vertically or horizontally)” 2.