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What we know… Wegener collected evidence and proposed the theory of Continental Drift. Hess found the “mechanism” to support the idea with his theory of Sea-Floor Spreading. Put the two together… we get the Theory of Plate Tectonics. Plate Tectonics The Earth’s crust and rigid upper mantle are broken into huge slabs called tectonic plates (about a dozen major plates, and several smaller ones). Tectonic plates interact at places called plate boundaries. This is where MOST of the action is really taking place. Divergent Boundaries The place where two tectonic plates are pulling away from each other (not always, but usually occurs on the seafloor). A common feature associated with a divergent boundary is a ridge/rift. A good example of a divergent boundary is the MidAtlantic Ridge. Another Picture of a Divergent Boundary – A Mid-Ocean Ridge Arguably the best example of a divergent boundary – The Mid-Atlantic Ridge Convergent Boundaries : The place where two tectonic plates are pushing against each other. – Oceanic/Continental: Basalt/Granite (a trench and a continental arc are formed) – Oceanic/Oceanic: Basalt/Basalt (a trench and an island arc are formed) – Continental/Continental: Granite/Granite (folded mountains are formed) Convergent Boundaries Cont… A common feature associated with a convergent boundary is a subduction. A good example of a convergent boundary is the Mariana Trench / Mariana Island. Transform Boundaries The place where two tectonic plates are sliding past one another. A good example of a transform boundary is the San Andres Fault in California. More Photos of the San Andreas Fault The San Andreas near San Francisco … in Central California