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Plate Tectonics How the Earth moves The Definition • The theory of plate tectonics states that the Earth’s lithosphere is divided into pieces called tectonic plates that move around on top of the asthensophere • Plates are either oceanic or continental These plates drift very slowly— centimeters per year • Once, the world had one giant supercontinent, called Pangea and one giant ocean. • Then, over time, the supercontinent spread out, forming the 7 continents we know today—this process is described as continental drift Sea-floor Spreading • Magma rises from the mantle and solidifies—forming new oceanic lithosphere. • Once the magma has solidified into the rocky crust, it spreads out in either direction Divergent Boundary • This is the boundary between two plates that are spreading away from one another – Like the ones found where sea-floor spreading occurs Mid-Atlantic Ridge: A Divergent boundary Convergent Boundary • A convergent boundary is formed where two plates collide with each other • Three combinations exist: – Continental/continental – Continental/oceanic – Oceanic/oceanic The Three Collisions: Convergent Boundaries Transform Boundaries • A transform boundary can be found where two plates are sliding past each other horizontally – The most famous of which is the San Andreas Fault The San Andreas Fault—A Transform Boundary How Mountains Form Types of Mountains— Folded • Folded mountains form at convergent boundaries where plates collide • Rock squeezes against rock, pushing it upward • These form the highest mountains in the world Types of Mountains— Fault-Block • When the crust is tense, the rock can break along a series of faults, causing large rock blocks to drop down lower than other rocks • Volcanic mountains form when oceanic crust sinks into the asthenosphere by subduction • These rock melts, forming magma • This magma, under pressure, eventually rises and erupts through volcanic mountains Types of Mountains— Volcanic The Ring of Fire