 Tectonic Plates – the massive, irregularly shaped slabs of rock that make up the Earth’s lithosphere  One plate cannot shift without affecting the others nearby  Plates can move apart, push together, or scrape past each other

 Theory of Plate Tectonics – states that Earth’s plates are in slow, constant motion, driven by convection currents in the mantle

 Scientists use satellites to measure plate motion precisely  The plates move very slowly – from 1 to 12 centimeters per year (about the same rate as a growing fingernail)

 As plates move, they change Earth’s surface, producing earthquakes, volcanoes, mountain ranges, and deep ocean trenches

 Geologists have evidence that before Pangaea existed, other supercontinents formed and split apart over the last billion years

 A plate boundary is where the edges of two plates meet  Three types of boundaries: 1. Divergent Boundary 2. Convergent Boundary 3. Transform Boundary

 Divergent Boundary – occurs where plates move apart

Features Found Along a Divergent Plate Boundary:  Mid-Ocean Ridges  Rift Valleys  Magnetic Stripes

 The Boundary begins to form when hot material rises from deep in the mantle and the heat causes the crust to bulge upward  The crust cracks as it is stretched and a rift valley forms

 Magma rises through the cracked, thinned crust forming volcanoes  As the rift valley grows wider, the continent begins to split apart and the thinned valley floor sinks lower and lower until it is below sea level and fills with water

Examples:  Mid-Atlantic Ridge  The world’s longest ridge  Runs the length of the Atlantic Ocean

 Iceland  It sits on top of the Mid-Atlantic Ridge  The country is being split in half because half of it sits on the North American Plate and half of it sits on the Eurasian Plate

 The Great Rift Valley  It is found in Africa and it runs through 4 countries: Ethiopia, Kenya, Tanzania, and Mozambique  One of the only divergent boundaries currently found above sea level

 Convergent Boundary – occurs where plates push together  At convergent boundaries crust is either folded or destroyed

Features Found Along a Convergent Plate Boundary:  Inner-Continent Mountains  Coastal Mountains  Deep-Ocean Trenches (Subduction)  Island Arcs

 Continental- Continental Collision – occurs when two plates carrying continental crust push together  Because both crusts are the same density, neither plate can sink beneath the other, so their edges crumple and fold

 The folded crust can be pushed up high enough to form mountains  Example: Himalaya Mountains – fossils of sea life found in the highest peaks from when the edges of the two continents collided

 Oceanic-Oceanic Subduction – occurs where one plate with oceanic crust sinks, or subducts, under another plate with oceanic crust  The older plate sinks because it is colder and denser than the younger plate

 Most are found in the Pacific Ocean  Example: Mariana Trench – deepest trench on Earth found in the Pacific Ocean where the Pacific Plate subducts into the Mariana Plate

Island Arcs  Chains of volcanic islands that form on the top plate, parallel to a deep-ocean trench  As oceanic crust of the sinking plate melts, magma rises through the top plate and over time the flows build up a series of islands  Examples: Aleutian Islands of Alaska and the islands of Japan

 Oceanic- Continental Subduction – occurs when ocean crust sinks under continental crust  The oceanic crust sink because it is colder and denser than the continental crust

Deep-Ocean Trenches  Some of the youngest trenches are in the eastern Pacific Ocean where the Pacific Plate is sinking under the North American Plate  As the ocean crust moves, it often causes underwater earthquakes

Coastal Mountains  As oceanic crust sinks under a continent, the continental crust buckles to form a range of mountains parallel to the deep-ocean trench  Some of these mountains are volcanoes  Examples: the Cascade Mountains in Oregon and Washington (Mount St. Helen’s)

 Transform Boundary – occurs where plates scrape past each other  At a transform boundary crust is neither created nor destroyed  Two plates move past each other in opposite directions and their edges scrape and grind against each other, causing earthquakes

 Transform boundaries occur mostly on the sea floor near the mid- ocean ridges, but they can occur on land where some are clearly visible as long cracks in Earth’s surface  Example: San Andreas Fault in California

 The theory helps geologists today explain Earth’s past and to predict what might happen along plate boundaries in the future  By studying rock layers and using the theory, geologists can uncover the history of any region on Earth

 The theory of plate tectonics also gives scientists a way to study and predict geologic events, such as earthquakes, volcanic activity, and mountain building