Chapter 8: Plate Tectonics 8.1: Earth has several layers 8.2: Continents change position over time 8.3: Plates move apart 8.4: Plates converge or scrape past each other

8.4 Plates converge or scrape past each other Before, you learned: Plates move apart at divergent boundaries In the oceans, divergent boundaries mark where the sea floor spreads apart On land, continents split apart at divergent boundaries Now, you will learn What happens when two continental plates converge What happens when an oceanic plate converges with another plate What happens when one plate scrapes past another plate

Tectonic plates push together at convergent boundaries Recall new crust forms at divergent boundaries At convergent boundaries, plates push together and crust is folded or destroyed A plate with older, denser oceanic crust will sink beneath another plate The crust melts in the athenosphere and is destroyed Subduction: when one plate sinks beneath another Sub-, “under” and ducere, “to lead” = “led under”

Continental-Continental Collision Both crusts are the same density, so neither plate can sink beneath the other The edges crumple and fold Push together two blocks of clay: one or both will buckle Folded crust may push up high enough to form a mountain Ex: European Alps: where the African and European Plates are colliding Ex: Himalaya: Indian Plate colliding with the European These mountains keep rising higher: plates are still moving

Oceanic-Oceanic Subduction Occurs where an older plate with oceanic crust sinks (subducts) under another with newer oceanic crust Older plate is colder and more dense than the younger plate – subducts into the asthenosphere

Oceanic-Oceanic Subduction Deep-Ocean Trenches: like deep canyons that form in the ocean floor as a plate sinks Most are found in the Pacific Ocean Ex: Marina Trench: Pacific Plate sinking under the Philippine Plate Deepest place in the world’s ocean: 11,000m (36,000ft) into the sea floor

Oceanic-Oceanic Subduction Island Arcs: chains of volcanic islands that form on the top plate, parallel to a deep-ocean trench Ocean crust of the sinking plate melts and magma rises through the top plate Overtime, flows can build up a series of islands Ex: Philippine Islands, Aleutian Islands (Alaska), islands of Japan

Oceanic-Continental Subductions Occurs when ocean crust sinks under continental crust: ocean crust is colder and denser than continental crust Deep-Ocean Trenches: In the Pacific Ocean, the Pacific Plate is sinking under the North American Plate: can cause underwater earthquakes

Oceanic-Continental Subductions Coastal Mountains: as oceanic crust sinks under a continent, the continental crust buckles to form a range of mountains Similar to island arcs, parallel a deep-ocean trench Some of theses mountains are volcanoes (when melted oceanic crust rises through the top plate) Ex: Cascade Mountains in Oregon and Wash. As the Juan de Fuca Plate began sinking under the North American Plate Mount St. Helens: active volcanoes

Tectonic Plate Boundaries Divergent Boundaries: new crust formed Convergent Boundaries: crust folds/destroyed Transform Boundaries… Plates move past each other in opposite directions Edges scrape and grind against each other Occur mostly on the sea floor near mid-ocean ridges Can occur on land: San Andreas Fault Runs from the Gulf of California to San Francisco Pacific Plate and North American Plate are moving opposite directions…Los Angeles will be a suburb of San Francisco in about 10 million years!!

The theory of plate tectonics helps geologists today Changed scientists view of Earth: the lithosphere has been in motion for millions of years…and can help predict what may happen in the future Eastern U.S: the deformed and folded rocks in the Appalachian Mountains are evidence of an ancient convergent boundary These rocks are the same as those in northwest Africa! Indicate these mountains formed when North America collided with Africa and Eurasia as part of Pangaea As they pulled apart: formed rift valleys (U.S. east coast) Predictions: more earthquakes are likely where plates slide past each other volcanic activity where plates are sinking beneath other plates mountains will continue to rise where plates push together