1. 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

2. 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

3. 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”

4. 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

6. 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

7. 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

8. 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

9. 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

10. 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

12. 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!!

16. 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