Interactions at Plate Boundaries Chapter 5 Lesson 1 Interactions at Plate Boundaries Main Idea: There are three different types of plate boundaries, where stresses cause rocks to deform.

Sometimes rocks can bend under stress without breaking. Rock Deformation Sometimes rocks can bend under stress without breaking. Plastic deformation: When rocks are stressed at high temperatures and pressures, they change shape by permanently folding.

Elastic deformation: Sometimes rocks can snap back to their original shape when the stress is removed. Just like a piece of wood can bend when force is applied.

There are three types of stress: Tension Stress pulls the rocks apart Compression Stress squeezes the rocks together. Shear Rocks slide horizontally in opposite directions.

Fracture: Sometimes rocks can not take the stress and they will break, or crack this is called a fracture.

Fault: If the rocks on one side of a fracture have moved relative to the rocks on the other side. Notice how the rock layers have moved.

Faults The type of fault depends on the type of stress applied to the rock.

Normal Reverse Strike slip Basin & Range Himalayas San Andreas, Calif. African Rift Rocky Mountains N. Anatolian, Turkey See notes section of previous slide for supporting information on types of faults.

There are 3 types of plate Boundaries This is where everything takes place. Convergent When 2 plates move toward each other. Divergent When 2 plates move away from each other. Transform When 2 plates move pass each other.

Where are these plate boundaries are located? Divergent Transform Convergent VIDEO LECTURE: “Plate boundaries” on Teachers on the Leading Edge web site under “Videos” under the topic “Introduction to Plate Tectonics and Earthquakes”. URL http://multimedia2.up.edu/Physics/TOLE/Plates&Earthquakes/VideoLectures/TypesOfBoundaries.mov Graphics from “This Dynamic Planet, World Map of Volcanoes, Earthquakes, Impact Craters, and Plate Tectonics.” A Smithsonian, USGS, US Naval Research lab publication. Available at http://mineralsciences.si.edu/tdpmap/ (Note that this material is copyright protected; the content may only be used for personal, educational or noncommercial purposes.) USGS Graphics

Produce: Volcanoes Mountains Islands Convergent Zones Produce: Volcanoes Mountains Islands An oceanic plate dives into the deeper mantle at a convergent plate boundary in a process called “subduction”. When introducing subduction, it is important to reinforce that earthquakes occur only in brittle rocks that are cold enough to fracture when exposed to large forces. If rocks are too warm, they will not be brittle and cannot break to produce earthquakes. Most rocks at depths greater than 100 km are too warm to be brittle. So intermediate (100 - 300 km depth) and deep (300 - 700 km depth) earthquakes only occur within the subducting oceanic plate. This plate is still cold enough to fracture at these depths because it takes millions of years for it to warm up as it subducts into the deeper mantle. The asthenospheric mantle cannot generate earthquakes because it is too warm to be brittle. Instead it is “viscoelastic”, like silly putty. Colors match the color scheme of Slides 5 and 6; yellow is shallow, green is intermediate, blue is deep VIDEO LECTURE: “Brittle fracture” and “Brittle and ductile deformation” on Teachers on the Leading Edge web site under “Videos” under the topic “Introduction to Plate Tectonics and Earthquakes”. Earthquakes occur within subduction zones.

Convergent Plate Boundaries Oceanic /Oceanic (Marianas) The most complex kind of boundary is convergent plate boundaries. We distinguish three kinds of convergent boundaries: 1. Two oceanic plates converge and one plate subducts (as in the Marianas Trench of the western Pacific Ocean). 2. An oceanic plate converges with a continent plate and subducts beneath the “leading edge” of the continent (as in the Cascadia subduction zone of the Pacific Northwest or at the Peru – Chile Trench along the western side of South America). 3. Two continental plates converge resulting in a “geologic train wreck” where continental crust piles up to form a mountain range (as in the Himalayas where the Indian and Asian plates collide). Remember that rocks of the continental crust are lower density than mantle rocks, so continental crust cannot “subduct” into the mantle. If continental rocks are pushed into the mantle, they will pop up again like a piece of Styrofoam pushed into a swimming pool. Oceanic/Continental (Cascades) Continental/Continental (Himalayas)

Cascade Mountain Ridge An oceanic plate dives into the deeper mantle at a convergent plate boundary in a process called “subduction”. When introducing subduction, it is important to reinforce that earthquakes occur only in brittle rocks that are cold enough to fracture when exposed to large forces. If rocks are too warm, they will not be brittle and cannot break to produce earthquakes. Most rocks at depths greater than 100 km are too warm to be brittle. So intermediate (100 - 300 km depth) and deep (300 - 700 km depth) earthquakes only occur within the subducting oceanic plate. This plate is still cold enough to fracture at these depths because it takes millions of years for it to warm up as it subducts into the deeper mantle. The asthenospheric mantle cannot generate earthquakes because it is too warm to be brittle. Instead it is “viscoelastic”, like silly putty. Colors match the color scheme of Slides 5 and 6; yellow is shallow, green is intermediate, blue is deep VIDEO LECTURE: “Brittle fracture” and “Brittle and ductile deformation” on Teachers on the Leading Edge web site under “Videos” under the topic “Introduction to Plate Tectonics and Earthquakes”.

Convergent Zones An oceanic plate dives into the deeper mantle at a convergent plate boundary in a process called “subduction”. When introducing subduction, it is important to reinforce that earthquakes occur only in brittle rocks that are cold enough to fracture when exposed to large forces. If rocks are too warm, they will not be brittle and cannot break to produce earthquakes. Most rocks at depths greater than 100 km are too warm to be brittle. So intermediate (100 - 300 km depth) and deep (300 - 700 km depth) earthquakes only occur within the subducting oceanic plate. This plate is still cold enough to fracture at these depths because it takes millions of years for it to warm up as it subducts into the deeper mantle. The asthenospheric mantle cannot generate earthquakes because it is too warm to be brittle. Instead it is “viscoelastic”, like silly putty. Colors match the color scheme of Slides 5 and 6; yellow is shallow, green is intermediate, blue is deep VIDEO LECTURE: “Brittle fracture” and “Brittle and ductile deformation” on Teachers on the Leading Edge web site under “Videos” under the topic “Introduction to Plate Tectonics and Earthquakes”.

Marianas Islands trench An oceanic plate dives into the deeper mantle at a convergent plate boundary in a process called “subduction”. When introducing subduction, it is important to reinforce that earthquakes occur only in brittle rocks that are cold enough to fracture when exposed to large forces. If rocks are too warm, they will not be brittle and cannot break to produce earthquakes. Most rocks at depths greater than 100 km are too warm to be brittle. So intermediate (100 - 300 km depth) and deep (300 - 700 km depth) earthquakes only occur within the subducting oceanic plate. This plate is still cold enough to fracture at these depths because it takes millions of years for it to warm up as it subducts into the deeper mantle. The asthenospheric mantle cannot generate earthquakes because it is too warm to be brittle. Instead it is “viscoelastic”, like silly putty. Colors match the color scheme of Slides 5 and 6; yellow is shallow, green is intermediate, blue is deep VIDEO LECTURE: “Brittle fracture” and “Brittle and ductile deformation” on Teachers on the Leading Edge web site under “Videos” under the topic “Introduction to Plate Tectonics and Earthquakes”.

Divergent Plate Boundaries A) Plates move away from each other. In land it will create a rift valley. The most complex kind of boundary is convergent plate boundaries. We distinguish three kinds of convergent boundaries: 1. Two oceanic plates converge and one plate subducts (as in the Marianas Trench of the western Pacific Ocean). 2. An oceanic plate converges with a continent plate and subducts beneath the “leading edge” of the continent (as in the Cascadia subduction zone of the Pacific Northwest or at the Peru – Chile Trench along the western side of South America). 3. Two continental plates converge resulting in a “geologic train wreck” where continental crust piles up to form a mountain range (as in the Himalayas where the Indian and Asian plates collide). Remember that rocks of the continental crust are lower density than mantle rocks, so continental crust cannot “subduct” into the mantle. If continental rocks are pushed into the mantle, they will pop up again like a piece of Styrofoam pushed into a swimming pool.

Divergent Plate Boundaries An example is the East African rift valley. The most complex kind of boundary is convergent plate boundaries. We distinguish three kinds of convergent boundaries: 1. Two oceanic plates converge and one plate subducts (as in the Marianas Trench of the western Pacific Ocean). 2. An oceanic plate converges with a continent plate and subducts beneath the “leading edge” of the continent (as in the Cascadia subduction zone of the Pacific Northwest or at the Peru – Chile Trench along the western side of South America). 3. Two continental plates converge resulting in a “geologic train wreck” where continental crust piles up to form a mountain range (as in the Himalayas where the Indian and Asian plates collide). Remember that rocks of the continental crust are lower density than mantle rocks, so continental crust cannot “subduct” into the mantle. If continental rocks are pushed into the mantle, they will pop up again like a piece of Styrofoam pushed into a swimming pool.

Divergent Plate Boundaries In the ocean it will create a rift valley. The most complex kind of boundary is convergent plate boundaries. We distinguish three kinds of convergent boundaries: 1. Two oceanic plates converge and one plate subducts (as in the Marianas Trench of the western Pacific Ocean). 2. An oceanic plate converges with a continent plate and subducts beneath the “leading edge” of the continent (as in the Cascadia subduction zone of the Pacific Northwest or at the Peru – Chile Trench along the western side of South America). 3. Two continental plates converge resulting in a “geologic train wreck” where continental crust piles up to form a mountain range (as in the Himalayas where the Indian and Asian plates collide). Remember that rocks of the continental crust are lower density than mantle rocks, so continental crust cannot “subduct” into the mantle. If continental rocks are pushed into the mantle, they will pop up again like a piece of Styrofoam pushed into a swimming pool.

Transform Boundaries the plates slide horizontally past each other. Motion of plates across transform boundaries is purely horizontal. Along a transform boundary, plates are moving PARALLEL to the boundary between them. Earthquakes occur as the plates grind in “jerky” motions past each other. The strain buildup is greater on transform faults separating plates with continental crust (like the San Andreas Fault) than on transform faults faults separating plates with oceanic crust (like on the hundreds of transform boundaries that offset segments of spreading oceanic ridges). A note about “Fracture Zones” in the oceans: When considering oceanic plates, remember that “older” means “colder” and “colder” means lower elevation because the plate shrinks as it cools. So the top of an older piece of an oceanic plate is deeper than the top of a younger piece of an oceanic plate. This means that there can be a small difference in elevation (a “scarp”) where older and younger parts of an oceanic plate are adjacent to each other (e.g. across the transform fault on the right diagram of this slide). Along most parts of active transform faults in the oceans, there are offsets in the depth of the ocean floor. Oceanic fracture zones (like the Mendocino Fracture Zone off the coast of northern California) are the locations of presently active (or formerly active) transform faults that separate pieces of oceanic plates of different ages and therefore different elevations. Strike-slip fault Strike-slip fault between two spreading ridges allows the two plates to move apart. Example: San Andreas Fault, California