Plate Tectonics: Plate Boundaries Note: If ‘Snack Tectonics’ activity was done the previous day, relate the following slides to the ‘Snack Tectonics’ activity.

3 Types of Boundaries Divergent Convergent Transform This is an overview to introduce plate motion. Without going heavily into convection and other driving forces, explain the boundary types: Plates come together, and crust is destroyed; Plates move apart, and new crust is formed; Plates move past each other, and crust is not created or destroyed at that boundary. Image from USGS ‘This Dynamic Earth’ online book: http://pubs.usgs.gov/gip/dynamic/understanding.html

boundary where tectonic plates are moving apart Divergent Boundary: boundary where tectonic plates are moving apart The two divergent boundaries are circled. First, in the middle of the slide, is a spreading center where two ocean plates are moving apart. A mid-ocean ridge forms at this boundary. Also, less common, is a continental rift zone. Here, one plate is breaking apart into two – it is a newly forming plate boundary. Mid-ocean ridge Continental Rift Zone

boundary where tectonic plates are colliding together Convergent Boundary: boundary where tectonic plates are colliding together There are two types of convergent boundaries. Both examples of convergent boundaries on this slide are subduction zones (def. on slide). In one case an ocean plate is subducting beneath a continental plate. A trench forms at the boundary and volcanoes form on land. The volcanoes form as the less elements of the ocean crust, including wet sediments, rise back to the surface, while the more dense lithosphere sinks down. In the case where an ocean plate is subducting under another ocean plate, there is again a trench and volcanoes, but the volcanoes form an island arc. Subduction: convergent boundary where one tectonic plates sinks below the other into the mantle

boundary where tectonic plates are colliding together Convergent Boundary: boundary where tectonic plates are colliding together The second type of convergent boundary is a continental collision. When two continental plates collide the crust, which is thick and buoyant crunches together and piles up into mountains. Image from USGS ‘This Dynamic Earth’ online book: http://pubs.usgs.gov/gip/dynamic/understanding.html Continental Collision

On land Transform Boundary: boundary where tectonic plates are sliding past each other horizontally Lastly we have transform boundaries (def. on slide). They form on land, such as the San Andreas Fault in California, or at sea between two segments of mid-ocean ridge. Here, as plates move past each other, “Offsets” of features form: for instance a stream which crosses the boundary in a straight line (blue line on the image) will be moved one way on one side, and the opposite way on the other side, leaving an ‘offset’ in the stream. Image on the right: http://tasaclips.com/illustrations.html On land Between ridge segments

Plate Tectonics: Stresses where plates meet Stress: Force per unit area Demo Force Vs. Stress: Example: give a small light push to a student volunteer, then a poke with about the same amount of ‘force’ – the area that got poked feels the same force, but concentrated on a smaller area, so it hurts more. Another example of force vs. stress is high heals. With flat shoes, a person’s weight (which is a downward force) is spread over the whole feet. With high heals, the force is concentrated to a smaller area, and feet get more stressed and sore sooner.

Tension: Stress that pulls apart or stretches a body Types of stress Tension: Stress that pulls apart or stretches a body Compression: Stress that squeezes or shortens a body Ask students for examples of each Tension: rubber bands Compression: two cars in a collision Shear: ‘snakebite’ – where one person puts both hands on another’s forearm and twists opposite directions. Could *gently* demo *Tension stress is also called ‘Extentional stress’ in geosciences Shear: Stress that pushes parts in opposite directions

Dominant Stress at Boundary Types: Divergent: Tension Convergent: Compression Note: All stress types can occur at all boundaries. Transform: Shear