Plate Tectonics EQ: How does the Theory of Plate Tectonics account for the development of new earth structures?

Questions (Based on Page G78-G83) 1. How is a divergent plate boundary defined? 2. What is a RIFT VALLEY? 3. Describe a convergent plate boundary. 4. Why do convergent plate boundaries cause subduction to occur?

Answers 1. Divergent Plate Boundary: A plate boundary where two plates move away from one another. 2. A RIFT VALLEY is a large, long valley on a continent, formed where the continent is pulled apart by forces produced when mantle material rises up beneath the continent.

Answers (cont.) 3. A convergent plate boundary happens when two plates move toward one another resulting in subduction. 4. Subduction occurs when one plate moves downward into the mantle beneath the edge of the other plate. The plate that goes below is always oceanic lithosphere. The plate that stays at the surface can have either oceanic or continental lithosphere.

Questions (cont.) 5. Explain a suture zone. When does it occur? 6. What is the only good example of a suture zone on today’s Earth? 7. Describe a transform plate boundary. 8. What is a transform fault?

5. A suture zone on the Earth’s surface is where two continents have collided and have been welded together to form a single continent. 6. The only good example of a suture zone on today’s Earth is the Indian Plate colliding with the Eurasian Plate and is still working its way underneath it.

7. A transform plate boundary is where two plates slide parallel to one another. 8. A transform fault is where two plates slide against each other.

Questions (cont.) 9. Name the three types of boundaries between lithospheric plates. 10. How and where are rift valleys formed? 11. How can ocean basins change in size? 12. Convergent plat boundaries can be three different settings, what are they?

Answers (cont) 9. Divergent boundaries (mid-ocean spreading ridges), convergent boundaries (subduction zones), and transform boundaries (transform faults) 10. Rift valleys are formed when a changed pattern of mantle convection causes doming and fracturing of existing continental lithosphere. This could happen anywhere on the continents in the future.

Answers (cont) 11. Ocean basins can grow by the creation of new oceanic lithosphere at mid-ocean spreading ridges and can shrink by consumption of oceanic lithosphere at subduction zones.

Answers (cont) 12. (a) Ocean-ocean subduction zones, where one oceanic lithospheric plate is subducted under another oceanic lithospheric plate. (b) Ocean-continent subduction zones, where an oceanic lithospheric plate is subducted under a continental lithospheric plate. (c) Continent-continent collision zones, where two continental lithospheric plates have collided.

Questions (cont.) 13. Describe subduction. 14. Why is it that transform faults can be used to figure out the direction of plate movements? Why can’t subduction zones be used for that also? 15. What happens when two continents collide along a convergent plate boundary?

Answers (cont) 13. The subducting plate bends downward as it reaches the subduction zone, to form a deep oceanic trench. This plate travels far down into the mantle, eventually to be absorbed and become part of the mantle once again. Earthquakes occur on the down going plate because initially the plate is stretched as it is pulled down. Volcanoes are produced because magma is generated by release of water from the subducting plate and the rise of that water into the overlaying mantle, which causes melting.

Answers (Cont) 14. The movement of plates along transform faults is always parallel to the transform fault itself. Movement of a plate down a suduction zone can be at some angle with the trend of the subduction zone that is different from 90 degrees.

Answers (cont) 15. Because the continents are of lower density than the mantle, continental lithosphere is buoyant and resists being subducted. Instead, one of the plates is shoved horizontally under the other, resulting in thickening of the lithosphere and creation of high plateaus and mountains.