How and why does subduction occur?

1. How and why does subduction occur? • Convergence • One plate sinks beneath the other • Dense oceanic crust sinks into mantle

2. What and where is the Wadati-Benioff zone? • Inclined array of earthquakes at subduction zone • Correspond to upper edge of plate

3. How is magma produced at arcs? • Water from subducting slab lowers the melting point of overlying mantle • Note: water also makes for explosive eruptions…

4. Where and how do sediments accumulate in arc environments? • Sediments accumulate in accretionary wedges at the trench • Two sources: 1) volcanic arc 2) ocean sediments

5. Describe deformation of the sediment in the trench • ‘Bulldozer action’ scrapes ocean sediments and thrusts material into a wedge

6. Briefly explain the orogenic cycle using India as an example • Gondwana rifts into pieces incuding India • India diverges from antarctica but converges on Asia • India sutures to Asia

7. How can continents accumulate additional terrain via plate convergence? • ‘collage’ tectonics • Example; The berkshires

8. Have the continents always looked like they do today? • Continents have grown by lateral accretion • Average age of continents 2 billion • Average age of oceans 100 million • Appalachian not at plate boundary

9. Has the mid-continent always been flat? • Continental shield have the oldest rocks. • Composed of sutured belts of mountains, which were eroded to platforms long ago • These platforms have harbored shallow seas

10. What happens to the weak upper layers of crust during collision? • Compression results in thrust faulting and folding into nappes. • Deformation intensifies in the core of the orogen (higher T and P)

11. What happens to the weak upper layers of crust during collision? • Compression results in thrust faulting and folding into nappes. • Deformation intensifies in the core of the orogen (higher T and P)