How and why does subduction occur?

 Convergence  One plate sinks beneath the other  Dense oceanic crust sinks into mantle

What and where is the Wadati-Benioff zone?

 Inclined array of earthquakes at subduction zone  Correspond to upper edge of plate

How is magma produced at arcs?

 Water from subducting slab lowers the melting point of overlying mantle  Note: water also makes for explosive eruptions…

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

Describe deformation of the sediment in the trench

 ‘Bulldozer action’ scrapes ocean sediments and thrusts material into a wedge

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

How can continents accumulate additional terrain via plate convergence?

 ‘collage’ tectonics  Example; The berkshires

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

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

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)

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)