1

Plate boundaries and rates

handout

Plate rates

Plate rates

2

Plate rates

Fracture zonesOld ridge orientation~40 Ma

New ridge orientation~28 Ma, rapid change

North Atlantic Spreading Jumped to Iceland Hotspot

N. Am. Grld Europe

N. Am. Grld Europe

3

Driving forces

Plate rates

No strong relationship between plate size and rate.Suggests asthenospheric push/drag is not very important.

Area forces are weak drivers

Plate rates

No strong relationship between ridge length and rate.Suggests Ridge Push is not very important.

4

Strong positive relationship between circumference of plateas subduction zone and rate. Negative buoyancy is important.

Plate rates - Edge forces

Rate Handout

Pacific fast. Lots of trenchIndia fast. Still responding to subducted ocean

Plate rates

Strong negative relationship between continental area ofplate and rate.

5

Continents are slow. WhyDeep keels (drag) or lack of trenches ?

Summary85% of the driving force of plate tectonics is slabsubduction (negative buoyancy).10% of the driving force is trench pull (coming up).5% other forces

The cooling effect associated with ridge systems is thekey factor responsible for plate tectonics.

Subduction Angle

6

Subduction Angle

Trench suction

Ridge hits trench

7

San Andreas Evolution

Slab Window