1. Recall Continental Drift?

2. Seafloor Spreading Nazi hunting and better maps of the ocean floor – Discovery of Mid-Atlantic Ridge Magnetism and more Nazi hunting – Discovery of magnetic stripes of alternate polarities on the ocean floor Even more Nazi hunting – Seafloor sediment thickens with distance from the MAR

4. The Crust Can’t Grow on and on… If new oceanic crust is constantly added at mid- ocean ridges, why doesn’t Earth grow? Look to the maps… – Earthquake locations

6. Odd Patterns… Scientists in Japan noticed odd patterns when they plotted earthquake foci – Appeared to lie along a down-sloping plane Volcanoes and ocean trenches coincided with these earthquakes It seemed like the ocean crust was being pushed down into the mantle, creating earthquakes as it did so

7. Red lines represent Ocean trenches

8. How Could This Happen? Remember the Two Types of Crust? – Oceanic crust Dense, thin (~4-7 km), iron-rich Oldest oceanic crust is 180 million years old – Continental crust Less dense, thick (35-70 km), silicon-rich Oldest continental crust is 4 billion years old – Ocean plate forced to sink when encountering material of lesser density (ocean/continental)

9. The Grand Unifying Theory…of Geology Theory of Plate Tectonics The rigid lithosphere is broken up into ~16 tectonic plates – May include both oceanic and continental crust – Each plate moves as one giant mass – Collisions responsible for earthquakes and volcanoes

10. How do plates interact?

11. Three main types of interactions Divergent plate margins: – Plates move apart, new lithosphere is created Convergent plate margins: – Plates come together and one plate always subducts beneath the other back into the mantle Transform-fault margins: – Two plates slide past one another

12. Divergent Plate Margins Within an ocean basin, marked by a mid- ocean ridge that exhibits volcanism and earthquakes Also known as spreading centers

13. Ocean-Ocean Convergent Margin When two ocean plates meet, one descends or subducts beneath the other ► Depends on which is older  colder  denser

14. Subduction Leads to Melting As plate enters the asthenosphere – Increase in pressure + temp forces out water into asthenosphere above it causing it to melt – This creates a chain of volcanoes (island arc)

15. Ocean-Continent Convergence Plates with continental edges override ocean edges because they are less dense – Again, the ocean crust melts as it subducts, giving water to the asthenosphere which also melts – Coastal trench, huge earthquakes/volcanoes on land – Melting along continent edges richer in silica (Si) which makes them viscous and explosive

16. Continent-Continent Convergence Neither plate edge can subduct Neither plate edge can subduct – Creates a sort of double crust forming world’s highest mountain ranges The Himalayas

17. Transform-Fault Margins When two plates slide past each other lithosphere is not created or destroyed – The rocks on either side of the faults are often different ages due to displacement – Large earthquakes

18. Hot Spots? Some volcanoes occur at distances from plate boundaries… – Hot columns of rock from the lower mantle – Act like a blowtorch on plate moving above – Mostly volcanism – Ex: Hawaii Hawaii 0.8-present Maui <1.0 Molokai 1.3-1.8 Oahu 2.2-3.3 Kauai 3.8-5.6 Direction of plate motion Decreasing age Hot Spot Loihi