Plate Tectonics II: Making Mountains & Volcanism

1. Plate Tectonics II:Making Mountains & Volcanism • Presented by Dr. Sridhar Anandakrishnan • The Pennsylvania State University

4. The World’s Volcanoes

5. The World’s Ocean Trenches

6. The World’s Big Earthquakes

7. Overview • Ocean floor material made at spreading ridges • Moved off to the side by mantle convection • Collides with continental crust... • Subduction & accretion • Volcanoes/mountains • Trenches • Hot spots

8. Review • Mantle made of hot, soft rocks (asthenosphere) • Upper mantle + crust are rigid (lithosphere) • Lithosphere broken into plates • Plates move on mantle convection cells • Convection cells bring up mantle material and it freezes at pull-apart ridges

9. Oceanic Crust • Let mantle material rise and freeze: Basalt • Basalt is Silica (SiO4) + Iron + Magnesium. • Dark in color, relatively dense. • Ocean floor is mostly basalt (formed at ridges) • Earth isn’t getting bigger - oceanic crust has to be destroyed/recycled.

10. Subduction • As oceanic crust moves away from the ridge, it cools, grows more dense, starts to sink back down. (think of the lava lamp...) • As it is moving sideways, it will run into a continent. • If it is cold enough, already sinking, we get a subduction zone. • If it is warm and buoyant, we get anaccretion zone (Olympics)

11. The Fate of the Seafloor • As it (ocean crust) sinks down, it carries seawater and sediments (ocean bottom mud). • As it sinks, it gets hot again (remember, it’s hot inside the earth). • BUT, the added water and sediments help it to melt. • Most things melt better in the presence of water and impurities.

12. Volcanic Arcs... • That melted mix of ocean floor basalt, water, and sediments is low-density, which rises, creating volcanoes at the surface of the Earth. • This type of volcanic rock is called Andesite (from the Andes Mountains).

13. Earthquakes... • Occasionally that downgoing slab gets stuck and then breaks free with a huge earthquake. • Sumatra, 2004; Peru 1960; Alaska, 1964 • The deepest earthquakes aren’t well understood: not like the “stick-slip” we talked about. More like a sudden rearrangement of the solid (phase change).

14. Trenches • Where the slab is subducting, it deforms the over-riding plate - a linear trench is formed. If these trenches are near land, they fill up with sediments from land. • Those in midocean are the deepest places on the planet.

15. Trenches Near Land... • Trenches near continents aren’t as deep as midocean trenches. Sediment washed out by rivers fills them up quickly. • Midocean trenches are deeper than Mt. Everest is high! • Marianas Trench off the Philippines is 35,000’ deep.

16. First (best!) Science-fiction • 20,000 Leagues Under the Sea by Jules Verne • The 20,000 leagues (about 60,000 miles) is the length of the trip, not how deep it dives - Verne knew better!

18. Density... • Continents are lowest density (“lightest”) • Mainly silica, v. little iron. Light colored rocks. • Seafloor is heavier. Basalt. • Silica (SiO4) + iron + magnesium. • Mantle is heavier. • But cold seafloor is heavier than hot mantle. That’s why seafloor sinks at subduction zones. • Core is heaviest. Mostly iron.

19. Accretion • As seafloor runs into continents, the sediment is scraped off and smeared on the continent. • Most of that sediment came from the continent, so the continents grow slowly if at all. • Sometimes, an oceanic mountain/volcano runs into the continent, and the continent grows. • Oceans are never really old (oldest is 160 million years old). • Continents are old! 4 billion years old.

20. Review... • Mantle is hot and flows in convection cells, called the asthenosphere. • Upper mantle and crust is rigid and broken into a few plates. This is called the lithosphere. • Plates meet at pull-apart, push-together, and slide-past boundaries. Mountains built here (mostly). • Heat (from radioactive decay) drivesthe whole thing.

22. Subduction Volcanoes • When basalt + water + sediments heat up, they melt and rise, forming andesitic volcanoes. • The magma tends to polymerize. It makes stringy, lumpy rocks. As the magma comes out the top of the volcano, it solidifies right away. • The volcano is tall, steep, and symmetrical • A stratovolcano.

23. Subduction Volcanoes • The magma doesn’t polymerize inside the earth because the water and CO2 in the magma keeps the magma fluid. • When it gets to the surface of the earth, the water and gas escapes and the rock quickly solidifies. • Sometimes, the rock forms a cap. • The pressure builds.

24. Mt. St. Helens, 20 May, 1980

25. Mt. St. Helens, 20 May, 1980

26. Stratovolcano Eruption • Magma, containing water, CO2 rises. • Earlier flows have capped the volcano. • Pressure builds in the magma pocket. • Something (small earthquake?) cracks the top of the volcano. • Pressure drops... releasing gas/water... which cracks open the top some more... which dropsthe pressure... releasing gas/water...

27. Hot Spots • Sometimes a plume of magma will rise up from deep inside the asthenosphere. • As deep down as core/mantle boundary? • The magma has lots of iron, so it doesn’t polymerize. At the surface, the magma spreads out making a broad gently-sloped mountain. • Hawaii is a shield volcano. Broad, gently sloped (like a gladiator’s shield).

28. Hot Spots • The location of the hot spot is fixed in the asthenosphere... but because the plate moves over it, the surface location of the hot spot moves in a straight line (plates move straight - usually).

29. Hot Spots

31. Hazards • Don’t be on the volcano. • Gasses are hot (300+); cloud blast is fast (100+ miles/hr); gasses are heavy, so they flow along the ground • nuee ardente • Ashes and cinder: pyroclastic flows • Landslide/avalanche

32. Hazards • Lake Nyos, Cameroon is on a hot spot. • CO2 seeped into the lake and built up on the bottom of the lake. • Something (small earthquake?) disturbed the lake and the CO2 escaped. • Suffocated 100s of people living downhill from the lake. • Now they pump the CO2 out

33. Tsunamis • Move a big volume of water, and it will create a wave that travels across the ocean, eventually striking land. • Landslides, volcano eruptions, even meteorite strike • In subduction zone, during an earthquake, the overlying plate will “snap” upwards, moving water.

34. 26 Dec 2004

35. 26 Dec 2004

36. Shorelines are Inundated • As the wave gets closer to shore, the sea bottom is rising (water is getting shallower). • The wave gets bigger and bigger. • Strikes with great force, as well as flooding and washing out villages & people. • Roads, infrastructure gone... disease, hunger...