Plate Tectonics III: Making Mountains, Obduction, & Tsunamis

1. Plate Tectonics III:Making Mountains, Obduction, & Tsunamis • Presented by Dr. Sridhar Anandakrishnan • The Pennsylvania State University

2. Go Dog Go PJ Eastman, 1961

3. Appalachians

4. Sideling Hill, West VA

5. Review • Tectonics is driven by heat. • The plates (8 major ones, few small ones) move on the surface of the Earth. • Oceanic plates are basaltic. • Initially hot and buoyant... later cool and sink • Continental plates are silica-rich, low-density, and buoyant.

6. Review (2) • When cold ocean plates collide with continental plates, they “dive under” the continents. • Subduction leads to • Stratovolcano chains (Andes, Cascades, Aleutians) • Trenches (unless filled by sediments) • Deep earthquakes • Tsunamis

7. When Continents Collide? • Appalachian mountain range from Newfoundland to Alabama, and again in Oklahoma. • Continents are rarely destroyed - so the story gets very complicated. • A colleague’s office - he never throws anything away, so the piles of papers and books get jumbled up • Oceanic crust is created, then destroyed...

9. Susquehanna Valley from Space Image courtesy NASA, MISR

10. Appalachians Complicated • About 300 Million years ago, N. America collided with Europe/Africa to create a chain of mountains (perhaps 15,000 ft high?). • Similar to what is going on today with India and Asia.

11. Continent-continent Collision

12. Mt. Everest, looking N Image courtesy USGS, photo by Gimmy P Li

13. How to Shorten a Continent... • Thrust fault • Shorten up a continent by sliding one part up and over another part. • This is what happened in Great Smokies area.

14. How to Shorten...(2) • Farther north (around here), the rocks “wrinkled up” like a kicked rug. • The “rug” is layered (hard and soft layers). • As time goes by, the soft layers get eroded and the hard layersform the ridges. Susquehanna valley from space, image courtesy NASA, MISR

15. Cross Section Through SC

16. Eventually the Collision Stopped • This area spread apart (similar to Death Valley pull-apart. • Atlantic Ocean formed... still spreading. • Mountains stopped being pushed up. • Erosion scrapes away the tops of mountains and deposits the remains (sediment) in the low areas or in the ocean.

17. Appalachians Still High... • But, mountains have “deep roots.” • By the principle of isostasy, mountains that stick up high above the landscape also have a thickened crust below them that sticks down into the mantle.

18. Isostasy, Erosion, Icebergs... • As these mountains are eroded, they remain high because material from below is rising up. • Like an iceberg floating in water. • Chop off the part above water • Part below water will rise up. • It will be almost as high. • Depends on the density difference.

19. Plate Collisions... • Pull-apart (saw that first week - Death Valley) • Subduction (saw that last time - Crater Lake) • Obduction (this time) • Slide-past (San Andreas fault) • Saw this briefly when we talked about earthquakes

20. Slide-past Tectonics • Sometimes 2 plates slide past each other. • Usually not smoothly (stick slip... and offset of fences, etc.) • If there is a “kink” in the boundary, then the sliding-past behavior can make mountains. • The 2 plates push together at the kink rather than sliding past (as they do on the straight bits) • Or the 2 plates pull apart at the kink...

21. Project Habakuk • During WW2, proposal to build an aircraft carrier from an iceberg. • Mixture of wood pulp and ice. • Small one built on Lake Louise in Canada.

22. Rocky Mtns - Leading Idea • Collision of N. America with oceanic crust. • Normally oceanic crust will subduct under continental crust. • But, if the oceanic crust is hot, it will slide under the continent, but scrape along the bottom. • Continent gets deformed way inland • Rockies are 1500 miles from the Pacific

23. Rocky Mtns - Leading Idea • Collision of N. America with oceanic crust. • Normally oceanic crust will subduct under continental crust. • But, if the oceanic crust is hot, it will slide under the continent, but scrape along the bottom. • Continent gets deformed way inland • Rockies are 1500miles from the Pacific.

24. Cross Section of West Push up far inland Warm Used to be far offshore Cold

25. Metamorphic Rocks • Some rocks in the Rockies and Great Smokies are metamorphic rocks. • Means they have been changed from their original form. • Under the heat, pressure, and chemical action, they change from (usually) sedimentary rocks to a harder, more-resistant rock. • They have been “cooked” • The look different - folded, squeezed, with pretty colors (and funny names).

26. Metamorphic Rocks

27. Igneous Rocks Pele’s Hair, courtesy USGS Andesite, courtesy USGS

28. Why are They at the Surface? • As the overlying rocks get eroded and removed, the deeper rocks rise up. • The deeper rocks rise up because of isostasy. • Like the iceberg rising up as the part above water is lopped off, and the alien emerging...

29. Rock Cycle Courtesy USGS

30. Types of Rocks • Igneous rocks • Formed by melting and then solidifying of magma • Or during a volcano • Sedimentary rocks • Formed by erosion of other rocks and then deposition • Metamorphic rocks • Formed by “cooking” rocks

31. Hazards • Most of us will die of old age. • About 1% or fewer die of “outside causes” • Car crashes and handgun deaths. • The main geologic hazards. • Tsunamis, volcanoes, earthquakes, landslides. Wear a seatbelt

32. Tsunami Warning • Because the water wave can take minutes to hours to get across the ocean to land, there is time for warning. • The earthquake/landslide/meteor strike can be detected almost immediately, so we can send out warning by radio/internet/smoke signals.