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PLATE TECHTONICS and formation of ocean basins part II Recall: “Layered Earth” •Hot Core (heat source for melting) •Chemical composition and physical conditions (temp, pressure) combine to create “physical” layers •Lithosphere (=Brittle, solid) & Asthenosphere (=plastic, can flow)- Another view: • Different Layer names based on: • 1) chemical properties = ~ how much silica vs. metal content (Core, Mantel, Crust) • 2) “physical properties”= liquid, solid, or “plastic” (mesosphere, asthenosphere, lithosphere) PLATE TECHTONICS review from last time: 2. “Distillation”, and resulting rock types Light Rocks vs. Heavy Rocks Granites and similar rocks (silica fewer metals): ~ 2.7 g/cm3 Basalt (silica, but lots of metals) ~ 2.8 g/cm3 Basically two types of crust based on these rock types • Oceanic crust – Mostly basalt – More Dense (3 gm/cm3) – Thin (~5-10 km) • Continental crust – Mostly granites – Less dense (2.7 gm/cm3) – Thick (~30-50 km) PLATE TECHTONICS review: 1. Earth's internal structure revisited 2. Distillation, and resulting rock types 3. Mantel Convection = subduction/ spreading systems Convection Cells of oceanic crust TODAY: A. some more Plate Tectonic Fundamentals 1. Why do plates move? Convection model: basic idea we have talked about ~ density driven Alternate model: ridge push/slab pull ~gravity driven Note: two models are not mutually exclusive- both probably operate 2) What exactly is a “Plate” Plates are: Broken chunks of solid lithosphere (either ocean or continental crust) “floating along” on the plastic mantel layer below (Asthenosphere). Plates of the world N. Amer. Plate= Mix of Ocean & Cont. Pac. Plate= All Ocean floor 1) Recall: Plates can be made of ocean crust or continental crust, or BOTH. Plates “float” on the semi-solid mantle (& Oceanic crust is very different than continental!) Recall: • This is why the different “physical properties” (= liquid, solid, or “plastic”) becomes very important! Result: currently about a dozen independent plates 3) So why are there “ocean basins”? Ie: why are there these “deep” areas between continental crust and ocean crust that has filled up with water? The model: lighter continental rock vs. denser ocean crust both floating on a common liquid layer below forms the ocean basins A Thought experiment: 1) Think about putting ice cubes in bucket of water. 2) Then imagine putting some blocks of dense wood in a bucket of water, and also some blocks of Styrofoam. All will float- right? But not at same level! What determines how much is above water vs. underwater for each? Actually, both density and volume determine “depth” at which something would float in water- And also the amount if it sticking up out of the water! Nice website: http://discoverourearth.org/student/topography/isostasy.html General principle is called “Isostatic equilibrium” Based mostly on relative density So density and volume of crust that is floating in liquid mantel determine depth of ocean basins! continents “float “higher, ocean crust “floats” lower) Nice website: http://discoverourearth.org/student/topography/isostasy.html Plate Elevations/ Gravity forces. Note that oceanic plate slopes downwards from ridge to subduction zone - gradient or slope causes plate to move downhill. The forces that are available to drive the plates include the following: ridge push, trench pull, slab pull, viscous drag at the base of the plate, and the Eötvös force. The graph above indicates that the fastest plates on Earth, like the Juan de Fuca, Pacific, Philippines, and Cocos Plates, are oceanic plates with a significant percentage of their plate boundaries being subducted. The Australian, Indian, and Arabian Plates all include a modest amount of continental crust that is attached to a subducting slab of significant proportions. Plates with a large amount of continental crust, like the Americas, Africa, Antarctica, and Eurasia, move significantly slower than most oceanic plates. Data from Gripp and Gordon, 1990, Geophysical Research Letters, v. 17, p. 1109-1112. Next topic: What are the Consequences of plate boundaries? THREE BASIC TYPES of plate interactions a) Two plates collide (“converge”) b) Two plates slide along each other (“faulting”) c) Two plates pull apart (“diverge”) Basic Boundaries: Divergent, Convergent &Faulting A. Convergent = “subduction zones” 1) ocean crust meets continental crust? “Subduction Zone” Ocean floor (Basalt and Sediments) is being subducted under a continent– shoved under and re-melted- Consequences can have some or all in given location “Subduction” 1. Volcanism- often explosive - volcanic mountain ranges formed on land where re-melted sea floor bubbles back up 2. Trenches and “fore-arc” basins -can be formed by sediments shoved up CASCADE MOUNTAINS Stretch all along coastal W. Coastfrom Southern BC (Canada) into N. California ANDESITES Common Rocks in Cascades Typical Volcanic Rocks on active margins = Re-melted and reformed Seafloor (basalts) + … (whatever got subducted with The Plate) Cascade Volcanoes Did you know We have a Volcano “Chain”.. All the way down W CoastWhy? Cascade Volcanoes Mt St Helens (Oregon) May 18, 1980 Cascade Mountain Range We’ve also got a “local” volcano within weekend driving range! Lassen Lassen Peak: the last volcano in the cascade range Like a mini-Yellowstone- but 5 hrs away.. And still active.. (Last blew: 1914) 2) ocean crust meets ocean crust “Subduction” Typical consequence: “Island Arc and Trench” system “Subduction” Back-Arc basin +Trench Kurile Island Arcs and trenches Aleutian Japan Izu Bonin Marianas Caribbean Marianas Trench + Island system behind: classic Island Arc/Trench 3) Continental crust meets Continental crust Result: Massive mountain building Himalayas today B. Plates Moving past each other= “Transform Faulting” Essentially relieves stress in crust, when forces of plate movement don’t line up perfectly ( & rare they would ever line up perfectly..) Transform faulting often seen on mid-ocean ridges Can see major transform faults clearly on global map of mid-ocean rid Transforms on global ridge system A detailed map of East Pacific Rise- spreading center E-pac rise Do these kind of faults also occur on land? Anyone know of any especially relevant examples to where you are now living? Local San Andreas Fault: Huge transform fault Wider view- major faults and “fractures” along w.coast *Only currently active subduction zone is off Wa. State / Canada *Each of these major faults is relieving pressure built up by uneven plate movement *FYI: eventually, Baja and S. Cal WILL become a very big island.. (very good news for Riverside County property values. If you can wait ..) San Andreas Fault- near San Luis Obisbo Downtown Santa Cruz- Pacific Ave in 1989 (After Loma Prieta Quake) San Francisco C. Divergent boundary = Sea floor spreading (new ocean crust formed) Major spreading centers occur at “mid ocean ridges”- main zones for formation of New Ocean crust Global map of mid-ocean ridges: seafloor spreading creates the MAJOR feature of open ocean deep sea floor RIDGE SYSTEMS How ridges create ocean floor hydrothermal Vents Hydrothermal activity on top of these underwater mountain ranges represent some of most wild and unexpected ecosystems in ocean ! Q: Are there spreading centers on land? Yes: “Rift valleys” Major Examples today of continents pulling apart to form new oceans in the future are in: 1) Iceland and 2) Africa (the “great rift valley” Rift valley evolution- a new sea will soon be created.. C. Some final “Big Picture” consequences of Tectonics Major global consequence is: leads to “Recycling” of Ocean Floor! (but not continents!) C. Some final “Big Picture” consequences of Tectonics 1. Global Earthquake Patterns and Volcanoes 2. Recycling of seafloor – age of ocean floor vs. continents 3. Over Geologic time: “dance of the continents” Look at Major plate boundaries Now look at map of global earthquakes. (look familiar?) “Ring of FIRE” Global earthquake locations tend to go with volcanoesmost can be predicted by plate boundaries (A “ring of fire” surrounds the Pacific ocean) The “ring of fire” Note that major parts of “ring of fire” are volcanic island chains (island arc systems) 2) Subduction and age of ocean floor (Seafloor recycling) Very oldest sea floor rock is ~ 200 MY- most much younger. In contrast, continental rocks can be BILLIONS of yrs old.. Dark Blue: Older than 2.5 BILLION yrs Yellow: older than 500 Million yrs Why? Can you use ocean floor ages to tell how can how fast plates are spreading over time? Age of Ocean Floor Age of seafloor Young crust (zero yrs!) is at mid ocean ridges- OLDEST is just before subuction Old Young Can show that ridges spread at very different rates.. Some fast, some much slower Fast sea-floor spreading (10-15 cm/year) Slow Slow sea-floor sea-floor spreading spreading (a (a few few cm/year) cm/year) 3) Over Geologic Time.. “Dance of the continents” Continent shapes, and ocean basins (what oceans exist!) are constantly changing- Evolution of continents / seas over just last 250 Million years.. (not very long on geologic time scale!) Finally- an important exception : Sometimes you find volcanism on ocean floorbut far away from plate boundaries.. See any plate boundaries in center of pacific? Anyone heard about any volcanic islands there? “Hot Spot” Island Chains Stationary “hot”area in mantel- plate moves over it.. Volcanic islands “punch through” at regular intervals over geologic time HAWAIIAN ISLANDS/ sea mount chain! Chain of Islands Hawaii Note: very different types of volcanic eruptions • Hot spots: Basalt – relatively low silica concentrationtypically flows “gently” • Subduction zonesViscous- full of gas/ steam (subducted seafloor!) explodes Can see change in apparent plate motion! Emperor seamount chain from Chain of Islands space “Loihi”- is next Hawaiian island on its way up.. Next time: MARINE PROVINCES