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Transcript
Honors 1360 Planet Earth 29 September 2008 Read for Mon: 213-241 Last time: Exam Date: Oct 10 Obs: Volcanism at plate boundaries Obs: “Normal” geotherm < melt temperature Hyp: Volcanism in rift zones geotherm raised by rock “rising to fill” vacated space Hyp: Hotspots: Some volcanoes far from plate boundaries temperature raised by mantle plumes (convection) Pred: Should see “hotspot tracks” in direction of plate motion Obs: Volcanoes over 100-km depth contour of subducting slab Hyp: Water released by slab lowers melt temperature Today: • Mountain Building • Rock Cycle & Geologic Time Plate Tectonics & Mountains Global Elevation (Shuttle Radar Topography Mission) Global Plate Motions Two major processes by which mountains develop: Orogeny: At convergent plate boundaries, get: -- Thrust faulting in the near-surface -- Flow of weak crustal rocks at depth -- Isostatic response to thicker (buoyant) crust Mountains (Himalaya, Andes) Epeirogeny: At divergent plate boundaries and continental rifts, get: -- Thinning of the crust and lithosphere -- Hot rock brought nearer the Earth’s surface -- Isostatic response to (buoyant) higher temperature rock Mountains (Basin & Range, Rio Grande Rift, mid-ocean ridges) An easy way to tell the difference is by crustal thickness: Orogeny: Himalayan Plateau 65 km 40 km Crust Mantle Epeirogeny: Mid-Ocean Ridge The Rest of the Rock Cycle: I. Weathering, Erosion, & Deposition Weathering describes all the processes that break up / alter / remove rock from the surface: • Includes mechanical processes of fracturing/jointing, freeze/thaw, glacial “scraping”, plant roots; • chemical/biological processes (acids in rainwater and biological waste products) Erosion is the transport process: • Mass-wasting, glaciation, wind and water Deposition is the accumulation process wherever it ends up! Weathering… Physical Biological Chemical Erosion… 200,000 tons of soil removed per year, per dot Results in Sedimentary Rocks Note These Rocks Contain Clues to the Past!