Honors 1360 Planet Earth Last time: 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 29 September 2008 Read for Mon: Exam Date: Oct 10

Global Elevation (Shuttle Radar Topography Mission) Global Plate Motions Plate Tectonics & Mountains

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 40 km 65 km CrustMantle Epeirogeny: Mid-Ocean Ridge

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! The Rest of the Rock Cycle:

Weathering… Biological Physical Chemical

Erosion… 200,000 tons of soil removed per year, per dot

Results in Sedimentary Rocks Note These Rocks Contain Clues to the Past!