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 slabHyp: Water released by slab lowers melt temperature

Today:• Mountain Building• Rock Cycle & Geologic Time

29 September 2008

Read for Mon: 213-241Exam 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 km65 km

Crust Mantle

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!