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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!