Course News

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Course News

• Recitation - EarthFirst! essay due 29 SEPTEMBER
  (not November!)
• First Hour Exam
• being graded
• exam return next Tuesday, I hope

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Lecture 9Geosphere and Plate Tectonics

• The long-timescale, big-reservoir player in the
  climate system
• provides context for recent and anthropogenic
  climate effects
• provides lessons about Earths possible states
• To make this point once more yet again

3
Solid Earth Things to Explain

• Topography
• Why so flat?!
• Why are the bumps and lumps where they are?
• Ubiquitous rock types granite, basalt, sediments
• Dynamic rock cycle
• Earth not just a cold static rock

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Earths Topography
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Three Dominant Surface Rocks Granite

• light-colored, coarse-grained, less-dense rocks
• found on continents
• quartz, feldspars, micas
• rich in potassium, sodium, aluminum

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Three Dominant Surface Rocks Basalt

• dark-colored, fine-grained, more-dense rocks
• found on ocean floor
• olivine, not quartz
• rich in iron, magnesium, calcium

basaltic lava
photomicrograph of thin section
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Three Dominant Surface Rocks Sedimentary

• rocks formed from eroded grains of other rock
  types
• found in variety of settings
• can also be of chemical origin (e.g., limestone,
  salts)

polished slab, conglomerate
fossiliferous limestone
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Rock Cycle
We see evidence of links among rock-forming
processes
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Basic Earth Facts

• Radius 6371 km
• 4.55 billion years in age
• Differentiated chemically into
• Core (Iron-Nickel)
• Mantle (magnesium, iron silicates)
• Crust (Ca, Na, K silicates n stuff)
• Evidence largely seismological
• Whole Earth composition close to that of stony
  meteorites

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Layering Schemes Compositional

• Granitic continents, density 2.85, age up to
  4.0 Byr
• Basaltic ocean crust, density 3.2, age lt 180
  Myr
• Dense mantle (3.3)
• Crust floats on mantle (isostasy)

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Layering Schemes Rheological

• Lithosphere is cold, brittle
• Asthenosphere has partial melt
• Asthenosphere, mesophere are warm, deform
  plastically

Rheology the details of how matter deforms and
flows
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Plate Tectonics
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Three Types of Plate Boundaries

• Convergence (subduction) destroys lithosphere
  (e.g. under Andes, Cascades)
• Divergence (sea-floor spreading) creates
  lithosphere (e.g., Iceland)
• Conservative (strike-slip or transform faulting)
  preserves lithosphere (e.g. San Andreas)

14
Action at Plate Boundaries

• Plate boundaries the site of earthquakes,
  volcanoes, most mountains, hazards
• Interiors or cratons are stable and ancient

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Plate-Tectonic System
Driving forces? Energy sources?
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Earth as Chemical Factory
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2
3
1

• Partial decompression melting to basalt
• 1300C interaction with seawater brine
• Partial hydrous melting to granite during
  subduction
• Chemical weathering at surface
• Voila! Rock cycle the continents explained!

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Whole-Earth Dynamics
Rigid plates drive convection when they
sink Plates thus organize the flow system
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Plate-Tectonics, Summary

• Rigid lithospheric plates in motion above ductile
  asthenosphere
• New lithosphere made at spreading centers
• Matter and heat transferred, interior to exterior
• Old lithosphere recycled via subduction
• Matter to interior (subduction) matter and heat
  to exterior (volcanism)
• Continuous chemical cycling driven by internal
  heat
• Plates organize the flow in solid Earth

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Geosphere and Global Change

• Earth has evolved from a cold, homogeneous planet
  to be differentiated and dynamic
• Internal energy (radiogenic heat) drives
  solid-Earth processes
• Earth is an immense chemical factory that
• serves as an immense reservoir for several
  important elements
• constantly recycles these elements
• Geosphere is a major player in environmental
  systems, if at long time scales