Earth's Structure, Plate Tectonics, and the Rock Cycle

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Our Hazardous EnvironmentGEOG 1110Dr. Thieme

• Lecture 2
• Earth's Structure, Plate Tectonics, and the Rock
  Cycle

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Structure of Earth
Average density of the solid Earth 5.5 g/cm3
Rocks of continental crust average 2.5 to 3.0
g/cm3
Dense (Fe, Ni) core required by planetary motions
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What Is Inside Earth?
Upper layer is crust two types continental
oceanic
Thickest layer mantle
Lowest layer iron-nickel core (molten outer
core solid inner core)
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• Relief and Tectonics
• Ocean Basins are spreading away from mid-ocean
  ridges
• Continental collisions build mountains and
  increase land surface elevation
• ocean-ocean
• ocean-continent
• continent-continent

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Earth's crust is divided into 7 major and 20
smaller plates
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Plate boundaries are defined by areas of seismic
activity
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Tectonic Cycle Earth's crust is constantly
being recycled as lithosphere is created at
spreading ridges, rides on aesthenosphere, and is
subducted into the mantle
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• Tectonic Cycle
• New Ocean Crust is produced at spreading ridges
• Different spreading rates along the ridge are
  accomodated by Transform Faults
• Compressional Stress at Convergent Boundaries
  results in Folding and Faulting of Crustal Rocks
• Earthquakes are responses to tectonic stresses

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Divergence at Spreading Ridges
- most important area for creation of new crust -
sea floor spreading apart at gradual and constant
rate - ocean floor subducted at trenches, remains
geologically young
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Ocean-Continent Convergence
- deep sea trenches on the ocean floor - denser
plate of oceanic crust is "subducted" beneath
continental crust - ocean floor remains
geologically young
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Convergent Boundary - Oceanic-Oceanic

• both plates are the same density
• Aleutian, Mariana, and Tonga Islands in Pacific
• Antilles in Caribbean

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Convergent Boundary - Continent-Continent

• both plates are the same density
• edges of colliding plates crumple into mountains
  
• Himalayan Mountains in Nepal and India

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divergent plate boundary
convergent plate boundary
transform faults/ triple junctions
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Strike-slip (Transform) fault
Reverse (Thrust) fault
Normal fault
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• Transform Boundary
• most transforms offset spreading ridges of
  oceanic crust
• the San Andreas fault zone in California is a
  transform plate boundary
• the Pacific plate is sliding horizontally to the
  northwest past a segment of the North American
  plate
• transform plate motions result in the strongest
  shaking by earthquakes at the Earth's surface

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Continental Transform Fault
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San Andreas Transform Fault running across the
Carrizo Plain in California
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Figure 1.18a
San Andreas Fault impounds drainage used by palm
trees
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Figure 1.18b
Marsh in Pool impounded by San Andreas Fault
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• Rock Cycle
• all types of rocks can be changed into other
  types by
• time
• heat
• pressure
• beginning the cycle with igneous rocks (high
  temperature) is arbitrary

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Igneous Rocks - crystallized from magma which
either intruded deep beneath the surface
("plutonic") or extruded to the surface
("volcanic") Sediment - particles and dissolved
substances which settle out of a liquid (oceans,
river floodplains, swamps), also windblown
(dunes) Sedimentary Rocks - sediment compacted
by weight of overlying layers, cemented by
percolating water, or chemically
precipitated. Metamorphic Rocks - Igneous rocks,
sediment, or sedimentary rocks altered by being
subjected to temperature or pressure conditions
above those at the Earth's surface.