lecture 3: earth's structure, plate tectonics, and the rock cycle our hazardous environment...
TRANSCRIPT
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Lecture 3: Earth's Structure, Plate Tectonics, and the Rock
Cycle
Our Hazardous Environment
GEOG 1110Dr. Thieme
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Structure of Earth
Average density ofthe 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 What Is Inside Earth?Earth?
Thickest Thickest layer: layer:
mantlemantle
Upper layer is Upper layer is crust; crust; two types:two types: continental continental oceanicoceanic
Lowest layer: iron-nickel core Lowest layer: iron-nickel core (molten outer core; solid inner (molten outer core; solid inner
core)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
Normalfault
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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.