Lithosphere broken into Plates
Move over the Asthenosphere
PLATE BOUNDARIES
u Boundaries marked by earthquakes
u Divergent Boundaries
u Transform Boundaries
u Convergent boundaries
DIVERGENT BOUNDARIES
u Mostly coincide with MID-OCEANIC RIDGES
u Tensional cracks tap into MAGMA in asthenosphere
- magma solidifies in cracks and erupts on sea floor
u New cracks develop and the process is repeated--oceanic crust is continuously created
TRANSFORM BOUNDARIES
u Two plates slide past each other
- Example: the San Andreas Fault
CONVERGENT BOUNDARIES
u Plates move toward each other
u One plate overrides the other
- SUBDUCTION ZONE
u Magma created at depth
- works its way surfaceward
» extrusive igneous rock- volcanoes
» intrusive igneous rock
u Unmelted rock becomes METAMORPHIC rock
Scientific Method & Plate Tectonics
u Some facts to explain:
- pattern of earthquakes
- pattern of rocks in mountain ranges
- deep sea trenches
- mid-oceanic ridges
u Separate hypotheses proposed for separate problems
u Some facts to explain:
- pattern of earthquakes
- pattern of rocks in mountain ranges
- deep sea trenches
- mid-oceanic ridges
u Separate hypotheses proposed for separate problems
u Hypothesis of Continental Drift
u Hypothesis of Sea floor Spreading
Hypothesis of Continental Drift
u Proposed in early 1900s (by A. Wegener)
u Fit of continental margins
- Pangea (Laurasia, Gondwanaland)
u Rock matches across continents
u Fossil evidence
u Glaciation
u Mountain belts
u Earthquakes due to continued movement
u Problem of mechanism
- Hypotheses not taken seriously by most geologists
u Later supporting evidence
- Paleomagnetism
- Computer reconstruction
- Better rock & age correlation
- Better fossil evidence
Hypothesis of Sea Floor Spreading
u Mid-oceanic ridges high and hot
u Volcanism
u Rift valley at crest
- Tensional forces
u Continental Drift and Sea floor spreading combined into hypothesis of PLATE TECTONICS
Scientific Method--Plate Tectonics
u Predictions
- If we could measure distance between continents we should see changes in time
- Oceanic crust should get older away from mid-oceanic ridges
u Testing
- Deep sea drilling determines age of sea floor
- Measuring distances between continents using satellites
u Another Verification
- Magnetic Reversals
» Record of past magnetic field frozen into igneous rocks at the time they solidified.
- Pattern of marine magnetic anomalies
» Symmetrical with respect to ocean ridge
- Can predict age of any part of ocean floor
In Depth- Divergent Boundaries
u Continent:
- rift valley
- shallow earthquakes
- basalt eruptions
- example: African rift valleys
u Narrow sea as continent separates
- rift valley, quakes
- basalt forms oceanic crust
- example: Red Sea
In Depth- Divergent Boundaries
u Wide sea
- mid-oceanic ridge
- quakes, rift valley, basalt
- example- Mid-Atlantic Ridge
Transform Boundaries
u Fracture zones offset ridges at sea
u Shallow quakes
- between offsets only--Transform fault
u San Andreas fault
- separates North American Plate from Pacific Plate
Convergent Boundaries
u Oceanic-Continental Convergence
- Subduction of oceanic lithosphere
- Trench
- Shallow to deep quakes
- Volcanoes near edge of continent
- Large intrusive bodies (batholiths)
- Young mountain ranges at edge of continent
- Explains young sea floor
» <200 m.y. old vs. up to 4 b.y. for continents
u Oceanic-Continental Convergence (continued)
- Cordilleran belt
» Alaska to South America
» Includes: Rockies, Andes, Sierra Nevada, Coast Ranges, etc.
Convergent Boundaries
u Oceanic-Oceanic Convergence
- subduction
- trench
- quake
- volcanoes as an Island Arc
- example: Aleutian Islands off Alaska
Convergent Boundaries
u Continental-Continental convergence
- disappearing ocean
- mountains in middle of continent
- shallow quakes
- examples: Himalaya, Appalachians
Driving Force for Plate Tectonics
u Convection in mantle
- in deep mantle
- plates riding along or causing convection?
u Ridge-push & Slab-pull
u Mantle Plumes
- beneath "hot spots"
- may break up continents
- cause of intra-plate volcanoes
- e.g. Hawaiian volcanoes