Download NSTA Geology Reading 1 • Plate Tectonics

NSTA Geology Reading 1
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Plate Tectonics
- What We Mean by Plate Tectonics
‣ Plate tectonics = more of the Earth moves than just continents
‣ Outer part of Earth = relatively thing rigid pieces called plates, and the plates continually
move (slowly)
‣ Each plate about 100km think and surface area of thousands of square kilometers
‣ Each plate’s velocity is about 1-20 cm/year
- Developments of a Unifying Theory
‣ Plate tectonics provides a unifying theory that explains large scaled geologic change
over long stretches of time
‣ Continental drift first proposed as scientific theory by F.B. Taylor and Alfred Wegener in
early 1900s, but was not universally accepted in the early 1900s
‣ Evidence that supports includes:
* The apparent fit between continental coastlines
* Distribution of fossils of plants and animals that were unlikely to have crossed
oceans
* Ancient magnetic patters preserved in rocks
- Fossil Records
‣ Distribution of organisms across continents could be best explained by one large
continent
- Paleomagnetism (Ancient Magnetism)
‣ Geologists have demonstrated hat continents moved over time a significant distance
and continue to move
- Ages of Seafloor Rocks
‣ Rift valley = crack or series of cracks in Earth’s crust that occurs in places where the
plates are being pulled apart; lie roughly midway between the continents in the Atlantic
and Indian Oceans
- Seafloor Spreading
‣ Age of rocks from seafloor increased moving away from the ridges and toward the
continents
‣ If seafloor is moving, then places and continents must also be moving
‣ When new oceanic crust from at mid-ocean ridges, both the older oceanic crust and the
continents riding atop the plates move
- Convection Cells - Cause of Plate Motion?
‣ An accepted cause for plate motion is the idea that convection currents or cells within
Earth’s interior provide the driving mechanism for plate movement
‣ Past few years, increasing uncertainty that convection cells could result in the observed
distribution and movement of lithospheric plates
- Ridge Push and Slab Pull
‣ Ridge push = gravity causes the plate to slide down inclined surface over the plastic
asthenosphere and push on the rest of the plate; many believe ridge push is insufficient
to move a plate
‣ Slab pull = cold, dense, plate edge sinks into the mantle and pulls on the remainder of
the plate; considered tho be main driving force for plate tectonics
- Plate Boundary Types
NSTA Geology Reading 1
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Interactions at plate boundaries result in earthquakes, volcanoes, deep-sea trenches,
destruction of seafloor, uplift of continents, rifting of continents, formation of new
seafloor, and deformation and metamorphism of rocks
‣ 3 types of boundaries
* Divergent boundary = two plates moving away from each other; earthquakes and
volcanoes
* Convergent boundary = two plates move toward each other; mountain range,
earthquakes, and volcanoes
* Transform fault boundary = lateral movement along the fault dominates; San
Andreas Fault in California
Conclusions
‣ Lithospheric plates exist, move, and interact at plate boundaries
‣ Exact mechanism of plate movement is still uncertain but the slab-pull theory is favored
‣ Heat from Earth’s interior makes plate tectonics “work”
‣ Theory of plate tectonics provides unifying explanation for many geological phenomena
that were not well understood in the past