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Transcript of lec 2 notes
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Role ofPressureReducing the pressure
lowers the meltingtemperaturethe rock
probably melts
RIDGE: When
confining pressuresdrop, decompression
melting occurs
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Basalts forming in rifts and Mid Oceanic
Ridges (MORs)
Decompression Melting:
Magma under lithosphere heats and cracks it.
Mantle rock is exposed to low pressures itpartially melts
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Role of
Temperature
Increase in temperaturecan occur because of the
upward intrusion of
magma in the mantle.
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Divergent
Convergent
Transform
USGS Graphics
Using hands to show relative motion
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Spreading ridges
As plates move apart new material is erupted to fill
the gap
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Divergent boundaries are also present under
continents during the early stages of continentalbreakup
Beneath a
continent,magma wells
up, and
the crust is
initially
elevated,
stretched
and thinned
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The stretching produces fractures and rift
valleys.
During this stage,
magma typically
intrudes into thefractures
and flows ontothe valley floor
Example: EastAfrican Rift Valley
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As spreading proceeds, some rift valleys
will continue to lengthen and deepen until
the continentalcrust eventuallybreaks
a narrow linearsea is formed,
separating two
continental blocks Examples:
Red Sea
Gulf of California
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Iceland has a divergent plate
boundary running through its
middle
: An example of continental rifting
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Called SUBDUCTION
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An oceanic-continental plate boundary
occurs when a denser oceanic plate subducts under less dense continental lithosphere
Magma generated by subduction rises into the continental crust to form large
igneous bodies
or eruptsto form avolcanicarc ofandesitic
volcanoes Example:
Pacificcoast ofSouthAmerica
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How can former subduction zonesbe recognized in the rock record? Andesitic magma erupted,
forming island arc volcanoes and continental
volcanoes The subduction complex results in
a zone of intensely deformed rocks between thetrench and the area of igneous activity
Sediments and submarine rocks are folded, faultedand metamorphosed making a chaotic mixture ofrocks termed a mlange
Slices of oceanic lithosphere may be accreted to the continent edge and are called ophiolites
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Ophiolites consist
of layers representingparts of theoceanic crust andupper mantle.
The sedimentsinclude
graywackes
black shales
cherts
Ophiolites arekey to detectingold subductionzones
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Forms mountains,e.g. European Alps, Himalayas
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Two approaching continents are initially
separated by ocean floor that is being subductedunder one of them, which, thus, has a volcanic arc
When the 2 continents collide the continental lithosphere cannot subduct
Its density istoo low,
although
onecontinentmay partlyslide under
the other
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Two kinds of arcs are generally observed
on Earth:
A) Island arc- formed on oceaniclithosphere
Ex. Tonga isalnd arc and Mariana
B) Continental arc formed on the
continent
Ex. Cascade Volcanic arc
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Transform Boundaries
Lithosphere is neither produced nor destroyed as the
plates slide horizontally past each other.
Example:San Andreas Fault, California
Strike-slip fault Strike-slip fault betweentwo spreading ridgesallows the two plates tomove apart.
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In addition to a thermal convection system, some geologists think that movement may be aided
by slab-pull
the slab is cold anddense and pulls theplate
ridge-push
rising magma pushesthe ridges up
gravity pushes theoceanic lithosphereaway from the ridge
and toward thetrench
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PLATE TECTONICS
http://en.wikipedia.org/wiki/File:Oceanic_spreading.svg