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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