Earth Sciences 1022A/B Lecture 2: Earth Science Lecture January 14 2016

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JANUARY 14 2016 LECTURE
IGNEOUS ROCKS
They crystallize from molten silicate material called magma that forms at high
temperatures and pressures deep in the Earth
oMagma rises through the crust and either reaches the surface by
volcanoes or cools below the surface
Known as extrusive if it makes its way by volcanoes and intrusive if
it below the surface
How Magma Evolves
According to Bowen’s Reaction Series:
oDiscontinuous
Different molecular structures of olivine [single tetrahedra],
pyroxene [single chains], amphibole [double chains], biotite [sheets]
Continuous (same plagioclase structure)
Repeated crystal settling in a magma chamber results in the melt crystallizing
through a series of minerals according to their decreasing melting temperatures
oRemaining crystals react with the melt that is enriched in lighter elements
Magmatic differentiation
oDark crystals form early, settle and take heavy elements with them
Light crystals then form from remaining light elements eventually to
quartz
Thus granite can ultimately evolve from an original magma
having a basaltic composition
Magma composition can also change through assimilation and magma mixing
where one body of magma invades another
oThis produces a composition intermediate between the two
Assimilation is when pieces of host rock fall into the magma and
melt
Rock Composition
Dark
oMafic
Contains less silica
Light
oFelsic
Contains more silica
Texture
oAphanitic
Fine
Formed by rapid cooling at or near the ground surface
oPhaneritic
Coarse
Formed by slow cooling below the surface
oPorphyritic
Larger crystals surrounded by smaller ones
oGlassy
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