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Lecture 3

ESS 101 Lecture Notes - Lecture 3: Tuff, Amphibole, Aphanite


Department
Earth and Space Sciences
Course Code
ESS 101
Professor
Swanson Terry
Lecture
3

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January 25th, 2017
Magma is composed of liquid, solid (mineral crystals), and gas
Magma composition is largely controlled by its source
Magmas are subdivided largely by silica content
o As silica content increases, iron and magnesium content decreases
o Note that lighter elements such as sodium and potassium content follow the
silica trends
Elemental composition of magmas and rocks are described in terms of oxide
composition because of their common bonds with oxygen
Viscosity
The viscosity (resistance to flow) of magma is controlled by silica content and
temperature
o High silica content and low temperature magmas tend to have higher viscosities
Rhyolite/dacite flows will retain steep front slopes because of their high viscosity
Basaltic composition magma forms at four different tectonic settings and is always
derived from a partial melt of the asthenosphere
o Partial melting of the asthenosphere occurs at a depth of 100-350km where the
geothermal gradient intersects the melting temperature curve for the upper
mantle rock (garnet peridotite)
o NOTE: the geothermal gradient is dependent
on pressure (depth) while the melting curve is
dependent on pressure (depth) AND the
composition of the substance
Basaltic magma is a dry melt (little dissolved water)
and its melting temperature decreases with
decreasing pressure (as the magma rises)
As the basaltic magma further melts, its density
decreases, causing it to continue to rise until it
reaches the surface
o NOTE: the temperature of rising magma is
~200°C higher than its melting temperature at
the surface (more liquid)
Basaltic composition magma has a relatively low
viscosity and will flow great distances from its vent
o It is dark colored because of its mafic mineral
content (largely pyroxene and Ca-rich
plagioclase)
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Pahoehoe (ropey texture) basalt flows have a lower viscosity than aa (blocky textured)
flows which have degassed and cooled
Granitic magma forms from a partial melt of continental crust, which contains dissolved
water
Dissolved water content in a magma reduces its melting temperature with increasing
pressure, water molecules will inhibit the silicate tetrahedra from forming bonds
o NOTE: the melting temperature curve for a wet granitic melt increases with
decreasing temperature (opposite of basaltic dry melt
o Melting occurs at a depth of 35-45km within the continental crust
As granitic magma rises, it solidifies
as its melting temperature increases
while the geothermal gradient
(actual temperature) decreases
Granitic composition magmas rarely
reach the surface as volcanic rhyolite
flows because of the high water
content and corresponding increase
in melting temperature as it rises
towards the surface
Granitic composition magma is
produced at continental collision
margins
As the continental crust thickens, it
begins to partially melt at depth
Igneous intrusions (plutons) form
below the mountain belts
Volcanism is rare in continental
collision boundaries
As collisional tectonic mountain
ranges are uplifted, the overlying
marine sedimentary and
metamorphic rocs are eroded, exposing the underlying granitic plutons
The granitic rocks of New Hampshire and Vermont represent old granitic plutons that
were intruded when the Appalachian Mountains formed 300Mya as the North American
continent collided with the proto-European continent
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