GEOG 1350 Chapter Notes - Chapter 5: Igneous Rock, Pyroclastic Rock, Silicate Minerals

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16 Aug 2016
GEOG1350 Katy Lemaire
September 29th, 2015.
5.1 Introduction to Volcanoes
-volcanic activity is directly related to plate tectonics and most active volcanoes are located near
plate boundaries
-mid-ocean ridges and subduction zones are sites where molten rock or magma reaches the
surface and erupts as lava
-2/3 are located in the Pacific Ring which surrounds the Pacific Ocean
How magma forms: the planet is composed of solid rock, most magmas come from the
asthenosphere where rock is close to its melting temperature. Silicate rocks can melt by
decompression, addition of volatiles or addition of heat
(1) DECOMPRESSION: Pressure exerted on rock within the asthenosphere is reduced at
a melting temperature of 1200 degrees C. Pressure increases with depth generated
by the weight of overlying rock that keeps Earth’s mantle in a solid state, happens at
divergent boundaries, continental rifts, the lithosphere is thinned which causes the
mantle to come up to the surface where pressures are lower, causing
decompression melting.
(2) VOLATILES: chemical compounds that exist in a gaseous state at Earth’s surface and
evaporate easily, addition of volatiles lowers the melting temperature of rocks by
helping to break chemical bonds within silicate minerals, commonly incorporated
into minerals formed on the seafloor within oceanic lithosphere, released volatiles
rise upward from subducting slab and interact with dry lithosphere to induce
(3) ADDITION OF HEAT: If temperature of rocks exceeds temperature of melting point
of silicate rocks at that depth, as magma rises from asthenosphere it transfers heat
to surrounding rocks.
Magma Properties:
(a) Composition: composed of melted silicate minerals and dissolved gases, main
elements are oxygen, silicon, aluminum, iron, magnesium, potassium, sodium,
calcium. When silicon and oxygen (S and O) combine, they are called silica. Magma
contains small but significant amounts of dissolved gases mostly water vapour and
(b) Viscosity: a fluid’s resistance to flow, silica-rich magma does not flow easy and has a
high viscosity, however regular magma flows easily with low viscosity. Viscosity is
affected by temperature and composition.
(c) Volatile Content and Eruptive Behaviour: high concentration of volatiles within
magma will cause an explosive eruption when the melt is decompressed upon
reaching the surface of Earth, volatile content of magma increases with increasing
silica content, volcanoes fed by andesitic or rhyolitic magma are more prone to
eruption, the rapid bubble formation that breaks up molten material into small
fragments violently ejected from the volcano are referred to as tephra, an
accumulation of tephra is called pyroclastic deposit that can be fused to for
pyroclastic rock.
Volcanic Types:
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