GEOL1002 Lecture Notes - Lecture 11: Harzburgite, Pyroxene, Olivine
5 Jul 2018
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Basalts and Granites
Plate tectonic context of igneous activity
Melting only occurs in certain locations where conditions lead to: decompression,
flux-addition and/or heat transfer.
The conditions that lead to melting can develop in four major geological settings: at
hot spots, along volcanic arcs bordering oceanic trenches, mid-ocean ridges and
within continental rifts.
Products of hot spots
Researchers associate hot-spot igneous activity with mantle plumes, columns or
streams of hot mantle rock rising from deeper in the mantle.
According to the plume hypothesis, the plume itself does not consist of magma. The
plume is solid rock that is hot enough to flow plastically at rates of a few centimetres
a year.
Decompression causes partial melting within the plume and generates mafic magma.
At oceanic hot spots, much of the mafic magma erupts at the surface as basalt.
At continental hot spots, some mafic magma erupts as basalt, however some
transfers heat to the continental crust, which itself, then partially melts, producing
felsic magmas and erupt rhyolite.
Products of subduction
A chain of volcanoes (volcanic arc) forms on the overriding plate adjacent to the
deep-ocean trenches that mark convergent plate boundaries.
Continental arcs develop where oceanic lithosphere subjects under continental
lithosphere.
Island arcs rise up from the ocean floor where one oceanic plate is subjected under
another oceanic plate.
Beneath arcs, a range of intrusions (plutons, dykes, sills) develop.
Some minerals in oceanic crust rocks contain volatile compounds (mostly water). At
shallow depths, these volatiles are bonded to other elements within mineral crystals.
During subduction, the crust going downward warms up and at a depth of about
100-150km it becomes so hot that volatiles separate and diffuse up into the
overlying hot ultramafic rock (peridotite) of the asthenosphere in the mantle wedge.
Volatiles a flux of melting of peridotite. Since silica preferentially goes into the partial
melt, the process gives mafic magma which rises to form basaltic sills and dikes in
the crust and some extrudes as basaltic lava.
In continental volcanic arcs, not all mantle-derived basaltic magma rises directly to
the surface.
Fractional crystallisation and assimilation may cause some of the magma to become
progressively more felsic.
The hot, mantle-derived magma transfers heat into the adjacent continental crust
causing partial melting of the continental crust. The continental crust is originally
mafic to intermediate in composition which causes the resulting magma to have an
intermediate to felsic composition.
Continental derived magma rises and cools in the crust as plutons or erupts as
andesitic to rhyolitic lava.
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Document Summary
Melting only occurs in certain locations where conditions lead to: decompression, flux-addition and/or heat transfer. The conditions that lead to melting can develop in four major geological settings: at hot spots, along volcanic arcs bordering oceanic trenches, mid-ocean ridges and within continental rifts. Researchers associate hot-spot igneous activity with mantle plumes, columns or streams of hot mantle rock rising from deeper in the mantle. According to the plume hypothesis, the plume itself does not consist of magma. The plume is solid rock that is hot enough to flow plastically at rates of a few centimetres a year. Decompression causes partial melting within the plume and generates mafic magma. At oceanic hot spots, much of the mafic magma erupts at the surface as basalt. At continental hot spots, some mafic magma erupts as basalt, however some transfers heat to the continental crust, which itself, then partially melts, producing felsic magmas and erupt rhyolite.
