Chapter 6 notes – Igneous rocks
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Igneous rocks are formed from the solidification of a melt
molten rock belowground is called magma (intrusive)
molten rock aboveground is called lava (extrusive)
what causes melting?
o Decompression melting – hot rock rising towards surface >
less pressure > causes rock to start melting
o Addition of volatile substances such as CO2 or H2O
o Heat transfer melting
 What are the different types of magma?
o silicic
 >=70% silica, little Mg or Fe
o intermediate
 ~ 55% silica
o mafic
 <50% silica, lots of Mg or Fe
o ultramafic
 < 40% silica
 What influences the composition of magma?
o source rock composition
 partial melting – only part of the source rock melts
 minerals have different melting points that affect the
composition of the magma
 silica-rich minerals minerals melt at lower
temperatures than mafic minerals
 magma contamination
 fractional crystallization
 minerals solidify at different temperatures
 mafic minerals freeze at higher temps so they
settle out first and pull out the iron and
magnesium with them. This tends to make the
magma progressively more silica-rich as it
cools.
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Bowen’s Reaction series
o Minerals crystallizing out at different tempeatures in
succession
 From first (high temps) to last (cool temps):
 Discontinuous series: olivine > pyroxene > amphibole
> biotite > quartz, muscovite, K-feldspar.
 Continuous series: Ca-rich plagioclase > Na-rich
plagioclase where Ca+ ions are in the mineral lattice
structure at high temps and Na+ ions at lower temps.
Forces driving magma movement
o Viscosity
 Temperature (heat breaks bonds between molecules)
 Silica content (structure of silica tetrahedron makes
magma “sticky”)
 Volatiles (ditto as for temp)
magma is more buoyant that surrounding, cooler country rock
pressure causes magma to squeeze upward
magma erupting at the surface solidifies into extrusive igneous
rock
o pyroclastic debris – glass, ash
o lava flows
magma solidifying belowground is intrusive igneous rock
o dikes
o sills
o plutons
 many = batholiths (ex: Sierra Nevada mtns)
o boundary between country or wall rock and magma
intrusion called an intrusive contact
o stoping – blocks of country rock fall into the magma
 xenoliths – blocks of rock that didn’t melt
 how fast does magma cool?
o Depth of magma intrusion or chamber
o Shape and size of magma chamber
o Presence of circulating groundwater
 Igneous rock textures – depends on cooling rate
o Fast cooling > fine-grained (aphanitic), ex: rhyolite. Instant
cooling = glassy – ex: obsidian, pumice
o Slow cooling > coarse-grained (phaneritic), ex: granite,
gabbro
o Exception to above: pegmatite dikes, cool quickly but are
water-rich so atoms can move around fast enough to form
very large crystal structures
 igneous rocks form at volcanic arcs, a result of subduction
o continental volcanic arcs – ex: Andes mtns
o oceanic volcanic island arcs – ex:Aleutians
o sinking oceanic plates contain volatiles which bubble off at
depth and cause the asthenosphere to partially melt,
producing a basaltic magma. This magma can either rise
directly to form a basaltic volcano or can undergo fractional
crystallization before erupting to create silica-rich
magma/lava. If magma stalls in the crust, the heat from it
starts to melt the surrounding country rock to produce a
more silicic magma. This kind of magma body may stop
short of the surface and cool underground to form plutons
or batholiths.
 hot-spot volcanoes – rising mantle plumes
o 100 currently active hot-spot volcanoes
 Hawaii, Yellowstone, East Africa, Iceland