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GEOLOGY 12
IGNEOUS ROCKS
April, 2004
Unit # 21
rock = a naturally formed consolidated materials usually composed of grains one or
more minerals
- earth’s crust constantly changes because of the external and internal heat engines
- Why is the earth’s crust not at equilibrium? (constant plate movement, erosion and
weathering)
- Rock Cycle : changes to rock material into sedimentary, metamorphic and igneous rock
(page 50 Figure 3.1)
magma = molten rock
igneous rock = when molten magma solidifies it turns into igneous rock
- magma brought to surface by volcanic eruptions
- magma brought to surface by extrusives
- magma may solidify below surface = intrusive brought to surface at latter time
- exposed igneous rock undergoes erosion and weathering
erosion = The physical removal of rock by an agent such as running water,
glacial ice, or the wind
weathering = The group of processes that change rock at or near the earth’s
surface. These processes break down the chemical properties of the rock that is
stationary.
Weathering breaks down rock that is stationary
Erosion physically removes particles of rock
Transportation moves the particles of rock
- igneous rock weathered and eroded and transported into sedimentary rock
transported and deposited; building sedimentary strata
build up of strata forces layers into the crust = increases heat and pressure =
metamorphic processes (several kms into the crust & takes millions of years)
tectonic forces also contribute to changing the structure of the rock: ductile, elastic,
brittle
- lava = basalt = igneous rock (very small crystals)
- lave is magma on the earth’s surface
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Igneous rock extrusive if lava solidifies on earths surface
Igneous rock intrusive if magma solidifies under the earth’s surface
- coarse grains = bigger then 1 mm
- granite composed mainly of feldspar and quartz and most abundant intrusive rock on
continents
- volcanic rocks are usually fined grained due to rapid cooling
- intrusives are usually coarse grained due to slow cooling process
- a large body of magma requires about a million years to solidify
- the greater the amount of water molecules in igneous rock the higher the temperature
must be to melt the rock
Texture
texture = refers to the rock’s appearance with respect to the size, shape and arrangement
of the grains or other constituents of the rock
fine grained rocks = grains smaller then 1 mm (cooled rapidly)
basalt, andesite and rhyolite are fine grained rocks (extrusives)
plutonic rocks = igneous rocks that have formed several kms down and have coarse
grained texture due to slower cooling processes
porphyritic rocks = larger crystals imbedded in finer matrix (finer crystals)
(Page 55 Fig. 3.7)
Identification of Igneous Rocks: (Page 52 Chpt. 3)
- igneous rock names are based on texture (grain size) and mineralogical composition
Refer to page 546 = Key for identifying Common Igneous Rocks
Chemistry of Igneous Rocks (Page 54 Chpt 3)
- chemical composition of magma determines which minerals and how much each will
crystallize when an igneous rock forms
mafic rocks = silica content of rock is 50% or less (e.g. basalt and gabro)
Felsic rocks = silica is 65% or more of rock = silica rich igneous rocks (e.g. ryholite and
granite = low ferromagnesium minerals)
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Intermediate rocks = silica content between 50% - 65% (e.g. andesite)
ultramafic rocks = composed of entirely or almost entirely of ferromagnesium minerals
(no feldspars or quartz) (e.g. Peridotite) rocks contain less then 45% silica
Intrusive Bodies: (Page 57 Chpt 3)
Names of intrusives are based on size and shape and relationship to surrounding rocks
- 1) size (large or small)
- 2) particular shape
- 3) shallow or deep formed
- 4) does it follow layering in country rock (host rock into which intrusive goes)
Shallow Intrusives:
- igneous rocks that solidify near surface (less the 2 kms down)
- solidified in ‘plumbing system of volcanoes
- relatively small structures
- tend to be fine grained
(page 57, Fig. 3.8)
e.g. of intrusive = volcanic neck (refer back to Devils Peak)
Dikes and Sills:
Dike = intrusive not parallel to any rock layers, usually vertical in nature
- discordant = goes across rock layers
Sill = intrusive that is parallel to rock (country rock) layers, usually horizontal
- concordant = goes along rock layers
(page 58, Fig. 3.9)
Intrusives that crystallize at depth:
Pluton = body of rock that has crystallized at great depth, no particular shape,
- mostly composed of granite (Page 59, Fig. 3.12)
- most intrusives are plutonic - large crystals - slow cooling
Stock = pluton less than 100 sq kms
Batholith = large plutons greater then 100 sq kms
- no specific shape
- some are very large: example = in Canadian mtn range = 1800 kms
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(see inside cover of text ‘Granite Pluton’ also note NS and
Applachian Mtns.)
diapers = blobs of magma that work their way towards surface - less dense then
surrounding rock (forced up) - like lava lamp - (Page 60, Fig. 3.14)
- rhyolite (volcanic) = granite (intrusive) counterpart
Distribution of Plutonic Rocks:
- most of central part of North American continent, its bedrock, is a remnant of old
mountains eroded and overlaid by sedimentary rock approximately 1 km thick
How Magma forms:
- rock can melt at temperatures as low as 625 C
- factors that effect melting = 1) pressure, 2) amount of gas present (e.g. water),
3) mix of minerals present
- Where does heat come from = from earth’s core (5000 C)
- heat conducted to surface (refer to circulation of heat in earth
as already discussed)
geothermal gradient = the increase of temperature with depth increase
= 30 C/km depth
(Page 61, Fig. 3.15)
mantle plumes = narrow areas of heat zones going toward surface
e.g. plume in Pacific leads to building up of Hawaiian Islands,
Factors that control Melting Temperatures:
1) melting point of minerals increases with increased pressure
(the deeper the rock is, the greater the pressure, the greater the temp to melt)
2) if water present, this lowers melting temperature of minerals
3) type and amount of minerals present effects melting point
Why Igneous Rocks Vary in Composition:
- basalt magmas are associated with oceanic crust
- granitic magmas are associated with continental crust
Bowen’s Reaction Series = sequence in which minerals crystallize (Page 63, Fig. 3.18)
= sequence in which minerals melt
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- from Bowen’s Reaction Series two concepts obtained:
1) Differentiation = different ingredients separate from original mix
(e.g. cream from milk) (Page 64, Fig. 3.19)
- differentiation happens due to crystal settling
- ore deposits exist due to differentiation i.e. crystals settle out
2) Partial Melting = occurs as only a portion of the rock melts due to temperature
differences between areas of rock (closer to center = hotter, closer to surface = cooler)
3) Assimilation = as hot magma forces its way through country rock, the host
rock itself is melted (assimilated) (Page 65, Fig. 3.20)
4) Mixing of Magmas = two plumes of magma bump into each other and mix
(Page 65, Fig. 3.21)
Igneous Activity By Plate Tectonics: (Page 66, Chpt. 3)
- divergent boundaries = ocean plates = basalt and gabbro
- convergent boundaries = continental plates = granite and andesite
Divergent Boundaries and Igneous Rocks:
- basaltic magmas due to partial melting of asthenoshpere
- as asthenoshphere rises = less pressure = partial melting = mafic (silica 50% or less)
- as plates spread, new magma fills empty areas = production of mafic ocean crust
(Page 67, Fig. 3.22)
Convergent Boundaries and Igneous Rocks:
- subducted plate partially melts asthenoshpere = mafic magma
- as subduction continues, where two plates meet, assimilation, differentiation occurs
melting and mixing the mafic magma occurs resulting in an intermediate magma which is
different from magma at divergent boundaries (Page 68, Fig. 3.24)
- partial melting of continental crust occurs, and the crust contains much silica
- silica maintains a low melting point, melt, pockets of mafic magma rises and partially
melt the bottom of the continental crust - as a result diapers form rising and cooling as
they rise (Page 69, Fig. 3.26, 3.25)
Assignment Questions: Page 71: #’s 2, 3, 7, 9, 11, 12, 14, 16, 19, 20, 21, 22, 23