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IGNEOUS ROCKS
rock = mixture of minerals, mineraloids, glass, or
organic matter bound together in some way
1. magma is parent material for all rocks
2. three major types: igneous, sedimentary, &
metamorphic
igneous rock = forms when
magma cools and hardens
Two Categories: classified according to where
magma cools and hardens
1. intrusive igneous rock = cooling & solidification of
magma deep below crust within Earth
a. can intrude and cut across into other rock
masses below surface
b. also called plutonic rocks
c. pluton = intrusive igneous
rock mass
d. cools slowly → large crystals →
called mineral grains → coarse grained
e. most common type [granite most common]
2. extrusive igneous rock = formed when lava cools
on Earth’s surface
a. also called volcanic rocks
b. cool quickly → fine-grained
c. often grains are too small to see with naked eye
d. if cools within a few hours → no grains →
volcanic glass
Ex: obsidian, pumice, scoria
magma = mixture of molten rock, gases, and mineral
crystals
1. composition = oxygen, silicon, aluminum, iron,
magnesium, calcium, potassium, & sodium
2. sources of Earth’s thermal energy = remaining
energy from Earth’s molten formation & heat
generated from decay of radioactive elements
3. factors affecting formation:
a. temperature: generally increases with depth in
Earth’s crust = geothermal gradient
b. pressure: increases with depth in crust
1) due to weight of overlying rock
2) as pressure on rock increases, its melting
point also increases
c. water content: as amount of water content
increases, melting point decreases
d. mineral content: different minerals have different
melting points
Rock Melting/Formation:
1. partial melting = process where some minerals
melt at low temperatures while other minerals
remain solid
a. as each group of minerals melts it is added to
the magma
b. if temperatures are not high enough to melt all
minerals in rock then
magma will have different
composition from that of
the rock
2. fractional crystallization = process where different
minerals form at different temperatures
a. can change composition of magma
b. change due to minerals that crystallize being
removed from magma
3. Bowen’s reaction series = shows relationship
between cooling magma and crystal formation
a. developed by N. L. Bowen (Canadian)
[Figure 5-6 p.104 text]
b. describes how when magma cools, minerals
from in predictable patterns
c. first pattern: described by a continuous, gradual
change of mineral compositions in feldspar group
1) right branch of reaction series
2) undergo continuous change in composition
3) first to form are rich in calcium → react with
magma to form sodium rich
4) if cools too quickly can have zones of calcium
and sodium
d. second pattern: described by abrupt change
of mineral type in iron-magnesium groups
1) left branch of reaction series
2) olivine first to form next to form is pyroxene, etc.
e. as series proceeds elements are removed from the
magma until silicon and oxygen are left
4. minerals from into distinct bands in order shown in
reactions series in some magma bodies
a. forms a layered intrusion (Figure 5-9. p.106)
b. settling of crystals, flowing currents in magma,
temperature gradients within magma chamber
all role in formation
c. some may have very high concentrations of
specific elements [Ex: platinum]
Classification: three main groups = felsic, mafic,
intermediate
composition = determine by chemical composition of
magma
1. felsic rocks = light-colored, high silica contents,
contain quartz, feldspars orthoclase, &
plagioclase
a. light coloring
1) main mineral components =
quartz, orthoclase, feldspar
2) low density
b. rocks include: [granite family]
1) granite = coarse grains
2) rhyolite = fine grains, light gray to pink
3) felsite = fine grains, light-colored
4) obsidian = no grains, dark brown or black,
5) pumice = full of holes, resembles a sponge,
light enough to float
2. mafic rocks = dark-colored, lower silica contents,
rich in Fe & Mg
a. dark coloring
1) main mineral components = plagioclase,
biotite, amphibole, pyroxene, & olivine
2) also include ferromagnesian minerals
3) heavy, higher density
b. rocks include: [gabbro family]
1) basalt = fine grains, most common,
2) gabbro = coarse grains
3) daibase = grain size between fine & coarse,
4) basalt glass = no grains
5) scoria = full of holes, dense, not float
3. intermediate rock = characteristics of both felsic &
mafic
a. main mineral components = moderate amounts
of plagioclase, hornblende, augite, biotite
b. have little or no quartz
c. color commonly medium grays or greens
d. rocks include: [diorite family]
1) diorite = coarse grains
2) andesite = fine grains
4. ultramafic rocks = low silica content, very high
levels of Fe & Mg
a. include peridotite & dunite
b. usually coarse grained
texture = refers to size, shape, and arrangement of
mineral grains or crystals
1. intrusive:
a. large, well formed crystalline grains
b. medium to coarse grain
c. can see individual grains of different colors
d. core of continental crust = granite
2. extrusive:
a. no grains to very fine grains
b. grains cannot seen with unaided eye
c. make up oceanic crust = basalt
d. obsidian (volcanic glass ) =extremely rapid
cooling allows no grain to form
e. pumice = gases trapped in material form small
bubbles leaving holes
3. porphyritic = mixture of large and small crystals
a. due to magma cooling slowly then quickly
producing both large and small mineral grains
b. large crystals appear embedded into smaller
grains
Ore Deposits:
1. veins = hot mineral solutions spread through many
small fractures depositing minerals in bands
a. lode = large number of thick mineral veins
b. many can be metal-rich quartz veins
2. pegmatites = veins of extremely large-grained
minerals
a. find ores of lithium & beryllium
b. can produce beautiful crystals
since minerals grow into voids
and retain their shape
3. kimberlites = ultramafic rock
that contains diamonds
a. form deep in the crust 150 –
300 km below or in the mantle
b. form only under very high
pressure