Download GEOL261-IgRocks-2013 (1)

The Rock CycleMinerals form rocks
All rocks can be transformed into other rock types
Rocks are divided into 3 categories
Igneous- crystalline- forms as liquid cools
Metamorphic- crystalline-forms as rocks are
heated and squeezed
Sedimentary- non-crystalline- smaller pieces
or chemicals from other rocks
Erosion
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Eruption or near surface
cooling & crystallization
(Heat & Pressure)
Igneous formed from
Magma and Lava
Magma
• molten rock below Earth's surface.
L ava
•
magma on the Earth's surface.
Pyroclastic material
• (pyro = fire, clastic = debris)
• Airborne lava
— cools as it falls
Composition of the magma
• Analogous to what makes up the “stew"
• What chemical elements are present
• What material has the magma moved
through
Temperature of the melt
• Not only how hot, but how long it stays that hot
• also relates to pressure of the molten rock
Cooling environment
• fast vs slow
• Internal vs External
Water content
Bowen's Reaction Series- Important !
Plagioclase
(Ca-feldspar)
Olivine
Pyroxene
Amphibole
Biotite
Plagioclase
(Na-feldspar)
Orthoclase
(K-feldspar)
Muscovite
Quartz
What things might you
describe when looking at
an igneous rock?
Composition of Igneous rocks
Felsic or Sialic magma
• Si-rich (> 65%)
• rich in K, and Al
• little Ca, Fe, and Mg.
Intermediate magma
• between the two extremes in Si
content and other atoms.
Mafic magma
• Si - poor (< 35%)
• richer in Ca, Fe, and Mg.
Crystal Settling
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Magma Mixing
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magma
Assimilation
Ways of Changing
Magma Composition
Viscosity of Magma/ Lava
Viscosity- important for volcanic activity
• the resistance of a liquid to flow
— high viscosity = thick and stiff
— low viscosity = thin and "runny".
Related to:
• amount of water (H2O) in magma
• amount of silica (Si) in magma
• Mafic
— thin, low viscosity
• Felsic
— thick, high viscosity
Igneous Rock TexturesPhaneritic Texture
how big are the
minerals?
• Visible Mineral Grains
distinguishable as different colored interlocking
shapes
Aphanitic Texture
• Mineral grains too small to be seen
— microscopic
Porphyritic Texture
• Two distinct sizes of mineral grains
• Large and Small
— Large = Phenocrysts
— Small = Groundmass or Matrix
• Groundmass
— Grains may be either visible or not
Porphyritic texture
Large white crystals
are phenocryts
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Aphanitic basalt (mafic
composition) constitutes
the groundmass
Cooling Histories
Minerals need time and space to grow
• More time = Bigger crystals
— visible mineral grains
P & T control cooling rates of magma
• Temp
— Earth is a good insulator
» holds in heat
» keeps out cool
— Atmosphere is a relatively bad insulator
» transfers heat easily
• Earth has pressure
— Weight of overlying rocks
— Magma trying to push up (density)
— water vapor (steam), wants to expand
Categories of Igneous EOF
Granite
Intrusive rocks
• cool beneath Earth's surface
• cool very slowly
• higher P & T
— Phaneritic textures
Extrusive rocks
Rhyolite
• cool on the Earth's Surface
• cool relatively fast
• lower T & P
— Aphanitic textures
Basalt
— Pyroclastic textures
porphyry
Complex
• Partially cools below and above
— Porphyritic textures
Igneous rock names
• determined by texture
— size and arrangement of mineral grains
• AND by mineral composition
— minerals affect rock color and indicate temperature of creation
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Pegmatite
A Pegmatite is a very coarse-grained igneous rock.
Crystals are >2 cm, often larger.
Most are granitic, although mafic pegmatites can form.
Biotite mineral
grain
Feldspar
mineral
grain
Quartz mineral
grain
Granite & Rhyolite
Phaneritic Texture
Felsic magma
Granite
What
are the textures in these two
rocks ?
Aphanitic Texture
Felsic magma
Rhyolite
Andesite & Diorite
Aphanitic texture
forms from intermediate
composition magma
Phaneritic texture
-forms from intermediate
composition magma
What are the textures in these two
rocks ?
Basalt & Gabbro
aphanitic texture
forms from mafic magma
phaneritic texture
forms from mafic magma
Basalt
Gabbro
»
Peridotite- the abundance (more than 40%)
of Olivine crystals makes the rock take on a
green appearance
Obsidian- conchoidal fracture
And association with volcanic rocks
Volcanic
Glass
Tuffs &Tuffs
Breccias
- look like sedimentary rock
& Breccias
But they are not….it is volcanic ash that is lithified
Tuffs - may be welded or not
Always consists of fragments smaller than
2mm in diameter
Welded means they were still hot enough that
they partially recrystallized they were
emplaced and consolidated.
If they are not welded, it means they were
not hot enough to partially recrystallize when
they settled on the landscape
Breccias - have a tuffaceous matrix but have
a large percentage of coarse-grained angular
fragment. They may be welded or not too.
How do we tell where the
igneous rocks formed?
What can we derive from
the rocks about the
conditions of formation?
Pyroclastics
Lava
flow
Volcanic
neck
Igneous
dike
Igneous
Sill
Batholith
Sierra Nevada
Batholith- Home to
Yosemite National
Park and a lot of
Granites and
Grano-diorites
Sierra Nevada Batholith
Areal extent of the Idaho
Batholith- a huge (15,400
square miles) intrusive body
of primarily felsic
composition (granites)
igneous rocks
An igneous dikeDiscordant with
surrounding rock
It cuts across
other rocks
An igneous sillConcordant with
surrounding rock
It runs parallel
to other rocks
Volcanic Necks & Igneous Dikes
Neck
Dike
igneous
Igneous sill
sill
Lava flows
Flood basalts create lava cliffs on the Columbia Plateau