Download 3 geology ign rx0

Earth Materials:
Chp 3:
Igneous Rocks
Convergent
Boundary
Divergent
Boundary
Convergent
Boundary
• Magma: molten rock below Earth’s surface
• Lava: molten rock at the surface
Composition
Type of magma Silica Location Viscosity Colour
content
felsic = silisic
>65% cont’l cr thick
light
Intermediate
53–65% cont-ocean
mafic = basaltic 45-52% ocean-cont
ultra mafic
<45%
ocean
thin
dark
How magma forms
• 1. Lower pressure
Pressure
solid
liquid
Temperature
• Caused by decrease in pressure as plates
move apart at divergent boundaries
High pressure
Low pressure High pressure
Pressure
• 2. Higher Temperatures at magma
plumes: localized heating caused by
increased radioactivity or the “blanket
effect” (large thick continental plates
insulate resulting in heat build up)
Continental plate
Magma
plume
Magma Plume
• 3. Water (yes water) lowers the melting
temperature of rocks
wet
pressure
dry rock
solid
liquid
Water leaks
down here
1100 C
temperature 1500 C
Cont’l plate
Oceanic plate
Water leaks down here
Magma Melts: wet vs. dry
Modification of Magma
= how magma becomes more felsic/silisic
1. Fractional Crystallization (crystal
settling)
• High temeprature minerals high in Mg, Fe;
low in SiO2 form first.
• The minerals then sink/settle out of the
magma (crystal settling). The magma
becomes more felsic/silsic as the plume
rises.
Bowen’s Reaction Series
Crystallize first
Magma Modification:
Fractional
Crystalization
2. Wall Rock Assimilation
• As hot magma rises it melts and mixes
with silisic/felsic country rock become
more silisicous.
• Xenolith = fragment of country rock that
falls into the magma and doesn’t melt
(that’s how diamonds reach the surface!)
Silisic country
rock
Mafic
Magma Modification: Wall Rock Assimilation
Xenoltih
3. Magma mixing
• When two magmas of different
compositions (more/less silisic) join and
mix to form an intermediate mixture.
• rare
Igneous Rock
TYPES:
1. Intrusive (plutonic)
• Solidify slowly underground
• Large crystals (visible to naked eye) –
PHANERITIC texture
2. Extrusive (volcanic)
• Solidify quickly at or near surface
• Small crystals = aphanitic texture
Igneous Rock Types
Extrusive
aphanitic
PHANERITIC
Intrusive
TEXTURE
1. PHANERITIC (coarse) (5 rocks)
• Minerals can be seen without
magnification
• Slow cooling = larger crystals
• Intrusive
• 4 intrusive rocks ID with chart
1b Pegmatic: very large crystals
(+5 mm) (1 rock = pegmatite)
• Very slow cooling
• Usually granitic composition
Granite
Granite
Diorite + Dalmation Rock
Gabbro
Peridotite
Pegmatite
2. aphanitic (fine):
minerals too small
to be seen without
magnification
(need lens/scope)
• Fast cooling
• Extrusive
• 3 extrusive rocks
ID with chart
• 2a. Volcanic glass (obsidian) cools veery
fast
• 2b. Vesicular (scoria & pumice) rocks that
freezes before the gas can escape leaving
numerous small vesicles (cavities) =
“frozen milkshake”
• 2c. Pyroclastic = fragmental texture
(Volcanic breccia & welded tuff)
• Material blown out of volcano, settles and
become rock
Composition
Texture
Vesicular
(bubbly)
Glassy
Pyroclastic
Felsic
Pumice
“styrofoam”
Mafic
Scoria/vesicular
basalt
Obsidian
(volcanic glass,usually black)
Volcanic Breccia
(welded cinders, blocks & bombs)
Tuff/Welded Tuff
(welded ash)
2d. Porphyritic
(porphyry)
• Large crystals
(phenocrysts)
surrounded by a small
crystals (groundmass)
Forms by:
a)magma cools slowly at depth as an intrusive body
forming initially the large crystals
b)Then magma is quickly extruded to the surface
where it cools quickly freezing the large crystals in
a fine ground mass
Rhyolite
Andesite
Basalt
Volcanic Glass = Obsidian
(Glassy Texture)
Pumice (Vesicular Texture)
Scoria = Vesicular Basalt (Vesicular
Texture)
Welded Tuff
(Pyroclastic Texture)
Look for layering that
formed as ash was laid
down
Volcanic Breccia = “Dog Puke”
(Pyroclastic Texture)
Porphyry = Choc-chip cookie
(Porphyritic Texture)
• Therefore all igneous rocks are classified
by:
• 1. composition (ultra-mafic
felsic) =
minerals present
• 2. texture (PHANERITIC
aphanitic)
HW
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Do WS 3.2
Do Lab 4.1