Download Ch._5_IGNEOUS_ROCKS

The 3 Types of Rocks
Rock: A geologic material composed of 2 or more
different minerals (rarely just 1 mineral). Rocks
are mixtures. (remember that minerals are
chemical elements or compounds.)
_______________________________
Igneous- Formed when minerals solidify (crystallize) from
liquid rock (magma or lava).
Sedimentary- Formed when minerals precipitate from water
or when mineral fragments get cemented by water.
Metamorphic- Formed when igneous or sedimentary rocks
get “baked” at higher temperature and pressure due to
contact with magma or due to tectonic plate collisions.
Rock vs. Mineral
Granite is a rock made of 3 main minerals: alkali
feldspar, quartz, and micas (biotite and muscovite)
Igneous Rocks – Chapter 5
Igneous rocks are rocks that crystallize from liquid rock.
Magma- Molten rock below Earth’s surface in the crust or
mantle. It is composed mostly of Si and O (silica, SiO2) and
dissolved gases.
Where is the image of magma?
Lava- Molten rock material at Earth’s surface.
Magmas can become lavas when they extrude at
the surface through fissures (cracks) in the
surface of the earth or through volcanoes.
Igneous rock- A silicate-rich rock that forms
when molten rock (magma or lava) solidifies
and crystallizes. (liquid  solid, so freezing).
Two main types of
igneous rocks:
1) Intrusive/PlutonicFormed when magma
solidifies deep
underground. Includes
granite (the main rock of
the continental crust).
Never directly witnessed!
2) Extrusive/VolcanicFormed when lava
solidifies at the Earth’s
surface. Includes basalt
(the main rock of oceanic
crust).
Where does the heat inside the
Earth come from?
• Original heat still left over from
earth’s original formation 4.6
billion years ago. Earth is still
cooling.
• Heat produced from the decay of
radioactive elements inside the
earth’s mantle and crust.
(radioactive elements like.
uranium and thorium release heat)
• Frictional heat caused by the
gravitational pull of the moon on
the earth.
Heat moves outward towards the
surface of the earth
• As heat moves
upward, it causes the
mantle to convect (hot
material rises, cold
material sinks) like
wax in a lava lamp.
• Convection moves
heat that causes
melting.
• Mantle convection
also cause the tectonic
plates to move!
What Favors Melting of Rock?
1) Increase the Temperature! The temperature of
the Earth increases from crust to core at
approximately 25 C/km (the geothermal
gradient). The core temperature is > 5000 C, so heat
moves upward and melts the upper mantle and crust.
What Favors Melting or Rock?
2) Decrease the pressure! Pressure favors solids
and less pressure favors liquids. Decompression
melting occurs when hot mantle rock moves upward
by convection.
Liquid
What Favors Melting of Rock?
3) Add water! Water reduces the melting point of
rock. Minerals with water in them help melting of
rock to occur.
Dry granite
melting
temperature:
900°C.
Wet granite
melting
temperature:
700°C.
What Favors Melting of Rock?
4) Mineral Content!
• Rocks are mixtures of
minerals and different
minerals have different
melting points.
• Rocks with minerals that
have more iron and
magnesium and less silicon
and oxygen (mafic rocks)
generally melt at higher
temperatures that rocks that
contain more silicon and
oxygen (felsic rocks).
• Granite melts at a lower
temperature than basalt.
Rocks do not melt at one specific melting point.
They melt over a range of temperatures.
• Rocks are chemical mixtures
so the different minerals in
them retain their own unique
properties like melting point.
• Each mineral in a rock melts
at its own melting point.
This leads to partial melting
of rocks when heated.
• The minerals with the lowest
melting points melt first,
followed by those with
higher melting points. The
entire rock may not melt!
A household example of partial
melting…..
Rock-Forming Minerals in Igneous Rocks
Mineral
Chemical Composition
Melting Temperature
SiO2
Lowest
(600C)
Alkali Feldspar
(K,Na)AlSi3O8
Low
(700C)
Muscovite Mica
KAl2(AlSi3O10)(F,OH)2
Low
(750C)
Biotite Mica
K(Mg,Fe)3AlSi3O10(F,OH)2
Medium
(800C)
Amphibole
Ca2(Mg,Fe,Al)5(Al,Si)8O22(OH)2
Med-High
(850C)
Plagioclase
NaAlSi3O8 – CaAl2Si2O8
Med-High
(800-1000)
Pyroxene
(Ca,Mg,Fe)2Si2O6
High
(1000C)
Olivine
(Mg,Fe)2SiO4
Highest
(1100C)
Quartz
Partial melting of source rock will produce
magma with a different chemical composition.
Source Rock
Minerals richer in silicon
and oxygen (like quartz
and feldspars) melt at
lower temperatures,
leaving minerals richer in
iron and magnesium (like
amphibole, pyroxene, or
olivine) behind.
So, the magma produced
(the partial melt) will be
felsic, richer in silica
(SiO2) and the residue
will be more mafic,
poorer in silica (SiO2).
Partial melting produces magmas of different
compositions which form the different types
of igneous rocks found on the earth.
Deeper melting in the Earth (say in the
mantle) leads to more mafic magmas
richer in magneisum (Mg) and iron (Fe)
and poorer in silica (Si and O), since the
source rocks are richer in these
elements.
Shallower melting in the earth (say in
the crust) leads to more felsic magmas
richer in silica (Si and O) and in sodium
and potassium, since the source rocks
are richer in these elements.
Partial melting is the norm on Earth!
Partial melting created the crust from the
mantle.
Quartz (SiO2)
Alkali Feldspar ((K,Na)AlSi3O8)
Muscovite Mica (KAl2(AlSi3O10)(F,OH)2)
Biotite Mica (K(Mg,Fe)3AlSi3O10(F,OH)2)
Amphibole
(Ca2(Mg,Fe,Al)5(Al,Si)8O22(OH)2)
Plagioclase Feldspar
(NaAlSi3O8 – CaAl2Si2O8)
Pyroxene ((Ca,Mg,Fe)2Si2O6)
Olivine ((Mg,Fe)2SiO4)
What happens to a partial melt or
magma once it forms??
• It can travel up and away from the
source rocks because it is less
dense and will rise.
• It can melt other rocks into itself
as it moves up towards the surface.
• The farther it moves from the
source rock, the more chemically
different it becomes compared to
the source rock.
• It will eventually crystallize back
to solid rock, but it will be
chemically different than the
source rock that partially melted to
produce it.
Bowen’s Reaction Series: Tells you the sequence of crystallization of
minerals from a magma or lava (including a partial melt). Minerals crystallize
in a predictable order as the magma or lava cools. Minerals crystallize in an
order that is the opposite of the order they melted when partial melting occurred.
So, last melted = first to crystallize.
Bowen’s Reaction Series
Bowen’s Reaction Series: Two Paths
The left side is the
discontinuous series. As
the magma cools and
crystallizes, earlier formed,
iron-magnesium rich
minerals react with the
remaining magma to form
new minerals.
Alkali
The right side is the
continuous series. The
right side represents the
continuous crystallization of
plagioclase feldspar, which
starts out calcium-rich and
ends up more sodium-rich.
Mineral Textures of Igneous Rocks show
Bowen’s Reaction Series in Action
An olivine crystal surrounded by
pyroxene in an extrusive (volcanic)
igneous rock.
A zoned crystal of plagioclase in an
igneous rock. The center (core) is Carich and the edge (rim) is Na-rich.
Alkali feldspar, muscovite mica, and
quartz crystallize last
Quartz
Plagioclase
feldspar
Alkali
feldspar
Amphibole
Lessons from Bowen’s Reaction Series
• 1) The chemical composition of a magma controls
the types of minerals and kind of igneous rock that
can form from it.
• 2) The first minerals to solidify at high temperature
are olivine, pyroxene, Ca-rich plagioclase. This
produces a mafic rock (rich in Fe, Mg, Ca) like
basalt or gabbro.
• 3) At lower temperatures, minerals like quartz and
alkali feldspar solidify. This produces a felsic rock
(rich in Si and Al) like granite or rhyolite.
• 4) At intermediate temperatures, minerals like
amphibole, biotite, and plagioclase crystallize
together. This produces an intermediate rock like
diorite or andesite.
Fractional Crystallization
Fractional crystallization is the separation of previously-formed
minerals from the rest of a magma during cooling and crystallization.
This often occurs as dense, iron-magnesium rich minerals like olivine
and pyroxene settle out under the influence of gravity.
Fractional crystallization is
important because if these ironmagnesium rich minerals are
removed, they can no longer
react with the magma along the
discontinous side of Bowen’s
reaction series. Once their
elements are removed, the
magma becomes more highly
concentrated in silicon and
aluminum and so is now more
felsic than before.
Fractional Crystallization and Crystal
Settling in the Palisades Sill in the
Hudson River Valley of New York
Quartz “Veins”
• Represent the most
felsic (SiO2 – rich) bit
of magma left at the
end of fractional
crystallization.
• Quartz veins often cut
across other igneous
rocks because they
solidify last.
• Quartz veins often
contain rare elements
(like gold) that don’t
fit into any other
crystal structures.
They are often mined.
Summary Questions:
1) How are partial melting and
fractional crytallization similar?
2) How are partial melting and
fractional crystallization different?
3) How do partial melting and
fractional crystallization both lead to
the formation of igneous rocks?
Partial Melting vs. Fractional Crystallization
Classification of Igneous Rocks –
Ch. 5.2
The most useful system for classifying igneous rocks
utilizes texture and composition (color).
Type of
Igneous
Rock
Felsic
(light)
Intermediate
(medium)
Mafic
(dark)
Intrusive /
Plutonic
(coarsegrained)
Extrusive /
Volcanic
(finegrained)
It is a binary classification system!
Ultramafic
What is Texture?
• The size, shape and arrangement of crystal
grains within a rock.
• Igneous rocks have an interlocking texture
formed by growth of minerals from a melt.
• Crystal size is controlled by cooling-rate.
Intrusive or Plutonic Textures
Intrusive/plutonic rocks cooled slowly, deep within the
Earth. These are coarse-grained or phaneritic (most
crystals >1 to 10 mm across) because crystals have time
to grow large.
Extrusive or Volcanic Textures
Extrusive/volcanic rocks cooled quickly at the Earth’s
surface (so less time for crystal growth). These are finegrained or aphanitic (most crystals <1 mm across).
Extrusive Textures Cont.
An extreme case occurs when
rocks are glassy (no crystals
formed due to extremely rapid
cooling). An example is
volcanic glass or obsidian.
Volcanic rocks with a frothy or
foamy appearance are glassy
too! The magma just had a lot
of gases dissolved in it, and the
gases came out when the lava
erupted at Earth’s surface.
Examples are pumice and
scoria.
Rock Sort Part I
Using Texture-Grain Size to Infer
Origin
• Sort your rocks into 3 piles now:
- Intrusive / Plutonic (coarse-grained)
- Extrusive / Volcanic (fine-grained)
- Extrusive / Volcanic (glassy or
frothy)
Igneous Rocks are also Classified
by Composition!
4 compositional categories:
Felsic (Granitic)
Rich in silicon (Si) and Oxygen (O)
and Aluminum (Al)
Mafic (Basaltic)
Rich in magnesium (Mg) and iron
(Fe); less Si and O than granitic
Intermediate (Andesitic)
In-between felsic and mafic
Ultramafic (Ultrabasic)
Extremely rich in magnesium and
iron compared to mafic
A rock's color tells us about the minerals
present and overall composition.
Mineral Composition vs. Color
• Minerals With Light
Elements (Si, O, Al,
Na, K)…
• Minerals With Heavier
Elements (Ca, Mg,
Fe)…
• Tend to be
__________ colored.
• Tend to be
__________ colored.
Felsic (Granitic) Rocks Are:
• Rich in light-colored
minerals (quartz, alkali
feldspar, micas, and some
plagioclase feldspar).
• Compositionally rich in Si,
Na, Al, and K (and poor in
Fe and Mg).
• The dark-colored minerals
like biotite and amphibole
are present, but only in
small amounts). No
olivine or pyroxene.
Mafic (Basaltic) Rocks Are:
• Rich in dark-colored
minerals (plagioclase
feldspar, olivine, and
pyroxene with rare to no
amphibole and biotite).
• Compositionally rich in
Ca, Fe, and Mg (lower in
Si, Na, Al, and K).
• There is no alkali feldspar,
muscovite mica, or quartz!
Intermediate Rocks Are:
• Composed of roughly equal amounts of dark- and lightcolored minerals so they tend to have a “salt and pepper”
color. Little to no quartz, no olivine, little pyroxene
present. Amphibole, plagioclase feldspar, and biotite are
common. Quartz and some alkali feldspar may be
present too, in lesser amounts.
Ultramafic Rocks Are:
• Composed almost
exclusively of Fe
and Mg-rich
minerals from the
mantle (olivine and
pyroxene, but no
feldspar or quartz).
They are called
peridotite.
• Compositionally
rich in Fe, Mg, and
Ca, but poorer in Si
compared to mafic
rocks.
Rock Sort Part II –
Using Color to Infer Composition
• Sort your rocks into 3 piles now:
- Felsic (Granitic) - light colored
- Intermediate (Andesitic) – medium
colored (salt and pepper)
- Mafic (Basaltic) – dark colored
Classification of Igneous Rocks
The binary system for classifying igneous rocks
utilizes texture and composition (color).
Type of
Igneous
Rock
Intrusive /
Plutonic
(coarsegrained)
Extrusive /
Volcanic
(finegrained)
Felsic
(light)
Intermediate
(medium)
Mafic
(dark)
Ultramafic
Classification of Igneous Rocks
The most useful system for classifying igneous rocks
utilizes texture and composition (color).
Type of
Igneous
Rock
Felsic
(light)
Intermediate
(medium)
Mafic
(dark)
Ultramafic
Intrusive /
Plutonic
(coarsegrained)
Granite
Diorite
Gabbro
Peridotite
Extrusive /
Volcanic
(finegrained)
Rhyolite
Andesite
Basalt
Komatiite
Bowen’s Reaction Series: Tells you the sequence of crystallization of
minerals from a magma or lava (including a partial melt). Minerals crystallize
in a predictable order as the magma or lava cools. Minerals crystallize in an
order that is the opposite of the order they melted when partial melting occurred.
So, last melted = first to crystallize.
Classification of Igneous Rocks
Special Igneous Textures
Xenolith: A fragment of rock within an igneous rock that
differs compositionally from the host rock. The host rock
and zenolith inclusions formed from different magmas.
Vesicules: A bubble or hole formed by escaping gas
(common in basalts). Igneous rocks with vesicles in them
are called vesicular.
Special Igneous Textures
Porphyry: Igneous rock with large crystals (called phenocrysts)
in a fine-grained matrix. Rhyolite and andesite are often
porphyritic.
Porphyritic rocks represent a two-state cooling history:
1) slow cooling at depth followed by...
2) rapid ascent and fast cooling of magma or lave near or at Earth's
surface.
Special Igneous Textures
Pegmatite: A very coarse grained igneous
rock (crystal sizes > 10 cm) in which crystal
growth was enhanced by the presence of
fluids. Igneous rocks with very coarsegrained crystals are called pegmatitic.
Intrusive Rock Bodies
Intrusive rocks exist in intrusions that penetrate or cut through preexisting country rock.
The names of intrusive bodies are based on 1) size, 2) shape and 3)
geometric relationship to the country rock.
The two basic types of intrusions are:
A. Shallow intrusions (formed < 2 km beneath Earth’s
surface). These cool and solidify fairly quickly resulting in
relatively fine-grained rocks (but coarser than extrusive rocks).
B. Deep intrusions (formed > 2 km beneath Earth's
surface). These cool and solidify slowly resulting in coarse-grained
rocks.
Intrusive Rock Bodies
Types of shallow
intrusions:
Dike: Tabular structure that cuts
across the layering in the
country rock.
Intrusive Rock Bodies
Types of shallow
intrusions:
Sill: Tabular structure that
parallels layering in the country
rock.
Intrusive Rock Bodies
Types of shallow
intrusions:
Volcanic Neck: Shallow intrusion
formed when magma solidifies in
the throat of a volcano (Ship Rock,
New Mexico).
Types of deep intrusions.
Plutons are large, blob-shaped intrusive bodies formed when
rising blobs of magma (diapirs) get trapped within the crust.
Stock: A small plutons
(exposed over <100
km2).
Batholith: A large
pluton (exposed over
>100 km2).
The interfaces between
instrusions and country
rock are called contacts.
Rapid cooling of igneous
rock near the contact
(called a chill zone)
often results in a smaller
crystal size near the
contact.
Where Does Most Igneous Activity
Occur?
• Mainly at or near tectonic plate boundaries.
Types of Igneous Rocks relate to
where they are formed
Igneous rocks usually form at tectonic plate margins where plates
are converging (moving together) or diverging (moving apart).
Different types of igneous rocks form at different types of plate
margins.
Mafic Rocks are common at
divergent boundaries.
• The magma comes from the upper mantle
(asthenosphere) and rises through thin crust.
• Little opportunity to differentiated  mafic.
Felsic Rocks are Common on
Continents.
• Possibly form above subduction zones by
“magmatic underplating”.
• Lots of magma differentiation  felsic.
Intermediate Rocks.
• Common at convergent boundaries.
• Partial melting of subducted ocean crust
(mafic) produces basaltic magma; this
evolves into more felsic magma by the
assimilation of felsic crust.
Intermediate igneous rocks commonly form at convergent
boundaries.
Here, partial melting of subducted asthenosphere produces basaltic
magma which evolves into more felsic magma by the assimilation of
felsic crust.
Some igneous rocks form within plates (not at a plate
boundary). Rising mantle plumes (of controversial
origin) can produce localized hotspots and volcanoes
as they rise through continental or oceanic crust.
End of Chapter 5
The Rock Cycle
• A rock is a naturally formed,
consolidated material usually
composed of grains of one or
more minerals
• The rock cycle shows how one
type of rocky material gets
transformed into another
– Representation of how rocks are
formed, broken down, and processed
in response to changing conditions
– Processes may involve interactions
of geosphere with hydrosphere,
atmosphere and/or biosphere
– Arrows indicate possible process
paths within the cycle
The Rock Cycle and Plate Tectonics
• Magma is created by melting of rock
above a subduction zone
• Less dense magma rises and cools
to form igneous rock
• Igneous rock exposed at surface
gets weathered into sediment
Convergent plate boundary
• Sediments transported to low areas,
buried and hardened into sedimentary rock
• Sedimentary rock heated and squeezed at depth to form
metamorphic rock
• Metamorphic rock may heat up and melt at depth to form magma