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Chp 3 Igneous Rocks
Igneous Rocks-derived from volcanic processes
-volcanoes
-submarine and sub-aerial
Derived from magma chambers deep beneath the
surface of the Earth
Chp 3 Igneous Rocks
Magma –
a mobile, silicate melt formed in the upper mantle or lower crust,
as much as 100 to 300 kilometers below the surface.
- It accumulates at depths in reservoirs called magma chambers.
Magma chambers may be only a few km below the surface at
spreading centers and below the oceanic crust or only a few tens of km
below oceanic subduction zones or continental plates.
-Magma may slowly cool in place forming intrusive igneous rocks.
Or, magma may breech the surface in the form of a volcano forming
extrusive igneous rocks.
-Lava is the term for magma that has breeched the earth’s surface.
Granite rocks in Yosemite
National ParkPart of the Sierra Nevada
Batholith
Fig. 3-CO, p.60
Mount Rushmore-carved from Harney Peak Granite
Fig. 3-1a, p.62
Crazy Horse Monument, also being carved from Harney Peak Granite
Fig. 3-1b, p.62
Magma Types
Table 3-1, p.63
Fig. 3-2, p.64
Magma Types:
A. Ultramafic
-Comprises the upper mantle (asthenosphere)
-Very low in silica (45% or less) & low viscosity
-Does not breech the earth’s surface usually
-Only forms intrusive igneous rocks
-High in ultramafic minerals such as olivines
-Forms dark greenish-colored rocks
B. Basaltic or Mafic Magma
-Low viscosity; fluid-like and flows easily
-Low silica content (47 – 50% silica)
-Temperature range of 9000 – 12000C
-High in mafic minerals: amphibole, pyroxene, olivine
-Forms dark colored rocks
Magma Types:
C. Intermediate Magma
-A “mixture” of mafic and sialic magmas
-Medium viscosity
-Silica Content of 50 – 59%
-Temperature range of 8000 – 10000C
-May contain both mafic and sialic intermediate minerals
-Forms medium, grayish-colored rocks
D. Felsic or Sialic Magma
-Higher viscosity, thicker, gooier
-High silica content (65 – 70% silica)
-Temperature range of less than 8000C
-High in sialic minerals: quartz, microcline, muscovite
-Forms light colored rocks
Fig. 3-3, p.65
Bowen’s Reaction Series A series of reactions based upon fractional crystallization.
Mafic minerals have a higher point of crystallization and crystallize first, followed
by intermediates, followed by sialics,as the magma cools.
Discontinuous Branch – a succession of ferromagnesian silicates crystallize as the
temperature of the magma drops: Olivine to pyroxene to amphibole to biotite.
Continuous Branch – Plagioclase feldspars with increasing amounts of sodium crystalliz
This leaves the higher silicate minerals to crystallize last, at the lowest temperatures:
potassium feldspars, muscovite, and quartz.
1. Effects of Silica Content on Magma
Silica tetrahedra in magma link together to form polymers
The more silica polymers, the thicker, gooier the magma
The thicker, gooier the magma, the more explosive a volcano may be..
The more silica polymers, the more sialic the magma creating lighter
colored rocks
The more sialic the magma, the lower the temperature at which it
crystallizes (less than 8000C)
The reverse is true for more mafic magmas…
The fewer the silica polymers, the thinner, less viscous the magma and
the more free-flowing the magma
The less viscous the magma, the less explosive the volcano
The more mafic the magma is creates darker ferromagnesian minerals
and darker rocks
The more mafic the magma, the higher the temperature at which it
crystallizes (800 – 10000C)
2.
Effects of Pressure on Magma
In the earth, pressure on rocks (or a magma body) from the surrounding
rocks (or overburden – the weight of the rocks above the structure)
keeps it from expanding and prevents melting.
a. A drop in pressure causes hot rocks to melt.
-A decrease in pressure can arise from the erosion or removal of the
overburden, causing the rocks to melt.
b. An increase in pressure can arise from tectonic activity or an
increase in pressure from the magma chamber.
-An increase in pressure causes melting rocks to slow their melting.
As magma is rising upwards through the rocks, pressure is decreasing
as it nears the surface, preventing it from solidifying and thus possibly
forming a volcano.
3. Effects of Water Content on Magma
a. Water lowers the melting point of magma
“Dry Magmas” have a water content of less than 10%
“Wet Magmas” have a water content of 10 – 15% water
b. Water at high temperatures is very volatile:
At high temperatures water tends to escape as a gas (superheated water vapor)
c. High pressure keeps water from escaping
Cracks in the overburden may allow water to escape
d. Sialic (Granitic) magmas usually solidify below the surface intrusively
Mafic magmas being less viscous and many times reach the surface as lava
The water content in most mafic magmas is very low: 1- 2%
e. Pressure keeps water from expanding
Near “mafic” melts usually contain 1 – 2 % H2O, causing it to remain molten and it easi
reaches the surface.
Fig. 3-5, p.67
Fig. 3-6, p.68
Fig. 3-7, p.69
Aphanitic=rapid cooling
Phaneritic= slower cooling
Porphyritic= complex
cooling history
g= Obsidian (glassy)
h= Pumice (vesicular)
i= microscopic view of
Fragmental texture
Fig. 3-8, p.70
Igneous Rock Textures
The texture of an igneous rock refers to the size, shape and arrangement of the constituent
mineral grains and reflects the rate of cooling.
IGNEOUS ROCK TEXTURES:
I. Glassy Texture –
-“resembling man-made glass”
-having “concoidal fracture”
-possessing a “randomness” of crystal lattices
Ions have no time to migrate to form crystals
-Indicates a rapid rate of extrusive cooling
Example = Obsidian, Volcanic Glass
II. Aphanitic Texture –
“a” meaning “without”; “phaneros” meaning “visible” – overall meaning is that the rock has
crystals, but they can not be identified with the naked eye.
Tiny crystals have formed, but require magnification to identify
Indicates relatively “quick” extrusive cooling but at a rate slower than that required for a
glassy texture
Cooled at, or near the earth’s surface.
Example = Rhyolite, Andesite, or Basalt
Aphanitic= Basalt
Fig. 3-11a, p.72
III. Phaneritic Texture –
“Phaneros” meaning “visible” – whereby there are large crystals present that are easily
identified with the naked eye.
An individual large crystal is referred to as a Phenocryst.
The presence of phenocrysts indicates a slow rate of intrusive cooling occurring deep within
the earth’s crust.
Pegmatites are igneous rocks that cool extremely slow resulting in “giant” phenocrysts.
Examples = Granite, Diorite, Gabbro, or Peridotite
Phaneritic= Gabbro (note light reflecting from crystal faces).
IV. Porphyritic Texture –
This texture indicates that the igneous rock had two cooling periods –
the first one slow and the second one quicker.
The larger phenocrysts form during the first cooling period while the magma is at an intrusive
depth in the crust. If it were to continue cooling at this ratet, it would have formed a phaneritic
texture. Something occurred to move the still molten magma (containing the first formed
phenocrysts) closer to the surface whereby the still molten material cools at a faster rate.
This gives the rock two distinct crystal sizes: the larger phenocrysts that formed first are set
in a finer “groundmass” (matrix) of either porphyritic, aphanitic, or glassy textures.
Rock texture may be porphyritic/aphanitic, meaning that phenocrysts formed first and the
still molten ground mass rapidly cooled. Or, the rock may be porphyritic/phaneritic meaning
the the phenocrysts formed and the ground mass cooled having a phaneritic texture.
Examples: Basalt Porphyry, Granite Porphry
Fig. 3-11b, p.72
V. Pyroclastic Texture –
Means “fire-broken” – formed from volcanic ejecta: the ash, cinders, and “bombs” expelled
from
eruptions of volcanoes.
All pyroclastic textured rocks are extrusive being produced by explosive volcanic eruptions.
Aside from any pre-formed crystals, pyroclastics are generally categorized as to the particle
size:
fine ash X < 0.06 mm
coarse ash -
0.06mm – 2.0mm
cinders -
2.0mm – 64.0mm
“bombs” -
X > 64.0mm
Being thrown into the air and later settling out on the ground, they may cool before they settle
outforming ”unwelded” pyroclastics, or they may remain glowing hot as they settle out
forming “welded” pyroclastics.
“Tuffaceous rocks” is the term for the rocks resulting from the settling of pyroclastic particles,
and may result in the formation of “unwelded tuffs” or “welded tuffs”.
Felsic Igneous Rocks:
a. Rhyolite:
b. Granite
Typically light colored because
they contain non ferromagnesian
Silicate minerals.
Dark spots in granite are biotite
mica. White and pink minerals
Are feldspars.
Fig. 3-13, p.73
Fig. 3-15, p.75
Fig. 3-16, p.75
Plymouth Rock:
granodiorite
p.76
Igneous Intrusive Bodies – “Plutons”
Pluton – an intrusive igneous body that cools and crystallizes deep
within the earth’s crust.
The geometric shape of plutons may be:massive or irregular
Tabular, cylindrical, mushroom shaped
Plutons are also described as to whether they are concordant or
dis-concordant.
Concordant pluton - has boundaries parallel to the to the layering
of the country rock (the surrounding rock)
Disconcordant pluton – has boundaries that cut across the layering
of the country rock.
p.76
Pluton Types:
I.
Dikes – discordant intrusive bodies usually emplaced in pre-existing fractures cutting across
the country rock as the magma rises.
Characteristics:
They are discordant cutting across the layering of the country rock in zones of weakness, such
as cracks.
Most are 1 to 2 meters wide, can range from a few centimeters to more than 100 meters thick.
They form whenever magma is forced into pre-existing fractures of the country rock, or when the
fluid pressure in the dike itself creates its own fractures.
Many can form “wall-like” structures radiating outward from volcanoes like the spokes on a wheel.
II.
Sills – concordant intrusive bodies that are sheet-like emplaced between layers of the
country rock.
Characteristics:
Sills are concordant emplaced whenever fluid pressure is so great that it lifts the overlying rocks,
filling in with magma in a horizontal manner.
They are tabular or disk-like in shape with many usually a meter or less thick. Some are much
thicker, up to 300 meters or more (i.e. the Palisades of NY and New Jersey)
Most have intruded into sedimentary rock, but many are also commonly found injected into piles
of volcanic rock.
Sill inflation prior to a volcano erupting may account for volcanoes swelling just before exploding.
p.76
Shiprock, in New Mexico:
a. Volcanic neck
b. Dikes intruded laterally
from the volcanic neck, now
exposed at surface due to
erosion.
p.81b
Pegmatite Dike in
Black Hills of So
Dakota (a)
C: Tourmaline
crystals from
Pegmatite in
Maine
Fig. 3-14, p.74
III. Laccoliths – sill-like in that they are concordant, but with a “mushroom shape”.
Characteristics:
Laccoliths are concordant mushroom-shaped intrusive bodies.
They tend to have a flat floor with a domed up center.
Like sills, they lift up the overlying strata of the country rock, but usually being larger than sills,
the overlying strata bends to conform to the curved shape.
They also are relatively shallow intrusions.
IV. Volcanic Pipes and Necks – discordant cylindrical conduits of volcanoes.
Characteristics:
A volcanic pipe is the term for the actual conduit of magma upward from the magma chamber
deep below.
Through this structure magma rises to the surface.
When a volcano ceases to erupt, surface processes begin to erode the cone while the once molten
volcanic pipe solidifies.
Whenever the solidified volcanic pipe is exposed by erosion, it is termed a volcanic neck.
(i.e. ”Shiprock” in northwestern New Mexico)
Volcanic neck in Le Puy,
France.
Volcanic neck= the pathway
through which magma escapes
from Earth’s interior upwards,
eventually leading to volcanoes
or sills/dikes.
p.81a
Devil’s Tower: in Wyoming
a. Maybe a volcanic neck
b. or eroded laccolith
Vertical ‘lines’ are
fractures, described
as columnar
joints.
p.81c
V. Batholiths and Stocks – are the largest of all plutons.
Characteristics:
These are very large intrusions created by repeated, forceful injections
and voluminous intrusions of magma in the same area. Many times
these intrusions continue for millions upon millions of years (i.e. the
coastal batholiths of Peru took about 60 million years; and the Llano Uplif
of Central Texas).
To be called a batholith, the body must be greater than 100 km2 of total
surface area.
A stock is similar in formation but has a surface area less than 100 km2.
Some stocks are simply parts of large plutons that once exposed by erosion
are batholiths (the tip of the iceberg”)
Most are granite in composition, but some may be diorite. (mostly
sialic magmas, with someintermediate magmas)
Most are formed near continental margins during episodes of mountain
building or great uplift (during an orogeny or tectonic activity).
Fig. 3-18, p.82
Batholiths and Stocks – are the largest of all plutons.
Characteristics:
V.
As the solutions at the tops of the intrusions penetrate cracks in the
overlying strata, concentrations of minerals dissolved in the solutions may
become concentrated. (i.e. gold, copper, silver)
Granitization – the process whereby the surrounding country rock is
transformed into granite in a severe form of metamorphism. This may
account for the great amounts of granite formed in some batholiths that
shows a gradation from granite into some other rock at the borders.
Some batholiths show a direct igneous origin of its granite since its borde
are “sharp” at the transition from granite to country rock.
The presence of inclusions of country rock especially at the top of
batholiths indicates that it was igneous in origin.
Batholith emplacement through time: parts of host rock, termed
‘country rock’ are detached and engulfed by magma as they
Intrude into the host rock.
Fig. 3-19, p.83
Berkeley Pit Mine in Butte, Montana: nearly 1.5 billion tons
of copper have been removed from rocks adjacent to the
Boulder batholith.
Fig. 3-17, p.82
Classification of Igneous Rocks
Fig. 3-9, p.71
Chapter 3 Summary: Igneous Rocks
a.
Magma – source of igneous rocks
-3 things effect magma: pressure, temperature, water
b.
Textures of igneous rocks: function of cooling and composition; 5 types
1. aphanitic
2. phaneritic
3. porphyritic
4. glassy
5. pyroclastic
c.
Intrusive igneous bodies are known as ‘Plutons’
1. Dikes
2. Sills
3. Laccoliths
4. Volcanic necks
5. Batholiths and stocks
Fig. 3-4, p.66
Ultramafic= Peridotite
Fig. 3-10, p.71
Intermediate: Andesite
Fig. 3-12a, p.72
Intermediate: Diorite (note salt and pepper appearance)
Fig. 3-12b, p.72