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3-1
Igneous Rocks and Processes
Classification of Igneous Rocks
If a rock is subjected to a high enough temperature it
will melt.
Igneous rocks are classified on the basis of two
characteristics:
Molten rock beneath the surface is called magma.
1. Composition
2. Texture
Magma which reaches the surface is called lava.
As this material cools, mineral crystals will form.
As this material completely solidifies the solid
formed from the crystals will be an igneous rock.
1. Composition
Magma’s on Earth typically contain the most
abundant elements on Earth: O, Si, Al, Fe...
However, they do not all have the exact same
composition and thus they form igneous rocks of
different compositions.
Rocks which are relatively poor in silica (45% –
55% silica content) are called mafic (derived from
magnesium and ferrum for iron).
Those which are very poor in silica (<40%) are
called ultramafic.
Rocks rich in silica (>65%) are called felsic.
Note that the terms mafic and felsic can also be used
to refer to the composition of a magma.
2. Texture
Texture refers to the size, shape, and orientation of
the mineral grains within an igneous rock.
These depend mostly on the rate of cooling.
The two main categories are:
1. Coarse Grained Rocks (phaneritic):
The crystals can be clearly seen by the
naked eye (> 1mm in size).
What does this suggest about the rate of
cooling and where they formed?
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2. Fine Grained Rocks (aphanitic):
Igneous Rocks:
What does this suggest about the rate of
cooling and where they formed?
While most igneous rocks are phaneritic or aphanitic
there are other classifications:
Coarse Grained Fine Grained
felsic
Granite
Rhyolite
light
colored
intermediate
Diorite
Andesite
medium
grey or
green
mafic
Gabbro
Basalt
dark
grey to
black
ultramafic
Peridotite
Komatiite
dark
green to
black
4. Porphyritic—
Some igneous rocks contain crystals of
widely differing sizes. Some large, some
fine grained.
3. Glassy—
These are rocks which cooled so quickly
they did not have time to form crystals.
How might such a rock form?
Obsidian
Pumice
What does this suggest about the
formation environment for these rocks?
5. Pyroclastic—Tephra: volcanic dust, ash,
cinders,
bombs
Fragments of rocks which have been
blasted apart in an explosive volcanic
eruption. Either preexisting rock or lava
which cools rapidly in various shapes
and sizes.
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Creation of Magma
2. Pressure
What processes cause rock to melt?
Most materials melt at a higher temperature as the
pressure is increased.
1. Heat
Conversely, if pressure is released, the melting point
will be lowered and may cause melting.
Raise the temperature enough and rock will melt.
Radioactive elements within the Earth release energy
in the planet’s interior:
This factor is particularly important at the midoceanic ridge:
==> The temperature increases with depth into the
Earth’s surface.
Some radioactive elements are enhanced in the crust
(uranium, potassium) leading to a great deal of
heating in the lower crust.
This heat can lead to melting at depths of ~250km.
3. Water
The introduction of water generally lowers the
melting point of rocks.
This is one of the processes which creates magma at
subduction zones:
Bowen’s Reaction Series
Magma (or lava) eventually cools down and
solidifies.
Exactly how does this occur? What minerals form?
Would you expect all the material in a melt to
solidify at the same time?
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Problem:
What would happen to the composition of the
magma if this occurred?
Most magmas are silica poor (mafic). How can silica
rich rocks be generated from them?
While under ideal conditions the entire sequence
may occur, often it does not.
As crystals form they may not remain with the
magma.
If they are denser than the magma they may settle
out of the magma.
This process is called fractional crystallization or
differentiation.
Or they may stick to the walls of the magma
chamber.
Or even be physically filtered out as the magma
continues to flow towards the surface.
What would happen if one were to reverse the
process—heating and melting a rock?
Intrusive Igneous Structures
While volcanoes may be more spectacular, most
igneous rocks are formed underground.
These are called intrusive rocks because they intrude
into the preexisting rock.
The preexisting rock into which the intrusive rocks
are emplaced is called country rock.
As noted before, rocks buried underground tend to
cool slowly ==> coarse grained rocks.
However, this is not always the case.
What types of structures do they form?
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Shallow Igneous Structures:
If it follows the layering found in the country rock it
is said to be concordant and is called a sill.
Tabular Structures
If the intrusive rock does not follow the preexisting
layering—it cuts across them—it is said to be
discordant.
In some cases we see igneous rocks which have
squeezed between other rock layers (or perhaps
forced their way between them).
These often form features which are long and wide
but not very thick.
Tabular igneous structures are given different names
depending on their orientation with respect to the
country rock.
These features are called dikes.
Suppose we had a lava flow over preexisting rock.
Lava flow was then buried under further layers.
What would it look like?
How can we distinguish between a sill and a volcanic
flow which was later buried?
3. Look for evidence of heating of the surrounding
rock—if seen in rock atop an igneous formation this
would suggest what type of feature?
There are several characteristics which can help
distinguish these cases:
1. Vesicular structure—lavas which flow on the
surface tend to have a frothy appearance on its top
layer caused by escaping gases.
2. Xenoliths—xenoliths are foreign rock
incorporated in the pluton.
Sometimes xenoliths of the overlying rock are
present within the igneous layer.
If so would this suggest an intrusive or volcanic
feature?
4. Signs of weathering—Lava flow would be
exposed at the surface for some time and thus the
top edge might show signs of weathering.
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Tabular features are thought to be formed relatively
near the Earth’s surface.
Dikes are often the result of magma solidifying
within the conduits of a volcano.
Sills tend to form by intruding in spaces between
layers of rocks.
Deep Intrusive Structures
Sills, dikes, laccoliths, and lopoliths are all features
seen near the Earth’s surface (within a few
kilometers).
At too large a depth any cracks in the layers are
closed by the high pressure of the overlying material.
Sometimes igneous material can force its way into
the country rock causing it to bulge upwards.
However, other structures are seen.
Creates a dome shaped feature called a laccolith.
If the intruding magma is heavier than the
surrounding rock its weight may push down on the
underlying rock.
This will yield an upside down dome shaped
structure called a lopolith.
These are called plutons.
These underground structures can be enormous
(1000s of km).
Vast plutonic structures are called batholiths.
Deep intrusive structures are almost always felsic in
composition.
Smaller features exposed at the surface and covering
areas less than 100 square kilometers are called
stocks.
Most are granite.
Though some are diorite.
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Volcanoes
Basaltic Volcanoes
Volcanic eruptions can be quite spectacular:
The main factor which determines a volcano’s
eruptive style is the composition of the lava it erupts.
Mt. St. Helens
The island of Krakatau literally blew itself up.
However, some volcanoes erupt in a quite peaceful
manner, quietly flowing lava out onto the surface.
Mafic (low silica content) magmas are much less
viscous than felsic (high silica content) magmas.
Thus it is relatively easy for gases in such magmas
to escape.
Thus are large pressures likely to develop?
Clearly there are a variety of types of volcanoes.
Why do they differ in form?
Are they likely to be explosive?
Why are some explosive and others not?
Basalt volcanoes are the most common.
Seen in two main geologic settings:
1. Mid-oceanic ridge
Volcanism at Hot Spots
In addition to the mid-ocean ridges, basaltic
volcanoes are seen at intra-plate volcanoes overlying
hot spots.
The Hawaiian Islands are the classic example.
==> Ocean floor almost entirely made of basalt and
gabbro.
2. “Hot Spots”
Because the lava generated is of low viscosity, would
you expect the lava to be able to flow over relatively
large or small distances?
Would you expect such volcanoes to be gently or
steeply sloped?
These are called shield volcanoes.
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Types of Lava
Underwater we often see pillow basalt.
Much of our knowledge of basaltic lava flows come
from the Hawaiian islands.
The water quickly cools the lava as it emerges.
Thus, our names for types of lava come from
Polynesian roots.
Outside layers solidify instantly.
pahoehoe—“ropy” lava.
Lava then continues to ooze out within the solidified
skin.
Low viscosity and thus flows quickly yielding a ropy
appearance.
Typically seen near the eruption site.
aa—
Looks much as if someone were squeezing
toothpaste out of its container.
Looks much like pillows stacked on top of each
other.
Higher viscosity (cooler) lava. Moves more slowly
and has a more blocky jagged look to it.
Andesitic Volcanoes
How do these volcanoes differ from those erupting
more mafic material?
Volcanoes which occur at subduction zone
boundaries tend to exhibit a more felsic composition.
The higher silica content makes the lava more
viscous.
Why?
Will lava flow as far?
As with more mafic volcanism, it is thought that the
magma is created from melted mantle.
What about the slope of the volcano?
What makes it more felsic?
Finally how would you expect the eruptive style to
differ from more mafic volcanoes?
3-9
Some volcanoes are on the border between being
explosive or effusing lava quietly.
Rhyolite Volcanoes
They may do both at various times.
Very felsic volcanoes are rare.
These types of volcanoes create composite cones or
stratovolcanoes.
However, when they do occur the lava is very viscous
and thus they tend to be quite explosive.
Often form volcanic domes.
Both basaltic and andesitic volcanoes may also form
pyroclastic cones or cinder cones.
Why are they rare?
These types of volcanoes tend to be small.
The unconsolidated pyroclastic material also tends to
be eroded quickly.