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GEOLOGY NOTES
PETROLOGY
INTRODUCTION
Petrology is the branch of geology that studies rocks, and the conditions in which rocks form
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Petrology utilizes the classical fields of mineralogy, petrography, optical mineralogy, and
chemical analyses to describe the composition and texture of rocks
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There are three branches of petrology, corresponding to the three types of rocks: igneous,
metamorphic, and sedimentary, and dealing with experimental techniques:
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Igneous petrology focuses on the composition and texture of igneous rocks (rocks such as
granite or basalt which have crystallized from molten rock or magma)
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Sedimentary petrology focuses on the composition and texture of sedimentary rocks
(rocks such as sandstone, shale, or limestone which consist of pieces or particles derived
from other rocks or biological or chemical deposits, and are usually bound together in a
matrix of finer material)
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Experiments are particularly useful for investigating
rocks of the lower crust and upper mantle that rarely survive the journey to the surface in
pristine condition
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IGNEOUS ROCKS
Igneous rocks begin as hot, fluid material, and the word "igneous" comes from the Latin for fire
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Rock formed of lava is called extrusive, rock
from shallow magma is called intrusive and rock from deep magma is called plutonic
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People commonly think of lava and magma as a liquid, like molten metal, but geologists find that
magma is usually a mush — a liquid carrying a load of mineral crystals
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Not just that, but when they
crystallize, they leave the remaining liquid with a changed chemical composition
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This makes igneous petrology a very complex field
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Shallow generally refers to depths less than about 1 km
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Hypabyssal igneous rocks are formed at a depth in between the plutonic and volcanic rocks
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Hypabyssal rocks are less common than plutonic or volcanic rocks and often
form dikes, sills, laccoliths, lopoliths , or phacoliths
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Discordant means that they cut
across pre-existing structures
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Sills are also small (<50 m thick)
shallow intrusions that show a
concordant relationship with the
rocks that they intrude
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Laccoliths are somewhat large
intrusions that result in an uplift
and folding of the pre-existing
rocks above the intrusion
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Plutons
Plutons are generally much larger intrusive bodies that have intruded much deeper in the
crust
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Lopoliths are relatively small plutons that
usually show a concave downward upper surface
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Batholiths are very large intrusive bodies, usually so large that their bottoms are rarely exposed
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Stocks are smaller bodies that are likely to be fed from deeper level batholiths
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Phacolith
A phacolith is a pluton parallel to the bedding plane or foliation of folded
country rock
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In rare cases the body
may extend as a sill from the crest of an anticline through the trough of an adjacent
syncline, such that in cross section it has an S shape
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PICTORIAL VIEW OF THE VARIOUS FEATURES

STRUCTURES
Versicular Structure
A structure that is common in many volcanic rocks and which forms when magma is brought to
or near the earth's surface
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Amygdaloidal Structure
Igneous Rocks can be of two types extrusive and intrusive
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As the gases stored inside the magma have no time to escape out they form voids inside the rock
mass
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Most columns are straight with parallel sides and diameters from a few centimetres
to 3 m
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Columns can reach heights of 30 m
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Most columns tend to have 5 or 6 sides but have as few as 3 and as many as 7 sides
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Pillow structures
These are aggregates of ovoid masses, resembling pillows or grain-filled sacks in size and shape
that occur in many basic volcanic rocks
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The interiors ordinarily are coarsergrained and less vesicular
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As additional lava is fed into each bud, it grows into a
pillow and continues to enlarge
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The rate of nucleation of new crystals - the rate at which enough of the chemical
constituents of a crystal can come together in one place without dissolving
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This depends largely on the diffusion rate of the
molecules of concern
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Once
a nucleus forms, the chemical constituents must diffuse through the liquid to arrive at the
surface of the growing crystal
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All of these rates are strongly dependent on the temperature of the system
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Shown below are hypothetical nucleation and growth rate curves based
on experiments in simple systems
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Three cases are
shown
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A few crystals
will form and grow at a moderate rate
until they run into each other
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This would be called a phaneritic
texture
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This will result in many crystals all growing rapidly, but because there are so many
crystals, they will run into each other before they have time to grow and the resulting texture
will be a fine grained texture
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At high degrees of under cooling, both the growth rate and nucleation rate will be low
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The resulting texture will be

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glassy, with a few tiny crystals called microlites
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Two stages of cooling, i
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slow cooling to grow a
few large crystals, followed by rapid cooling to
grow many smaller crystals could result in a
porphyritic texture, a texture with two or more
distinct sizes of grains
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In a porphyritic
texture, the larger grains are called phenocrysts and
the material surrounding the the phenocrysts is
called groundmass or matrix
<1 mm
In a rock with a phaneritic texture, where all grains are
about the same size, we use the grain size ranges shown 1 - 5 mm
to the right to describe the texture:
5 - 3 cm

fine grained
medium grained
coarse grained

> 3 cm

In a rock with a porphyritic texture, we use
the above table to define the grain size of the
groundmass or matrix, and this table to
describe the phenocrysts:

very coarse grained

0
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3 mm

Microphenocrysts

0
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Fabric refers to the mutual relationship between the grains
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If most of the grains are euhedral - that is they are bounded by well-formed crystal
face

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