Transcript 9 Igneous rocks.ppsx

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PETROLOGY
•Is a branch of geology, which deals with study of rocks (Petro=rock, Logos=study)
ROCKPETROLOGYS
IGNEOUS
-most abundant
-primary rocks
-source is magma or
lava
SEDIMENTARY
-thin veneer above the Sial
and Sima in Oceanic
and Continental
Crusts
-secondary rocks
METAMORPHIC
-proportion is similar to
that of Igneous rocks
-change of forms of
Igneous and
Sedimentary due to
Temperature, Pressure
and Chemical Fluids
Here is another version of the Rock Cycle
Importance of petrology in civil Engg.
• It provides an opportunity to interpret the physical properties of
individual rocks, likewise: texture, structure, mineral composition,
chemical composition etc.
• This helps in knowing the strength, durability, colour, appearance,
workability etc.
• These properties are very important for CE to know because
different rocks are suitable for different purposes and no rock is
ideal or best suited for all purpose.
•Granite: hard, competent, durable = suitable for foundation
Limestone: comparatively soft= best for flooring
Marble: soft and attractive= flooring/sculpturing etc.
Sandstone: sculpturing, wall etc.
Igneous Rocks
IGNEOUS ROCKS: The rocks formed through volcanic activity
OR
The rocks which are derived from a molten
mass “magma or lava”
SOURCE OF IGNEOUS ROCKS:
Magma:
Molten mass comprising most abundant elements in
earth – Si, Al, Fe, Ca, Mg, K, H & O. Where the SiO2
is most abundant among all.
Temperature of Magma- 10400 to 12000 C
FLUIDITY OF MAGMA
Fluidity or Viscosity of magma depends on content (%) of Silica
Silica Rich
-known as Acidic magma
-More viscous, so do not spreads
and piles up at one place
Silica poor
-Known as Basic magma
-Less viscous, moves faster
and occupies larger area
However, the viscosity of magma is considerably influenced by
temperature too. When temperature is less-more viscous and when
High temperature- less viscous.
BROAD CLASSIFICATION OF IGNEOUS ROCKS
Volcanic rocks
(Extrusive rocks)
igneous rocks that form
from volcanic activity (at
or near surface). Extrusive
rocks are usually finegrained
-Lava or Magma flows
-Pyroclastic flows
Intermediate rocks
(Hypabyssal rocks)
Igneous rock that form
at an intermediate depth,
Usually consisting of both
Fine and coarse grain
Plutonic rocks
(Intrusive rocks)
igneous rocks that
form from cooling
magma at depth,
Plutonic rocks are
usually coarse-grained
-dykes, sills,
batholiths, laccoliths
etc.
Name comes from
Greek god of the
underworld - Pluto
Igneous Structures
• Intrusive (Plutonic)
– Magma cools slowly at depth
– Characteristic rock texture
– Characteristic structures
• Extrusive (Volcanic)
– Magma cools quickly at surface
– Characteristic rock textures
– Characteristic structures
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These samples represent igneous rocks
which have formed at depth within the
Earth, although each exhibit different
textures.
Structures of Igneous Rocks
Structures due to mobility of lava
Flow structures: are defined by the development of parallel layers or bands in the
body of an igneous rock. Sometimes the already crystallised particles within the
magma are arranged parallel to the direction of flow of the lava. They naturally
indicate the direction of flowing of the mass, prior to its consolidation. These are
also known as directional structure or more commonly flow structure.
Pillow structures:
Fluid lava erupted under water may form a special structure called pillow
lava. Such structures form when molten lava breaks through the thin walls
of underwater tubes, squeezes out like toothpaste, and quickly solidifies as
irregular, tongue-like protrusions. This process is repeated countless times,
and the resulting protrusions stack one upon another as the lava flow
advances underwater. Generally low viscosity, low gas content effusive lava
flows (basalt) at the divergent margin (mid-ocean ridge) settings produces
rounded “pillows” of lava with glassy outer rim, modern examples:Loihi,
Hawaii.
Ropy Lava: is more
mobile i.e. less viscous, it
has a wrinkled but smooth
and shiny surface on
cooling.
Pahoehoe
(pronounced
"pah-hoyhoy" - a Hawaiian term), is
a very fluid lava flow, that
in solidified form, is
characterized by a smooth,
ropy surfaces.
Vesicles are more in
number, small-spherical in
shape.
Block Lava: is less mobile
i.e. more viscous and has a
Rough and irregular surface.
Vesicles are few and
irregular in shape. Aa
(pronounced "ah-ah" - a
Hawaiian term), is lava that
has a rough, jagged, spiny,
and
generally
clinkery
surface. In thick aa flows,
the rubble surface of loose
clinkers and blocks hides a
massive, relatively dense
interior.
Spherulitic structure:
Its essential feature is
simultaneous crystallization
of fibres with radiating
arrangement about a
common centre. The large
spherulites are known as
'Lithophyse‘.
Orbicular structure:
These
are
spherical
segregations consisting
of concentric shells of
different
mineral
composition and texture,
which occasionally occurs
in granitic rocks.
Structures due to
cooling
Mural Joints
These are due to jointing. In
granites, three mutually
perpendicular, equally spaced
joints, which are taken into
advantage while producing
cubical blocks, are known as
'mural jointing. But for
processing of the blocks
down to smaller dimensions,
the mutually perpendicular
closely spaced joints (one
horizontal and the other
vertical) are taken into
advantages. These joints are
known as Mural joints.
Columnar structure:
As a consequence of
contraction
due
to
cooling, a few sets of
vertical joints develop
similar
to
those
developing in mud. Such
cracks continue depth
wise for considerable
depth and bring about
the
formation
of
columns, which may be
square,
rectangular,
rhombic or hexagonal in
outline.
• When the lava cools, it contracts. This is
because hot things generally take up
more space than cool things. When
objects contract, they often crack or
fracture. When contraction occurs at
centers which are equally spaced (see the
diagram), then a hexagonal fracture
pattern will develop. If the contraction is
not evenly spaced, then other
geometries of fractures, such as 5-sided
or 7-sided fractures, may occur.
Contraction may not be equally spaced if,
for
example,
the
thickness
or
composition of the lava flow varies. The
fracture pattern that forms at the cooling
surface will tend to be propagated down
the lava as it cools, forming long,
geometric columns. Thus, as lava cools to
form basalt, it may crack in a hexagonal
(or other) shape and form columns.
Vesicular
and
amygdaloidal
structures: When lavas heavily
charged with gases and other
volatiles are erupted on the surface,
the gaseous constituent's escapes
from the magma as there is a
decrease in the pressure. Thus,
near the top of flows, empty
cavities of variable dimensions are
formed. The individual openings are
known as vesicles and the structure
as a whole is known as vesicular
structure.
Amygdaloidal means having a
cellular or vesicular structure, in
which spherical or almond-shaped
cavities were formed by the
expansion of steam contained in
the rocks at the time of its
consolidation, and which have later
become filled with various minerals,
especially quartz, calcite, or the
zeolites. The rock having this
character is called an amygdaloid.
Reaction rims:
When the reaction between an
already crystallized mineral and
the rest of the magma is
incomplete, the corroded crystals
are found surrounded by the
products of reaction i.e., some new
mineral. Such zones are known as
reaction-rim.
Xenolithic structure:
Occurrence of foreign rock
fragments within an igneous rock
gives rise to xenolithic structure.
The xenoliths are said to be
'cognate' when they are genetically
related to enclosing rocks and
'accidental', when they are
fragments of country- rocks
without having any genetic relation
with the enclosing rock.
Large grey xenoliths in granite, Ingonish, Nova
Scotia
Texture of Igneous Rocks
Texture is defined as the mutual relationship of
 different mineralogical constituents. It describes the
appearance of an igneous rock, based on the size,
shape and arrangement of interlocking crystals.
 gives clues to the type of environment in which the
molten rock crystallized.
Five common textures in igneous rocks include;
1) Coarse Grain (Phaneritic)
4) Glassy
2) Fine Grain (Aphanitic)
5) Vesicular
3) Porphyritic
Igneous Rock Textures
Coarse-grained (Phaneric)
Glassy
Fine-grained (aphinitic)
Vesicular
Porphyritic
Pyroclastic
Factors explaining Textures
Granularity
• In a rock with a phaneritic texture, where all grains are about
the same size, we use the grain size ranges to describe the
texture:
• Fine grained: <1 mm
• Medium grained: 1 - 5 mm
• Coarse grained: 3-5 cm coarse grained, > 5 cm very coarse
grained
Degree of crystallization
• Holocrystalline: all the constituent minerals are distinctly
crystallized
• Holohyalline: all the constituents are very fine in size and
glassy.
• Fabric
• Fabric: Another aspect of texture, particularly in medium to coarse grained
rocks is referred to as fabric. Fabric refers to the mutual relationship
between the grains. Three types of fabric are commonly referred to:
• Idiomorphic: If most of the grains are euhedral - that is they are bounded by
well-formed crystal faces. The fabric is said to be idiomorphic granular.
Hypidiomorphic : If most of the grains are subhedral - that is they are
bounded by only a few well-formed crystal faces, the fabric is said to be
hypediomorphic.
• Allotriomorphic: If most of the grains are anhedral - that is they are
generally not bounded by crystal faces, the fabric is said to be
allotriomorphic.
Types of Textures
• Equigranular texture
• Inequigranular texture
• Intergranular texture
• Directive texture
• Intergrowth texture
Types of Textures
• Equigranular texture : A texture in which majority of constituents
crystals of a rock are broadly equal in size. These textures are
commonly shown by granite and felsites and often named as
Granitic and felsitic texture.
• Inequigranular texture: A texture in which majority of constituent
minerals show a marked difference in their relative grain size.
• Porphyritic Texture and Poikilitic texture are examples of
Inequigranular texture .
• Porphyritic Texture
• this texture results when magma with crystals already formed
escape to the surface and cools quickly forming a fine grained
igneous rock with large crystals inside.
Porphyritic Texture
Poiklitic
Texture:
This
texture
is
characterized by the presence of fine
grained crystals within the body of larger
size crystals. It is just reverse of the
porphyritic texture. smaller grains of one
mineral are completely enclosed in large,
optically continuous grains of another
mineral.
A poiklitic texture in which the host mineral
is augite and the inclusions are of
plagioclase feldspar, the poiklitic texture is
further distinguished as Ophitic texture.
Intergrowth texture: Sometimes during crystallization of igneous
rocks two or more mineral may crystallize simultaneously in a
limited space so that the resulting crystals are mixed up or
intergrown. Graphic texture:- where intergrowth is most
conspicuous between quartz and feldspar is known as Graphic
texture.
Intergranular texture: a texture in which the angular
interstices between plagioclase grains are occupied by
grains of ferromagnesium minerals such as olivine,
pyroxene, or iron titanium oxides.
Directive Texture
Those textures that indicate the result of flow of
magma during the formation of rocks are known as
directive textures. These exhibit perfect or semiperfect parallelism of crystals. Trachytic or
trachytoid textures are examples of this texture
Forms of Igneous Rocks
• Intrusive
• Concordant
– Sill
– Lopolith: dish-shaped layered
– intrusive
– Laccolith:
– Phacoloiths
Discordant
Dike
Batholith >100 km2 in map
area
Volcanic necks
Stock <100 km2 in map area
• Extrusive
–
–
–
–
–
Lava flow or plateau
Volcano (many types)
Crater
Caldera
Fissure
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Intrusive Igneous Structures
• Contacts (boundary
between two rock bodies)
can be:
–Concordant
•Does not cross cut country rock
(surrounding rock) structure, bedding,
or metamorphic fabric
• Ex: laccolith, sill
–Discordant
• Cross cuts country rock structure
•Ex: dike, batholith, stock
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Concordant Igneous Body
Sill: A "sill" is a sheet like
igneous body which runs
parallel to the bedding planes
of the pre-existing strata. They
may be horizontal, inclined or
vertical depending upon the
attitude of strata in which they
are intruded. Sills vary in
thickness
from
a
few
centimeters
to
several
hundred meters but they are
always thin as compared to
their length along beds. Sills
are commonly made up of
dolerites and basalts.
Bamburgh Castle, Northumbria, stands on a
sill of dolerite called the Great Whin Sill
dolerite
Carboniferous sandstone
Phacolith
Phacoliths are crescent
shaped bodies of
igneous rocks. They
occupy crests and
troughs of folded strata.
Phacolith are formed
when igneous material
invades the folded
region. The igneous
material accumulates at
the crests and troughs of
folds because these are
the zones of minimum
stress.
Concordant Igneous Body
Lopolith: A "lapolith" is a saucer shaped
concordant igneous body which is bent
downward into a basin like shape . Its
diameter is usually 10 to 20 times its
thickness. Thus lapoliths are very much
larger than laccoliths. The composition of
lapoliths is commonly basic.
Laccolith: A "laccolith" is a lens shaped
intrusive igneous body which causes the
overlying beds to arch in the form of a
dome. It has a flat base and domed top.
A laccolith may be 2 to 3 kilometers in
diameter and several hundred meters in
thickness. It differs from a batholith in
being much smaller and having a known
floor.
Dyke: A "dyke" is a wall like igneous body that cuts
across the strata of the pre-exsisting rocks. Dykes
are often vertical or steeply inclined. Their thickness
varies from a few centimeters to a hundred meter or
more. Dykes tend to occur in groups where they run
parallel to one direction or are radial to a centre. A
dyke having a circular outcrop and a conical form is
called a "ring dyke". Those which have inverted
conical form and circular outcrops are described as
"cone-sheets". Dykes probably represent a crustal
fracture into which the magma was injected.
Volcanic PIug: A volcanic plug is a vertical
cylindrically shaped igneous body which has a
roughly oval or circular cross-section . It represents
the vent of an extinct volcano.
Discordant Igneous Body
Dyke in S. Arran
cutting through
red sandstone.
Chilled margin in dyke,
probably basalt.
Baked margin in sandstones
0.75m
Dolerite dyke
Some dykes weather faster than the country rock
around.
Red sandstone
baked margin
dolerite
Corrie Shore, Arran
Describe the
baked margin
and say how it
has been altered
by the intrusion
of the dolerite
dyke.
This dyke in Tenerife cuts
across the country rocks
which are basalt lava flows.
Discordant Igneous Body
Batholith: Batholiths are large
intrusive igneous bodies which
have transgressive relation with the
adjacent country rocks. Their
diameter is usually 100 km or more
and their outcrop at the surface is
roughly circular or oval. In crosssection batholiths possess steep
outwardly dipping contacts and they
are thought to be bottomless. The
composition of batholiths is usually
granitic or granodioritic.
Stock and Boss: Irregular igneous
masses of batholithic habit are
called'‘ stocks". They are of smaller
size and their diameter is usually
between l0 to 20 kilometers. The
term "boss" is applied to those
stocks
which
have
an
approximately circular outcrop.
How do we tell where the igneous rocks formed?
What can we derive from the rocks about the conditions of
formation?
Pyroclastics
Lava
flow
Volcanic
neck
Igneous
dike
Igneous
Sill
Batholith
Sierra Nevada BatholithHome to Yosemite
National Park and a lot of
Granites and Granodiorites
Sierra Nevada Batholith
Areal extent of the Idaho
Batholith- a huge (15,400
square miles) intrusive body
of primarily felsic
composition (granites)
igneous rocks
Classification of Igneous Rocks
• Igneous rocks are classified on the basis of:
• (a)Composition of the magma: Magmas are
divided into two broad groups:
• (i) acid magma.
• (ii) basic magma.
• The "acid magma" is rich in Si, Na and K, and poor in
Ca, Mg and Fe.
• The "basic magma" on the other hand, is rich in Ca,
Mg and Fe, and poor in Si, Na and K.
Classification based on silica percentage
• On the basis of silica prcentage igneous rocks are classified into the
following groups.
• (i) Ultrabasic Rocks: These contain less than 45% silica, also known as
Undersaturated rocks. e.g. Peridotite.
• (ii) Basic Rocks: These contain silica between 45% and 55%, also known as
Saturated rocks. e.g. gabbro and basalt.
• (iii) Intermediate Rocks: These contain silica between 55% and 66%,
e.g. diorite.
• (iv) Acid Rocks: These contain more than 66% silica, also known as
Oversaturated Rocks. e.g. Granite.
Acid igneous rocks
• In general, acid igneous rocks
are light in colour.
• low in specific gravity (about
2.7)
and
have
high
proportion of minerals like
quartz and alkali felspars.
• Acid rocks are also called the
"felsic rocks". An example of
acid rock is granite
Basic rocks
• Basic rocks, on the other
hand, are usually dark in
colour (often black).
• Relatively high in specific
gravity (about 3.2).
• Contain mainly silica poor
minerals, such as olivine,
pyroxene, hornblende or
biotite and little or no quartz.
• Basic rocks are also called
"mafic rocks" as they contain
a high percentage of
ferromagnesian minerals.
• An example of basic rock is
basalt.
Feldspar
•
•
•
•
•
Feldspars (KAlSi3O8 – NaAlSi3O8 – CaAl2Si2O8) are a group of rock-forming minerals which make up
as much as 60% of the Earth’s crust.
Compositions of major elements in common feldspars can be expressed in terms of three end
members.
Potassium Feldspar (K- feldspar) end member KAlSi3O8
Albite end member NaAlSi3O8
Anorthite end member Ca Al2Si2O8
•
Solid solutions between K-feldspar and albite are called alkali-feldspar. Solid solutions between
albite and anorthite are called Plagioclase, or more properly plagioclase feldspar.
•
•
•
•
•
•
•
Alkali feldspars
The alkali feldsparare as follows
Orthoclase (monoclinic)- KAlSi3O8
Sanidine (monoclinic)- (K, Na)AlSi3O8
Microcline (triclinic)— KAlSi3O8
Anorthoclase (triclinic) — (Na,K)AlSi3O8
Sanidine is stable at the highest temperatures, and microcline at the lowest.
• Plagioclase feldspars
• The plagioclase feldspars are triclinic. The plagioclase
series follows (with percent anorthite in parentheses):
• Albite (0 to 10) — NaAlSi3O8
• Oligoclase (10 to 30) — (Na,Ca)(Al,Si)AlSi2O8
• Andesine (30 to 50) — NaAlSi3O8 — CaAl2Si2O8
• Labradorite (50 to 70) — (Ca,Na)Al(Al,Si)Si2O8
• Bytownite (70 to 90) — (NaSi,CaAl)AlSi2O8
• Anorthite (90 to 100) — CaAl2Si2O8
Classification of Igneous Rocks
• Many schemes have been proposed for the classification of igneous
rocks but the most useful for the beginners is based on mineralogy
and texture. In this scheme the various criteria that are considered
in classifying igneous rocks are as follows:
• 1) Relative Silica content: presence of quartz in an igneous rock
indicates excess of silica whereas felspathoids indicate deficiency of
it.
• (2) Kinds of Felspar: Determination of relative amount of
alkalifelspar and plagioclase helps greatly in classifying igneous
rocks.
• (3) Mafic Minerals: The relative amount and type of mafic minerals
present in an igneous rock are determined. This information is
valuable from the point of view of classification of igneous rocks.
• (4) Texture: Texture of an igneous rock is an important criterion of
classification.
Tabular Classification of Igneous Rocks
Granite: Quartz, feldspar, mica,
plagioclase, Phaneritic texture
Rhyolite: Quartz, feldspar, mica,
plagioclase, Aphanitic texture
Diorite: Quartz, plagioclase,
amphibole, pyroxene, mica
Phaneritic texture
Andesite: Quartz, plagioclase,
amphibole, pyroxene, mica
Aphanitic texture
Gabbro: Plagioclase, amphibole,
pyroxene, olivine
Phaneritic texture
Basalt; Plagioclase,
amphibole, pyroxene, olivine
Aphanitic texture