Earth’s Building Blocks 1:
Igneous Rocks
Atoms/
elements
As stated in the last couple lectures,
atoms of elements bond together to
form minerals.
minerals
Solid aggregates of minerals (and
some other things) are what we call
ROCKS.”
rocks
Just as minerals are the “building
blocks of rocks” of the geosphere,
rocks are the basic “building blocks”
of the geosphere.
rocks
geosphere
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Basic Classification of Rocks
Rocks are classified into three
main categories (igneous,
sedimentary and
metamorphic) based on the
mode of their origin (i.e. in
terms of the geological
processes that form them).
In addition there are rock
types which combine features
of two of the three main types
(pyroclastic and diagenetic
rocks and migmatites). For
the most part, we will focus on
the three main categories in
this course.
Igneous
Pyroclastic (I-S)
Sedimentary
Diagenetic (S-M)
Metamorphic
Migmatites (M-I)
Igneous Rocks
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How Are Igneous Rocks Formed ?
The term “igneous” is derived from the Latin ignis = “fire”
As indicated by their name, igneous rocks are formed from hot
material.
The hot, molten, parent material of igneous rocks is called magma.
Magma generally is comprised by the following components:
1) Liquid (“melt”) which corresponds to molten, non-volatile mineral
matter.
2) Gases (“volatiles”) of various sorts including water vapour and
carbon dioxide (these stay in the magma at depth due to high
confining pressures).
3) As magma cools it will also begin to contain small mineral
crystals.
Lava is magma that has erupted onto the Earth’s surface.
Where Does Magma Come From ?
Magma is generated by the partial melting of pre-existing rock in
Earth’s crust and mantle.
The source of the heat energy required to melt the rock is derived
largely from the decay of radioactive materials in the core.
Consequently, as a general rule, temperature increases with
increasing depth in Earth’s lithosphere. Pressure also increase
with increasing depth (more material on top, pushing down).
Increased temperature acts to melt rock but increased pressure,
counteracts the melting effect of higher temperatures (atoms in
the magma are pushed closer together, tending to forms solids).
The melting point of a rock will consequently tend to increase with
increasing depth. So how can rocks be melted ?
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One way: Decompression Melting
(allowing atoms to move more freely)
In places where hot, near-liquid,
material of the asthenosphere is
allowed to rise toward Earth’s
surface, the pressure acting on this
material is lowered (lower pressure
closer to surface).
Lithosphere
This material, still very hot (thermally
buoyant), but is under less pressure.
Reduction of pressure allows the
material to be completely molten.
Asthenosphere
This magma-forming mechanism
typically occurs where lithospheric
plates are moving apart (e.g. at mid
ocean ridges or rifts).
Another way: Hydration-Related Melting
(wet rocks melt easier than dry rocks)
In some places on Earth, one
lithospheric plate is pushed under
another.
This process is known as subduction.
Note that the crust underlying the
oceans (oceanic crust) contains lots of
water.
As the plate descends, it is heated, and
releases the water in the form of vapour
(dehydrates). The vapour then hydrates
the rock in the asthenosphere.
Dehydration and metamorphic changes
in the rock at depth lower its melting
point, allowing it to melt into magma.
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What Happens to the Magma ?
volcanoes
volcanic
fissures
Lithosphere
magma
chambers
Asthenosphere
Magma, now a mixture of liquid melt and gas, is less dense (therefore more
buoyant) than the rock from which is was derived and possibly also from the
predominant rock of its surroundings (“country rock”).
As a result, the magma will rise to a point at which its buoyancy is neutralized
(density equalized) or where it can not proceed farther due to an obstruction.
Unless it comes to the surface, magma will collect in large subterranean
pockets known as “magma chambers” or “plutons”. When magma reaches the
Earth’s surface, the development of volcanoes is the usual result.
Crystallization Below the Surface
Now closer to the surface than before, the magma starts to
cool, allowing minerals to crystallize from chemical
components of the melt.
If magma remains trapped in a magma chamber, it cools
very slowly (the magma is completely surrounded by rock
and dissipation of heat from the magma occurs at a very low
rate), allowing the crystals to grow to large sizes.
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Intrusive Igneous Rocks
Rocks that form from the cooling of magma below the
surface are commonly called plutonic igneous rocks (named
after Pluto, Roman god of the underworld).
Because they are formed from magma intruded into preexisting rocks of the crust, they are also called intrusive
igneous rocks.
Depending on the characteristics of the rocks into which
magma is intruded, intrusive bodies can assume different
shapes and sizes
Batholith
Magma can be intruded as big blobs (batholiths, laccoliths) or sheet-like bodies
(vertical dykes and horizontal sills) into pre-existing layered rocks.
The resulting intrusive rock bodies can be exposed at the surface if erosion
removes the overlying “country rock” (the rock into which the bodies were
intruded).
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Batholith: Very large mass of
igneous rock (usually formed at
depth) formed from cooling of one
or more large intrusions.
Dyke: A tabular-shaped igneous
body oriented at a significant angle
to layers of pre-existing rocks.
Sill: A tabular-shaped igneous
body intruded parallel to layers of
pre-existing rocks.
Laccolith: A blister-like igneous
body (usually shallow) that has
produced some upwarping of
overlying layers.
Batholith
Volcanic neck: The remnant of
igneous material that once
occupied the vent of a volcano.
The Aesthetic Beauty of Igneous Intrusions
The awesome landforms in Yosemite Valley, California were naturally
carved by erosion from igneous intrusions that comprise the enormous
Sierra Nevada Batholith. Intrusions were emplaced between 120 and 90
million years ago.
Yosemite
Valley
These landforms were featured prominently in the photographs of Ansel
Adams (1902-1984).
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Photographs of Ansel Adams
“Monolith – The Face of Half Dome,
Yosemite National Park, California,
1927”
“El Capitan, Winter Sunrise
Yosemite National Park, 1968”
Igneous Intrusions and the Group of Seven
The smooth, glacially
scoured and weatherworn exposures of much
older igneous intrusions
(well over 1.5 billion years
old) north of Georgian
Bay deeply inspired the
Group of Seven particularly A.Y. Jackson
and Tom Thomson.
This type of landscape
occurs throughout
Canadian Shield
A.Y. Jackson
“Night, Pine Island”
1924
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Texture of Plutonic/Intrusive Igneous Rocks
Because intrusive igneous rocks cool
below Earth’s surface, mineral
crystals have ample time to grow.
As a result, the mineral crystals of
intrusive igneous rocks are large
enough to be observed with the
unaided eye.
The result is what is termed a
phaneritic texture.
(Greek; phaner = visible, apparent)
Example of phaneritic texture
Plutonic/Intrusive Igneous Rocks: Uses
Phaneritic igneous rocks, particularly granites and granitoids are
commonly used in facing stones of buildings, headstones, expensive
countertops, and stone carvings due to their attractive “salt and
pepper” appearance.
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Extrusive Igneous Rocks
Rocks that form from the cooling of magma above the
surface are commonly called volcanic igneous rocks
(named after Vulcan, Roman god of Fire, Blacksmiths (the
forge) and Craftsmanship (particularly of the weapons of
war)– equivalent to Greek god Hephaestus).
As volcanic rocks are formed from magma that has come
out of the ground (i.e. that has extruded from the ground),
they are also called extrusive igneous rocks.
Depending on the characteristics of the magma, and
conditions at the surface, a number of features may result.
Lava
Lava is extruded from a volcanic vent due to the expansion of
volatiles as the magma approaches the surface. This expansion
is due to the relatively rapid decrease in confining pressure close
to the surface (similar to what happens when you open a pop
bottle that has been shaken).
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Types and Nature of Eruptions
Several types of volcanic eruptions are recognized.
The nature of a lava eruption will depend upon its temperature,
the composition and resultant physical properties of the melt and
volatile content of the magma
(we will discuss this in greater detail later in the term).
Lava can be extruded quietly…
…or violently
Volcanic/Extrusive Igneous Textures: Aphanitic
Extrusive igneous rocks cool above
Earth’s surface and consequently,
mineral crystals have little time to
grow (due to lack of insulation at the
surface, dissipation of heat occurs
rapidly).
As a result, the mineral crystals of
extrusive igneous rocks are
generally too small to be observed
with the unaided eye.
The result in most extrusive igneous
rocks is what is termed a aphanitic
texture.
(Greek; a= not, phaner = visible,
apparent)
andesite
Magnified about 30 x
under crosspolarized light
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Volcanic/Extrusive Igneous Textures: Porphyritic
In some cases, a magma will cool slowly at depth and then
travel rapidly toward the surface where it will cool at a higher
rate.
This two-stage cooling can result in a rock that has aphanitic
matrix, but contains some large crystals called phenocrysts
(apparent crystals).
The resulting texture is called a porphyritic texture.
Igneous rock
showing porphyritic
texture
Volcanic/Extrusive Igneous Textures: Glassy
In some instances, lava solidifies
too quickly for any significant
crystallization to occur.
Unordered ions remain essentially
“frozen” before they can unite to
form crystals.
The result is a glassy texture.
30 x magnification, cross-polarized light
Obsidian is a common rock
exhibiting such a texture.
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Volcanic/Extrusive Igneous Textures: Glassy
Obsidian was a valuable material for making arrow and
spear points and cutting tools due to the sharp edges
produced when knapped (intersecting conchoidal
fractures).
Volcanic/Extrusive Igneous Textures: Vesicular glassy
Pumice is another volcanic glass (the
result of nearly instant cooling). The
macroscopic texture of pumice differs
considerably from obsidian, being
highly porous.
The pores, known as vesicles
represent areas where volatiles (gas
bubbles) were trapped in the original
magma.
Pumice is highly frothy (mostly air)
due to the speed with which it was
erupted and cooled. Most varieties
float on water as a result.
Pumice is commonly used as an
exfoliant. When broken, the glassy
vesicular walls of pumice are very
sharp – ideal for removing loose
flakes of skin.
Natural pumice
“Pumice stone”
Pumice floating
in water
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Volcanic/Extrusive Igneous Textures: Vesicular
Vesicular igneous rocks have
been used to make various
implements in many cultures due
to the good grinding surface
provided by the holes.
For example the familiar mortar
(molcajete) and pestle (tejolote)
used in traditional Latin American
cooking-formerly used for grinding
corn, but now used mostly for the
grinding of spices and preparation
of moles (sauces/stews) and
salsas are made from the
extrusive rock basalt.
Volcanic/Extrusive Igneous Textures: Pyroclastic
Lava is not the only material
produced by volcanoes.
Eruptions that are exceptionally
violent can eject dust-sized
particles (ash) to fist- or even carsized bodies (volcanic bombs).
These particles include blobs of
molten material (consolidate in
flight) and pulverized bits of rock.
Rocks composed of this
fragmented material are said to
have a pyroclastic texture
(Greek; pyro= fire, klastos= broken)
A rock showing pyroclastic
texture (note large angular
fragments of rock surrounded
by fine-grained ash)
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Volcanic/Extrusive Igneous Textures: Pyroclastic
An item of cultural significance:
The fine-grained pyroclastic
debris (ash) generated by the
eruption of Mount Vesuvius buried
the residents of Pompeii,Italy in
79 AD, preserving an unparalleled
record of the culture of that time.
The ability of pyroclastic rocks to
preserve “organic remains” in this
manner and the layered nature of
their deposits are among the most
significant similarites between
pyroclastic and sedimentary
rocks.
Back to Minerals (for a minute)
Most igneous rocks are principally
made of silicate minerals (although
other types of minerals do occur in
smaller quantities).
As their name implies, silicate
minerals all contain the elements
silicon (Si) and Oxygen (O).
Silicates other than the quartz group
also contain one or more metals in
their chemical makeup.
Most silicates are based on the silica
tetrahedron (central silicon,
surrounded by 4 oxygen atoms as
shown).
Typical Silicate ion.
(small blue ball: silicon;
“charge”= +4)
(large balls: oxygen;
“charge”= -2X4= -8)
Note:only one bond of each
oxygen atom are connected to
the silicon, so silicate molecule
(ion) has a net –4 charge.
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Ferromagnesian Minerals
Among the most important silicate minerals used in the identification of
igneous rocks are the ferromagnesian minerals.
Ferromagnesian minerals are those silicate minerals containing iron
(indicated by the “ferro” part of the name) and magnesium (indicated by
the “magnesian” part of the name). Due to their iron/magnesium
content, they are dark in colour (green, brown or black).
The main ones are:
Olivine
Pyroxene
(Mg,Fe)2SiO4
(Mg,Fe)SiO3
Amphibole
Biotite Mica
Ca2(Mg,Fe)5Si8022(OH)2 K(Mg,Fe)3AlSi3O10(OH)2
Note: you need not remember chemical formulae- but do remember the names
Ferromagnesian Minerals
These minerals can have isolated silica tetrahedra or
have silica tetrahedra linked into chains or sheets,
forming “structural units” of silica.
These “units” are ionically bonded with metal ions (mainly
iron and magnesium-silicate ion has strong negative
charge, metals have positive charges) to form the
ferromagnesian minerals. Strong silicate chains may be
formed due to covalent bonding of the silica tetrahedra.
Planes of weakness in the mineral structure relate to
regions across which relatively weak ionic bonding
occurs, between silicate structural units (e.g. in micas).
Due to variation in the the geometry of ionic bonds as well
as the stacking arrangement of silica units, varieties of
ferromagnesian mineral vary in such characteristics as
cleavage.
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Ionically
Structural units of silica bonded with positive
(SiO4)4 metal ions
Mg2+,
Fe
2+
Mg2+, Fe 2+
Ca2+, Mg2+,
Fe 2+
Make the
ferromagnesian
mineral…
Olivine
(no cleavage)
Pyroxene
(2 cleavages at 90o)
Amphibole
(2 cleavages at
120oand 60o)
K+, Mg2+,
Fe 2+, Al3+
Biotite Mica
(1 cleavage)
Weak ionic bonding between structural units is
responsible for cleavage.
Quartz
Quartz, the second most common
mineral type in igneous rocks, is the
only common igneous silicate mineral
consisting entirely of silicon and
oxygen.
Because of the complete sharing of
electrons by oxygen by adjacent
silicon atoms, all of the bonds
(covalent) in quartz are equally
strong. As a consequence, quartz is
structurally quite homogeneous and
does not exhibit cleavage. It
fractures conchoidally. Pure quartz is
clear and colourless.
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Feldspar Minerals
Feldspar is the most common mineral group, forming under a very wide
range of temperatures and pressures.
Like quartz, the structure of feldspar
minerals is basically a three
dimensional framework of silica units
with aluminum atoms substituting for
some of the silicon atoms.
To offset the net negative charge of
the silica-dominant framework, ions
of sodium (+1), calcium (+2) or
potassium (+1) are incorporated into
the structure.
Bonds are less uniform in strength
than in quartz similar to condition in
amphiboles and pyroxenes.
Cleavage planes (2 at right angles)
mark planes along which bonding is
weakest.
Feldspar Minerals
The two most common types of feldspar
are plagioclase and potassium
feldspar:
1. Plagioclase contains ions of sodium
and/or calcium. Colour ranges from
white to bluish grey.
2. Potassium feldspar, contain ions of
potassium. Colour ranges from creamy
white to salmon pink.
Plagioclase
Both mineral groups have two sets of
cleavage meeting at 90o, reflecting the
same pattern of bonding weakness
within the crystal structure of the
feldspar mineral group.
Potassium feldspar
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Why Aren’t All Igneous Rocks Just Made of One Mineral ?
Different minerals crystallize at
different temperatures. Thus, as
a magma cools, minerals
crystallize in a distinct sequence
(expressed as Bowen’s Reaction
Series).
High temperature
The ferromagnesium minerals
remain distinct in terms of their
composition at different
temperatures. Thus they crystallize
in a discontinuous sequence.
Plagioclase changes gradually in
composition with changing
temperature (six different minerals
within the series). Thus, plagioclase
crystallizes in a continuous
sequence.
Low temperature
Why Aren’t All Igneous Rocks Just Made of One Mineral ?
Note that during each stage in cooling, the resulting rock will
preserve a different suite of minerals.
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Igneous Rock Types: Composition
On a very basic level, the composition of igneous
rocks can be estimated on the basis of colour.
Mafic rocks: Igneous rocks containing a high
concentrations of dark minerals (especially
ferromagnesian minerals) are called mafic rocks
(“ma” referring to magnesium and “f” referring to
ferrum or iron). These rocks are generally very
dark in colour.
Felsic rocks: Igneous rocks that contain small
amounts of dark minerals (especially
ferromagnesian minerals) are called felsic rocks
(“fel” referring to the abundance of light-coloured
feldspar, and the “si” referring to silica (quartz)).
These rocks are generally light in colour.
Intermediate rocks: These are rocks that have a
colour in-between mafic and felsic.
Mafic igneous rock
Felsic igneous rock
Intermediate
igneous rock
Why Igneous Rock Names are Important
Note that the terms “felsic,” “intermediate,” and “mafic” only
reflect the composition of an igneous rock. They do not
indicate anything about texture.
Specific names are applied to rocks to make communication
easier among geologists (but it does take a bit of getting
used to).
A rock name is extremely informative as it tells something
about a rock’s composition and texture (without the user
having to spell out the details every time he/she refers to
that specific rock).
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Naming igneous rocks
Here is a very basic scheme on how 6 common igneous rock names relate to
their composition and texture.
composition
Mafic
Intermediate
Felsic
(dark coloured)
(“medium”-coloured)
(light coloured)
Name: Gabbro
Name: Diorite
Name: Granite
Name: Basalt
Name: Andesite
Name: Rhyolite
texture
Phaneritic
(large
crystals)
Aphanitic
(very small
crystals)
Igneous Rocks and Plate Tectonics
Not all magmas have the same composition. This is partly because of
differences in their environment of formation.
Magmas produced by hydration tend to be formed at lower temperatures
than those produced by decompression melting.
Under relatively low temperatures, low-temperature felsic minerals are
preferentially melted, enriching the magma in silica. As a result, magmas
produced at subduction zones tend to be felsic to intermediate in
composition.
volcanoes
magma
chambers
Extrusive igneous
rocks:
rhyolite and andesite
Intrusive igneous
rocks:
granite and diorite
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Igneous Rocks and Plate Tectonics
In contrast, magmas generated by decompression melting of the
mantle (in places where plates are spreading apart) are produced by
the melting of both low and high temperature minerals, and therefore
have a more mafic composition (i.e. the magma contains silica, but also
a large amount of iron and magnesium).
volcanic
fissures
Extrusive
igneous rock:
basalt
Lithosphere
Intrusive
igneous rock:
gabbro
Asthenosphere
END OF LECTURE
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Next on the agenda: sedimentary and metamorphic rocks
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