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Rocks
Igneous rocks make up the
majority of the Earth’s crust.
Sedimentary rocks dominate
the Earth’s surface.
Igneous Rocks
Sedimentary Rocks
Metamorphic Rocks
Igneous Rocks
All rocks that form from cooling of a mass of molten rock
(melt or magma).
Includes crystalline rocks (interlocking mineral crystals)
and glasses (lacking crystalline minerals).
Phaneritic: mineral grains can be seen with the unaided eye.
Aphanitic: mineral grains cannot be seen with the unaided eye.
Coarse-grained igneous rocks are formed as intrusive
rock bodies:
They crystallize relatively slowly within the Earth’s crust.
First order classification: based on average crystal size
(termed texture).
Fine-grained igneous rocks are formed as extrusive rock
bodies:
Coarse-grained: 1 mm or larger.
They crystallize relatively quickly at or very near the
surface of the Earth.
Fine-grained: less than 1 mm.
Magma that is extruded to the Earth’s surface is called
lava.
Many lavas crystallize so quickly that there is no time for
the organized structure of crystals to develop.
In general, the size of the crystals depends on the rate of
cooling; the slower the rate of cooling the larger the
crystals that form.
Gas trapped in the magma
when it cools quickly forms
bubbles that remain after
cooling and solidification; the
resulting void spaces in the rock
are termed vesicules
The texture of such rocks is termed glassy and the rocks
lack discrete minerals.
Obsidian is an igneous rock
that cooled very quickly at the
Earth’s surface and displays a
glassy texture and conchoidal
fracture.
Pumice is a glassy igneous rock that
is characterized by many small
vesicules.
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Additional terms related to crystal size:
The world’s largest crystals: gypsum crystals found in caves near
a zinc and silver mine in Mexico.
Pegmatite: very coarsegrained igneous rock with
crystals exceeding 2.5 cm
in size.
Porphyritic: large crystals set
in a matrix of finer crystals.
The large crystals are termed phenocrysts.
Igneous rocks are classified more precisely on the basis of
the relative proportions of their minerals.
Silicic or Felsic rocks: white, grey or pink in colour;
rich in quartz, potassium feldspars and sodium
plagioclase feldspars and biotite/muscovite.
Intermediate rocks: salt and pepper for coarsegrained rocks, dark grey for fine-grained rocks; rich
in amphiboles and calcium plagioclase feldspars.
Mafic rocks: dark grey to black in colour; rich in
calcium plagioclase feldspars and pyroxene.
Ultramafic rocks: green to black in colour; rich in
olivine.
Igneous rock names based on texture and composition.
Coarse-grained
Fine-grained
Granite
Rhyolite
Silicic or
felsic
Diorite
Andesite
Intermediate
Gabbro
Basalt
Mafic
Peridotite
Ultramafic
Komatiite
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Crystallization from Magma
The melting or crystallization temperature depends on:
Magmas begin deep within the crust or the upper mantle
where temperatures are high enough to melt rock.
Geothermal Gradient:
the rate of increase in
temperature with depth
beneath the Earth’s
surface.
On average:
3 °C per 100 m depth.
Melting/Crystallization temperature increases with depth
beneath the Earth’s surface (if the rocks are dry) due to
the increase in pressure with depth.
Melting of “dry” rocks will
normally not occur beneath
continents because
temperature do not become
sufficiently high.
The pressure exerted on the material
(which depends on the depth of burial).
The amount of water that is present within
the magma.
The chemical composition of the magma.
Water, under pressure, substantially reduces the melting
temperature.
The greater the pressure that is exerted on the water the
lower the melting temperature.
In the presence of water
melting will take place
beneath continents.
Melting/crystallization temperature varies widely
depending on the composition of the magma.
The melting temperature increases with decreasing quartz
to 1300 °C for pure K-spar.
Complete melting of a mixture of potassium feldspar (Kspar) and quartz occurs at a minimum of 1000 °C when
there is 42% Quartz and 58 % K-spar.
The melting temperature increases with decreasing K-spar
to over 1500 °C for pure Quartz.
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Some magmas begin within the mantle as semisolid masses.
The melting or crystallization temperature depends on:
The pressure exerted on the material
(which depends on the depth of burial).
Even though the temperature is very high the extreme
pressure inhibits melting.
These masses may slowly rise towards the crust due to
convection within the mantle.
The amount of water that is present within
the magma.
The chemical composition of the magma.
At a depth of about 50 km from the Earth’s surface
pressure is low enough to allow melting to form a magma.
The rising plume of magma remains hotter than the
ambient mantle, retaining heat from greater depths.
The plume rises into the overlying crust and continues to
migrate upwards.
It continues to cool as it moves through the crust.
Higher in the crust temperatures are lower and the magma
cools and crystallizes into a body of igneous rock.
The temperature at which a mineral crystallizes from a
magma depends on its composition.
If it doesn’t cool within the crust it reaches the surface to
form a volcano.
Bowen’s Reaction Series describes the sequence in which
minerals will crystallize with decreasing temperature in
the magma or melt.
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The rock type that forms from the crystallization of a
magma depends on:
When a rock heats up the minerals melt in the reverse
order to Bowen’s Reaction Series.
The initial composition of the magma.
The stage at which the minerals crystallized.
The wide variety of igneous rocks is due to three primary processes:
1. Crystal settling and magmatic differentiation.
2. Assimilation of host rock.
3. Magma mixing.
Time 2.
As the first crystals begin to form in the magma (olivine) they
remove iron and magnesium from the magma, changing the
composition of the magma as the crystals settle to the bottom of the
magma chamber.
1. Crystal settling and magmatic differentiation.
Time 1
While the magma body is first
emplaced into the crust it has an
initial composition.
The first igneous rocks may be
mafic rocks with abundant iron
and magnesium.
Time 3
As successive minerals crystallize, following Bowen’s series, the
composition of the magma continues to change or differentiate.
2. Assimilation of host rock: if the rock into which the magma has
intruded is melted by the high temperatures, its inclusion in the
magma will change its composition; the rock type that forms will
similarly change.
Over the period of crystallization of the magma the types of
igneous rock change due to the changing chemical composition of
the magma.
The last rocks to
form with have a
felsic or silicic
composition,
reflecting the
composition of the
differentiated
magma.
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3. Magma mixing: if two magmas with different compositions
become mixed, the resulting magma will have a different
composition and different rocks will crystallize from it.
Igneous Structures
Volcanoes are structures that are produced by extrusive
igneous activity (when magma is extruded to the surface).
Plutons are solitary masses of igneous rock within the crust.
Over millions of years the surface of the crust is
eroded away.
When many plutons are emplaced into the crust they
coalesce to form a larger structure called a batholith.
If the surface rocks are softer than the igneous rocks of
the pluton it will form a topographic high as it resists
erosion.
Batholiths form extensive masses of igneous rock that
may become exposed at the surface following erosion of
the land surface.
Exposed batholiths form broad uplands when they are
exposed by erosion.
Mt. Evans Batholith, Colorado
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Mt. Rushmore is likely the best known batholith!
Smaller intrusive structures commonly extend away from
major bodies such as batholiths and plutons.
Dikes cut across layered strata that they intrude.
Sills intrude along planes that are parallel to associated
strata.
Sedimentary Rocks
Sedimentary rocks include those that are made up of
discrete particles of minerals or rock fragments (termed
clastic sedimentary rocks) and those made up of
interlocking crystals (termed chemical sedimentary rocks).
A vertical dike forms a
resistant ridge of igneous rock
that intruded softer
sedimentary rocks.
Individual grains in clastic rocks are surrounded by
cement (normally of calcite, dolomite or quartz)
Igneous Rock
Clastic Sedimentary Rock
Image by Dr. Roger Bain.
http://enterprise.cc.uakron.edu/geology/natscigeo/Lectures/igneous/volcano2.htm#intrusions
Weathering, transport and deposition of sediment
Thin sections are 30 micron (30/1000 mm) thick slices of rock
through which light can be transmitted.
Click here to see how a thin section is made.
http://faculty.gg.uwyo.edu/heller/Sed%20Strat%20Class/SedStratL1/thin_section_mov.htm
Sedimentary rocks: composed of
the products of weathering of
source or parent rocks.
Weathering: the process by
which a rock breaks down when
exposed at or near the Earth’s
surface.
Physical or mechanical
weathering involves the physical
breakdown of the source rock.
Frost wedging, unloading
expansion, thermal expansion,
biological activity.
Solid particles are produced.
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When a granitic pluton is deep
within the crust it is compressed by
the great weight of overlying rock.
Frost wedging produced the
scree or talus at the base of this
mountain in the Northwest
Territories.
When erosion of the land surface
exposes the pluton the weight is
removed and it expands.
As it expands it “exfoliates” like the
skin of an onion into sheets of rock.
Tree roots can grow into the
fractures in rocks. As they
grow they exert considerable
pressure and cause the
fractures to expand.
Eventually the roots may
break the surface rocks
entirely into large boulders.
Chemical weathering takes place when the source rock undergoes
chemical reactions with surface water in contact with it.
Chemical weathering produces:
Solutions.
Stable mineral grains (e.g.,
quartz) as detrital grains.
New minerals grains (e.g., clay
minerals, oxides).
The resistance of igneous minerals to weathering is similar to the
Bowen’s Reaction Series.
The most stable minerals are those that crystallize last (quartz, kspar and muscovite).
Minerals that crystallize under high temperature are more prone to
chemical weathering.
Solid grains may be transported by:
Rivers
Wind
Glaciers
Ocean currents
Volcanic explosions
Solutions are transported
largely by rivers.
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Clastic sediment becomes a sedimentary rock following
compaction and cementation.
Clastic sediment: made up of the
solid products of weathering.
Compaction involves the pushing together of the particles by the
weight of overlying sediment that is subsequently deposited.
Deposition takes place when
medium ceases to move the
particles.
Cementation involves the precipitation (crystallization) of minerals
that are in solution in waters flowing through the sediment.
The precipitate forms a cement in the void spaces between particles
and binds them together.
Clastic sedimentary rocks
include:
Sandstone
Calcite and quartz are
common cements in
sedimentary rocks.
Conglomerate
Shale
Chemical sediment: made up of material that is transported in
solution.
Chemical sediment is deposited
when material in solution is
precipitated
Clastic sedimentary rocks are classified on the basis of their average
grain size.
Sediment Name
Rock Name
(particle shape)
Gravel
Conglomerate (rounded)
Breccia (angular)
Sand
Sandstone/Arenite
Silt
Siltstone/Lutite
Clay
Claystone/shale
Precipitation may take place:
Due to changes in water
chemistry.
Average Grain Size
> 2 mm
Due to evaporation (e.g., halite).
Due to shell production by
organisms.
Many limestones are made up of
calcite produced by organisms.
Conglomerate is made up of
well-rounded gravel.
2 – 0.0625 mm
0.0635 -0.004 mm
<0.004 mm
Sandstone:
Individual grains can be seen with the naked eye.
Breccia is characterized by
angular gravel.
Siltstone is very fine-grained
but feels “gritty” to the touch.
The shape of the gravel indicates
that it has not traveled far from
where it formed.
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Shale is smooth to the touch and weathers into thin flat slabs.
Chemical sediments are classified on the basis of their chemical
composition.
Halite (NaCl) and Gypsum (CaSO4 +H20) form by precipitation of
salt water.
Ions dissolved in water form crystals as the water evaporates.
Halite accumulations in
Death Valley
Halite
hopper
crystal
Gypsum formed in a playa lake.
Limestone (CaCO3) forms most commonly by the accumulation of
whole and/or broken shell material.
Dolomite (MgCO3) commonly forms from limestone when a
magnesium ion replaces the calcium ion bonded to the carbonate
ion.
Gypsum rosettes
Limestone and dolomite
are commonly very
fossiliferous.
Primary Sedimentary Structures
The reaction of limestone to hydrochloric acid.
CaCO3 + 2H+ → Ca2+ + CO2 (gas) + H2O
Many clastic rocks, limestones and dolomites display structures that
formed at the time that the sediment was deposited.
Sun cracks: formed when previously wet muds dry out to form a
polygonal pattern of cracks.
Modern sun cracks
Sun cracks on an ancient sandstone
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Wave ripples are straight-crested, symmetrical mounds of sand that
form when waves act on the water above a deposit of sand.
They indicate that the sediment
was laid down in an environment
that was influenced by waves (a
lake or sea).
Current ripples are asymmetric in cross section and have short,
curved crests.
The upstream side has a gentle slope whereas the downstream side
is steep.
Wave ripples on a vertical rock face.
Metamorphic Rocks
Metamorphic rocks form when pre-existing rocks are subjected to
high temperatures and/or pressure and interaction with chemically
active fluids.
Metamorphic Grade is reflected by Index minerals: minerals that
form under a limited range of pressures and temperatures.
Original minerals may not be stable under the changed P/T
conditions so new minerals form that are stable.
Metamorphic Grade: a
measure of the degree to which
a rock has changed during
metamorphism.
Types of Metamorphism
Burial metamorphism: occurs when rocks become buried within the
crust due to subsequent deposition.
Contact metamorphism: takes place when an igneous intrusion heats
up the rocks into which it intrudes.
Only rocks near the intrusion are affected.
As they are buried deeper the temperature and pressure increases.
Metamorphism begins at
temperatures above 200 C (about 8
km depth).
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The type of rock that forms with contact metamorphism varies with
the composition of the original rock and the distance from the
intrusion (cooling away from the intrusion).
The zone of contact metamorphism is termed a metamorphic aureole.
Sandstone
Quartzite
Regional Metamorphism:
Limestone
Marble
Under a directed pressure crystals that grow will grow more readily
in the direction that is perpendicular to the applied force.
The most common metamorphic rocks; formed over extensive areas
due to high temperatures and pressures associated with the
interaction between tectonic plates.
Unlike burial metamorphism, pressures have a preferred direction.
Foliation: the tendency in regional metamorphosed rocks to have
minerals that are preferentially oriented parallel to each other.
A directed pressure may
align minerals into an
orientation that is
perpendicular to the
applied force.
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Pressure and temperature ranges for
different types of metamorphism.
Burial and Contact metamorphic rocks
are not foliated and include:
Igneous Rocks
View pictures of the metamorphic rocks described below at:
http://www.gpc.edu/~pgore/geology/geo101/meta.htm
A site created by Pamela J.W. Gore of Georgia Perimeter College
Sedimentary Rocks
The geologic cycle
Metamorphic Rocks
The geologic cycle….
A concept that relates the three rock types through
processes that act in their formation.
•Weathering: the breakdown of a rock exposed at
the Earth’s surface.
Involves:
•Cooling of magma (to form igneous rocks).
•Transport of weathering products (e.g., by
rivers).
•Heat and pressure inside the earth
(metamorphism or melting to form a new magma).
•Uplift of buried rocks by tectonic processes (e.g.,
mountain building).
•Deposition of transported material (as loose
sediment) to where it can no longer be
transported.
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The geologic cycle….
•Burial and compaction: covered by subsequent
deposition and pushed into close contact due to the
weight of overlying sediment.
•Cementation: the binding together of
sedimentary particles by minerals that act as a
cement.
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