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Rocks: Earth’s Rocks 2:
Sedimentary and Metamorphic Environments
Sedimentary Rocks
Sedimentary rocks are formed by the accumulation and
hardening of sediment.
Three kinds of sediment:
Clastic sediment - consisting of particles derived from preexisting rocks (e.g. sand)
Chemical sediment - consisting of mineral matter precipitated
from a solution (e.g. salt)
Biogenic sediment- consisting of materials produced by
organisms (e.g. shells, bone, teeth, leaves, wood, etc.)
Clastic Sediment:
Weathering
When exposed at Earth’s surface, rocks are broken down
by processes of weathering
Mechanical weathering: physical breakup or disintegration of
rocks without changes in their composition. This is
accomplished mainly by physical agents such as water, wind
and ice but can be aided by biological factors (e.g. widening of
cracks in bedrock by tree roots).
Chemical weathering: breakdown or decomposition of minerals
due to chemical reaction of minerals with water or gases in the
air.
Clastic Sediment:
Mechanical Weathering
Mechanical weathering basically serves to break rock into smaller
particles.
In areas where the degree of chemical weathering is very low (e.g. in
cold, dry, regions of the Canadian Arctic), clastic sediment can consist
almost entirely of small fragments of rock with no change in mineral
makeup.
Clastic Sediment:
Chemical Weathering
Some minerals are more susceptible to weathering than
others.
For minerals in igneous rocks, resistance to weathering
basically follows the reverse trend of same trend as the order
of crystallization in Bowen’s Reaction Series.
This is because high-temperature minerals are less stable at
Earth’s surface than low-temperature minerals.
Note that Earth’s surface is a lot cooler than the environments
in which minerals form from magma; low-temperature
minerals are most stable at Earth’s surface.
Bowen’s Reaction Series Revisited
Note that olivine (crystallized at high temperature) will tend to
weather more readily than quartz (crystallized at low
temperature). It is for this reason that quartz is very abundant in
sedimentary rocks, whereas olivine is very rare.
Products of Chemical Weathering
Chemical weathering produces a number of minerals and free ions.
The primary mineral products of weathering are quartz and clay – these
products form the bulk of sedimentary particles. Iron oxides (hematite)
can also be left behind as residue.
The remaining material is dissolved in water, in the form of ions.
Climate and Sediment Composition
Climate has a strong effect on the characteristics of clastic
sediment.
Sediment produced in cold, dry areas (where mechanical
weathering dominates) tends to contain rock fragments of
variable mineral content.
Sediment produced in warm, wet areas (where chemical
weathering dominates) tends to be composed largely of
quartz, clay and iron oxides.
Clastic Sediment:
Transportation
Once dislodged (eroded), sedimentary particles can be
transported away from their source area by:
Gravity
Water
Wind
Ice
(Mass Wasting)
(Rivers)
(e.g. Dust Storms)
(Glaciers)
Clastic Sediment:
Deposition
Sedimentary particles ultimately come to rest once the transporting medium
can no longer carry them.
As a general rule, smaller/lighter particles are deposited in less-agitated
conditions than larger/heavier particles when transported by wind or water.
Sedimentary particles can therefore experience some degree of sorting.
Beach sand tends to be well sorted, due to constant wave action.
Sedimentary particles transported and deposited rapidly by events such as
mudflows tend be poorly sorted.
beach
mudflow
Well-sorted
sediment
Poorly sorted
sediment
Depositional Environments
Clastic sediments can be
deposited in a wide
variety of settingscharacteristics of clastic
sedimentary rocks can
provide information on
where their constituent
sediments were
originally deposited.
We will look at some of
these environments in
greater detail as the term
progresses.
Sediment to Sedimentary Rock: Lithification
Once buried, sediment undergoes changes that transform it
into rock. This transformation is called lithification (lithos =
stone)
Compaction: As sediment layers are buried to deeper and
deeper levels under successive sediment layers,
sedimentary particles are squeezed together as the spaces
between them decrease in size. The material thus becomes
more rock-like.
Cementation: Sediment grains can also become cemented
together by minerals that precipitate from water remaining in
the pore spaces between the grains. Grains therefore
effectively become glued together.
Common Clastic Sedimentary Rocks
Clastic sedimentary rocks are classified primarily by grain size.
Sediment type
Rock Type
Gravel
(pebbles, cobbles, boulders)
Conglomerate
Sand
(sand-sized particles that are
obviously gritty)
Sandstone
Silt
(particles barely discernible
under low magnification)
Siltstone
Clay
(particles too small to be seen
under low magnification)
Shale
Chemical Sediment
Water on Earth’s surface contains dissolved ions (electrically
charged atoms or groups of atoms of various elements) principally
derived from weathered minerals and volcanic gases.
Dissolved Components in Seawater (Percent by Weight)
Positive Ions
Sodium (Na+): 30.61 %
Magnesium (Mg 2+ ): 3.69 %
Calcium (Ca2+): 1.16 %
Potassium (K+): 1.10 %
Negative Ions
Chloride (Cl-): 55.04 %
Sulphate (SO42-): 7.68 %
Others: 0.72 %
Include trace amounts of metals
(gold, silver, zinc, lead, copper, etc.)
For example…
HF HCl
CO2
Carried in water vapour
HF HCl
SO2 Reacts with water in atmosphere H2SO4
H2O
Acid rain
Pyroclastic debris
(pulverized rock)
Ions dissolved in seawater
H+
SO42Cl-
F-
Chemical Sedimentary Rocks: Evaporites
Due to evaporation, dissolved ions can
become too concentrated for the water
to hold, so positive and negative ions
join together and precipitate as
minerals.
Evaporite deposits accumulate in
basins isolated from main sea
seawater flows into basin, becomes
concentrated in dissolved ions and
sinks.
The saltier water cannot escape back
to the ocean, and becomes further
concentrated to the point that
“evaporite minerals” are deposited.
Two common minerals in evaporite deposits are:
Gypsum (calcium sulphate) and Halite (sodium chloride).
In some cases, rocks can be composed exclusively of one of these
minerals.
A rock formed exclusively of
halite is called rock salt
(used as table salt)
A rock formed exclusively of
gypsum is called alabaster or
“rock gypsum”
(commonly used in sculpture)
Chemical Sedimentary Rocks: Chemical Limestones
Limestone is a sedimentary rock
dominated by the mineral calcite.
In some caves and hotsprings,
concentrations of calcium ions (Ca2+) and
the complex ion carbonate (CO3)2- can
reach sufficient levels to allow the
precipitation of calcite (CaCO3).
A travertine sample
This forms a banded variety of limestone
called travertine.
Some forms of travertine also display
pores that are produced from the liberation
of gases by bacteria.
Travertine with abundant pores
Travertine Structures
Travertine stalactites on
the ceiling of a cave
(Lehman Caves,
Nevada, U.S.A.)
Travertine terraces in
Yellowstone National Park
(Wyoming, U.S.A.)
Biogenic Sediment
Biochemical sediment consists of materials that are
produced by chemical processes associated with biological
activity.
So one can think of biochemical sediment as being chemical sediment
formed through biological activities.
Biogenic components include:
Shells
Bones
Teeth
Plant Remains
Biogenic Sedimentary Rocks
Two common rocks that are formed from biogenic sediment
are fossiliferous limestone, chert, and coal.
Fossiliferous limestone is composed almost entirely of calcite
shells (skeletons) of organisms. In many cases, the identity of
the former owners of skeletal material can be identified.
This piece of limestone contains
broken “stem” segments of animal
called a crinoid.
Biogenic Sedimentary Rocks
A familiar rock that, in a loose sense, can also be considered
a variety of fossiliferous limestone is chalk, which is made of
microscopic skeletons of algae.
Chert is another common biochemical sedimentary rock.
A few organisms, such as some microscopic planktonic
organisms (as well as some sponges), have a skeleton
made of silica. The silica can dissolve and form a “gel” on
the seafloor. When this gel solidifies, it forms a finely
crystalline rock called chert. As it is basically made of very
fine-grained quartz crystals, it is very hard.
Chert
Silica skeletons of
planktonic organisms
Coal is a special type of biogenic sedimentary rock that is
largely composed of organic matter from plants. Coal
seams represent large accumulations of organic matter that
were deposited in swamps and were subsequently buried.
swamp
coal seam
coal sample
Metamorphic Rocks
Metamorphic rocks are formed by the transformation of preexisting rocks under the influence of high temperatures and
pressures and chemically active fluids
Three basic types of metamorphism:
Contact metamorphism
Regional metamorphism
Metasomatism
Contact metamorphism: occurs when a pre-existing rock
is baked under relatively low pressures
Commonly occurs when rock is heated by igneous intrusion, forming
a metamorphic halo or “aureole” in the adjacent rock – generally local
in extent
Mineral grains recrystallize in random orientations. Overall composition of
the rock basically remains the same.
Regional metamorphism: occurs when pre-existing rock is
subjected to heat and pressure on a regional scale.
Commonly associated with mountain building events in which rocks
are lowered to great depths and squeezed by compressive forces.
Metamorphism: Foliation
In contact metamorphism, pressure
is uniform. As a result, grains of
platy minerals such as mica, and
elongate minerals such as pyroxene
and amphibole retain a random
orientation.
Regional metamorphism generally
occurs in areas where two
lithospheric plates are pressing
against one another, rocks are
subjected to differential stress.
In response to this stress,
platy/elongate minerals line up to
produce a foliated texture (folium =
leaf)
Common Metamorphic Rocks
Non-foliated rocks
Some metamorphic rocks, whether formed by contact or
regional metamorphism always have a non-foliated (also
called granoblastic) texture. This is because they lack
platy/elongate minerals required to define foliation.
Such rocks include quartzite (formed via metamorphism of
quartz sandstone), and marble (formed via metamorphism of
limestone).
In both cases, mineral grains grow in size and form an
interlocking texture. Fossils once present in the original
sedimentary rock are obliterated due to this recrystallization.
Common non-foliated (granoblastic) metamorphic rocks
Heat (+ pressure)
Limestone
Marble
Heat (+ pressure)
Quartz sandstone
Quartzite
Common Foliated Metamorphic Rocks
Foliated rocks
Rocks containing platy/elongate minerals that subjected to
regional metamorphism (and therefore) affected by
differential pressure are typically foliated.
Increasing intensity of metamorphism (called “grade”) results
in increased size of mineral grains, and the development of
distinct types of foliation.
With increasing metamorphism, shale changes into the
following rock types:
Shale  Slate  Phyllite  Schist  Gneiss
Common foliated rocks
Slate: very slight
sheen due to
growth of platy
mica grains
Phyllite: distinct
sheen due to
continued growth
of platy mica
grains
Schist: very
sparkly
appearance due
to large mica
grains
Gneiss: banded
appearance due to
separation of darkand light-coloured
minerals
Metasomatism: occurs when fluids (generally water or
carbon dioxide) react with a pre-existing rock and alter the
chemical composition of minerals within the rock. In some
cases, the fluid itself is involved. In others, substances
dissolved in the fluid are involved.
For example, the mineral olivine (which occurs in blocky
crystals) reacts with water to form the mineral serpentine
(with platy to fibrous crystals).
2Mg2SiO4
olivine
+
2H2O 
water
Mg3Si2O5(OH)4
serpentine
+ MgO
(in solution)
Distribution of Rock Types
As indicated on this map, different types of rocks occur in different areas
depending on the dominant mode of rock formation (even more complex when
one considers variations within the three rock families). This means that the
chemical components of soils also vary geographically.
Geologic processes controlling this distribution will be detailed in the next
couple lectures.
END OF LECTURE