Weathering and Erosion:
The Formation of Sediments and Soil
I. Differences between the earth and the moon:
Earth is tectonically active
continual uplift, folding, and breaking of the earth’s surface.
Subsequently, it is “t
orn down” by the surface processes of
weathering and erosion
The earth has a strong enough
an atmosphere and surface water.
drives most of the surface processes
of weathering and erosion.
II. Define Wea
thering and Erosion
“The decomposition and disintegration of rocks and
minerals at the Earth’s surface by
processes. Weathering processes involve
(or removal of)
decomposed earth materia
of weathered rocks and minerals from
the place where they formed.
Forces or transporting agents
involved in moving disintegrated earth materials are:
running water such as streams, rivers, etc.
tornadic storms, sea breezes, etc.
The influence of gravity causing landslides,
in the form of glaciers
III. Modification of the Earth’s Surface
Weathering, erosion, and transportation of earth materials
sses continually wear away rocks and landforms
In geologic time they combine to wear away entire mountain
ranges, reducing them to flat, low
IV. Types of General Weathering
Mechanical (or Physical) Weathering
ation of rock into smaller and smaller pieces. The
chemical composition of the rocks and minerals are
The particles formed are called
occurs when air and water react
chemically with rocks t
o alter their composition and mineral
content. The final products not only differ physically from the
parent material, but they are different chemical substances. (i.e.
Limestone dissolving by acid rain releasing its calcite content as
Rocks weather by
chemical processes occurring together. Since rocks are not
homogenous in composition, usually parts weather at different
rates. This is called
resulting in an uneven
Because of differential weathering, the
surface of rocks is many times sharp and angular, or cuboidal.
These corners formed on the rocks are “attacked” from all three
sides resulting in a “
” of the angular piece. This is
V. Types of Mechanical (Physical) Weathering
upon freezing. If water seeps
into cracks in the rock and freezes, the ice formed exerts pressure
along the crack, expanding the crack, or breaking off
a piece of
rock. Many times the broken piece remains in place until the
spring thaw, resulting in areas (such as mountain passes) of
. The loose angular rock debris at the base of
mountains and cliffs is termed
never salt water evaporates, the salts
reform crystals. If water containing dissolved salts enters a crack
in the rock and then evaporates, the pressure created by the
newly forming salt crystals can break the rock. This is
and is common in
deserts and shorelines. This is why it
is not a good idea to salt driveways or sidewalks to rid them of
ice. The concrete will eventually break apart.
This is the mechanical wearing and grinding on rock
friction and impact
her rock materials. This
gives the rocks a rounded appearance. This occurs in flowing
water, wind actions (i.e. natural sand blasting), and glaciers.
Plant roots can crack rock material by the
associated with root g
rowth. Also, burrowing
animals can contribute to rock disintegration.
Pressure Release Fracturing
Rocks buried deep within the
earth are under the pressure of the overburden (country rock). As
the overburden is eroded away, the internal pressures of
pluton cause it to expand. This causes the surface of the granite
to split and crack forming sheets and blocks of rock at the surface
in a process known as
. This may also occur in rocks
that are porous such as feldspar rich granites
. Water may be
“absorbed” by the feldspars causing them to swell and crack.
This process of swelling by the addition of water is called
. This is one of the processes that can turn feldspars
, a major clay
Expansion and Contraction
causes matter to
causes matter to contract. Surface rocks
exposed to the intense heat of the daytime sun heat up and
expand. At night when it is cooler, the rocks contract. This
constant expansion and co
ntraction over many years causes the
rocks to break apart. Enchanted Rock in central Texas was
named so because of the cracking sounds it is supposed to make
during this process.
VI. Types of Chemical Weathering
reactions with ox
4 Fe + 3 O
iron + oxygen = iron oxide
Oxidization reactions are common in nature and usually turns
useful material into wastes. This is most common in iron bearing
mafic minerals such as olivine, amphibole, and biotite.
reactions involving oxygen, water, and CO
the air and water. Combinations of these can cause corrosive
chemical conditions that can chemically weather rocks.
Weathering by Solution
dissolution whereby ions disperse
into water. I.e. rivers
flowing across limestone can dissolve Ca
and carry these ions away.
Acids and Bases
are solutions with an abundance of
free hydrogen ions (H
are solutions that have an
abundance of free hydroxyl ions (OH
). Acids and base
minerals by pulling atoms out of crystals.
is formed in abundance in nature whenever CO
dissolves in some rivers and streams.
is formed by
lightning breaking apart N
he atmosphere into N + N. This
combines with water to form nitric acid causing
naturally become slightly acidic with a pH of 5.5
Pollutants in the atmosphere such as sulfur dioxide gasses can
also contribute to acid rain.
Soil: One P
roduct of Mechanical and Chemical Weathering
I. The Components of Soil
the loose, unconsolidated, weathered rock overlying
the bedrock. Since different geographic locations have their own
unique geologic histories with different rock
chemistries, there are
resulting in a broad variety
of soil types worldwide.
= Greek for “soil”);
Earth material that has been
so modified and acted upon by chemical, physical, and biologic
ts that it will support rooted plants.
3. Soil terms
a mixture of sand, silt, and clay sized particles,
along with organic matter.
plant or animal matter before decay processes.
term for when litter decomposes sufficiently th
can no longer be identified.
4. Soil Profiles
the uppermost layer of a mature soil that is
composed largely of litter and humus with relatively small
amounts of minerals.
is a mixture of humus and minerals in the form of
nd, silt, and clay. Layers “O” and “A” horizons are referred
is a transitional zone between the topsoil and
the weathered bedrock below. Roots and other organic
matter may be present but generally the organic content is
this lies directly on unweathered “parent”
and consists of partially weathered rock.
5. Dissolved Material
movement of dissolved minerals
by downward moving water (i.e. rainwater)
Zone of Leeching
the “A” horizon is called the zone of
leeching where clay and dissolved ions are removed.
Zone of Accumulation
the “B” horizon is called the zone
of accumulation where clay, dissolved ions, and water
the nature of the soil is partially dependent
on the nature of its parent rock, including the texture of the
soil and its nutrients.
It has been estimated that for the creation of 1 inch
of topsoil, natural processes need around 100 years.
the upward migration of water by evaporation,
root absorption, and capillary action are all factors
determining the soil type in areas that have different
climates. Soils worldwide are categorized into three main
soil types as to the three main climate
s condusive to soil
formation. A very many distinct soil types exist in the world.
desert soils where there is an
accumulation of dissolved minerals, calcium,
magnesium, and sodium. Deserts typically receive
10 inches of rainfall per year, and
many times all at
once over a couple of days. This carries dissolved
minerals downward forming
layers comprised of calcium carbonate. This also
of the soil (an accumulation of
salts) which limits that amounts of v
can grow. This in turn reduces the ability to form a
good “O” horizon.
humid soils of a more temperate
climate. There is a complete loss of the more
soluble ions of calcium, potassium, magnesium, and
sodium. Less soluble ions
are left such as aluminum
tropical soils formed in areas of great
amounts of rainfall. All of the silicon and soluble ions
are removed leaving only aluminum, oxygen, and
water. The mineraloid
(a major aluminum
ore) forms her
7. Rates of Growth and Decay of Organic Matter
This is related
to the accumulation of humus:
are ecologically balanced that
thick layers of humus occurs resulting in the
have so much water t
decomposition by bacteria, mold and other fungi that
decomposition is so rapid that very little humus level
have so much salinization that abundant plant
life cannot be supported so very little or no humus
dusive to such slow plant growth
that little humus forms.
Slope Angle and Aspect
Valley floors have the deepest and
richest soils due to the fact that soils tend to “creep” down slope.
Exposure of a slope to the sun also affects soil formation.
rosion and Agricultural Systems
Rates of erosion are dependent upon vegetation, litter,
humus, and amounts of rainfall. Erosion increases by
the removal of the ground cover (usually vegetation).
Deforestation results in the loss of topsoil due to run
erosion rates exceed the rate of topsoil
production by about 35%
in the world’s croplands.
Silt runoff into major river systems causes near
continent oceanic waters to become turgid, reducing
the amount of photosynthesis by phytoplankton.
Sediments and Sedimentary Rocks
are found in sedimentary rocks. This is because the
organic remains of organisms are usually destroyed by the high
temperatures associated with igneous activity or the processes of
metamorphism. The type
of sedimentary rock formed in an area reflects
environment in which it was deposited
. The term used by
geologist to describe this aspect of sedimentary beds is “
can be learned about the ancient environments of the earth by studying
ious characteristics of sedimentary rocks.
All rocks form initially with the solidification of molten
These newly formed
rocks are subsequently subjected to the
surface processes of weathering and erosion (the destructive actions
running water, wind, glaciers, etc.) These rock fragments eventually
settle out somewhere to form “
”. These sediments can
become compacted to form
. If these “new”
sedimentary rocks are subjected to enough heat and pressure
, they may
become changed into “
” rocks. If the sedimentary rocks
are completely melted by geologic processes, they revert back into a
type of igneous rock upon cooling.
I. The Rock Cycle:
The rock cycle is the conversion of one rock typ
e into another by
melting, pressure deformation, and weathering and erosion. All rocks
are initially igneous (The word “
” means “
born of fire
processes can then weather and erode these igneous rocks into
sediments that can form sediment
ary rocks. Both igneous and
sedimentary rocks being subjected to intense heat and pressure can
form metamorphic rocks. All three rock types after being subjected to
intense temperature can reform igneous rocks.
II. Rock Types:
up 90% by volume of the earth's
crust. Igneous rocks are formed directly from molten
material having its origin in the interior of the earth. As this
molten material cools in some areas, it solidifies and
hardens to become rock.
below the surface of the earth.
form from molten material that has been forced out onto
the surface of the earth (i.e. volcanoes).
form from the accumulation of eroded
debris of other rocks or chemically f
rom elements in
seawater. Sedimentary rocks make up 75% of all of the
at the earth's surface and are where most
all fossilized remains are found. This makes sedimentary
rocks useful in interpreting the earth's geologic history.
are formed from pre
that have been altered as the result of intense heat and
pressure. Metamorphism increases the “
hardness of the rock; sandstone changes to quartzite;
shale changes to slate, and limestone change
s to marble.
III. Types of Sedimentary Rocks:
Since the facies of sedimentary beds tells the geologists so much
information about the geologic past (paleoenvironments, paleoclimates,
and past life forms),
are emphasized in Historical
Geology. There are
2 basic groups of sedimentary rocks
1. Chemical Precipitates
from the evaporation of seawater, or
from the concentration of ions in water. These include rocks such as
limestone and various salts such as Halite (NaCl), Sylvite (KCl)
), etc. The salts usually indicate periods of massive evaporation
of aqueous environments.
2. Clastic Sedimentary Rocks
are formed from the accumulation
of debris from the weathering and erosion of other rocks. The 4 stages
of the form
sedimentary rocks (“
” means "
are described on the following pages.
IV. The Four Steps for Formation of Sedimentary
Physical and Chemical Weathering
of the “
source rock from which the clast
ic material is being derived). Physical
weathering includes the breaking apart of the parent rock by freezing
and thawing, wind erosion, etc. Chemical weathering includes
dissolution of the parent rock by chemicals in the water (i.e. acid rain).
is the stage where the clastics are
”) from the source area by
water, wind, gravity,
. The terrain determines the area of transportation. The distance
the particles are moved depends on the amount of energy operating
the environment. It would take more energy to move a boulder than a
grain of sand. The larger the sediment size, the more energy is needed
to move it.
would include white water
mountain streams that are capable of moving al
most all sizes of
include lagoons, lakes, deltas,
swamps, etc., that are capable of moving only the smaller particles.
is the stage where the sediment is
particular geographic environment,
which constitutes the
. As in transportation, the area of deposition is also
determined by terrain. For example, large rocks formed on a mountain
range would be carried down the steep gradient and deposited at the
base of the moun
tain if the energy of the stream carrying them
decreased when it reached the base of the mountain. Since the stream
no longer has the high energy from the gradient, the large rocks are
deposited in a manner indicative of a mountain stream environment.
dimentary rocks can be interpreted to find out the environment in
which they formed.
can be divided into several categories:
Shoreline and Coastal Environments
” or Stream, River, and Delta Environments
at the bases of
” or “wind
There are numerous other sedimentary environments that your
instructor will inform you of at the appropriate time
in the formation of a sediment
rock. At this stage the sediments are compacted due to the weight of
(overlying sediments) and can be eventually “
(turned to stone) as the particles are cemented together with substances
, or forms of
), among other compounds..
V. Properties of Clastic Sediments:
These include certain characteristics of the sedimentary rock that give
specific information about the environment of deposition. These include
size, degree of roundness, degree of sorting, and color.
Clastic sediments are found in various sizes ranging
from <1/256 mm to >256 mm. Refer to
Figure 1. The Wentworth
Scale of Particle Sizes.
The name of a particular sediment si
based on its particle size rather than its chemical composition. For
example, "sand" refers to particles having a size range between
0.5mm. There can be quartz sand such as that found along
the Gulf Coast or there may be feldspar sands, g
ypsum sands, etc.
Remember that sediment size indicates the amount of energy operating
in the depositional environment and is therefore a useful clue in
determining what the sedimentary environment was. Boulders represent
energy environment such
as a river channel while clays
represent a low energy environment such as a floodplain or swamp.
Wentworth Scale of Particle Sizes
that is a list of sediment
particle sizes and the names used to describe them:
The Wentworth Scale of Particle Sizes
Particle Name Approximate Particle Diameter in millimeters
greater than 256mm
Very Coarse Sand
Very Fine Sand
less than 1/256
This is simply how “round” (or smooth) the particles in
the rock are. Particles in rocks that are angular, irregular in shape, and
have sharp edges are called “
”. Particles that are
smooth and have no edges are called “
The degree of
roundness indicates either the amount of agitation the particles were
subjected to before deposition, or the length of time it took to transport
the particle. “Well rounded” particles indicate that the particles were
subjected to a high am
(bouncing along as they were
transported) or being transported for a very long distance such as from
the center of a continent to its shoreline. Both of these factors indicate
how much the rock particle was hit by other fragments or was
along the route of transportation. “Poorly rounded” sediments indicate
either a low amount of agitation, or a short distance of transportation
from the time the particle weathered or broke away from their
. A high
which allows for a
exposure to weathering, such as a beach or in a stream, is condusive to
the formation to the formation of “well
rounded” sediments. On the other
hand, a high
energy depositional environment that does not allow a long
iod of exposure to agitation, such as an alluvial fan, prevents the
sediments from becoming “well
refers to rock fragments separated according to particle
” sediment would contain particles of varying size.
This usually represents a rapid deposition as the result of a rapid
decrease in the energy of an environment. Poorly sorted sediments are
many times found in
at the base of a mountain. This
results in a "
" of sediments at th
e base of the mountain
energy to low
” sediment contains
material that is made up primarily of all the same sized particles. This
indicates that the rate of deposition is slow enough to allow the materials
to be separated. O
f course, the energy of the environment must be
sufficient to accomplish this. Beaches, such as those along the Texas
coast, allow sorting to occur. The high energy from the waves combined
with a proper depositional rate provides excellent conditions for
of the sediments. Sediment is said to be "
" if it is well rounded
and well sorted. Poorly sorted and poorly rounded sediment is said to
color of sediment can provide useful information about a
nvironment. In general, colors of sedimentary rocks can
be interpreted in the following manner:
Red, yellow, brown
oxidation conditions, probably marine in
Black, gray, greenish
reducing conditions, probably
marine except for fl
oodplains and swamps.
Light gray or white
little iron present, either marine or non
marine; other characteristics of the rock must be considered such
as the presence of fossils, the type of fossils, whether or not there
Chemically formed sediments are produced under various conditions,
but generally speaking, when seawater becomes saturated with
chemicals, they will
out of solution. This is similar to when a
lot of sugar is added to
hot tea and then it is allowed to cool. Some of
the sugar will "
" or settle out of solution because the tea was
"saturated" with sugar and it could not stay dissolved.
usually form only in
environments such as
environments. Chemical Precipitates would
be found in
Limestone and Dolostone
These “carbonate rocks result from the
concentration and precipitation of Ca
, and CO
ions in the sea.
forms offshore from the
precipitation of calcium and carbonate ions that have been dissolved off
of the continents. Limestones may also be formed from the
accumulation of microscopic calcareous tests (shells) of planktonic (or
aquatic level) micro
forms in a similar
manner, but contains magnesium as well as calcium. Dolostone may
start off as limestone and later is subjected to groundwater replacing Ca
, some dolostones indicate having formed the
calcium/magnesium carbonate all at once.
are formed by living organisms. Many aquatic
marine organisms produce
or other protective coverings by
calcium carbonate (l
. When these organisms die, their shells
accumulate along the sea floor forming layers of broken shell fragments.
Such material is
produced and is ultimately broken by
water action They are
then referred to as "
is a good example of a bioclastic deposit.
The availability of nutrients decreases the further from the shore
therefore most marine organisms live in the coastal, shallow water
as. As the distance from shore increases, generally the number of
marine organisms decreases. The
of bioclastic sediments such
as coquina usually indicates a
form as the result of organics (such as vegetative
low energy, reducing, anaerobic
environments such as swamps. The material does not rot quickly and
the volatiles are driven off leaving behind the carbon. A good example
of an organic rock is
. The first stage is called
. As the
gets compressed over time, it becomes
. As lignite becomes
compressed, it becomes
. As bituminous coal
becomes compressed, it forms the metamorphic rock
final stage of coal. Other types of organic rocks ma
y form from
accumulations of dead organisms (such as fish) in low energy lagoons.
VII. Bedding or Layering of Sedimentary Materials
Sedimentary rocks are deposited in layers known as "
". The type
of bedding will vary depending on the environmen
t of deposition. Under
normal conditions, beds are deposited in
bedding planes (the line of contact between the beds) parallel to one
" occurs when the surface of deposition is
inclined (i.e. a delta) or
a current is present (i.e. a stream). This type of
bedding is called "
" and is indicative of these
The types of currents that form cross
bedding strata are:
river and stream action
Types of cross
the bedding planes separating
bedded units are parallel,
the bedding planes are at
an angle to one another and form a wedge; and
planes separating the
bedded units are curved.
Thick planar or wedged
bedding always indicates an aeolian
(wind) deposit such as a sand dune in the desert.
Thin planar or
units may be
. Because of this,
other characteristics su
must be used to determine the
environment of deposition.
of water (and sometimes wind) can be
traced by the
left in some sedimentary rocks indicating
ancient river channels or beachfronts.
n also be
preserved indicating ancient low energy mud flats.
Another type of bedding is known as
. This is where
there is a gradation in the size of particles within a unit of deposition.
Larger particles are found on bottom with success
sediments on top. This type of bedding is formed by "
", which are the sudden flows of material down the continental
slopes. This causes the finer particles to be suspended in the water
while the larger particles fall out a
nd are deposited on the bottom with
smaller and finer sediment on top. This results in a "
particle size. The facies of graded bedding is deep water marine.
VIII. The Marine Lithofacies:
This refers to the depositional sequence found in
a cross section of a
shore to deep
water environment. The usual sequences of rock types
1. Sandstone formed on beach areas
2. Siltstone formed near
3. Claystone/Shale formed further out
4. Limestone formed even further out in de
A schematic of the typical marine lithofacies is as follows:
The Marine Lithofacies
the advancement of the sea onto the land because of
a worldwide increase in sea level or a subsidence of the landmass.
the retreat of the sea from the land due to a worldwide
drop in sea level or the uplift of the land.
Transgressional and Regressional sequences of strata can be used to
interpret and retrace ancient coastlines.
From the Greek
= to change, and
“The altering of rock characteristics and mineral
compositions due to heat and/or pressure, or other environmental
factors. This changing is a
Solid State Reaction
, meaning that the
rocks subjected to metamorphic processes
do not melt
upon cooling, they would form igneous rocks). It is thought to be a
relatively slow geologic process. A great many areas o
metamorphism yield abundant mineral reserves of gold, silver, copper,
lead, zinc, and other valuable minerals.
Metamorphic rocks are formed either by being exposed to
pressure, or chemically active fluids
, or a combination of these
factors to cre
ate a rock that has a
different texture and mineral content
” is the term for the rock prior to metamorphism. It
may be igneous, sedimentary, or another metamorphic rock. For
example, here are some parent rocks and the rock that they ma
metamorphose into under certain conditions:
The effect of metamorphism on rocks is analogous to baking a cake:
the resulting cake is dependent upon the ingredients, the amount of
temperature, and the length of time it was “baked”.
A great portion of the continents is metamorphic formed during
” during the formation of the Precambrian.
Metamorphics form the stable basement rocks called
upon which surface sedimentary rocks have been deposited.
Metamorphics also comprise a large portion of the
many mountain ranges.
Factors Involved in Metamorphism
The source of heat may be from a large intrusive body such
as a pluton, or heat from activities associated with s plate tectonics.
At temperatures below 200
C, only a small amount of fluid is
present in most rocks. As the temperature increases many
that was trapped in the rock or in
crystal lattices of its minerals. This pore fluid may become very
chemically reactive, altering the chemistry of the surrounding
On average the temperature of the
rocks in the earth increase
C per kilometer of depth
continental cratons, the average is 20
C/km. On the continental
boundaries it is 40
C/km. At subduction zones, it is 10
because heat is dissipated into the sea.
C, most rock components become “
” where many
times the pre
, or twist altering the texture
of the rock.
Under conditions of high heat, pressure, and chemically active
fluids, crystal lattices begin to break down, recreate new types of
crystal lattices, rearrange ions, and form new minerals in the
Some minerals only form at certain temperature and pressures.
If these are found in a metamorphic rock, the temperature of
formation can be deduced.
When rocks are buried, they are subjected to
that is the
sides by the
overburden weight of the country rock...(This is similar to the intense
pressure increases experienced by going deeper and deeper in water).
may exist whereby the pressures
exerted upon the rock are
not equal in all directions. This results
in a distortion or twisting effect on the rock.
or distortion may occur. This can cause
grains in the rock to stretch, rotate, bend, line up in rows, become
platy, etc. (i.e. micas forming i
n mica schists)
of metamorphic rocks is common around
areas of high
such as areas around tectonic
Chemically Active Fluids
Fluids released from igneous
intrusions, or other metamorphic processes ca
n cause a constant
interaction or exchange of ions altering the rocks.
carried away in solution
the introduction by fluids of ions from an external
e not directly associated with the intrusion.
changes due to migrating
superheated water and dissolved ions. Hydrothermal rocks many
times appear “bleached” because of the intense chemical reactions.
Sources of water
1. Juvenile Water
water given off by cooling magma.
2. Metamorphic Water
water already present the country
rock, which is given off during metamorphic processes.
3. Meteoric Water
“groundwater” contained in aquifers
encountered in the c
ountry rock during metamorphic processes.
many times form
of gold, copper, iron, lead, etc. This process is
also responsible for the “veining” (“mother loads”) of gold and
other valuable mineral
Volcanic activities such as
usually have associated
hydrothermal activities resulting in mineral enrichment.
The Three Sources for Chemically Active Fluids in
Water trapped in the pore spaces of sedimentary rocks as
ter arising as volatile fluid within magma
Water from the
bearing minerals such
as Selenite Gypsum: CaSO
O, and some clays.
Types of Metamorphism
Effects of Heat and Fluids
“Heat” is the driving for
in contact metamorphism.
Common where hot magmatic plutons come into
surrounding country rock.
The degree of metamorphism is related to the
of the intrusion, and the
chemically active fluid
the magma involved. Large intrusions such as
batholiths cool for long periods of time so there is usually a more
intense metamorphic change in the country rock.
Temperatures can reach
next to the intrusion.
As the heat and associated metamorphic c
hanges alter the
country rock, the country rock
to the intrusion is affected
, and the
from the intrusion is affected
This sets up a “
” or “
” in the country
rock around the intrusion.
is a grad
ation of degrees of metamorphism
surrounding the intrusion such as the following:
unaltered country rock
low grade metamorphism
between low and medium grade
medium grade metamorphism
high grade metamorphism
very high grade metamorphism
7. Melting occurs at above this temperature resulting in the
formation of an
of contact metamorphic rocks are recognized:
1. those resu
lting from the “
” of the country rock
2. those resulting from the actions of
” types have the texture of
if they contain
high amounts of clay such as shale. This effect is seen in the
firing of cerami
cs in a kiln.
is also common with contact
metamorphism resulting in an enrichment of valuable ore
deposits. This occurs during the final stages of cooling, whenever
the magma begins to crystallize. Large amounts of hot, watery
ions are released. This process usually occurs near the
surface of the earth, also resulting in the enrichment of minerals
such as gold, silver, copper, lead, etc..
II. Regional Burial
Effects of Lithostatic Pressure
Occurs over a v
ery broad area
Rocks are altered due to
(and the resulting
high temperatures), resulting in deformation within deeper
portions of the crust.
Very common along convergent and divergent plate boundaries.
are minerals that
are known to form only under
certain temperatures and pressures. The following is a sequence
of known minerals that form from low grade metamorphism to
(forms around 200
C), muscovite, biotite,
garnet, staurolite, kyanite (
forms around 500
Quartz and feldspars can be present in both igneous and
metamorphic rocks, but some minerals such as andalusite,
sillimanite, and kyanite (all 3 minerals are forms of Al
only from these metamorphic conditions.
The presence o
r absence of these minerals is an indication of the
degree of pressure (and resulting heat) in the formation of the
rock in question.
Examples of regional burial rocks are:
from quartz sandstone, and
III. Dynamic Metamorphism (“Dynamo
Usually associated with the pressures around
is the term used to describe rocks formed in this
Typically, the extent of metamorphism is restricted to narrow
rgins adjacent to faults.
Myolinites are hard, dense, fine
grained rocks, many of which
have laminations or layerings.
These also can be associated with tectonic settings.
Textures of Metamorphic Rocks
I. Foliated Textures
ly associated with
Minerals are arranged in a platy, parallel fashion.
The size and shape of the mineral grains determines if the
usually indicates a higher degree of heat such
as in gne
usually indicates a lower degree of heat such as
is very fine foliation exhibiting the lowest grade of contact
Examples of Foliated Textured Metamorphic Rocks:
has a very fine foliation due to i
t having formed at the
lowest grade of contact metamorphism. It possesses a
, easily cleaving or parting along the axis of layering. It
is used for pool tables, chalkboards, and building tiles for this
reason. The different colors of slate
s are due to the presence of
minerals such as chlorite (green), graphite (black), or iron oxide
similar to slate but coarser grained. It is more lustrous
or glossy due to tiny mica minerals. Grains are too small to be
identified with t
he unaided eye.
is most commonly produced by regional burial
metamorphism. It can also be produced by medium grade
contact metamorphism. Metamorphosed clay rich sedimentary
rocks typically produce schists (although other rocks may also
hem). All schists contain more than
50% platy and
all of which large enough to identify. The
reflects the temperature of formation: the
greater the temperature, the greater the degree of schistosity.
common in low to medium grade metamorphic
environments. Schists are named as to the most abundant
mineral: mica schist, talk schist, biotite schist, chlorite schist, etc.
is a streaked or has segregated bands of alternating
light and dark mineral
s. Quartz and feldspar are the major light
colored minerals and biotite and hornblende are the principle dark
colored minerals. Gneiss typically forms from regional
metamorphism of clay
rich sedimentary rocks, from contact
metamorphism of granites, or fr
om metamorphism of older
colored, slightly foliated rock consisting
primarily of hornblende and plagioclase. The metamorphism of
mafic rocks such as basalt produce amphibolites.
hese have characteristics
of both igneous and metamorphic rocks indicating very high heat
and pressure. Examples include the rocks
the very highest grade contact metamorphism. Most contain
granite components, or
lenses (small piece
s of other rocks)
and appear to have been twisted or wavy. This may be due to
of the country rock.
II. Nonfoliated Textures
These textures result from the metamorphosing of rocks whose
show a p
referred orientation, and therefore are
foliated rocks result from contact or regional burial of
rocks that are devoid of platy or elongated crystals.
Two Types of Nonfoliated rocks:
those composed of mainly one mineral (marble o
those composed of mineral grains that are too small to be
seen as in hornfels or greenstones.
Examples of Non
foliated Textured Metamorphic Rocks:
the parent rock is a limestone (mostly calcite) or
dolostone (mostly dolomite)
that was subjected to contact or
regional burial. It may be fine
grained to coarse
variation is due to impurities in the parent rock. Because of its
texture and softness, marble has been used extensively for
arent rock is a quartz sandstone subjected to
medium to high grade contact or regional burial resulting in a
grained compact rock. Pure quartzite is white but
impurities may alter the color. Since it is so hard from the re
the quartz, it is commonly used for the bases
of roads and buildings.
this is the name given to any compact, dark
green, altered, mafic igneous rock that formed under low to
high grade metamorphic conditions. The green color is due to
nerals chlorite, epidote, and hornblende. These are
commonly the rocks found in “
” along the
transitional zones of sialic continental plates to mafic oceanic
grained, nonfoliated rock formed from contact
m. The grains are equidimensional with its
composition dependent upon the composition of the parent
rock. Most are formed from contact metamorphism of clay
sedimentary rocks or impure dolomites.
is a black, lustrous, hard coal that is h
carbon and low in volatiles. Its parent rock is bituminous coal
that was subjected to regional burial.
Metamorphic Zones or Facies
” is a group of metamorphic rocks
characterized by particular
ore than one
mineral is present) under the same broad temperature/pressure
Each facies is named after its most characteristic rock or mineral.
Metamorphic facies are usually are applied to areas whose
rocks were originally clay
tamorphic facies cannot be
applied to areas where the parent was pure limestone or pure
quartz sandstones because they would produce only marbles and
Examples of Metamorphic Facies:
forms whenever the rock
is rich in the
mineral chlorite and is subjected to relatively low temperatures
Granulite Facies and Amphibolite Facies
form under similar
chemistries but the pressures are significantly greater.
where, due to
the presence of seawater, the
temperature is low
, but because
of the tectonic activity,
the pressure is high
. This results in an
abundance of a blue
colored amphibole mineral named
. The presence of a blueschist facies indicate
the geologist the presence of ancient subduction zones.
the science of dating the earth and events in
There are two main types of dating techniques:
these techniques determine t
he order of
events…which one happened first, second, third, etc. Relative
tell you how many years ago
the event took
Absolute (Radiometric) Dating
these techniques use the
decay rates of radioactive isotopes found in rocks to dete
the precise number of years ago the rock in question formed.
II. Founders of Geochronology and Relative Dating
Archbishop Ussher (1600’s)
conscribed by the Pope at the
time to figure the age of the earth. He took the Bible and going
ations backwards to Genesis, and ascribing a standard
life span to the peoples mentioned, he figured that the earth was
created on October 26
, 4004BC, at 9:00 ante meridiem (a.m.)
(post meridiem is for “p.m.”). The last “4” in 4004 is to
a four year mistake whereby it is mentioned in the
Bible that the Magi (Wise Men) traveled to Bethlehem by way of
King Herod’s castle. Herod died 4 BC (“before Christ”) by today’s
calendar, making the birth of Christ around 4 BC! So, the second
m AD (Anno Domini = Latin “Year of our Lord”) was in
the year 1996, not 2000.
He was a Danish physician for the
Duke of Tuscany. When not attending the Duke, he hiked around
the countryside making notes of his observations of
streams eroded hills, how rocks were deposited, etc. He
proposed three ideas that are known as
in any sequence of undisturbed strata,
the oldest is on bottom and they are progressively younger to the
As rock layers are being
deposited, they are first deposited in a horizontal fashion and then
later uplifted, folded, or broken. (He did not take into
bedded layers or the near vertical la
In a sequence of strata,
particular rock layer does not go on forever laterally. There are
a. The depositing body, such as a river, may run out of
b. There may be a geographic barrier (i.e. a mountain
range on either side of a river valley) that prevents lateral
expansive deposition of a layer.
c. The conditions of the energy of deposition may
change such as larger parti
cles can be carried by the river’s
headwaters, but only sand and clay in the river’s path across the
coastal plains. This allows for a “feathering out” transition
between rock layer types.
James Hutton (1726
1797, Scottish Geologist)
His concept of “
” states that all of
the chemical and physical processes that go on today’s earth
(mountain building, volcanoes, erosion, deposition, etc.), also
went on in the geologic past. This meant that the earth mu
older than the 6000 years accepted by the Church. His Book
Theory of the Earth
describes that the earth must be millions of
years old, not thousands.
a student of Hutton. He is considered to be the
“Father of Geochronology” because
of his amendments to
uniformitarianism set forth in his book
Principles of Geology
Principle of Cross
states that any
intrusion or fault that cuts across a body must be younger than
the body it cuts. Another principle of his is
that states any rock included in another rock must be
older than the rock in which it is included (i.e. a sandstone may be
10 million years old, but the sand particles, inclusions, must be
older because they must have been weat
hered and eroded from
another older parent rock.
Thickness of sediment measurements
Both Hutton and Lyell
(as well as others) measured the outcrops of exposed, fossil
bearing, sedimentary rocks all over Europe. Supposing an
average sedimentation rate o
f 0.3 meters/1000years, and a total
thickness of 150,000 meters, they estimated the age of the earth
to be around 500 million years. The flaw with this idea is that they
were only measuring fossil
bearing strata of the Phanerozoic Eon.
They did not take
into consideration transgressive
sequences of the sea, interrupting depositional sequences. Also,
because of its sometimes inaccessibility for study, they did not
know of the vast amounts of
88% of earth’s dep
William “Strata” Smith
His concept of
Floral and Faunal Succession
states that fossil plants and
animals occur in the geologic record in a definite and
determinable order, and time periods can be recognized by thes
fossils. For every geologic time, there is a unique assemblage of
plant and animal fossils specific for that timeframe.
the “Father of Evolution”
In his book
Origin of the Species
, he laid down the concepts of natural
and evolution of life that in 1859 was accepted and
contributed to the acceptance that the earth was considerably
older than believed by the Church.
Baron Georges Cuvier
(1800’s French anatomist and
As an opponent of uniformitarianism,
believed that the Church’s accepted age of the earth was correct.
He believed in
(the “wrath of God”). This is the
concept of how mountains, valleys, crumpled rock layers, etc. was
by God unleashing some catastrophe upon the earth. He came
up with the concept of
to explain the age of the
He proposed that the earth is 90
million years old based on salinity measurements of the sea
compared to freshwater. He assumed that the seas were
freshwater. After measuring the (average) salinity of the
oceans (35ppt salts), he compared that to the average salinity
runoff of rivers. The 90
year estimate is an
approximation of the time required to make the sea have 35ppt
major flaw in this reasoning is that he did not consider
transgressions and regressions of the sea leaving vast amounts
of landlocked salts as evaporite rocks that would then again be
subjected to erosion…sea salts get “recycled”.
1907, English Physicist) tried to discredit
uniformitarianism by thermal (heat) studies. Assuming that the
earth was molten at the beginning, and, knowing the mass and
volume of the earth, and that the earth has continued to cool,
Kelvin figured that the
earth could not be younger than 20 million
years, or older than 400 million years. This broad range of ages
for the earth is due to his variability in his temperature data
collected in some deep mines in Europe. His major flaw is that he
did not take in
to account the heat created by the decay of
radioactive elements that has kept the earth’s interior from
cooling. These properties cause the earth not to lose heat at a
“Geologic time is continuous…deposition of rock l
ayers is not”
The surface processes of weathering and erosion erases
Deformation of once horizontal beds can create topographic
highpoints that are more apt to erode away creating
irregular erosional surfaces on the earth
re deposition can occur on top of these once
The irregular line between beds represents the
a hiatus or gap in depositional time
IV. Types of Unconformities
Sedimentary layers are deposited in a
ontal fashion. Then, at a later time, they are exposed to
erosion. Subsequently, the erosional surface that was formed
gets covered by more sedimentary rock layers.
Rock layers are deposited in a
horizontal fashion, then acted up
on by some diastrophic action
such as uplift of folding. As these layers erode, and are later
covered by more deposition on top, the layers at the bottom
remain angular or bent condition while the newer layers on top
is named so because the rock types do
across the erosional surface. If a granite pluton is
exposed by the erosion of the overburden or country rock, the
granite then begins to also erode. Later if sediment covers the
area, there is eroded ign
eous (or in some cases metamorphic)
rock on bottom with sedimentary rock on top.
is all about utilizing the above principles geologists
are able to “interpret” events of a particular outcrop to determine which
event came first, second, thi
rd, etc. Relative dating is
with the order of events,
not the ages
of those events.
V. Absolute Dating (Radiometric Dating)
92 naturally occurring elements
All matter is made up of chemical elements, with each bein
composed of extremely small particles called
The nucleus of an atom is comprised of positively charged particles
, neutrally charged particles called
negatively charged particles, called
eus in energy levels or
The number of protons in the nucleus of any atom of an element is
of that particular element. That is the basis of
the numbering of the elements on the Periodic Table of Elements.
For instance, t
has one proton in its nucleus.
Therefore it has an
atomic number of 1
has two protons
in its nucleus, and therefore has an
atomic number of 2
has 92 protons in its nucleus and therefore has an
atomic number of an element defines that element.
If an element
looses a proton
by some means, it is no longer that
element. Conversely, if an element
gains a proton
means, it is no longer that element.
Neutrons in the nucleus of an atom do not
affect its charge (since
neutrons are neutrally charged), but neutrons do affect the atomic
mass of the element.
of an element is the combined number of
protons and neutrons in the nucleus.
Not all atoms of the same element have the sa
me number of
neutrons in their nuclei. These variable forms of the same element
. For instance, hydrogen has an atomic
number of one: one proton and one electron. If a neutron is added
to the nucleus of hydrogen, it still has the same
atomic number, but
you have increased its atomic mass, forming the
. If another neutron is added to the
nucleus it becomes the isotope of Hydrogen called
three, Hydrogen, Deuterium, and Tritium all have a
number of one (one proton in the nucleus), but they are all
different isotopes of the same element
If you could continue to add neutrons to the nucleus of an atom, a
point would be reached that the nucleus would become very
unstable. Because of
our reality being “ruled” by the processes of
(whereby everything “wants” to be at its lowest point of
equilibrium, or its lowest “rest” state), atoms with unstable nuclei
(those with high neutron to proton ratios) begin to emit particles,
e refer to as
is the process whereby an unstable atomic
nucleus is spontaneously transformed into an atomic nucleus of a
There are three basic types of radioactive decay:
Electron Capture Decay
occurs when 2 protons and two neutrons are emitted
from the nucleus, resulting in a loss of 2 atomic numbers and 4
atomic mass numbers.
occurs when a neutron in the nucleus emits a fast
changing that neutron to a proton and
the atomic number by 1, with no resultant
atomic mass number change.
Electron capture decay
comes about by a proton in a nucleus
capturing an electron from an electron shell and thereby convert
into a neutron, resulting in the loss of one atomic number, but not
changing the atomic mass number.
Some elements undergo only one step to convert from an unstable
nucleus to a stable one. Others require several conversions until a
stable state is ac
hieved. For example, the element rubidium 87
decays to strontium 87 by a single beta emission, and potassium
40 decays to argon 40 by a single electron capture. Uranium 235
decays to lead 207 by seven alpha steps and six beta steps.
Uranium 238 decays t
o lead 206 by eight alpha and six beta steps.
of a radioactive element is the time it takes for one
half of the atoms of the original unstable
into atoms of a new, stable
element that an
element decays or
changes into. The half
life of radioactive elements is constant and
can be measured. Each different unstable radioactive element has
a different half
life that can range from less than a billionth of a
cond to 49 billion years.
All igneous rocks contain radioactive isotopes
. Whenever they
solidify (or cool) the radioactive parent isotope begins to decay into
the stable daughter element. So, whenever an igneous rock of
unknown age is found, a field sam
ple of it is taken, and the sample
is analyzed as to which radioactive isotope is present in
abundance. When that is determined, a survey of the daughter
element that particular radioactive element decays into is made
from the sample in question. This cr
radioactive parent isotope
the stable daughter isotope present in
what is called the
for that particular rock.
life (a measurement of time) for that particular radioactive
element found in abundance
in the field specimen is easily found in
physics and chemistry reference books. So, knowing the
percentage of the radioactive parent to the stable daughter element
present in the sample of igneous rock of unknown age, and
knowing the half
life for the rad
ioactive isotope in question, the
actual age of the igneous rock can be deduced.
only igneous rocks
can be dated using the following
procedures. For metamorphic rocks, only the age of the actual
metamorphism can be determined. In rare instances,
sedimentary rocks containing
radioactive potassium mineral found in some sedimentary deposits)
can give information on the age of deposition of the sedimentary
All igneous rocks can be dated using radiometric
Absolute dating techniques
involve the measurement of the
breaking down of certain radioactive elemental compounds in the
rock that have occurred over time. The rate of decay is known for
these radioactive elements from laboratory experimentation.
a geologist finds an igneous rock layer in the field and needs to
know the exact age of the rock, a sample is taken from the outcrop.
This sample is then sent to a laboratory that specializes in
radiometric dating techniques. There the rock is ground in
to a very
fine powder. This powder is then analyzed as to which radioactive
isotopes are present in the rock. This lab must be equipped with
an apparatus called a
. This analytical device
allows the geologist to project purified samples
of the rock in
question into a strong, fluctuating magnetic field that has sensors
that can detect the presence of different elements that have
different atomic masses. It works similarly to the following
scenario. If you turned on a strong fan and stoo
d in front of the fan
with a feather in one hand and a lead ball in the other, and
simultaneously let go of both, what would happen? The feather
would go shooting off because of its low weight (low mass) and the
lead ball would fall to the ground because
of its high weight (high
mass). It’s the same principle whenever the atoms of different
masses are projected through the magnetic field of the mass
spectrometer: the “lighter” elements “fall” through the magnetic field
differently than the “heavier” eleme
nts, there fore hitting the
sensors at different areas and different rates. This is how the
parent daughter ratio
is determined in an unknown sample.
To fully understand this technique, one must be familiar the
Varieties of t
he same element that have different
mass numbers. Their nuclei contain the same number of
protons but different numbers of neutrons.
the full amount of isotope in the newly
formed igneous rock.
(what the parent isotope
turn into) the amount of altered parent isotope over
last daughter isotope is stable.
The time it takes for one
half of the atoms of a
radioactive substance (Parent Material) to decay into another
element (Daughter Ma
terial). For example, Uranium 238
(Parent) decays to Lead 206 (Daughter). The rate of decay
is known for many of the naturally occurring radioactive
elements. So, if the rate of decay is known, and the ratio of
parent material to daughter material is me
asured in a rock,
then the age of the formation of the rock can be found.
the laboratory device used in
determining the relative amounts of residual radioactive
Parent and stable Daughter isotopes.
VI. Other Radiometric Dating Techniq
Carbon 14 Dating
There are three common isotopes of carbon:
In the upper atmosphere, nitrogen gas is bombarded by
cosmic radiation transforming it into radioactive
, forms. Along with this is
from other sources such as volcanic eruptions.
During photosynthesis in plants, CO
is taken in, and along
with water, sunlight, and some pigment such as chlorophyll,
sugars are made.
Of the sugars made, isotopes of carbon are in a ratio of 1/3
C, and 1/3
C. Other forms of life dependent on
sugars produced by photosynthesis for food. As the sugars
are eaten and digested, they become incorporated in to the
on containing compounds of their bodies. As long as
they live, the ratio of the carbon isotopes is 1:1:1. Whenever
organisms die, the
C begins to decay back into nitrogen at
life of 5730 years.
Any organic remains may be dated using this method
around 75,000 years ago making
C dating especially useful
As trees grow they create rings of xylem tissues
representing each year of growth. By counting the rings, the
age of the tree can be determined. By
growth patterns from different trees, a timeline backwards
can be established. This is particularly accurate back to
around 14,500 years ago, again greatly benefiting
Fission Track Dating
As radioactive elements i
n rocks decay, particles are emitted that
leave tiny, microscopic tracks in the crystals of minerals. The
older the rock, the more the tracks the crystals contain. By
counting the tracks, the ages of rocks formed between 40,000
years ago to 1.5 million y
ears ago can be determined. This
method is useful because this time frame is difficult to date: it is
too old for
C techniques, and many times too young for other
radioactive isotope techniques.
Examples of Radiometric Problems
1.) In the geologi
c past, a rock formed from cooling magma, containing 1
gram of radioactive Uranium 238 and no Lead 206. Many years later a
geologist who wants to find the exact age of this rock collects a sample.
If the half
life for U
is 4.5 billion years and after
analysis the Uranium
238 to Lead 206 ratio (parent/daughter ratio) was 1:1 (50% U & 50%
Pb), how old is the rock?
2.) A rock specimen was found that had a ratio of Potassium
40, which was 1:7 (1 part Potassium
40 to 7 parts of Argon
40 has a half
life of 1.3 billion years. How old is the rock?
3.) A geologist collects a piece of a meteorite rock in the field and wants
to know the exact age of the rock. After close examination of the
specimen, it was discovered that the s
pecimen contains sufficient
amounts of the potassium 40 to warrant using the K
Knowing that the half
life of K
is 1.3 billion years and that there was a
ratio of 1 part K
to 3 parts Ar
, how old is the rock?
4.) If a rock contained
a parent/daughter ratio of “parent element X” to
“daughter element Y” of 3:1, and the known age of the rock is 500
million years, what is the half
life of “element X”
VII. The Geologic Time Scale
This is a calendar of sorts stretching from the birth o
f the earth,
until today. It has taken the work of thousands of scientists and it
is still being updated every three years or so as dating techniques
become more and more precise.
Study the handout of the geologic time scale focusing on the
mentioned in lecture class.