Download Sediments and Sedimentary Rocks

Weathering and Erosion:
The Formation of Sediments and Soil
I. Differences between the earth and the moon:
 Earth is tectonically active – diastrophic movement is the
continual uplift, folding, and breaking of the earth’s surface.
Subsequently, it is “torn down” by the surface processes of
weathering and erosion.
 The earth has a strong enough gravitational force to retain
an atmosphere and surface water.
 The hydrologic cycle drives most of the surface processes
of weathering and erosion.
II. Define Weathering and Erosion Weathering - “The decomposition and disintegration of rocks and
minerals at the Earth’s surface by mechanical and physical
processes. Weathering processes involve very little or no movement
(or removal of) decomposed earth material.
Erosion – “The removal of weathered rocks and minerals from
the place where they formed. Forces or transporting agents
involved in moving disintegrated earth materials are:
1. Water – running water such as streams, rivers, etc.
2. Wind – prevailing winds, tornadic storms, sea breezes, etc.
3. Gravity – The influence of gravity causing landslides,
avalanches, etc.
4. Ice – in the form of glaciers
III. Modification of the Earth’s Surface  Weathering, erosion, and transportation of earth materials
 Surface processes continually wear away rocks and landforms
 In geologic time they combine to wear away entire mountain
ranges, reducing them to flat, low-lying plains.
IV. Types of General Weathering 1. Mechanical (or Physical) Weathering – The physical
disintegration of rock into smaller and smaller pieces. The
chemical composition of the rocks and minerals are not altered.
The particles formed are called clastics (meaning “broken”)
2. Chemical Weathering – occurs when air and water react
chemically with rocks to 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
ions.)
3. Differential weathering - Rocks weather by both mechanical and
chemical processes occurring together. Since rocks are not
homogenous in composition, usually parts weather at different
rates. This is called differential weathering resulting in an uneven
surface.
4. Spheriodal weathering – 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 “rounding” of the angular piece. This is
spheriodal weathering.
V. Types of Mechanical (Physical) Weathering 1. Frost wedging – Water expands 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 rock
fall hazard. The loose angular rock debris at the base of
mountains and cliffs is termed tallus.
2. Salt Cracking – Whenever 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 salt
cracking 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.
3. Abrasion – This is the mechanical wearing and grinding on rock
surfaces by friction and impact with other rock materials. This
gives the rocks a rounded appearance. This occurs in flowing
water, wind actions (i.e. natural sand blasting), and glaciers.
4. Organic Activity – Plant roots can crack rock material by the
hydraulic pressures associated with root growth. Also, burrowing
animals can contribute to rock disintegration.
5. 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 a granite
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 exfoliation. 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
hydration. This is one of the processes that can turn feldspars
into kaolinite, a major clay-forming mineral.
6. Thermal Expansion and Contraction - Heat causes matter to
expand and cold 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 contraction 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 1. Oxidation – reactions with oxygen – rusting:
4 Fe + 3 O2  2 Fe2O3
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.
2. Corrosion – reactions involving oxygen, water, and CO2 found in
the air and water. Combinations of these can cause corrosive
chemical conditions that can chemically weather rocks.
3. Weathering by Solution – dissolution whereby ions disperse
into water. I.e. rivers flowing across limestone can dissolve Ca+
and CO3- and carry these ions away.
4. Acids and Bases – Acids are solutions with an abundance of
free hydrogen ions (H+), while bases are solutions that have an
abundance of free hydroxyl ions (OH-). Acids and bases dissolve
minerals by pulling atoms out of crystals. Carbonic Acid
(H2CO3) is formed in abundance in nature whenever CO2
dissolves in some rivers and streams.
5. Acid Rain – During storms, Nitric Acid, (H2NO3) is formed by
lightning breaking apart N2 in the atmosphere into N + N. This
combines with water to form nitric acid causing rainwater to
naturally become slightly acidic with a pH of 5.5 – 6.5.
Pollutants in the atmosphere such as sulfur dioxide gasses can
also contribute to acid rain.
Soil: One Product of Mechanical and Chemical Weathering
I. The Components of Soil 1. Regolith - the loose, unconsolidated, weathered rock overlying
the bedrock. Since different geographic locations have their own
unique geologic histories with different rock chemistries, there are
different bedrocks and different regoliths resulting in a broad variety
of soil types worldwide.
2. Soil – (Pedal = Greek for “soil”); Earth material that has been
so modified and acted upon by chemical, physical, and biologic
agents that it will support rooted plants.
3. Soil terms –
 Loam – a mixture of sand, silt, and clay sized particles,
along with organic matter.
 Litter – plant or animal matter before decay processes.
 Humus – term for when litter decomposes sufficiently that it
can no longer be identified.
4. Soil Profiles –
 Horizon – the uppermost layer of a mature soil that is
composed largely of litter and humus with relatively small
amounts of minerals.
 A Horizon – is a mixture of humus and minerals in the form of
sand, silt, and clay. Layers “O” and “A” horizons are referred
to as topsoil.
 B Horizon – 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
low.
 C Horizon – this lies directly on unweathered “parent”
bedrock and consists of partially weathered rock.
5. Dissolved Material –
 Leeching – the downward 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
accumulates.
6. Soil-Forming Factors  Parent Rock – the nature of the soil is partially dependent
on the nature of its parent rock, including the texture of the
soil and its nutrients.
 Time – It has been estimated that for the creation of 1 inch
of topsoil, natural processes need around 100 years.
 Climate – 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 climates condusive to soil
formation. A very many distinct soil types exist in the world.
1. Pedocals – 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 caliche or hard pan
layers comprised of calcium carbonate. This also
causes salinization of the soil (an accumulation of
salts) which limits that amounts of vegetation that
can grow. This in turn reduces the ability to form a
good “O” horizon.
2. Pedalfers – 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
and iron.
3. Laterites – 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 bauxite (a major aluminum
ore) forms here.
7. Rates of Growth and Decay of Organic Matter - This is related
to the accumulation of humus:
 Temperate latitudes are ecologically balanced that
thick layers of humus occurs resulting in the most
fertile soils.
 Tropical latitudes have so much water that
decomposition by bacteria, mold and other fungi that
decomposition is so rapid that very little humus level
forms.
 Deserts have so much salinization that abundant plant
life cannot be supported so very little or no humus
develops.
 Polar regions are condusive to such slow plant growth
that little humus forms.
8. 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.
9. Soil Erosion 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 runoff.
Today, the 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
Most fossils 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
the environment in which it was deposited. The term used by
geologist to describe this aspect of sedimentary beds is “facies”. Much
can be learned about the ancient environments of the earth by studying
various characteristics of sedimentary rocks.
All rocks form initially with the solidification of molten magma or lava.
These newly formed igneous rocks are subsequently subjected to the
surface processes of weathering and erosion (the destructive actions of
running water, wind, glaciers, etc.) These rock fragments eventually
settle out somewhere to form “sediments”. These sediments can
become compacted to form sedimentary rocks. If these “new”
sedimentary rocks are subjected to enough heat and pressure, they may
become changed into “metamorphic” 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 type into another by
melting, pressure deformation, and weathering and erosion. All rocks
are initially igneous (The word “Igneous” means “born of fire”). Surface
processes can then weather and erode these igneous rocks into
sediments that can form sedimentary 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:
 Igneous rocks make 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. Intrusive igneous rock forms
below the surface of the earth. Extrusive igneous rocks
form from molten material that has been forced out onto
the surface of the earth (i.e. volcanoes).
 Sedimentary rocks form from the accumulation of eroded
debris of other rocks or chemically from elements in
seawater. Sedimentary rocks make up 75% of all of the
rocks exposed 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.
 Metamorphic rocks are formed from pre-existing rocks
that have been altered as the result of intense heat and
pressure. Metamorphism increases the “crystallinity“ and
hardness of the rock; sandstone changes to quartzite;
shale changes to slate, and limestone changes 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), sedimentary rocks 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), Gypsum
(CaSO4), 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 formation of clastic sedimentary rocks (“clastic” means "broken")
are described on the following pages.
IV. The Four Steps for Formation of Sedimentary
Rocks:
1. Physical and Chemical Weathering of the “Parent Rock” (the
source rock from which the clastic 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).
2. Transportation is the stage where the clastics are
"moved"(“transported”) from the source area by water, wind, gravity,
or ice. The terrain determines the area of transportation. The distance
the particles are moved depends on the amount of energy operating in
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. High- energy environments would include white water
mountain streams that are capable of moving almost all sizes of
particles. Low-energy environments include lagoons, lakes, deltas,
swamps, etc., that are capable of moving only the smaller particles.
3. Deposition is the stage where the sediment is deposited in a
particular geographic environment, which constitutes the sedimentary
environment. 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 mountain 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.
Sedimentary rocks can be interpreted to find out the environment in
which they formed.
Sedimentary Environments can be divided into several categories:
 Shoreline and Coastal Environments
 “Fluvial” or Stream, River, and Delta Environments
 Alluvial Fans or deposits at the bases of
mountains
 “Aeolian” or “wind-borne” deposits
There are numerous other sedimentary environments that your
instructor will inform you of at the appropriate time
4. Compaction is the final stage in the formation of a sedimentary
rock. At this stage the sediments are compacted due to the weight of
the overburden (overlying sediments) and can be eventually “lithified”
(turned to stone) as the particles are cemented together with substances
such as Calcite (CaCO3), Silica (SiO2), or forms of Iron Oxide (i.e.
Fe2O3), 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
particle size, degree of roundness, degree of sorting, and color.
1. Particle Size: 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 size is
based on its particle size rather than its chemical composition. For
example, "sand" refers to particles having a size range between
0.125mm – 0.5mm. There can be quartz sand such as that found along
the Gulf Coast or there may be feldspar sands, gypsum 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
a high- energy environment such as a river channel while clays
represent a low energy environment such as a floodplain or swamp.
The 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
Boulders
Cobbles
Approximate Particle Diameter in millimeters
greater than 256mm
128
64
32
Pebbles
16
8
4
Granules
2
Very Coarse Sand
1.0
Course Sand
0.5
Fractional Equivalents
1/2
0.25
1/4
0.125
0.0625
0.0313
0.0156
1/8
1/16
1/32
1/64
0.0078
0.0039
1/128
1/256
Medium Sand
Fine Sand
Very Fine Sand
Silt
Clay
less than 1/256
2. Roundness: 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 “poorly rounded”. Particles that are
smooth and have no edges are called “well rounded”. 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 amount of saltation (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 saltated
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 parent
rocks. A high-energy environment, which allows for a long period of
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
period of exposure to agitation, such as an alluvial fan, prevents the
sediments from becoming “well-rounded”
3. Sorting: refers to rock fragments separated according to particle
size. “poorly sorted” 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 alluvial fans at the base of a mountain. This
results in a "dumping effect" of sediments at the base of the mountain
(high- energy to low- energy). “Well Sorted” 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. Of 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 sorting
of the sediments. Sediment is said to be "Mature" if it is well rounded
and well sorted. Poorly sorted and poorly rounded sediment is said to
be "Immature".
4. Color: The color of sediment can provide useful information about a
sedimentary environment. In general, colors of sedimentary rocks can
be interpreted in the following manner:
a.) Red, yellow, brown - oxidation conditions, probably marine in
origin.
b.) Black, gray, greenish-gray - reducing conditions, probably
marine except for floodplains and swamps.
c.) Light gray or white - little iron present, either marine or nonmarine; other characteristics of the rock must be considered such
as the presence of fossils, the type of fossils, whether or not there
is cross-bedding, etc.
VI. Chemical Precipitates:
Chemically formed sediments are produced under various conditions,
but generally speaking, when seawater becomes saturated with
chemicals, they will precipitate 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 "crystallize" or settle out of solution because the tea was
"saturated" with sugar and it could not stay dissolved. Precipitates
usually form only in low energy environments such as lagoons or
deep-sea environments. Chemical Precipitates would not be found in
high- energy environments.
Limestone and Dolostone – These “carbonate rocks result from the
concentration and precipitation of Ca+, Mg+, and CO3- ions in the sea.
Limestone - Ca CO3 (primarily calcite)- 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
other aquatic level) micro-organisms.
Dolostone - Ca,Mg (CO3)2 (primarily dolomite)- 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+
with Mg+. Or, some dolostones indicate having formed the
calcium/magnesium carbonate all at once.
“Bioclastic sediments” are formed by living organisms. Many aquatic
marine organisms produce shells or other protective coverings by
secreting calcium carbonate (limestone) or calcium magnesium
carbonate (dolomite). When these organisms die, their shells
accumulate along the sea floor forming layers of broken shell fragments.
Such material is biochemically produced and is ultimately broken by
water action They are then referred to as "bioclastic sediments". The
sedimentary rock coquina 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
areas. As the distance from shore increases, generally the number of
marine organisms decreases. The facies of bioclastic sediments such
as coquina usually indicates a beachfront.
“Organic Rocks” form as the result of organics (such as vegetative
matter) accumulating in 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 coal. The first stage is called peat. As the peat
gets compressed over time, it becomes lignite coal. As lignite becomes
compressed, it becomes bituminous coal. As bituminous coal
becomes compressed, it forms the metamorphic rock anthracite, the
final stage of coal. Other types of organic rocks may 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 "beds". The type
of bedding will vary depending on the environment of deposition. Under
normal conditions, beds are deposited in horizontal layers with the
bedding planes (the line of contact between the beds) parallel to one
another. "Cross-bedding" 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 "cross-bedding" and is indicative of these
environments.
The types of currents that form cross-bedding strata are:
a. Aeolian - wind action
b. Fluvial - river and stream action
c. Marine in Origin - current action
Types of cross-bedding include planar - the bedding planes separating
the cross-bedded units are parallel, wedged - the bedding planes are at
an angle to one another and form a wedge; and trough - the bedding
planes separating the cross-bedded units are curved.
Thick planar or wedged cross-bedding always indicates an aeolian
(wind) deposit such as a sand dune in the desert. Thin planar or
wedged units may be aeolian, fluvial, or marine. Because of this,
other characteristics such as color must be used to determine the
environment of deposition.
Many times paleocurrents of water (and sometimes wind) can be
traced by the ripple marks left in some sedimentary rocks indicating
ancient river channels or beachfronts. Mud cracks can also be
preserved indicating ancient low energy mud flats.
Another type of bedding is known as graded bedding. This is where
there is a gradation in the size of particles within a unit of deposition.
Larger particles are found on bottom with successively smaller
sediments on top. This type of bedding is formed by "turbidity
currents", 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 and are deposited on the bottom with
smaller and finer sediment on top. This results in a "gradation" in
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
are:
1. Sandstone formed on beach areas
2. Siltstone formed near-shore
3. Claystone/Shale formed further out
4. Limestone formed even further out in deeper waters
A schematic of the typical marine lithofacies is as follows:
The Marine Lithofacies
Transgression: - the advancement of the sea onto the land because of
a worldwide increase in sea level or a subsidence of the landmass.
Regression: - 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.
Transgressional Sequence
Regressional Sequence
Metamorphism
Metamorphism – From the Greek “meta” = to change, and
“morpho” = shape.
Metamorphism – “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 (otherwise
upon cooling, they would form igneous rocks). It is thought to be a
relatively slow geologic process. A great many areas of
metamorphism yield abundant mineral reserves of gold, silver, copper,
lead, zinc, and other valuable minerals.
Metamorphic rocks are formed either by being exposed to heat,
pressure, or chemically active fluids, or a combination of these
factors to create a rock that has a different texture and mineral content.
The “parent rock” 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 may
metamorphose into under certain conditions:
 Limestone – marble
 Clay stone – slate
 Granite – gneiss, etc.
The effect of metamorphism on rocks is analogous to baking a cake:
the resulting cake is dependent upon the ingredients, the amount of
fluids, the temperature, and the length of time it was “baked”.
A great portion of the continents is metamorphic formed during
“continental accretion” during the formation of the Precambrian.
Metamorphics form the stable basement rocks called “continental
shields” upon which surface sedimentary rocks have been deposited.
Metamorphics also comprise a large portion of the crystalline core of
many mountain ranges.
Factors Involved in Metamorphism
I. Heat – 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 2000 C, only a small amount of fluid is
present in most rocks. As the temperature increases many
minerals release pore fluid 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
rocks.
 The Geothermal Gradient – On average the temperature of the
rocks in the earth increase 250C per kilometer of depth. On the
continental cratons, the average is 200C/km. On the continental
boundaries it is 400C/km. At subduction zones, it is 100C/km
because heat is dissipated into the sea.
 At 7000C, most rock components become “plastic” where many
times the pre-existing crystals rotate, 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
process.
 Some minerals only form at certain temperature and pressures.
If these are found in a metamorphic rock, the temperature of
formation can be deduced.
II. Pressure – When rocks are buried, they are subjected to
lithostatic pressure that is the pressures from all 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).
 Differential pressures – 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.
 Phenocryst rotation 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 in mica schists)
 Pressure distortion of metamorphic rocks is common around
areas of high lithologic stress such as areas around tectonic
boundaries.
III. Chemically Active Fluids – Fluids released from igneous
intrusions, or other metamorphic processes can cause a constant
interaction or exchange of ions altering the rocks.
i.e.
2Mg2SiO4 + 2H2O  Mg3Si2O5 + MgO
Olivine
Water
Serpentine
carried away in solution
Metasomatism – the introduction by fluids of ions from an external
source not directly associated with the intrusion.
Hydrothermal Metamorphism – 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 country rock during metamorphic processes.
 Hydrothermal activities – many times form economically rich
mineral deposits of gold, copper, iron, lead, etc. This process is
also responsible for the “veining” (“mother loads”) of gold and
other valuable minerals.
 Volcanic activities such as calderas usually have associated
hydrothermal activities resulting in mineral enrichment.
The Three Sources for Chemically Active Fluids in
Metamorphism:
1. Water trapped in the pore spaces of sedimentary rocks as
they form
2. Water arising as volatile fluid within magma
3. Water from the dehydration of water-bearing minerals such
as Selenite Gypsum: CaSO42H2O, and some clays.
Types of Metamorphism
I. Contact – Effects of Heat and Fluids
Characteristics:
 “Heat” is the driving force in contact metamorphism.
 Common where hot magmatic plutons come into contact with the
surrounding country rock.
 The degree of metamorphism is related to the temperature of the
magma, the size of the intrusion, and the chemically active fluid
content of 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 9000C next to the intrusion.
 As the heat and associated metamorphic changes alter the
country rock, the country rock closest to the intrusion is affected
most, and the furthest from the intrusion is affected least.
 This sets up a “metamorphic halo” or “aureole” in the country
rock around the intrusion.
 The aureole is a gradation of degrees of metamorphism
surrounding the intrusion such as the following:
1. Shale – unaltered country rock
2. Slate – low grade metamorphism
3. Phyllite –between low and medium grade
4. Schist – medium grade metamorphism
5. Gneiss – high grade metamorphism
6. Migmatite – very high grade metamorphism
7. Melting occurs at above this temperature resulting in the
formation of an igneous rock.
 Two types of contact metamorphic rocks are recognized:
1. those resulting from the “baking” of the country rock
2. those resulting from the actions of chemically active
fluids
 Many “baked” types have the texture of porcelain if they contain
high amounts of clay such as shale. This effect is seen in the
firing of ceramics in a kiln.
 Hydrothermal activity 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
solutions 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
Characteristics:
 Occurs over a very broad area
 Rocks are altered due to tremendous pressures (and the resulting
high temperatures), resulting in deformation within deeper
portions of the crust.
 Very common along convergent and divergent plate boundaries.
 Index minerals 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
high grade:
chlorite – (forms around 2000C), muscovite, biotite,
garnet, staurolite, kyanite (forms around 5000C)
 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 Al2SiO5) form
only from these metamorphic conditions.
 The presence or 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: marble from limestone,
quartzite from quartz sandstone, and argillite from clay.
III. Dynamic Metamorphism (“Dynamo-thermal”)Characteristics:
 Usually associated with the pressures around fault zones.
 “Mylonites” is the term used to describe rocks formed in this
way.
 Typically, the extent of metamorphism is restricted to narrow
margins 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 Characteristics:
 Typically associated with contact metamorphism.
 Minerals are arranged in a platy, parallel fashion.
 The size and shape of the mineral grains determines if the
foliation is fine or coarse.
 A coarse foliation usually indicates a higher degree of heat such
as in gneiss.
 A fine foliation usually indicates a lower degree of heat such as
in schist.
 Slate is very fine foliation exhibiting the lowest grade of contact
metamorphism.
Examples of Foliated Textured Metamorphic Rocks:
1. Slate –has a very fine foliation due to it having formed at the
lowest grade of contact metamorphism. It possesses a slaty
cleavage, 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 slates are due to the presence of
minerals such as chlorite (green), graphite (black), or iron oxide
(red).
2. Phyllite – 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 the unaided eye.
3. Schist – 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
produce them). All schists contain more than 50% platy and
elongated minerals all of which large enough to identify. The
degree of schistosity reflects the temperature of formation: the
greater the temperature, the greater the degree of schistosity.
Schists are 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.
4. Gneiss – is a streaked or has segregated bands of alternating
light and dark minerals. 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 from metamorphism of older
metamorphic rocks.
5. Amphibolite – a dark-colored, slightly foliated rock consisting
primarily of hornblende and plagioclase. The metamorphism of
mafic rocks such as basalt produce amphibolites.
6. Migmatites – “mixed metamorphics” – These have characteristics
of both igneous and metamorphic rocks indicating very high heat
and pressure. Examples include the rocks touching an intrusion:
the very highest grade contact metamorphism. Most contain
granite components, or lenses (small pieces of other rocks),
and appear to have been twisted or wavy. This may be due to
partial melting of the country rock.
II. Nonfoliated Textures Characteristics:
 These textures result from the metamorphosing of rocks whose
minerals do not show a preferred orientation, and therefore are
not foliated.
 Most non-foliated rocks result from contact or regional burial of
rocks that are devoid of platy or elongated crystals.
Two Types of Nonfoliated rocks:
1. those composed of mainly one mineral (marble or quartzite)
2. those composed of mineral grains that are too small to be
seen as in hornfels or greenstones.
Examples of Non-foliated Textured Metamorphic Rocks:
1. Marble – 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-grained. Color
variation is due to impurities in the parent rock. Because of its
texture and softness, marble has been used extensively for
sculpturing.
2. Quartzite – the parent rock is a quartz sandstone subjected to
medium to high grade contact or regional burial resulting in a
hard, coarse-grained compact rock. Pure quartzite is white but
impurities may alter the color. Since it is so hard from the recrystallization of the quartz, it is commonly used for the bases
of roads and buildings.
3. Greenstone – 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
the minerals chlorite, epidote, and hornblende. These are
commonly the rocks found in “greenstone belts” along the
transitional zones of sialic continental plates to mafic oceanic
plates.
4. Hornfels – fine-grained, nonfoliated rock formed from contact
metamorphism. The grains are equidimensional with its
composition dependent upon the composition of the parent
rock. Most are formed from contact metamorphism of clay-rich
sedimentary rocks or impure dolomites.
5. Anthracite – is a black, lustrous, hard coal that is high in
carbon and low in volatiles. Its parent rock is bituminous coal
that was subjected to regional burial.
Metamorphic Zones or Facies –
 A “metamorphic facies” is a group of metamorphic rocks
characterized by particular mineral assemblages (more than one
mineral is present) under the same broad temperature/pressure
conditions.
 Each facies is named after its most characteristic rock or mineral.
 Metamorphic facies are usually are applied to areas whose parent
rocks were originally clay-rich. Metamorphic facies cannot be
applied to areas where the parent was pure limestone or pure
quartz sandstones because they would produce only marbles and
quartzites respectively.
Examples of Metamorphic Facies:
1. Greenschist Facies – forms whenever the rock is rich in the
mineral chlorite and is subjected to relatively low temperatures
and pressures.
2. Granulite Facies and Amphibolite Facies – form under similar
chemistries but the pressures are significantly greater.
3. Blueschist Facies – form at subduction zones 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
glaucophane. The presence of a blueschist facies indicates to
the geologist the presence of ancient subduction zones.
Geologic Time
I. Geochronology – the science of dating the earth and events in
earth’s history.
There are two main types of dating techniques:
1. Relative Dating– these techniques determine the order of
events…which one happened first, second, third, etc. Relative
dating does not tell you how many years ago the event took
place.
2. Absolute (Radiometric) Dating – these techniques use the
decay rates of radioactive isotopes found in rocks to determine
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
from Revelations backwards to Genesis, and ascribing a standard
life span to the peoples mentioned, he figured that the earth was
created on October 26th, 4004BC, at 9:00 ante meridiem (a.m.)
(post meridiem is for “p.m.”). The last “4” in 4004 is to
compensate for 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
millennium AD (Anno Domini = Latin “Year of our Lord”) was in
the year 1996, not 2000.
 Nicholas Steno – (1600’s) – 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 geology: how
streams eroded hills, how rocks were deposited, etc. He
proposed three ideas that are known as Steno’s Principles.
1. Superposition – in any sequence of undisturbed strata,
the oldest is on bottom and they are progressively younger to the
top.
2. Original Horizontality – 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
consideration cross-bedded layers or the near vertical layering at
river deltas.)
3. Lateral Continuity – In a sequence of strata, one
particular rock layer does not go on forever laterally. There are
limiting factors:
a. The depositing body, such as a river, may run out of
sediments to deposit.
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 particles 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) – “the Father
of Geology” – His concept of “Uniformitarianism” 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 must be
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.
 Charles Lyell – 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.
His Principle of Cross-cutting Relationships states that any
intrusion or fault that cuts across a body must be younger than
the body it cuts. Another principle of his is the Principle of
Inclusions 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 weathered 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 of 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-regressive
sequences of the sea, interrupting depositional sequences. Also,
because of its sometimes inaccessibility for study, they did not
know of the vast amounts of Precambrian strata that represents
88% of earth’s depositional history.
William “Strata” Smith – (1800’s Geologist) – 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 these
fossils. For every geologic time, there is a unique assemblage of
plant and animal fossils specific for that timeframe.
Charles Darwin – the “Father of Evolution” – In his book The
Origin of the Species, he laid down the concepts of natural
selection 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
paleontologist) – As an opponent of uniformitarianism, he
believed that the Church’s accepted age of the earth was correct.
He believed in Deus Irae (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 Catastrophism to explain the age of the
earth.
John Joly – (1899) – He proposed that the earth is 90 – 100
million years old based on salinity measurements of the sea
compared to freshwater. He assumed that the seas were
originally freshwater. After measuring the (average) salinity of the
oceans (35ppt salts), he compared that to the average salinity
runoff of rivers. The 90 – 100 million-year estimate is an
approximation of the time required to make the sea have 35ppt
salts. The 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”.
 Lord Kelvin – (1824 – 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 into 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
regular rate.
III. Unconformities
“Geologic time is continuous…deposition of rock layers is not”
 The surface processes of weathering and erosion erases
depositional evidence.
 Deformation of once horizontal beds can create topographic
highpoints that are more apt to erode away creating
irregular erosional surfaces on the earth.
 Later, more deposition can occur on top of these once
erosional surfaces.
 The irregular line between beds represents the
Unconformity – a hiatus or gap in depositional time.
IV. Types of Unconformities
1. Disconformity – Sedimentary layers are deposited in a
horizontal 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.
2. Angular Unconformity – Rock layers are deposited in a
horizontal fashion, then acted upon 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
are horizontal.
3. Nonconformity – This is named so because the rock types do
not conform 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 igneous (or in some cases metamorphic)
rock on bottom with sedimentary rock on top.
Relative Dating is all about utilizing the above principles geologists
are able to “interpret” events of a particular outcrop to determine which
event came first, second, third, etc. Relative dating is only concerned
with the order of events, not the ages of those events.
V. Absolute Dating (Radiometric Dating)
 There are 92 naturally occurring elements in nature.
 All matter is made up of chemical elements, with each being
composed of extremely small particles called atoms.
 The nucleus of an atom is comprised of positively charged particles
called protons, neutrally charged particles called neutrons, with
negatively charged particles, called electrons encircling the
nucleus in energy levels or electron shells.
 The number of protons in the nucleus of any atom of an element is
the atomic number of that particular element. That is the basis of
the numbering of the elements on the Periodic Table of Elements.
For instance, the element Hydrogen has one proton in its nucleus.
Therefore it has an atomic number of 1; Helium has two protons
in its nucleus, and therefore has an atomic number of 2; Uranium
has 92 protons in its nucleus and therefore has an atomic number
of 92. The 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 by some
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.
 The atomic mass of an element is the combined number of
protons and neutrons in the nucleus.
 Not all atoms of the same element have the same number of
neutrons in their nuclei. These variable forms of the same element
are called isotopes. 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 isotope of
hydrogen called deuterium. If another neutron is added to the
nucleus it becomes the isotope of Hydrogen called tritium. All
three, Hydrogen, Deuterium, and Tritium all have an atomic
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
entropy (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,
which we refer to as radioactivity.
 Radioactive decay is the process whereby an unstable atomic
nucleus is spontaneously transformed into an atomic nucleus of a
different element.
 There are three basic types of radioactive decay:
1. Alpha Decay
2. Beta Decay
3. Electron Capture Decay
 Alpha 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.
 Beta decay occurs when a neutron in the nucleus emits a fastmoving electron, changing that neutron to a proton and
consequently increasing 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 converting
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 achieved. 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 to lead 206 by eight alpha and six beta steps.
 The half-life of a radioactive element is the time it takes for onehalf of the atoms of the original unstable parent element to decay
into atoms of a new, stable daughter element. The daughter
element is the stable element that an unstable 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
second 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 sample 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 creates a percentage of
radioactive parent isotope to the stable daughter isotope present in
what is called the parent/daughter ratio for that particular rock.
 The half-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 radioactive isotope in question, the
actual age of the igneous rock can be deduced.
 Usually, 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, some
sedimentary rocks containing Glauconite (a green-colored,
radioactive potassium mineral found in some sedimentary deposits)
can give information on the age of deposition of the sedimentary
beds. All igneous rocks can be dated using radiometric
techniques.
 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.
 If 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 into 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 mass spectrometer. 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 stood 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” elements, 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
following terms:
1. Isotope - Varieties of the same element that have different
mass numbers. Their nuclei contain the same number of
protons but different numbers of neutrons.
2. Parent Isotope - the full amount of isotope in the newly
formed igneous rock.
3. Daughter Isotope - (what the parent isotope will eventually
turn into) the amount of altered parent isotope over time. The
last daughter isotope is stable.
4. Half-life - The time it takes for one-half of the atoms of a
radioactive substance (Parent Material) to decay into another
element (Daughter Material). 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 measured in a rock,
then the age of the formation of the rock can be found.
5. Mass Spectrometer – the laboratory device used in
determining the relative amounts of residual radioactive
Parent and stable Daughter isotopes.
VI. Other Radiometric Dating Techniques
Carbon 14 Dating –
 There are three common isotopes of carbon: 12C, 13C, and
14C.
 In the upper atmosphere, nitrogen gas is bombarded by
cosmic radiation transforming it into radioactive 14C. Carbon
dioxide, 14CO2, forms. Along with this is 12CO2 and 13CO2
from other sources such as volcanic eruptions.
 During photosynthesis in plants, CO2 is taken in, and along
with water, sunlight, and some pigment such as chlorophyll,
sugars are made.
sunlight
6CO2 + 6H2O

C6H12O6 + 6O2
pigment
 Of the sugars made, isotopes of carbon are in a ratio of 1/3
12C, 1/3 13C, and 1/3 14C. 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
carbon containing compounds of their bodies. As long as
they live, the ratio of the carbon isotopes is 1:1:1. Whenever
organisms die, the 14C begins to decay back into nitrogen at
a half-life of 5730 years.
 Any organic remains may be dated using this method back to
around 75,000 years ago making 14C dating especially useful
for archeology.
Tree-ring Dating –
 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 cross-referencing
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
archeologists.
Fission Track Dating –
As radioactive elements in 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 years ago can be determined. This
method is useful because this time frame is difficult to date: it is
too old for 14C techniques, and many times too young for other
radioactive isotope techniques.
Examples of Radiometric Problems
1.) In the geologic 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 U238 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 to
Argon-40, which was 1:7 (1 part Potassium-40 to 7 parts of Argon-40).
Potassium-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 specimen contains sufficient
amounts of the potassium 40 to warrant using the K40 – Ar40 test.
Knowing that the half-life of K40 is 1.3 billion years and that there was a
ratio of 1 part K40 to 3 parts Ar40, 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 of the earth,
4.6BYA
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
points mentioned in lecture class.