Weathering and Erosion: The Formation of Sediments and Soil

trextemperΜηχανική

22 Φεβ 2014 (πριν από 3 χρόνια και 3 μήνες)

129 εμφανίσεις

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 “t
orn 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 Wea
thering 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 materia
l.



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 proce
sses 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
disintegr
ation 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 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
ions.)


3.

Differenti
al 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.

S
pheriodal 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
sp
heriodal 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


Whe
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
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 ot
her 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 g
rowth. 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 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
-


1.

Oxidation


reactions with ox
ygen


rusting:

4 Fe + 3 O
2



㈠䙥
2
O
3

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.

Corrosio
n


reactions involving oxygen, water, and CO
2

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 CO
3
-

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 base
s dissolve
minerals by pulling atoms out of crystals.
Carbonic Acid
(H
2
CO
3
)
is formed in abundance in nature whenever CO
2

dissolves in some rivers and streams.

5.

Acid Rain


During storms,
Nitric Acid
,
(H
2
NO
3
)
is formed by
lightning breaking apart N
2

in t
he 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 P
roduct 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
agen
ts 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 th
at 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
sa
nd, 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 climate
s 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 v
egetation 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 her
e.


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 t
hat
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 con
dusive 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 E
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
off.
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
var
ious 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 typ
e 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 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:




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 f
rom 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.





Metamorphi
c 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 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),
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
(CaSO
4
), 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
ation 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 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).



2.
Tra
nsportation

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 al
most 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 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.
Se
dimentary 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 sediment
ary
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

(CaCO
3
),
Silica

(SiO
2
)
, or forms of
Iron Oxide

(i.e.
Fe
2
O
3
), 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 si
ze 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, 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
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 Approximate Particle Diameter in millimeters




Boulders



greater than 256mm




Cobbles




128







64







32



Pebbles




16








8






4







Granules









2







Very Coarse Sand






1.0


Course Sand





Fractional
Equivalents






0.5



1/2


Medium Sand











0.25



1/4





Fine Sand







Very Fine Sand



0.125



1/8






0.0625



1/16






0.0313



1/32






0.0156



1/64



Silt






0.0078



1/128






0.0039



1/256






Clay







less than 1/256


2.
Round
ness:

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 am
ount 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
per
iod 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 th
e 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. 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

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 e
nvironment. 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 fl
oodplains and swamps.

c.)
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
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

o
r
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 CO
3
-

ions in the sea.


Limestone
-

Ca CO
3

(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 (CO
3
)
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 (l
imestone)

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
are
as. 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) a
ccumulating 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 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 "
beds
". The type
of bedding will vary depending on the environmen
t 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 su
ch 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

ca
n 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 success
ively 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 a
nd 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 de
eper 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 o
f
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 cre
ate 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 ma
y
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 200
0

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
25
0
C per kilometer of depth
.
On the
continental cratons, the average is 20
0
C/km. On the continental
boundaries it is 40
0
C/km. At subduction zones, it is 10
0
C/km
because heat is dissipated into the sea.



At 700
0
C, 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 i
n 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 ca
n cause a constant
interaction or exchange of ions altering the rocks.


i.e.


2Mg
2
SiO
4

+ 2H
2
O


Mg
3
Si
2
O
5

+ MgO




Olivine


Water


Serpentine


carried away in solution


Metasomatism


the introduction by fluids of ions from an external
sourc
e 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 c
ountry 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 mineral
s.



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.

Wa
ter arising as volatile fluid within magma

3.

Water from the
dehydration
of water
-
bearing minerals such
as Selenite Gypsum: CaSO
4

2H
2
O, and some clays.


Types of Metamorphism


I. Contact


Effects of Heat and Fluids

Characteristics:



“Heat” is the driving for
ce

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
900
0
C
next to the intrusion.



As the heat and associated metamorphic c
hanges 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 grad
ation 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 resu
lting 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 cerami
cs 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
solut
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

Characteristics:



Occurs over a v
ery 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 200
0
C), muscovite, biotite,
garnet, staurolite, kyanite (
forms around 500
0
C)



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
2
SiO
5
) form
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:
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
ma
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
-


Characteristics:



Typical
ly 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 gne
iss.



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 i
t 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 slate
s 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 t
he 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 t
hem). 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 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
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”


T
hese 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 piece
s 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 p
referred 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 o
r 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 p
arent 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 re
-
crystallization 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 mi
nerals 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
metamorphis
m. 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 h
igh 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

(m
ore 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
. Me
tamorphic 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 zone
s

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 indicate
s 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 t
he 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 dete
rmine
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 Revel
ations backwards to Genesis, and ascribing a standard
life span to the peoples mentioned, he figured that the earth was
created on October 26
th
, 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
millenniu
m 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 la
yering 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 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)


“the Fat
her
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 mu
st 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 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
-
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 dep
ositional 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 thes
e
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 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
regular rate.


III. Unconformities


“Geologic time is continuous…deposition of rock l
ayers 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, mo
re 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
horiz
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.


2.

Angular Unconformity


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
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 ign
eous (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, thi
rd, 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 bein
g
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
nucl
eus 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, t
he 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 sa
me 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 a
n 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 w
e 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 Deca
y

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 fast
-
moving 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 convert
ing
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.



The
half
-
life
of a radioactive element is the time it takes for one
-
half 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
se
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
eates 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 rad
ioactive 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
techni
ques
.



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 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
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 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
following terms:


1.

Isotope

-

Varieties of t
he 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 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.

5.

Mass Spectrometer



the laboratory device used in
determining the relative amounts of residual radioactive
Parent and stable Daughter isotopes.

VI. Other Radiometric Dating Techniq
ues


Carbon 14 Dating





There are three common isotopes of carbon:
12
C,
13
C, and
14
C.



In the upper atmosphere, nitrogen gas is bombarded by
cosmic radiation transforming it into radioactive
14
C. Carbon
dioxide,
14
CO
2
, forms. Along with this is
12
CO
2

a
nd
13
CO
2

from other sources such as volcanic eruptions.



During photosynthesis in plants, CO
2

is taken in, and along
with water, sunlight, and some pigment such as chlorophyll,
sugars are made.







sunlight










6CO
2

+ 6H
2
O



††
C
6
H
12
O
6

+ 6O
2


pigment




Of the sugars made, isotopes of carbon are in a ratio of 1/3
12
C, 1/3
13
C, and 1/3
14
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
carb
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
14
C 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
14
C 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 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
14
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
238

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).
P
otassium
-
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
40



Ar
40

test.
Knowing that the half
-
life of K
40

is 1.3 billion years and that there was a
ratio of 1 part K
40

to 3 parts Ar
40
, 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,
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.