adapted to HTML from lecture notes of Prof. Stephen A. Nelson Tulane
Relative and Absolute Age
In order to understand how geologists deal with time we first need to
understand the concepts of relative age and absolute age.
Relative age - Relative means that we can determine
if something is younger than or older than something else.
Relative time does not tell how old something is, all we know is the
sequence of events. For example: the sandstone in this area
is older than the limestone.
Absolute age- Absolute age means that we can more or less
precisely assign a number (in years, minutes, seconds, or some other
units of time) to the amount of time that has passed. Thus we can
say how old something is. For example: The sandstone is 300
million years old.
To better understand these concepts, let's look at an archeological
example: Imagine we are a group of archeologists studying two
different trash pits recently discovered. By carefully digging, we
have found that each trash pit shows a sequence of layers. Although
the types of trash in each pit is quite variable, each layer has a
distinctive kind of trash that distinguishes it from other layers in the
What can we say and learn from these excavations?
Relative age of trash layers - Because of the shape of the pits the
oldest layers of trash occur below younger layers i.e. the inhabitants
of the area likely deposited the trash by throwing it in from the top,
eventually filling the pits. Thus the relative age of the trash
layers is, in order from youngest to oldest.:
5.25" Disk Layer - Youngest
Al Cans Layer
Tin Cans Layer
Ceramic Cups Layer
Stone Tools Layer - Oldest
Notice that at this point we do not know exactly how old any layer really
Thus we do not know the absolute age of any given layer.
The civilizations that deposited the trash had a culture and
industrial capabilities that evolved through time. The oldest
inhabitants used primitive stone tools, later inhabitants used cups
made of ceramics, even later inhabitants eventually used tin cans and
then changed to Aluminum cans, and then they developed a technology
that used computers.
Similar cultures must have existed in both areas and lived at the
same time. Thus we can make correlation's between the layers
found at the different sites, by reasoning that layers containing
similar discarded items (artifacts) were deposited during the same
Because the Ceramic Cups layer is found at the Tulane site, but not
at the UNO site, the civilization that produced the Ceramic cups
probably did not live in the UNO area. Thus, we can recognize a
hiatus, or break in the depositional sequence at the UNO site.
The surface marking in the break in deposition would be called an
unconformity in geologic terms, and represents time missing from the
The trash pits contain some clues to absolute age:
The Tulane trash pit has an old license plate in the Tin Cans
layer. This plate shows a date of 1950, thus the Tin Cans layer
is about 48 years old.
The UNO trash pit has an old newspaper in the Al Cans layer.
The date on the newspaper is Oct. 1, 1978. Thus the Al Cans
layer is about 20 years old.
In geology, we use similar principles to determine relative ages,
correlations, and absolute ages.
Stratigraphy = the study of strata (layers) in the Earth's crust. Laws of Stratigraphy Original Horizontality - sedimentary strata are deposited in layers
that are horizontal or nearly horizontal, parallel to or nearly parallel
to the Earth's surface. Thus rocks that we now see inclined or
folded have been disturbed since their original deposition. Stratigraphic Superposition - Because of Earth's gravity,
deposition of sediment will occur depositing older layers first followed
by successively younger layers. Thus, in a sequence of layers that
have not been overturned by a later deformational event, the oldest layers
will be on the bottom. This is the same principle used to determine
relative age in the trash pits discussed previously. In fact,
sedimentary rocks are, in a sense, trash from the Earth's surface
deposited in basins.
Breaks in the Stratigraphic Record
Because the Earth's crust is continually changing, i.e due to uplift,
subsidence, and deformation, erosion is acting in some places and
deposition of sediment is occurring in other places. When sediment
is not being deposited, or when erosion is removing previously deposited
sediment, there will not be a continuous record of sedimentation preserved
in the rocks. We call such a break in the stratigraphic record a
hiatus (a hiatus was identified in our trash pit example by the
non-occurrence of the Ceramic Cups layer at the UNO site). When we
find evidence of a hiatus in the stratigraphic record we call it an
unconformity. An unconformity is a surface of erosion or
non-deposition. Three types of unconformities are recognized.
Because of the Laws of Stratigraphy, if we see a cross section like
this in a road cut or canyon wall where we can recognize an angular
unconformity, then we know the geologic history or sequence of events that
must have occurred in the area to produce the angular unconformity.
Angular unconformities are easy to recognize in the field because of the
angular relationship of layers that were originally deposited horizontally
Disconformities (called parallel unconformities in your lab book) are much
harder to recognize in the field, because often there is no angular
relationship between sets of layers. Disconformities are usually
recognized by correlating from one area to another and finding that some
strata is missing in one of the areas. The unconformity recognized in the
UNO trash pit is a disconformity.
Nonconformities occur where rocks that formed deep in the Earth, such as
intrusive igneous rocks or metamorphic rocks, are overlain by sedimentary
rocks formed at the Earth's surface. The nonconformity can only
occur if all of the rocks overlying the metamorphic or intrusive igneous
rocks have been removed by erosion.
Variation of unconformities
The nature of an unconformity can change with distance. Notice how
if we are only examining a small area in the figure above, we would
determine a different type of unconformity at each location, yet the
unconformity itself was caused by the same erosional event.
Two types of stratigraphic classification are used,
one based on physical characteristics or material properties of the
rocks - Rock Stratigraphic Units, and the other based on the
time over which the material was formed - Time Stratigraphic Units.
Rock Stratigraphic Units
Distinctive bodies of rocks that differ from the rocks above and below
in the general characteristics. The basic unit is a formation.
Time Stratigraphic Units
A bodies of rocks that were deposited during the same geologic time
interval. The basic unit is a period.
Correlation of Rock Units
In order for rock units to be correlated over wide areas, they must be
determined to be equivalent. Determination of equivalence is based on:
Relative Age - if two rock units are equivalent they must
have the same age relative to rocks that occur below and above.
The laws of stratigraphy enable this determination.
Physical Criteria - two rock units share similar physical
Similarity in rock type, but only if relative age is
equivalent. Key beds, like widespread volcanic ash layers that are the
same over wide areas are often used to establish equivalence.
fossils present - Fossils are key indicators of relative age (life has
evolved through time) and environments of deposition.
The Geologic Column
Over the past 150 years detailed studies of rocks throughout the
world based on stratigraphic, paleontologic, and correlation studies have
allowed geologists to correlate rock units throughout the world and break
them into time stratigraphic units. The result is the geologic
column, which breaks relative geologic time into units of known relative
age. Note that the geologic column was established and fairly well
known before geologists had a means of determining absolute ages.
Thus in the geologic column shown, the absolute ages in the far right-hand
column were not known until recently.
Absolute Geologic Time
Although geologists can easily establish relative ages of rocks based on
the principles of stratigraphy, knowing how much time a geologic Eon, Era,
Period, or Epoch represents is a more difficult problem without having
knowledge of absolute ages of rocks. In the early years of geology,
many attempts were made to establish some measure of absolute geologic
Age of Earth estimated on the basis of how long it would take the oceans
to obtain their present salt content. Assumes that we know the rate
at which the salts (Na, Cl, Ca, and CO3 ions) are input into the oceans by
rivers, and assumes that we know the rate at which these salts are removed
by chemical precipitation. Calculations in 1889 gave estimate for
the age of the Earth of 90 million years.
Age of Earth estimated from time required to cool from an initially molten
state. Assumptions include, the initial temperature of the Earth
when it formed, the present temperature throughout the interior of the
Earth, and that there are no internal sources of heat. Calculations
gave estimate of 100 million years for the age of the Earth.
In 1896 radioactivity was discovered, and it was soon learned that
radioactive decay occurs at a constant rate throughout time. With
this discovery, Radiometric dating techniques became possible, and gave us
a means of measuring absolute geologic time.
Radiometric dating relies on the fact that there are different types of
Radioactive Isotopes - isotopes (parent isotopes) that spontaneously
decay at a constant rate to another isotope.
Radiogenic Isotopes - isotopes that are formed by radioactive decay
The rate at which radioactive isotopes decay is often stated as the
half-life of the isotope (t1/2). The half-life is the amount of time
it takes for one half of the initial amount of the parent, radioactive
isotope, to decay to the daughter isotope. Thus, if we start
out with 1 gram of the parent isotope, after the passage of 1 half-life
there will be 0.5 gram of the parent isotope left.After the passage of two
half-lives only 0.25 gram will remain, and after 3 half lives only 0.125
will remain etc.
Some examples of isotope systems used to date geologic materials.
Type of Material
>10 million years
Igneous Rocks and Minerals
40Ar & 40Ca
>10 million years
100 - 70,000 years
Potassium - Argon (K-Ar) Dating In
nature there are three isotopes of potassium:
39K - non-radioactive (stable)
40K - radioactive with a half life of 1.3 billion years, 40K decays
to 40Ar and 40Ca, only the K-Ar branch is used in dating.
41K - non-radioactive (stable)
K is an element that goes into many minerals, like feldspars and
biotite. Ar, which is a noble gas, does not go into minerals
when they first crystallize from a magma because Ar does not bond with
any other atom.
When a K-bearing mineral crystallizes from a magma it will
contain K, but will not contain Ar. With passage of time, the
40K decays to 40Ar, but the 40Ar is now trapped in the crystal
structure where the 40K once was.
Thus, by measuring the amount of 40K and 40Ar now present in the
mineral, we can determine how many half lives have passed since the
igneous rock crystallized, and thus know the absolute age of the rock.
Radiocarbon (14C) Dating
Radiocarbon dating is different than the other methods of dating because
it cannot be used to directly date rocks, but can only be used to date
organic material produced by once living organisms.
14C is continually being produced in the Earth's upper atmosphere by
bombardment of 14N by cosmic rays. Thus the ratio of 14C to 14N in the
Earth's atmosphere is constant.
Living organisms continually exchange Carbon and Nitrogen with the
atmosphere by breathing, feeding, and photosynthesis. Thus, so long as
the organism is alive, it will have the same ratio of 14C to 14N as
When an organism dies, the 14C decays back to 14N, with a half-life
of 5,730 years. Measuring the amount of 14C in this dead
material thus enables the determination of the time elapsed since the
Radiocarbon dates are obtained from such things as bones, teeth,
charcoal, fossilized wood, and shells.
Because of the short half-life of 14C, it is only used to date
materials younger than about 70,000 years.
Absolute Dating and Geologic Time Scale
Using the methods of absolute dating, and cross-cutting relationships of
igneous rocks, geologists have been able to establish the absolute times
for the geologic column. For example, imagine some cross section
such as that shown here.From the cross-cutting relationships and
stratigraphy we can determine that:
The Oligocene rocks are younger than the 30 m.y old lava flow and
older than the 20 m.y. old lava flow.
The Eocene rocks are older than the 57 m.y. old dike and younger
than the 36 m.y. old dike that cuts through them.
The Paleocene rocks are older than both the 36 m.y. old dike and the
57 m.y. old dike (thus the Paleocene is younger than 57 m.y.
By examining relationships like these all over the world, the Geologic
Time scale has been very precisely correlated with the Geologic
Column. but, because the geologic column was established before
radiometric dating techniques were available, note that the lengths of the
different Periods and Epochs are variable
The Age of the Earth
Theoretically we should be able to determine the age of the Earth by
finding and dating the oldest rock that occurs. So far, the oldest
rock found and dated has an age of 3.96 billion years. But, is this
the age of the Earth? Probably not, because rocks exposed at
the Earth's surface are continually being eroded, and thus, it is unlikely
that the oldest rock will ever be found. But, we do have clues about
the age of the Earth from other sources:
Meteorites - These are pieces of planetary material that fall
from outer space to the surface of the Earth. Most of these
meteorites appear to have come from within our solar system and either
represent material that never condensed to form a planet or was once
in a planet that has since disintegrated. The ages of the most
primitive meteorites all cluster around 4.6 billion years.
Moon Rocks - The only other planetary body in our solar
system that we have samples of are moon rocks (samples of Mars rocks
have never been returned to Earth). The ages obtained on Moon
rocks are all within the range between 4.0 and 4.6 billion
years. Thus the solar system and the Earth must be at least 4.6
billion years old.