Where do scientists begin the story of Earth’s history? One place to start
is by looking at the age of the materials that make up the Earth itself.
When scientists are able to determine the age of rocks and fossils, they can
then tell a more accurate story of the Earth’s history.
When Geologists and other scientists refer to the age of the Earth, they are
using one of two methods to determine this. First, we must understand they
are dating the rocks and other materials found on the Earth to determine
this. There are two basic types of dating methods geologists rely on,
Relative Time Dating and Absolute Time Dating.
Relative Time Dating:
In Relative Time dating a specific age in years is not given. This type of
dating puts events in their correct chronological order based upon the
sedimentary layer in which an object is found. In the late 18th
century and early 19th century, James Hutton developed the idea
that the layers found in rock outcrops were representations of the story of
time on the planet. He concluded, the bottom most layers were deposited
first, thus making them the oldest and subsequent layers were progressively
younger in age. Since there is no “set” rate of deposit for these layers,
the actual length of time represented by these layers is a good guess.
Stratigraphy:
is the science or study of layers or strata of rocks. Sedimentary rocks
build up layers from the oldest deposits, at the bottom with the newest at
the uppermost layers. Individual layers are called Stratum. Biostratigraphy
is a specialized form of Stratigraphy, which uses plant and animal fossils
to establish the different layers. There are four basic principals of
Stratigraphy, which help us understand Earth’s time line.
The Principle of Original Horizontality:
Sediments are layered in a horizontal fashion. Non-horizontal layers have
been folded or tilted after the original sedimentation. This happens
because of upheaval, earthquakes, or other large earth movements.
The Principle of Lateral Continuity:
Because rock layers cover large areas of the Earth’s surface, scientists can
relate layers in one location to another. The correlations between one area
of the Earth and another are largely based on fossil deposits.
The Principle of Superposition:
As layers are formed, older layers are generally at the bottom of the
sedimentation with younger layers being laid over the older ones. There are
many ways these sediments can be formed. A volcanic ash buildup is one.
Lake or other water body sedimentation is another way these layers can be
formed.
The Principle of Faunal Succession:
An English scientist, William Smith is accredited with the discovery of this
idea. Smith noticed that specific forms of life were fossilized in
particular layers of rock, giving a time line story indicating when, in
time, events occurred. This progression of or evolution of life occurred in
a vertical fashion up through the layers. The same vertical changes in
fossils occur in different places all over the earth.
The major division of relative time can be and are expressed in a vertical
chart, progressing in age from the oldest, at the base, with the youngest
time found at the top of the chart. The following chart shows this concept
with a brief description of each age represented.
Absolute Time Dating:
The age of a rock in years is called its absolute age. The most common
types are based on the rate of decay of naturally occurring radioactive
elements. The age of the material being dated is commonly expressed in a
number of years. When rocks are formed, small amounts of radioactive
elements usually get included. As time passes, the "parent" radioactive
elements change at a regular rate into non‑radioactive "daughter" elements.
Thus, the older a rock is, the larger the number of daughter elements and
the smaller the number of parent elements is found in the rock.
Radiometric dating is another term used to talk about Absolute Time dating.
A chemical element consists of atoms with a specific number of protons in
their nuclei, but different atomic weights owing to variations in the number
of neutrons. Atoms of the same element with differing atomic weights are
called isotopes. Radioactive decay is a spontaneous
process in which an isotope (the parent) loses particles from its nucleus to
form an isotope of a new element (the daughter).
Most radioactive isotopes have a rate of decay that is much too fast to be
of any use in determining the geologic age of materials. Carbon-14 dating
is the best-known type of radiometric dating, but the half-life of carbon-14
is much too short to determine the age of things in terms of millions of
years. A few isotopes do decay at a, slow enough, rate to be able to date
things into the millions of years range, and can be effectively used as
geological clocks.
The table below indicates which parent isotopes decay slowest, what their
half-lives are, and the daughter isotope, which results from the decay
process.
PARENT
ISOTOPE
HALF‑LIFE
STABLE
DAUGHTER
Uranium‑235
704 Million Years
Lead‑207
Potassium‑40
1.25 Billion Years
Argon‑40
Uranium‑238
4.5 Billion Years
Lead‑206
Thorium‑232
14.0 Billion Years
Lead‑208
Lutetium‑176
35.9 Billion Years
Hafnium‑176
Rubidium‑87
48.8 Billion Years
Strontium‑87
Samarium‑147
106 Billion Years
Neodymium‑143
The mathematical expression that relates radioactive decay to geologic time
is called the age equation and is:
A typical “parent - daughter” combination used is Uranium to Lead decay.
The Uranium is trapped
in the rock as it forms and over time as the Uranium decays, it is replaced
by non-radioactive Lead.
Until
the advent or discovery of Radiometric dating, a type of Absolute time
dating, there was no way to verify the accuracy of the relative ages of
materials found in the various layers of sedimentary rocks. Rocks which
have been dated utilizing one of the various methods of actual Time dating,
can be incorporated into a relative time scale. With this combination of
the two types of dating methods, geologists can relatively, accurately
determine the date of earth events and much of the time line associated with
the evolution of life on the planet.
Geological time is traditionally divided into eons (Archaean or Archaeozoic,
Proterozoic, and Phanerozoic in ascending chronological order), which in
turn are subdivided into eras, periods, epochs, ages, and finally chrons.
The
chart to the right is a composite chart utilizing both relative and absolute
time dating techniques. It is a more accurate chronological time dating
method, than either of the two types of dating generally is on their own.
Links
to other versions of this time scale are listed below:
