*This packet is only available online.
The full text the USGS publication titled, Geologic Time is available online.
the close of the 18th century, the haze of fantasy and mysticism that
tended to obscure the true nature of the Earth was being swept away.
Careful studies by scientists showed that rocks had diverse origins.
Some rock layers, containing clearly identifiable fossil remains of
fish and other forms of aquatic animal and plant life, originally
formed in the ocean. Other layers, consisting of sand grains winnowed
clean by the pounding surf, obviously formed as beach deposits that
marked the shorelines of ancient seas. Certain layers are in the form
of sand bars and gravel banks -- rock debris spread over the land by
streams. Some rocks were once lava flows or beds of cinders and ash
thrown out of ancient volcanoes; others are portions of large masses of
once-molten rock that cooled very slowly far beneath the Earth's
surface. Other rocks were so transformed by heat and pressure during
the heaving and buckling of the Earth's crust in periods of mountain
building that their original features were obliterated.
the results of studies on the origins of the various kinds of rocks
(petrology), coupled with studies of rock layering (stratigraphy) and
the evolution of life (paleontology), today geologists reconstruct the
sequence of events that has shaped the Earth's surface. Their studies
show, for example, that during a particular episode the land surface
was raised in one part of the world to form high plateaus and mountain
ranges. After the uplift of the land, the forces of erosion attacked
the highlands and the eroded rock debris was transported and
redeposited in the lowlands. During the same interval of time in
another part of the world, the land surface subsided and was covered by
the seas. With the sinking of the land surface, sediments were
deposited on the ocean floor. The evidence of the pre-existence of
ancient mountain ranges lies in the nature of the eroded rock debris,
and the evidence of the seas' former presence is, in part, the fossil
forms of marine life that accumulated with the bottom sediments.
recurring events as mountain building and sea encroachment, of which
the rocks themselves are records, comprise units of geologic time even
though the actual dates of the events are unknown. By comparison, the
history of mankind is similarly organized into relative units of time.
We speak of human events as occurring either B.C. or A.D. -- broad
divisions of time. Shorter spans are measured by the dynasties of
ancient Egypt or by the reigns of kings and queens in Europe.
Geologists have done the same thing to geologic time by dividing the
Earth's history into Eras -- broad spans based on the general character
of life that existed during these times, and Periods -- shorter spans
based partly on evidence of major disturbances of the Earth's crust.
names used to designate the divisions of geologic time are a
fascinating mixture of works that mark highlights in the historical
development of geologic science over the past 200 years. Nearly every
name signifies the acceptance of a new scientific concept -- a new rung
in the ladder of geologic knowledge.
discovery of the natural radioactive decay of uranium in 1896 by Henry
Becquerel, the French physicist, opened new vistas in science. In 1905,
the British physicist Lord Rutherford -- after defining the structure
of the atom -- made the first clear suggestion for using radioactivity
as a tool for measuring geologic time directly; shortly thereafter, in
1907, Professor B. B. Boltwood, radiochemist of Yale University,
published a list of geologic ages based on radioactivity. Although
Boltwood's ages have since been revised, they did show correctly that
the duration of geologic time would be measured in terms of
hundreds-to-thousands of millions of years.
next 40 years was a period of expanding research on the nature and
behavior of atoms, leading to the development of nuclear fission and
fusion as energy sources. A byproduct of this atomic research has been
the development and continuing refinement of the various methods and
techniques used to measure the age of Earth materials. Precise dating
has been accomplished since 1950.
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). The rate of decay is
conveniently expressed in terms of an isotope's half-life, or the time
it takes for one-half of a particular radioactive isotope in a sample
to decay. Most radioactive isotopes have rapid rates of decay (that is,
short half-lives) and lose their radioactivity within a few days or
years. Some isotopes, however, decay slowly, and several of these are
used as geologic clocks. The parent isotopes and corresponding daughter
products most commonly used to determine the ages of ancient rocks are
||Stable Daughter Product
||Currently Accepted Half-life Values
||4.5 billion years
||704 million years
||14.0 billion years
||48.8 billion years
||1.25 billion years
||106 billion years
The mathematical expression that relates radioactive decay to geologic time is called the age equation and is:
t=1/delta ln(1 + D/P)
- t is the age of a rock or mineral specimen,
- D is the number of atoms of a daughter product today,
- P is the number of atoms of the parent product today,
- ln s the natural logarithm (logarithm to base e), and
- delta is the appropriate decay constant.
(The decay constant for each parent isotope is related to its half-life, t 1/2, by the following expression:
t 1/2 = ln2/delta
rocks by these radioactive timekeepers is simple in theory, but the
laboratory procedures are complex. The numbers of parent and daughter
isotopes in each specimen are determined by various kinds of analytical
methods. The principal difficulty lies in measuring precisely very
small amounts of isotopes.
Literally thousands of dated materials
are now available for use to bracket the various episodes in the
history of the Earth within specific time zones. Many points on the
time scale are being revised, however, as the behavior of isotopes in
the Earth's crust is more clearly understood. Thus the graphic
illustration of the geologic time scale, showing both relative time and
radiometric time, represents only the present state of knowledge.
Certainly, revisions and modifications will be forthcoming as research
continues to improve our knowledge of Earth history.