Zircon Crystal. Photo
via: National Geographic
How do we know the Age of the Earth?
The Earth (like other
terrestrial planets) is a constantly changing planet. Its crust is continually
being created, modified, and destroyed via Plate
Tectonics. As a result, rocks that record its earliest history- like
in the very beginning of Earths creation process- have not been found and
probably no longer exist. However, there is substantial evidence that the Earth
and the other bodies of the Solar System are 4.5-4.6 billion years old, and
that the Milky Way Galaxy and the Universe are older still. The principal
evidence for the antiquity of Earth and its cosmic surroundings is below:
The oldest rocks on Earth, found in western Greenland (Evidence Here), have been dated by four independent
radiometric dating methods at 3.7-3.8 billion years. Rocks 3.4-3.6 billion
years in age have been found in southern Africa (Evidence Here), western Australia (Evidence Here & Here), and the Great Lakes region of North
America (Evidence Here). These oldest rocks are Metamorphic
rocks but they originated as lava flows and Sedimentary
rocks. The debris from which the sedimentary rocks formed must
have come from even older crustal rocks. The oldest dated minerals (4.0-4.2
billion years) are tiny Zircon crystals found in sedimentary rocks in
western Australia.
The oldest Moon rocks: are from the lunar highlands and were formed when the
early lunar crust was partially or entirely molten. These rocks (Evidence
Here), of which only a few were returned by the Apollo missions,
have been dated by two methods at between 4.4-4.5 billion years in age.
The majority of dated meteorites: Meteorites have ages of 4.4 to 4.6 billion years.
Meteorites are fragments of asteroids, they are primitive material in
the solar system left over from the Accretion or
Creation of Earth. Meteorites have been dated by 5 independent
radiometric dating methods. (Types of Radiometric dating) (What is Accretion?)
Lead isotopes:
The "best" age for the Earth is based on the time required for
the Lead Isotopes in four very old lead ores
(galena) to have evolved from the composition of lead at the time the
Solar System formed, as recorded in the Canyon Diablo Meteorite. This "model
lead age" is 4.54 billion years.
The Velocity and Distance of Galaxies: The evidence for the antiquity of the Earth and the Solar
System is consistent with evidence for an even greater age for the Milky Way Galaxy and the
Universe.
b) The age of the Galaxy is estimated to be 14 to18 billion
years from the rate of evolution of stars in Globular clusters, which
are thought to be the oldest stars in the Galaxy- at least the oldest we know of. The age of the
elements in the Galaxy, based on the production ratios of Osmium Isotopes in Supernovae and the change in that ratio over time due to
radioactive decay, is 8.6 to 15.7 billion years. Theoretical considerations
indicate that the Galaxy formed within a billion years of the beginning
of the Universe.
c) Combining the data from a) and b), the "best,
i.e., most consistent, age of the universe is estimated to be around 14
billion years. For more current information on the age of the universe,
visit NASA's Planck Mission studies.
Spontaneous breakdown or decay of Atomic nuclei, termed
"Radioactive Decay", is the basis for all radiometric dating methods.
Radioactivity was discovered in 1896 by French physicist Henri Becquerel. By
1907, the study of the decay products of uranium (lead and intermediate
radioactive elements that decay to lead) demonstrated to B. B. Boltwood that
the lead/uranium ratio in uranium minerals increased with geologic age and
might provide a geological dating tool.
As radioactive Parent atoms decay to stable daughter atoms (as
uranium decays to lead) each disintegration results in one more atom of the
daughter than was initially present and one less atom of the parent. The
probability of a parent atom decaying in a fixed period of time is always the
same for all atoms of that type regardless of temperature, pressure, or
chemical conditions. This probability of decay is the "decay constant". The
time required for one-half of any original number of parent atoms to decay is
the "half-life", which is related to the decay constant by a simple
mathematical formula.
All rocks and minerals contain long-lived radioactive elements
that were incorporated into Earth when the Solar System formed. These
radioactive elements constitute independent clocks that allow geologists to
determine the age of the rocks in which they occur. The radioactive parent
elements used to date rocks and minerals are:
Parent
|
Daughter
|
Half-life
|
Uranium-235
|
Lead-207
|
0.704 billion years
|
Uranium-238
|
Lead-206
|
4.47
|
Potassium-40
|
Argon-40
|
1.25
|
Rubidium-87
|
Strontium-87
|
48.8
|
Samarium-147
|
Neodymium-143
|
106
|
Thorium-232
|
Lead-208
|
14.0
|
Rhenium-187
|
Osmium-187
|
43.0
|
Lutetium-176
|
Hafnium-176
|
35.9
|
|
Radiometric dating using the naturally-occurring radioactive elements
is simple in concept even though technically complex. -If we know the number
of radioactive parent atoms present when a rock formed and the number present
now, we can calculate the age of the rock using the decay constant. The
number of parent atoms originally present is simply the number present now
plus the number of daughter atoms formed by the decay, both of which are
quantities that can be measured-. Samples for dating are selected carefully to
avoid those that are altered, contaminated, or disturbed by later heating or
chemical events.
In addition to the ages of Earth, Moon, and meteorites,
radiometric dating has been used to determine ages of fossils, including
early man, timing of glaciations, ages of mineral deposits, recurrence rates
of earthquakes and volcanic eruptions, the history of reversals of Earth's
magnetic field, and the age and duration of a wide variety of other
geological events and processes.
Source: U.S. Department of the Interior | U.S. Geological Survey
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