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- by William L. Newman |
The Earth is very old --
4.5 billion years or more according to recent estimates. Most of the
evidence for an ancient Earth is contained in the rocks that form
the Earth's crust. The rock layers themselves -- like pages in a
long and complicated history -- record the surface-shaping events of
the past, and buried within them are traces of life --the plants and
animals that evolved from organic structures that existed perhaps 3
billion years ago. |
So far scientists have
not found a way to determine the exact age of the Earth directly from
Earth rocks because Earth's oldest rocks have been recycled and destroyed
by the process of plate tectonics. If there are any of Earth's primordial
rocks left in their original state, they have not yet been found.
Nevertheless, scientists have been able to determine the probable age of
the Solar System and to calculate an age for the Earth by assuming that
the Earth and the rest of the solid bodies in the Solar System formed at
the same time and are, therefore, of the same age.
The ages of
Earth and Moon rocks and of meteorites are measured by the decay of
long-lived radioactive isotopes of elements that occur naturally in rocks
and minerals and that decay with half lives of 700 million to more than
100 billion years to stable isotopes of other elements. These dating
techniques, which are firmly grounded in physics and are known
collectively as radiometric dating, are used to measure the last time that
the rock being dated was either melted or disturbed sufficiently to
rehomogenize its radioactive elements.
Ancient rocks exceeding 3.5
billion years in age are found on all of Earth's continents. The oldest
rocks on Earth found so far are the Acasta Gneisses in northwestern Canada
near Great Slave Lake (4.03 Ga) and the Isua Supracrustal rocks in West
Greenland (3.7 to 3.8 Ga), but well-studied rocks nearly as old are also
found in the Minnesota River Valley and northern Michigan (3.5-3.7 billion
years), in Swaziland (3.4-3.5 billion years), and in Western Australia
(3.4-3.6 billion years). These ancient rocks have been dated by a number
of radiometric dating methods and the consistency of the results give
scientists confidence that the ages are correct to within a few percent.
An interesting feature of these ancient rocks is that they are not from
any sort of "primordial crust" but are lava flows and sediments deposited
in shallow water, an indication that Earth history began well before these
rocks were deposited. In Western Australia, single zircon crystals found
in younger sedimentary rocks have radiometric ages of as much as 4.3
billion years, making these tiny crystals the oldest materials to be found
on Earth so far. The source rocks for these zircon crystals have not yet
been found. The ages measured for Earth's oldest rocks and oldest crystals
show that the Earth is at least 4.3 billion years in age but do not reveal
the exact age of Earth's formation.
The best age for the Earth
(4.54 Ga) is based on old, presumed single-stage leads coupled with the Pb
ratios in troilite from iron meteorites, specifically the Canyon Diablo
meteorite. In addition, mineral grains (zircon) with U-Pb ages of 4.4 Ga
have recently been reported from sedimentary rocks in west-central
Australia.
The Moon is a more primitive planet than Earth because
it has not been disturbed by plate tectonics; thus, some of its more
ancient rocks are more plentiful. Only the six Apollo and three Luna
missions returned a small number of rocks to Earth. These rocks vary
greatly in age, a reflection of their different ages of formation and
their subsequent histories. The oldest dated moon rocks, however, have
ages between 4.4 and 4.5 billion years and provide a minimum age for the
formation of our nearest planetary neighbour.
Thousands of
meteorites, which are fragments of asteroids that fall to Earth, have been
recovered. These primitive objects provide the best ages for the time of
formation of the Solar System. There are more than 70 meteorites, of
different types, whose ages have been measured using radiometric dating
techniques. The results show that the meteorites, and therefore the Solar
System, formed between 4.53 and 4.58 billion years ago. The best age for
the Earth comes not from dating individual rocks but by considering the
Earth and meteorites as part of the same evolving system in which the
isotopic composition of lead, specifically the ratio of lead-207 to
lead-206 changes over time owing to the decay of radioactive uranium-235
and uranium-238, respectively. Scientists have used this approach to
determine the time required for the isotopes in the Earth's oldest lead
ores, of which there are only a few, to evolve from its primordial
composition, as measured in uranium-free phases of iron meteorites, to its
compositions at the time these lead ores separated from their mantle
reservoirs. These calculations result in an age for the Earth and
meteorites, and hence the Solar System, of 4.54 billion years with an
uncertainty of less than 1 percent. To be precise, this age represents the
last time that lead isotopes were homogeneous throughout the inner Solar
System and the time that lead and uranium was incorporated into the solid
bodies of the Solar System.
The age of 4.54 billion years found
for the Solar System and Earth is consistent with current calculations of
11 to 13 billion years for the age of the Milky Way Galaxy (based on the
stage of evolution of globular cluster stars) and the age of 10 to 15
billion years for the age of the Universe (based on the recession of
distant galaxies).