Geological Dating

Geological Dating

For centuries people have argued about the age of the Earth; only recently has it been possible to come close to achieving reliable estimates. In the 19th century some geologists realized that the vast thicknesses of sedimentary rocks meant that the Earth must be at least hundreds of millions of years old. Charles Darwin reinforced this idea by pointing to the time that must have been required for the EVOLUTION of advanced life from primitive forms.

On the other hand, the great physicist Lord Kelvin vehemently objected and suggested that the Earth might only be a few tens of millions of years old, based on his calculations of its cooling history. These discussions were rendered obsolete by the discovery of radioactivity in 1896 by the French physicist Henri Becquerel. The existence of radioactivities of various kinds in rocks has enabled earth scientists to determine the age of the Earth, the moon, meteorites, mountain chains and ocean basins, and to draw up a reasonably accurate time scale of evolution. It has even been possible to work out a time scale of the reversals of the Earth's magnetic field. This "radiometric" approach has superseded all other techniques for determining absolute ages.

Radioactive Clock

The vast majority of atoms (each composed of a nucleus surrounded by electrons) are stable. Essentially, they will exist forever. A critical few, however, are unstable. Their nuclei tend to emit particles spontaneously - ie, they are radioactive. Because of this particle emission, the original radioactive parent atom changes its identity, becoming a different, stable daughter atom. This change takes place at a known rate determined by the half-life; ie, the time required for one-half of the original number of radioactive atoms to convert to the stable daughter product. The remaining number of radioactive atoms is halved every half-life. Radioactive elements of use in geological dating have relatively long half-lives. A good example is rubidium-87, which changes to strontium-87 at a rate of one-half every 50 billion years. Therefore, a rock can be dated by measuring how much of its original rubidium content has changed into strontium.

The other key dating techniques involve uranium-235 transforming to lead-207 at a rate of one-half every 713 million years, uranium-238 becoming lead-206 at one-half every 4.5 billion years, potassium changing to argon (and calcium) at one-half every 1.3 billion years and samarium-147 becoming neodymium-143 at one-half every 106 billion years. These radioactive processes present a set of natural clocks which reveal when the rock was formed, or when it was last heated severely. The well-known carbon-14 method involves the conversion of radioactive carbon-14 to stable nitrogen at a rate of one-half about every 5700 years. It can only be used to date organic matter, and is accurate only for materials younger than about 50 000 years (seeARCHAEOLOGY; GLACIATION).

Since 1950, radiometric methods have been developed to a very sophisticated level in several countries, including Canada. It has been demonstrated that when rocks which have led an undisturbed history are analysed, all methods reveal the same age. This uniformity demonstrates that the principle is reliable. When disturbed rocks are studied, the different techniques may give different readings, and much research has been carried out on how to interpret such results. It often proves possible to date even severely disturbed rocks.

Age of the Earth

To date the time of formation of a planet 12 740 km in diameter and 70.8% covered by water is not easy. Only the tiniest fraction of the Earth, the crust, is accessible. Those rocks available for analysis (ie, the oldest ones) have been heated and squeezed many times in their GEOLOGICAL HISTORY, because for billions of years continents have been drifting over the Earth's surface, colliding and producing mountains and new ocean floors.

Two approaches have been developed to circumvent these problems. The first involves sampling as much of the Earth's crust as possible and dating these rocks. The Earth certainly must be older than the oldest terrestrial rocks found. Samuel Bowring, now of the Massachussetts Institute of Technology, and his coworkers Ian Williams and William Compston of the Australian National University at Canberra have shown that a small area of metamorphic rock in northern Canada, known as the Acasta gneiss, is the oldest known intact solid piece of the Earth's crust. Using the uranium-lead technique they dated zircon crystals from the gneiss (located southeast of Great Bear Lake in the NWT) and showed that it was formed almost 4 billion years ago. Therefore it is clear that the Earth is over 4 billion years old. Consistent with this are the results of Stephen Moorbath and his colleagues at Oxford, who have shown that rocks near Godthaab in southwest Greenland either formed or were in existence approximately 3.8 billion years ago. These results have been confirmed and agreement has been found among the rubidium-strontium, uranium-lead and samarium-neodymium methods.

Rocks of almost this age have also been identified in other localities, including Labrador, Minnesota, Africa and India. Many scientists are searching for rocks older than these, and in 1983 Australian scientists claimed to have discovered minute zircon crystals 4.2 billion years old. They were found, however, in much younger sediment and it is not known where these zircons originated.

The second approach, which is more indirect but gives an answer currently believed correct, involves a comparison of the Earth with meteorites. They have clearly fallen to Earth from outside, often gouging out huge craters such as that called New Québec (61°17´ N, 73° 41' W). Rubidium-strontium, potassium-argon, uranium-lead and samarium-neodymium dating all show that the meteorites formed about 4.6 billion years ago. But detailed studies of lead isotopes in meteorites and terrestrial rocks strongly indicate that the Earth and meteorites formed at the same time.

Therefore, since the meteorites are very accurately dated at 4.6 billion years old, the Earth is also considered to be the same age. Dating of the lunar samples collected by the Apollo missions strongly indicates that the moon is of the same age. If the Earth, the moon and meteorites are all 4.6 billion years old, then so very probably is the whole solar system.

Age of Canadian Shield

The most ancient rocks of Canada comprise the Canadian SHIELD (seeGEOLOGICAL REGIONS). Many Canadian scientists, and others, have examined these rocks to work out the region's history. The Shield is made up of areas of rocks of distinctive ages. Apart from the Acasta gneiss, the oldest are found in Saglek Bay and near Hebron Fjord in Labrador and are about 3.6 billion years old. Other massive slabs are dated at 2.9-2.5 billion years, 1.8-1.7 billion years and 1.3-0.9 billion years.

Some of these areas represent the roots of what were ancient mountain chains, the upper parts of which were long ago removed by EROSION. Others represent volcanic belts, many of which have never been very deeply buried. Scientists are studying whether these portions of different age and geological history were always close together or were far apart until gathered together by continental drifting and PLATE TECTONICS. Although the igneous rocks of the Shield are very ancient, the formation of igneous rocks has been a continuing process in Canada. The rocks formed by the Aiyansh lava flow in BC are thought to be 90 to 350 years old, confirming legends of the Tsimshian people of the Nass River describing volcanic activity.

Time Scale of Biological Evolution

It seems probable that life has existed on the Earth for well over 3 billion years. FOSSIL bacteria have been tentatively identified in the Fig-Tree Sediments in South Africa. Volcanic rocks associated with these have been dated at 3.5 billion years by the samarium-neodymium method by scientists at Columbia University and by the potassium-argon method at the University of Toronto. Probably the oldest fossils in Canada (2.5 billion years old) are the stromatolite formations at Steep Rock Lake, Ont. An indisputably biogenic, highly diverse microfossil assemblage is present in the approximately 1.9-billion-year-old Gunflint cherts of southern Ontario. However, the fossil record is well documented only for the last 545 million years, for only during that period did organisms exist with the hard phosphate or calcium-carbonate components which make for good fossil preservation.

The final 545 million years of evolution have been divided by palaeontologists into 3 eras: Palaeozoic (ancient life), Mesozoic (middle life) and Cenozoic (present life). The eras are subdivided into periods. The numerical estimates of the time occupied by these periods were made mostly with the aid of potassium-argon, rubidium-strontium and uranium-lead dating of rocks, which could be correlated with the time scale.

Humans are creatures of only the last few million years. Fragmentary fossils of apelike, upright-walking potential human ancestors (Australopithecus anamensis) were found in 1995 in northern Kenya by an international expedition led by Meave Leakey of the National Museums of Kenya. Volcanic ash associated with these fossils has been dated at the Australian National University using a laser-probe dating technique developed at U of T. It is clear that creatures even remotely resembling modern humans have been on the Earth for less than one-tenth of 1% of our planet's history.