Earth's oldest rocks

The oldest-known rocks on Earth have absolute (radiometric) ages approaching 3.8 billion years. Although Earth apparently has no rocks resulting from the first 760 million years of its history, rocks with ages ranging back to the earliest age for the terrestrial planets, about 4.56 billion years, occur for many meteorites and are closely approached in age by some rocks from the moon.

Stratigraphic Time Scale

Present knowledge of the oldest rocks on Earth developed slowly and descriptively until the 1950s, when it became possible to measure the absolute (quantitative) ages of minerals and rocks by radiometric means. These means involve the instrumental (commonly, mass spectrometric) measurement of unstable (radioactive) and stable (radiogenic) isotopes, or species of elements that differ only in their masses.

Prior to the ability of physicists, chemists, and geologists to make absolute age determinations, the oldest rocks on Earth were known only through field relations. The main field relation used is stratigraphic sequence, which involves an application of the principle of superposition: In a sequence of undisturbed layered rocks such as sedimentary layers and lava flows, the oldest rock unit—that is, the first to be deposited—is at the bottom of the sequence. Another important field principle is the manner in which one rock is cut or cuts another rock unit. The obvious chronological conclusion is that the structure or rock that transects must be younger than the structure or rock that is transected. Through a combination of these stratigraphic and cross-cutting relationships, rock units studied and mapped can be assigned to a relative chronologic order and the geologic history of the mapped area worked out.

Accompanying the development of classical geologic principles was the understanding of the time dependence of biological evolutionary characteristics displayed by fossils found in the enclosing—primarily sedimentary—layers. Although it was early understood that fossil morphology changed through time from simpler to more complex forms, the time required for such evolutionary change could only be guessed. It is a tribute to early geologists and paleontologists that, before a quantitative measure of evolutionary scale was available, it was realized that enormous amounts of time probably were required between the deposition of rocks containing, for example, fossil collections of extinct marine animals such as trilobites and the deposition of those of containing fossils of horses.

The geologist's most important document, the stratigraphic column (the geologic time scale), was developed over the past several hundred years through the cumulative observations of field relations, paleontologic studies, and absolute dating methods. Its refinement will continue to be an important result of geologic endeavor. Correlation, the principal activity of the geologic study of stratigraphy, whereby rock units are related through their temporal and physical characteristics, was pioneered by William Smith in 1815. It enabled scientists to have some sense of the earth's oldest rocks, long before numbers of years could be assigned to paleontologic and physical geologic phenomena. Nevertheless, the Precambrian era—the vast period of geologic time that encompasses more than 85 percent of the known age of the earth—was not known to have harbored life or to have provided fossils until the past few decades. The discovery of widespread bacterial and stromatolitic fossils in Precambrian rocks has reversed this conclusion. Prior to these discoveries, the ages of the earliest rocks thus were surmised only through field relations and not through fossils.

Radiometric Ages

A misunderstanding of old rocks on Earth occurred because of the reasoning that the older the rock, the more opportunity for it to have altered, such as by weathering, tectonism (as in mountain building), or especially metamorphism. Thus, it was expected that the oldest rocks should be highly metamorphosed, as in much of the Precambrian terrain of Canada and other, commonly central continental areas of Precambrian rock (cratons or shields), and that essentially unaltered sediments and sedimentary rocks must be geologically young. Miscalculations in geologic age of billions of years occurred, owing to the incorrect correlation of rocks of similar petrology and metamorphic grade. Absolute age determinations, while not negating the essential premise of this theory, have nevertheless shown that some of the oldest rocks on Earth are not highly altered and that many young rocks may be highest-grade metamorphic and tectonized.

A major advance in geochronology has developed since radiometric ages were attached to points of the stratigraphic time scale and a quantitative framework for major fossil assemblages was established. Once the ages of characteristic, representative fossils are quantified through absolute age determinations, the ages of sedimentary rocks containing chronologically diagnostic fossils can be assigned by comparison of these “guide” fossils with points on the stratigraphic time scale. Thus, the field geologist may establish the approximate age of sediments or sedimentary rocks in his or her area of interest (and, through field relations, the qualitative ages of associated igneous and metamorphic rocks and of geologic structures), simply through fossil identification. Although fossils, especially diagnostic fossils, are rare in many sedimentary rocks (especially those sedimentary rocks that formed prior to about 600 million years) and are absent in most igneous and metamorphic rocks, the use of paleontology as a chronologic tool is routine and in most cases quicker and less expensive than are geochemical (radiometric) age determinations.

Antoine-Henri Becquerel presented his discovery of the phenomenon of radioactivity to the scientific community in Paris in 1895, laying the cornerstone for scientists' present understanding of Earth's oldest rocks. The finding was followed rapidly by the seminal work of Marie Curie in radioactivity, a term she was the first to use. Her discovery of the intensely radioactive radium as well as plutonium led Ernest Rutherford to distinguish three kinds of radioactivity—alpha, beta, and gamma—and, in 1910, with Frederick Soddy, to develop a theory of radioactive decay. Soddy later proposed the probability of isotopes, the existence of which was demonstrated on early mass spectrographs and mass spectrometers.

Rutherford and Soddy's theory of the time dependence of radioactive decay, followed by breakthroughs in instrumentation for the measurement of these unstable species and their radiogenic daughter nuclides by Francis William Aston, Arthur Jeffrey Dempster, and Alfred Otto Carl Nier, among others, caught the rapt attention of early geochronologists and had a revolutionary effect on the study of geology. In 1904, Rutherford proposed that geologic time might be measured by the breakdown of uranium in U-bearing minerals and, a few years later, Bertram Boltwood published the “absolute” ages of three samples of uranium minerals. The ages, of about half a billion years, indicated the antiquity of some earth materials, a finding enthusiastically developed by Arthur Holmes in his classic The Age of the Earth. Holmes's early time scale for Earth and his enthusiasm for the developing study of radioactive decay, although not met with immediate acceptance by geologists of his era, helped to set the stage for the acceptance of absolute age as the prime quantitative component in the study of geology and its many subdisciplines.

After the early study of the isotopes of uranium came the discovery of other unstable isotopes and the formulation of the radioactive decay schemes that have become the workhorses of geochronology, such as the rubidium-strontium, samarium-neodymium, potassium-argon, uranium-thorium-lead, and fission track methods. The formulation of the theory of radioactive decay of the parent, unstable nuclide (or the growth of the stable daughter nuclide), developed in the early 1900s, is the basis for the measurement of time, including geologic time, for any of these parent-daughter schemes used by geochronologists. Although each of the dating techniques is based on the formulation, differences occur in the kind of measurement and in the geochemical behavior of the several parent and daughter species. Thus, the geological interpretation of the data obtained is very different for the several chronometric schemes. These techniques for establishing absolute ages for minerals and rocks have been applied to the study of Earth's oldest rocks since the early 1900s and, with the development of mass spectrometry, more intensely since the 1950s.

Distribution of Earth's Oldest Rocks

Although not indigenous to Earth, the oldest rocks found on Earth (and also seen to fall to Earth) are the meteorites, many of which yield radiometric ages near 4.56 billion years, the accepted time of formation of many solar system materials. With respect to the oldest rocks indigenous to Earth, these rocks have the highest probability of being destroyed by ongoing geologic processes such as erosion, metamorphism, and subduction. It is no surprise that fewer and fewer outcrops are found as ages become older, deeper into Precambrian time. The oldest rocks are most commonly found in continental, cratonic regions because of geologic preservative features such as their protective superjacent rocks, their location in tectonically stable continental interiors, and their low density and thus lower propensity for subduction than the more common basaltic rocks.

Histograms of rock ages thus show fewer and fewer data the further back in time they go. Such figures also show a feature whose significance has not been immediately apparent: the clustering of ages in rather discrete groupings. These groupings correlate with regionally defined rock/tectonic units such as those of the Grenville and Superior provinces of the Canadian Shield and indicate the intense geologic activity that resulted in these Precambrian rocks. Many scientists believe that the “magic numbers” that mark the groupings represent geologic periodicity, perhaps a result of major, discrete plate tectonic episodes. Others, however, point out that many radiometric dates fall outside these groupings and that the picture is incomplete and thus misleading. Although certainly incomplete, the available data indicate to many that there is some patterning in both the chemical and chronologic analyses of these rocks.

Early radiometric results showed some ages far back in time, near 3 billion years, and further analyses confirmed their antiquity. The oldest rock was thought to be the Morton gneissin Minnesota, at about 3.2 billion years and questionably older, until several cratons yielded rocks with ages near 3.5 billion years. One such exposure, at North Pole, Australia, is of special significance, because of the concurrence on its age by several chronometric schemes (3.5 billion years) and especially because of its well-preserved bacterial and stromatolitic fossil assemblage, the earliest known. (Equivocal chemical evidence for organic life in even older rocks has been described; the existence of well-developed life at 3.5 billion years presupposes the existence of earlier life.) The oldest rocks, however, appear to be the well-studied Amîtsoq gneiss and contiguous, related rocks in the Godthaab area of western Greenland. Although there is incomplete agreement as to the exact range and significance of these earliest ages, several are close to or perhaps slightly greater than 3.8 billion years. In 2008 the oldest rock on earth was discovered in the Nuvvuagittuq greenstone belt on the coast of Hudson Bay, in northern Quebec, and is dated from 4.28 billion years old. Some of the disagreement with respect to these rocks, as well as for similar rocks around the world, results from incompletely known and undoubtedly variable diffusive and “freezing-in” behavior of the parent/daughter nuclides of the several chronometric systems. This varying behavior commonly results in different “ages” (dates) for the same analyzed rock specimen. A further uncertainty is whether the several isotopic systems can be completely reset, on the whole-rock scale, in metamorphic terrains that have been metamorphosed to lower physicochemical conditions.

Although not all scientists may agree, minerals of even older ages have been analyzed from Archean sandstones of Australia. Zircons (residual mineral phases from the final stages of crystallization of igneous rocks, especially granites) were separated from this stratigraphic unit and analyzed by uranium-thorium-lead dating using an innovative technique, the ion probe mass spectrometer. Although many of these zircons have been analyzed, only a few have exceptionally old ages. However, these ages, ranging back to almost 4.3 billion years, are especially important. Because they are detrital (fragmental) in their host sandstone, they must have eroded from even more ancient rocks, perhaps granites or granitic gneisses, whose age, composition, and petrologic features are important to an understanding of the development of Earth's earliest crust. So far, their provenance (parental rocks) has not been found; apparently they have been completely eroded, altered, or buried by younger rock. If one accepts these earliest ages, crustal rocks existed on Earth less than 300 million years after Earth accreted from the solar nebula.

Analogy with Extraterrestrial Materials

It is useful to place Earth's oldest rocks within the framework of the ages of other available solar system materials, especially meteorites. Although the formation of the Earth—that is, Earth's time of accretion—is accepted by most scientists as having occurred about 4.56 billion years ago, it is obvious from the discussion above that no terrestrial rocks have ages this old. Earth's absolute age, therefore, as well as that of other solid materials of the solar system except for the sun, is known only by analogy with meteorites. Many of the meteorites have been dated by the techniques discussed earlier and give formational ages near 4.56 billion years; some are thought to represent the oldest and most primitive material in the solar system with the possible exception of cometary material and cosmic dust. A few apparently unprocessed (primitive) meteorites yield radiometric age and initial isotopic composition data that suggest formational ages slightly older than 4.56 billion years. The terrestrial planets (Earth, Mercury, Venus, and Mars) are thought to have originated at the same time as did the meteorites.

A few meteorites have ages significantly younger than 4.56 billion years. These rocks are considered to have originated from parent bodies that were large enough to have maintained internal heat and, therefore, igneous processes significantly after 4.56 billion years, as did the earth, with its continuing volcanism and other geologic processes that have resulted in rocks of all ages from 4.56 billion years to the present. Several of these exotic rocks, collected from ice fields in Antarctica, were recognized almost immediately as pieces of the moon, owing to scientists' familiarity with the Apollo missions' lunar rock collections. Even more spectacularly, a small collection of meteorites, long known to be different from the main collection of meteorites, were found to have crystallization ages of about 1.3 billion years, much younger than the accepted accretion age for solar system materials. These rocks must have originated from a body large enough to have maintained geologic processes between 4.56 and 1.3 billion years, unlike the moon, which has so far yielded no rocks younger than about 3.0 billion years. This parent body is widely assumed by scientists to be Mars, a theory that is much strengthened by the compositional similarity of gases dissolved in glass from these meteorites and atmospheric compositions of present Mars, as measured from the Viking lander in the 1970s.

Rocks returned from the moon by U.S. and Soviet space programs yield ages from 0.8 to 4.54 billion years. Although the moon is thought to have originated at the same time as the earth, it is not massive enough to have provided a continuing internal heat source to drive volcanic or tectonic processes to the present time. Instead, significant igneous activity decreased substantially after 3.2 billion years ago until approximately 1 billion years ago. A current and popular theory is that the moon originated by accretion in Earth's orbit from material ejected from Earth after a grazing impact with a Mars-size object. This hypothesis explains why the moon has a composition similar to the Earth's mantle but is poor in volatile elements, and why the moon's core is so small. Such an impact would be responsible for resetting the radiometric dates on the moon and perhaps the earth as well.

A widespread though not fully accepted theory for the early moon is that it underwent a massive, perhaps global melting not long after formation (whether by nebular accretion or Earth impact). Upon cooling, plagioclase feldspar crystallized, floated, and formed the earliest lunar crust (anorthosite), which thus dates from some time after moon accretion. If this theory is correct, no rocks older than the anorthosite will be found; this rock has yielded ages of 4.44 billion years and, arguably, somewhat older. If the moon underwent significant or complete melting, it is possible or likely that the earth experienced the same event, in which case there also will be no Earth rocks representing its earliest history. Finally, owing to Earth's continuing history of constructive and destructive geologic processes, it seems unlikely that significant amounts of rock will be found that date from Earth's first 200 million years.

Geologic Applications

The absolute dating of geologic materials and events has had unprecedented influence on the evolution and understanding of geologic events on Earth, including Earth's origin and its oldest rocks, as well as other ancient minerals and rocks of the solar system. The ability of scientists to establish events in terms of actual years, rather than in relative terms such as “older than” or “younger than,” has led to a realistic knowledge of Earth's origin and its oldest rocks and has led to calibrated time scales for major geologic and biologic processes such as organic evolution. Owing to their usefulness in the precise determination of the ages of very old rocks, dating methods such as uranium-thorium-lead, rubidium-strontium, and samarium-neodymium continue to be of major use in refining the sequence and meaning of Earth's oldest rocks and extraterrestrial materials.

Principal Terms

absolute date/age: the numerical timing, in years or millions of years, of a geologic event, as contrasted with relative (stratigraphic) timing

geochronology: the study of the absolute ages of geologic samples and events

half-life: the time required for a radioactive isotope to decay by one half of its original weight

isotopes: species of an element that have the same numbers of protons but differing numbers of neutrons and, therefore, different atomic weights

mass spectrometry: the measurement of isotope abundances of elements, commonly separated by mass and charge in an evacuated electromagnetic field

nuclide: any observable association of protons and neutrons

radioactive decay: a natural process by which an unstable (radioactive) isotope transforms into a stable (radiogenic) isotope, yielding energy and subatomic particles

radiogenic isotope: an isotope resulting from radioactive decay of a radioactive isotope

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