Hadean and Archean Eons

The Hadean period is the earliest on the geologic time scale. Within this time, the Earth's basic structure and chemical composition evolved. Because the rocks of this period are complex and fragmentary, their history remained opaque until twentieth-century technological advances allowed scientists to study them with greater accuracy.

Evolution of Earth

The Hadean eon is the earliest in Earth's history and with the Archean, accounts for over 50 percent of all geologic time. The exact time of its beginning is unclear, but an estimate of 4.5 billion years ago is generally accepted. The Hadean eon continued until approximately four billion years ago. It was followed by the Archean eon, which continued until approximately 2.5 billion years ago.

The Hadean was a time of major evolution in the Earth's chemical and physical structure, which gave the planet its basic character. The current paradigm, first enunciated in 1905 by the American geologist Thomas C. Chamberlin and astronomer Forest R. Moulton at the University of Chicago, describes the Earth and the solar system in the beginning as a gas cloud rotating around a center point. Shock waves from two nearby and independent supernovas caused this cloud to collapse as the rate of rotation increased. With greater rotation, the cloud progressively flattened into a disk shape.

The dominant physical process at this time was the condensation of tiny particles consisting mostly of silicates and nickel-enriched iron. The resulting rotational eddies concentrated the particles, and they clustered at discrete distances from the proto-sun. The particles literally fell together under their own mutual gravitational attraction, creating larger particles, which, in turn, grew through impact with other masses. This process continued until planetesimal bodies (kilometers in size) accreted and acted as gravitational “dust mops” sweeping through space, collecting more mass. Astronomers believe that once accretion through gravitational attraction began to create density centers, the centers reached their present masses rather quickly, requiring something on the order of ten thousand years. The impact-accretion action resulted in the fragmentation and heating of the proto-earth, differentiating the preplanetary material. A segregated interior began to develop, and the process accelerated as larger masses retained more and more of the impacting fragments. When the proto-earth reached a sufficient mass, a segregation by density occurred similar to the overall density segregation of the solar system.

At or near this time, the sun ignited. Its gravitational influence, temperature gradient, and solar wind produced a strong chemical/density segregation among the planets. The “rocky” planets—Mercury, Venus, Earth, and Mars—formed close to the sun, while the frozen gas planets—Jupiter, Saturn, Uranus, and Neptune—formed in the outer orbits.

By 1954, Harrison Brown had proposed an impact hypothesis to account for the initial accretion and differentiation of the Earth into core, mantle, and crust. Following many gravitational impacts, the Earth accreted as a homogeneous mixture of silicates and iron-nickel. Radioactive heating caused the dense iron-nickel to melt and sink to the center of the Earth, where it formed the core. The remaining lighter silicates formed the mantle and the crust. A slightly different impact hypothesis, described by Robert Jastrow in 1963, requires the existence of a dense iron-nickel condensate phase at the Earth's core. A mantle and crust of silicates accreted around the core by gravitational impact. The actual mechanism of accretion and differentiation probably incorporates features from both hypotheses.

Meteorites

Meteorites may aid understanding of the Hadean eon. Their chemical composition suggests that the condensation or accretion from the nebular disk was not homogenous, falling instead into three general groups. These groups—called iron, stony-iron, and stony—appear to represent an early crystallization of two distinct chemical phases. Iron meteorites consist chiefly of iron with 4 to 20 percent alloyed nickel and small amounts of other elements such as chromium. The stony-irons, as the name suggests, consist of roughly equal amounts of rock and iron. Stony meteorites are largely silicate minerals. The marked difference in the two earliest known groups (4 to 4.5 billion years old) found in the primordial solar system increases the likelihood that the Earth accreted as a partially differentiated body.

A comparative study of Earth's moon strengthens the impact model of Earth's early history. Because of the close ratio of the two masses compared to other planet-satellite systems and the Earth and the moon revolve around a common center, astronomers often describe the Earth-moon relation as a dual planet system rather than a planet-satellite system. They assume that Earth and moon formed contemporaneously and near each other. The moon, however, lacks the destructive erosional and tectonic forces found on Earth. Therefore, it is a time capsule that mirrors an earlier Earth phase. A molten stage appeared during the later stages of the moon's evolution. The heat energy that caused this melting came from the impacts of the planetesimals and the decay of radioactive elements. Shortly after its crust cooled, intense meteoric bombardment left the moon with numerous craters, resulting in highland-type terrain. Near the end of this cratering period, several asteroid-sized objects (100 kilometers in diameter) struck the lunar surface, breaking through the thin crust. This allowed the darker-colored basaltic lava to escape to the surface, flooding the low-lying areas; these darker areas are the maria.

Equally large meteoroids must have struck Earth similarly during this time period. All maria basalts sampled so far have ages in three to four billion years and are contemporaneous with the oldest dated Earth rocks. In 1958, radioactive decay studies by L. T. Aldrich and G. W. Wetherill showed that the oldest surviving relics of terrestrial rocks date from about 3.8 billion years. Two explanations proposed in the late 1970s may account for the age differences of terrestrial rocks, lunar rocks (highlands) at 4.1 billion years, and meteorites at 4.5 billion years. According to J. V. Smith, the Earth's mantle was so cool after accretion (about 4.5 billion years ago) that it did not heat up sufficiently from gravitational pressure to produce magmas for another 700 million years. The second explanation is B. M. Jahn's and L. E. Nyquist's plate tectonics model of subduction. It requires the crust's continuous generation and destruction and its recycling into the mantle via convection currents until some of the crustal material became stable. Meteor bombardment may have destroyed some of this early thin crust, but tectonics was probably the dominant factor.

Impact Estimates

Geologists have experienced difficulty in estimating the number of meteors that struck Earth. They are also uncertain whether these meteors were of similar size and energy as those that formed the lunar craters in the Hadean period. R. A. F. Grieve and H. Frey have extrapolated much of the physical and statistical modeling for the Archean from the moon, whose early Archean crust equivalent is preserved. Maria lava dates obtained from the Apollo missions indicate an age of 3.1 to 3.8 billion years for these forty well-defined maria basin structures, suggesting that perhaps up to three thousand basins might have existed on Earth. These statistical models of the early 1980s project the formation of at least two thousand—and more likely twenty thousand—basins on Earth with ages between 3.9 and 4.4 billion years. Through time, the frequency of these impacts should have decreased nearly exponentially.

Meteors with diameters greater than 100 kilometers are the most significant geologic agents of the Hadean and early Archean. Geologists estimate that several thousands of 100-kilometer meteors impacted Earth's early Archean surface and converted 30 to 50 percent of the crust into impact basins. These impacts could produce walls three kilometers above the surrounding terrain, with depths of ten to twelve kilometers. An early ocean basin probably had this type of topography. The energy expended upon impact fractured the thin Archean crust to a depth of twenty-five kilometers. It allowed molten material from the mantle to escape to the surface and flood the basin. The resulting structure became an ideal trap for the accumulation of sediments. Its stratigraphy would have included several kilometers of impact melts, crustal breccia, volcanics, and highland sediments. Over time, the basins subsided, underwent a second partial melting, and produced a new generation of magma. If the recycling continued, the magmas could have produced rocks higher in silicon and aluminum, like continental cores. Geologists believe that once the basins became tectonically stable, their stratigraphy included a mixture of metamorphosed rock intruded by granites and capped by crustal sediments. Such an interpretation can explain the formation of the protocontinent nuclei.

Further tectonic development of the early crust led to the partial aggregation of nuclei, which, with the evolution of the greenstone belts, produced the familiar Archean shields. Greenstone belts are unique to the Archean eon. They consist largely of volcanic rocks and sedimentary rocks derived primarily from volcanics. Their stratigraphy is often metamorphosed, producing the mineral chlorite, which has the characteristic green color from which the belts derive their name. Because the composite features of the greenstone belts are without a counterpart in modern geology, the geologic conditions of their formation were very different from what is observed today. The Canadian Shield demonstrates the greenstone stratigraphy and tectonics. Characteristically, it exhibits alternating linear belts (compressed basins) of greenstone-granite and gneiss. It also contains a series of elliptically shaped basins, such as the Abitibi. While no large continents existed during the Archean eon, the nuclei necessary for their formation were present as protocontinents. These protocontinents were separated by numerous marine basins that accumulated lava and volcanic sediments. They later became greenstone belts. The thin Archean crust often broke under the active tectonic forces in the mantle and interjected magma into the protocontinents.

Geologists have had difficulty unraveling the true nature of the Archean stratigraphy. The two main problems are that the greenstone-granite terrains contain extensive metasedimentary sequences and that the combined igneous-metasedimentary successions grade laterally and vertically from intermediate-to high-grade metamorphic rocks. Large-scale magmatic intrusion and the structural response to the meteor impacts characterize the tectonics and stratigraphy of the Archean. The stratigraphic succession—the product of these tectonic forces—suggests that a deep crustal fracture system controlled the geology of this time and that meteorite impact produced the surface topography.

Study of the Hadean and Archean Eons

Scientists have used diverse analytical techniques to study the Hadean and Archean eons. Because tectonics or erosion has destroyed much of Earth's original Precambrian material, scientists look to the moon to sample and observe this early stage of planetary development. The imagery from various lunar orbiter missions of the 1970s yielded clarity about and perspective of the moon previously unknown. Few outside the field of geology realized that these images also functioned as a snapshot of the early Earth. Voyager photos of September 18, 1977, provided the first look at Earth and the moon as a dual-planet system. Moon rock samples obtained by Apollo astronauts during the missions of the early 1970s indicated chemical compositions and histories similar to the oldest of Earth's rocks. They provided the first evidence supporting a parallel history.

Geologists have a variety of tools and techniques to unravel the Precambrian story. These range from viewing thin sections of rock samples under a microscope to using remote-sensing, Earth-orbiting satellites beginning in July 1972. Because fossils are virtually nonexistent in the Archean, geologists rely on radioactive dating techniques to sequence events. Field mapping and sample collecting are their primary geological tools.

Principal Terms

accretion: the gradual accumulation of matter in one location, typically because of gravity

basalt: a fine-grained, dark mafic igneous rock composed chiefly of plagioclase feldspar and pyroxene

basin: a regionally depressed structure available for the collection of sediments

breccia: a rock formed by the amalgamation of various rock fragments

gneiss: a coarse-grained metamorphic rock that shows compositional banding and parallel alignment of minerals

granite: a coarse-grained, light-colored igneous rock composed of three types of minerals: two types of feldspars, quartz, and variable amounts of darker minerals

greenstone: a field term used to describe any altered basic igneous rock that owes its color to the presence of various green minerals

maria: Latin plural meaning “sea” that is used to describe the moon's dark areas; the light-colored areas of the moon are called the highlands

plate tectonics: the study of the movement and deformation of large segments or plates of the Earth's surface over the underlying mantle

silicate: a substance whose structure includes silicon surrounded by four oxygen atoms in the shape of a tetrahedron

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