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PRECAMBRIAN EARTH AND LIFE HISTORY THE ARCHEAN EON • The duration of geologic time is beyond comprehension. If 24 hours represented all geologic time, the Precambrian would be more than 21 hours long, more than 88% of the total. • Precambrian is a widely used term that refers to both time and rocks. In regard to time it includes all geologic time from Earth’s origin 4.6 billion years ago to the beginning of the Phanerozoic Eon 542 million years ago. • Precambrian encompasses all rocks lying below Cambrian-aged rocks. • Precambrian is difficult to interpret, particularly for the older part of the Precambrian because many of the rocks have been metamorphosed and deformed and in many areas they lie deeply buried beneath younger rocks. • Establishing formal subdivisions of the Precambrian is difficult because of the complexities and the fact that Precambrian contain few fossils and are of little use in stratigraphy. • Most Precambrian subdivisions are based on absolute ages rather than time-stratigraphic units. What Happened During the Eoarchean? • The Eoarchean refers to all geologic time from the Earth’s origin until the onset of the Paleoarchean 3.6 billion years ago. Also the oldest rocks on Earth are 3.96 billion years old, so we have no geologic record for most of the Eoarchean. Nevertheless some events took place during this time. • For one thing, it was during the Eoarchean that the Earth accreted from planetesimals and differentiated into a core and mantle, and at least some of the crust was present. • Earth was bombarded by comets and meteorites and volcanic activity. • An atmosphere formed but it was different from the oxygen-rich one we have now, and surface waters began to accumulate as the Earth cooled. • The oldest known rocks on Earth are 3.96 billion years—Acasta Gneiss in Canada and 3.8 billion- year old rocks from Montana and Greenland, indicating that some continental crust had evolved by the Eoarchean time. In addition some sedimentary rocks in Australia contain 4.4 billion year old zircons (ZrSiO4) so source rocks at least that old must have existed. • Shortly after Earth formed, it was exceedingly hot and volcanism was widespread. However, rather than being a fiery orb for half a billion years as was formerly accepted , some geologist think that the Earth cooled sufficiently by 4.4 billion years ago for surface waters to accumulate. • The first crust that formed was probably thin and made of ultramafic rock. Upwelling mantle currents of magma disrupted this early crust and numerous subduction zones developed to form the first island arcs. Collisions between island arcs eventually formed continental cores. Larger groups of merged island arcs, or protocontinents, grew faster by accretion along their margins and eventually the first continental nuclei or cratons formed. Continental Foundations—Shields, Platforms and Cratons • Continents consist of rocks with an overall composition similar to that of granite, and continental crust is thicker and less dense than oceanic crust, which is made of basalt and gabbro. • A Precambrian shield is found on all continents. Continuing outward from the shields are broard platforms of buried Precambrian rocks that underlie much of each continent. Collectively a shield and platform make up a craton—a continents ancient nucleus. • Cratons are the foundations of continents, and along their margins more continental crust was added as they evolved to their present sizes and shapes. • Both Archean and Proterozoic rocks are present in cratons. • In North America, the exposed part of the craton is the Canadian shield, which occupies most of northeastern Canada, a large part of Greenland, Adirondack Mountains of New York, and parts of Lake Superior region in Minnesota, Wisconsin, and Michigan. • The Canadian sheild as well as the adjacent platform are made up of numerous units of smaller cratons that amalgamated along deformation belts during the Paleoproterozoic. Archean Rocks • Only 22% of Earth’s exposed Precambrian crust is Archean, with the largest exposures in Africa and North America. • Archean crust is made up of a variety of rocks but are geologist characterize them as greenstone belts and granite-gneiss complexes. • Granite-gneiss complexes are actually composed of a variety of rocks with granitic gneiss and granitic plutonic rocks. Greenstone Belts • An ideal greenstone belt has three major rock associations; volcanic rocks are most common in the lower and middle parts whereas the upper rocks are mostly sedimentary. They typically have a synclinal structure. Evolution of Greenstone Belts • Greenstone belts probably developed in back-arc marginal basins. Back-arc marginal basins are found between continent and a volcanic island arc. • There is an early stage of extension when the back- arc marginal basin forms, which is accompanied by volcanism, emplacement of plutons and sedimentation, followed by an episode of compression when the basin closes. During this latter stage, the greenstone belt rocks are deformed, metamorphosed, and intruded by granitic magma. Archean Plate Tectonics and the Origin of Cratons • Most geologist are convinced that plate tectonics took place during the Archean but it differed in detail. • Because the Earth had more residual and radiogenic heat, plates moved faster and magma was generated more rapidly. As a result continents grew more rapidly along their margins by a process called continental accretion. The Atmosphere and Hydrosphere How Did the Atmosphere Form and Evolve? • Today Earth’s atmosphere is composed mostly of nitrogen and free oxygen, meaning oxygen not combined with other elements as in carbon dioxide (CO2 ) and water vapor (H2O). It also has small but important amounts of other gases such as ozone (O3) which blocks most of the Sun’s ultraviolet radiation. • Earth’s earliest atmosphere was probably composed of hydrogen and helium, the most abundant gasses in the universe. • This atmosphere would have quickly been lost into space because the Earth’s gravitational attraction is too weak to retain gasses with such low molecular weights. Second, the Earth at the time had no magnetic field—magnetosphere to keep solar winds from sweeping away the atmosphere. • Water vapor is the most common gas emitted by volcanoes today including carbon dioxide, sulfur dioxide, carbon monoxide, sulfur, hydrogen and nitrogen. Archean volcanoes emitted the same gases and thus an atmosphere developed, but an atmosphere lacking free oxygen. • Two processes account for introducing free oxygen into the atmosphere. • Photochemical dissociation—involves ultraviolet radiation from the Sun in the upper atmosphere disrupting water molecules, thus releasing their oxygen and hydrogen. • Photosynthesis—a metabolic process in which organisms use carbon dioxide and water to make organic molecules and oxygen is released as a waste product. Earth’s Surface Waters—The Hydrosphere • Water vapor is the most abundant gas released by volcanoes, so once the Earth had cooled sufficiently, water vapor condensed and began to accumulate, perhaps as early as 4.4 billion years ago. • Oceans were present during the Eoarchean. Although volumes and the geographic extent can not be determined. The Orgin of Life • Scientist have found fossils in 3.3 to 3.5 billion-year- old Archean rocks and chemical evidence in 3.85- billion-year-old rocks in Greenland that have convinced investigators that organisms were present by this early date. • There is unequivocal evidence for Archean organisms but compared to the present, the Archean was biologically impoverished. • How might have life originated? Abiogenesis—how life originated from nonliving matter. • Before discussing abiogenesis let us be clear on what is living and nonliving. In most cases the distinction is straightforward: dogs and trees are alive, but rocks and water are not. • Bacteria are living, but in some circumstances, they can go for long periods without showing signs of life and then go about living again. • Viruses behave like living organisms in the appropriate host cell, but when outside a host cell they neither metabolize nor reproduce. • Also, microspheres (carbon-based molecules) from spontaneously and grow and divide but these processess are more like random chemical reactions. • So what do viruses and microspheres have to do with the origin of life? First they show that living versus nonliving distinction is not always easy to make. Second, if life originated by natural processes from nonliving matter (abiogenesis), it must have passed through prebiotic stages—that is, stages in which the entities would have shown signs of living organisms but were not truly living. Abiogenesis holds that several small steps took place, each leading to an increase in organization and complexity. The Oldest Known Organisms • As far back as the early 1900’s, Charles Walcott described layered moundlike structures from Paleoproterozoic-aged Gunflint Iron Formation in Ontario, Canada. He proposed that these structures, now called stomatolites, represented reefs constructed by algae, but not until 1954 did paleontologist demonstrate that stromatolites are the product of organic activity. • Present-day stromatolites form and grow as sediment grains are trapped on sticky mats of photosynthesizing cyanobacteria (blue-green algae). • Currently the oldest known stromatolites are 3.0 billion years old in rocks found in South Africa.
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