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GEOLOGICAL HISTORY OF EARTH |
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by colorado-mall.com
In the picture above you can see where geologic history has been made at the west end of Horseshoe Park in Rocky Mountain National Park. A dam broke high in the mountains above this alluvial fan and the millions of gallons of water rushing down the mountain eroded out and carried rock debris to the foot of the mountain creating this alluvial fan. General Overview It is known that the history of the Earth spans a period of more than 4 billion years, although how the Earth formed is still a matter of speculation. Some experts suggest that it originated from condensing gas and dust. Others regard it as having developed from a cloud of meteorites and meteoric dust revolving around the Sun. Gravitational forces caused the particles to come together, or accrete, with heavier particles gravitating to the center and lighter ones collecting outside. This contraction of the mass generated heat, which caused melting and the gradual development of three layers: 1. core, 2. mantle, and 3. crust Geologic history began when the crust first formed, about 4.6 billion years ago, as determined by radiometric age-dating of the oldest rocks and of meteorites. This event must have been preceded by a considerably longer period of astronomical history during which the Solar System and, before that, the universe were created. Geologic History Divided Into Four Eras · The first, the Precambrian Era, began 4.6 billion years ago and includes the first 4 billion years of geologic history, about 85 percent of all geologic time. o Early Life-forms According to scientific theory, certain organic chemicals in the planet's primitive oceans combined--in reactions aided by electrical discharges and an atmosphere rich in hydrogen--to form self-reproducing structures. o The first chemical traces of life date from about 3.77 billion years ago. o Over the next two billion years, cellular life developed and separated into the predecessors of plants and animals. The first multicellular organisms appeared on land some 1.2 billion years ago. Living creatures began appearing in many forms and sizes. · The second, the Paleozoic Era , lasted from 600 to 225 million years ago, roughly 10 percent of geologic time. o The early Paleozoic Era saw an explosion of more complex life-forms, including arthropods and mollusks. Plants, insects, fish, and vertebrates appeared before the start of the Mesozoic Era 225 million years ago. · The third, the Mesozoic Era, lasted from 225 to 65 million years ago, about 4 percent of geologic time, was the Age of the Reptiles. Dinosaurs, including the largest land animals ever known, dominated the period. o The era ended with some great catastrophe--an asteroid collision, perhaps--that led to mass extinctions and ended the reign of the dinosaurs. · The fourth, the Cenozoic Era, embraces the last 65 million years, only about 1.5 percent of geologic time. Being the most recent, the Cenozoic is best known; the landscapes and life forms of modern times reached their present expression during this era. Precambrian Era Precambrian events occurred in four principal episodes, during the · first of which the primitive crust was formed. Presumably, the first crust was perhaps only hundreds of meters thick, which is analogous to that which forms on the surface of a lava flow. Its composition may have been similar to stony meteorites. Geologists suggest that the crust was heated from below by radiogenic heating and by gravitational compaction of the Earth's mass. If so, rain and surface water could not have existed until the crust had cooled below 100 degC (212 deg F), the boiling point of water. Water vapor escaping from below (as happens in volcanic eruptions) would have formed clouds in an atmosphere held by the Earth's gravity. Lightweight gases such as hydrogen and helium mainly escaped to outer space. Gradually, the crust thickened, partly by continued cooling but chiefly by accretion, a result of the gravitational rise of masses of light, molten rock similar to Granite. About 250 million years after it had first formed, the crust was cool enough to accept rain, running water, and primitive seas. Bulbous masses of rising magma formed granitic mountains, and running water washed away the weathering products and deposited them as sediments in the seas. With the addition of granitic rocks, the thickness of the crust may have grown to within a quarter of its present thickness. · The second stage of Earth's development, with the pushing up of granitic mountains; the appearance of primitive seas, drastically different in composition from the seas that later formed; the appearance of the first organisms, presumably in those seas; and the onset of weathering, which produced the first soils. · The third stage, oxygen began to play a major role. Some of it was released from the Earth's interior by Volcanoes, and some was produced by photosynthesis of sea plants (algae).
Paleozoic Era The transition from Precambrian to Paleozoic history is marked in the Fossil Record by the amazingly abrupt appearance of abundant marine fossils, hard-shelled animals preserved in sediments. All the great groups of marine invertebrate animals are represented in the Paleozoic fossil record, whereas no fossil shells have been found in Precambrian rocks. The complex physiology of these animals indicates that a long period of Evolution of soft-bodied invertebrate animals took place during the late Precambrian. The sudden development of hard shells may have resulted from a change in chemistry of the oceans. The composition of seawater may have passed a certain threshhold that enabled marine organisms to secrete layers of mineral matter about their bodies. Thus the fossil record does not indicate a population explosion, but rather a major change in the form of marine life. Moreover, this occurred while deposition in the geosynclines continued, with little or no interruption from late Precambrian into Paleozoic time. Most of the first hard parts of the organisms seem to have been composed of chitin, like the horny coats of many insects and lobsters. Shells of calcium carbonate are found early in the fossil record, however, perhaps indicating that the ocean's carbon-dioxide content reached some critical level that enabled animals to utilize it. Whatever the cause, the change in the fossil record at the beginning of the Paleozoic remains one of the great mysteries of geologic history. Additional species of animals and, later, of plants appeared during the Paleozoic. The first vertebrates, 1) Fishes, also are found in early Paleozoic rocks, although not in the oldest ones. 2) Amphibian fossils appear in rocks of the middle Paleozoic and reptiles in later formations. 3) Land plants, related to the ferns, also developed during the middle Paleozoic, and in late Paleozoic time they formed extensive forests in swamps. · Their woody deposits gave rise to the extensive coal beds (see Coal Mining) that characterize the late Paleozoic. Ancient Seas Ancient seas in which marine deposits accumulated were formed in geosyncline troughs, persisted through the Paleozoic, although with interruptions, until about 40,000 ft.(about 12,000 m ) of sediment had accumulated in them. · The ancient seas that occupied these troughs were quite shallow, and such great thicknesses of sediment could not have been deposited if the troughs had not gradually sunk as the sediment accumulated. · Toward the end of the Paleozoic the sides of the geosynclines were squeezed together by lateral movements in the crust (see Plate Tectonics), and the sediments, originally soft and wet, became consolidated into rock, uplifted, folded, and faulted (see Fold and Fault). · From these movements, Mountains slowly grew, just as slowly as the geosynclines sank. A foot of uplift in 1,000 years is not spectacular; many parts of the Earth, especially around the Pacific and areas bordering and extending east from the Mediterranean Sea, are now moving at faster rates than that. o Yet an uplift of 30 cm (1 ft) in 1,000 years, continuing for 10 million years (a brief period of geologic time), can create a mountain range 3,300 m (10,800 ft) high. This explains why fossil seashells are found high on some mountains. Sea Level never reached such heights. Rather, the marine sedimentary rocks with their fossils have been lifted up to form the mountains. Supercontinent (Pangaea) The crustal plates, constantly in motion brought all the world's landmasses together into one supercontinent (Pangaea) toward the end of the Paleozoic. They were not to come apart again until the Mesozoic, when present-day continents took shape. Moreover, the ancestral continent was located in the Southern Hemisphere, far removed from the area of greatest concentration of present-day landmasses. This is borne out by the fact that late Paleozoic coal deposits at temperate latitudes in Europe and North America contain evidence of tropical conditions, whereas correlative formations just south of the equator in Africa and South America contain evidence of continental glaciations. At the end of the Paleozoic, mountains again began to rise from the geosynclines, including the Appalachians in eastern North America. Also formed then were similar mountains ancestral to the present Rocky Mountains in western North America, and still others in Europe and North Africa ancestral to the mountains bordering the Mediterranean Sea. Shallow seas flooded the stable platforms of Europe and North America, depositing thick beds of salt over large areas. Mesozoic Era The flora and fauna preserved in Mesozoic rocks are wholly different from those of the Paleozoic. · In addition to the Dinosaurs, which are so characteristic of the era, the first birds appeared, and mammals, although small, thrived and became increasingly abundant. · Among the plants, conifers were dominant during the first part of the era; flowering plants, including broadleaf trees similar to modern ones, appeared later in the era. · Sea life was characterized by modern forms of corals, clams, and other kinds of shellfish, although the species differ from those of the Paleozoic. o One of the more important of these groups of shellfish is the Ammonites, marine animals having coiled shells like the nautilus, some of them 1 ft.(30 cm) or more in diameter. The ammonites are a good illustration of evolutionary change. They first appeared during the late Paleozoic and developed great diversity, but they became almost extinct at the end of that era. o A new stock appeared early in the Mesozoic and also thrived, although differently, and then became nearly extinct near the middle of the era. o A third stock with many new species developed during the late Mesozoic, and at the end of the era, it too had become almost extinct. Pangaea, the ancestral supercontinent, began to break up by about the middle of the Mesozoic (see Continental Drift). As a result, certain species of terrestrial mammals became isolated in such widely separated places as Antarctica, South America, Africa, and Australia. Each of the plates that broke apart to form the present continents, like the several segments of late Precambrian crust, consisted of three major structural parts: at the center were ancient Precambrian rocks; these were surrounded by a stable platform, also Precambrian but overlain by Paleozoic and younger sedimentary rocks; bordering the platform were mountains created by the uplift of geosynclinal deposits. Curiously, geosynclinal sediments were eroded not from the shields but from former mountains once located on what are now the ocean and continental margins. Drifting of the continents since the middle Mesozoic has created linear depressions analogous to geosynclines. These are located in front of the moving plates, especially in the oceanic crust peripheral to the Pacific Ocean. The sediments in the trenches, however, are being eroded from the continents and transported oceanward--just the reverse of sedimentation in the geosynclines. In the oceans volcanic ridges developed where viscous mantle rocks broke through the oceanic crust. The oceanic crust has spread laterally from these ridges, and as a result the crusts are progressively older away from the ridges. At the trenches crustal rocks are being shoved oceanward. At the Aleutian Trench, for example, along the south side of the Aleutian Islands, continental rocks are being shoved southward into the Pacific. This structural activity is marked both by volcanism and by Earthquakes. Cenozoic Era At the end of the Mesozoic and the beginning of the Cenozoic, several of the world's greatest mountain chains were uplifted 1. The Rockies 2. Pacific Coast Mountains in North America, 3. the Andes in South America, 4. the Alps, and 5. other mountains bordering the Mediterranean Sea, and their eastward extensions, 6. including the Himalayas of northern India. Beginning in the middle Cenozoic and continuing to the present, parts of some of the continents were broken into fault blocks, notably the western United States and Mexico. The fracturing seems
related to continental drifting. The Pacific coast of Mexico and the
United States, for example, has been and is moving northwestward relative
to the rest of the continent. Few Mesozoic animals, marine or terrestrial, survived into the Cenozoic, so another major paleontological discontinuity separates the eras. Cenozoic fossils serve as good examples of evolution among some of the vertebrates, notably the horse, for which a complete fossil record exists. The ancestral horse was little bigger than a dog and had five toes. By the middle Cenozoic the horse was larger, the number of toes reduced to three. The modern horse has
one toe; the splints on the sides of the legs are relics of the ancient
toes. Age of Man This minute part of geologic time is sometimes called the Age of Man. The earliest dated hominid skeletal remains are found in the rift valleys of Africa. From there, humans migrated to the other continents and evolved to the species Homo sapiens that dominates the world today (see Prehistoric Humans). During the last million years of the Cenozoic, human ancestors in Europe and Asia experienced five periods of glaciation. During the Ice Ages polar ice caps spread onto the continents--halfway to the Mediterranean in Europe and halfway to the Gulf of Mexico in North America, but humans apparently did not arrive in North America until the last glaciation. While the continental ice caps were spreading, small ice masses developed into glaciers (see Glacier And Glaciation) on the high mountains. At low latitudes these mountain glaciers formed only small caps on the highest peaks. Farther north, they extended halfway down the mountain sides, and still farther north they reached the base of the mountains and merged with the continental ice sheets.
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