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Unformatted text preview: AB iogr aph y of E arth Prepared by Ronald Parker
Earlham College Department of Geosciences Richmond, Indiana The Geologic Time Scale Know Era, Period, and Epoch Methods for Studying the Past Historic Earth changes are measured by... Orogenic events. Sea level. Climates. Living organisms. Continental positions. Plate boundaries. Chemistry. Atmosphere. Ocean. Depositional environments. These changes are recorded in rocks. Pb/Pb isochron age
derived from samples of the Earth and meteorites measurement of three isotopes of lead (Pb-206, Pb-207, and either Pb-208 or Pb-204) Over time, the amounts of Pb-206 and Pb-207 will change in some samples as these isotopes decay From the slope of the line we can compute the amount of time which has passed since the pool of matter became separated into individual objects. Methods for Studying the Past Paleoclimates Rocks preserve ancient climates. Tropical Extensive coral reefs. Subtropical Extensive deserts. Polar Extensive glacial deposits. Greenhouse Earth. Snowball Earth. Climatic belts expand and contract. O/16O isotopic ratios preserve ancient temperatures
18 Methods for Studying the Past Evolution Fossils preserve changes in Earth's life. Sedimentary rocks preserve fossil ecosystems. Organisms inhabiting Earth have obviously changed. Over geologic time, most species have exhibited both... Trends toward specialization. Catastrophic extinctions. The precise date of creation in years varies among Creationists. The Bishop of Ussher (1650) claimed that the first day of creation began at nightfall preceding Sunday, October 23, 4004 BC. H. M. Morris (1993) suggests that creation took place about 6,000 years ago and that elapsed time from creation to the flood was 1,656 years. By subtraction, this means that the flood took place about 2350 B.C., a time somewhat after the start of recorded Egyptian and Summerian history. J. D. Morris (1994) notes that some uncertainties exist between the length of the Biblical time and the historical record but "I suspect it is the secular chronology which needs revision." The Age of the Earth The oldest rocks on Earth's surface date to 3.96 Ga. Zircons in ancient sandstones date to 4.1-4.2 Ga. Age of Earth is 4.57 Ga based on correlation with... Meteorites. Moon rocks. Know Era, Period, and Epoch 5 Hadean The Hadean Eon After differentiation, Earth smashed a protoplanet. The size of Mars, this planet blasted... A sizeable chunk of Earth's mantle. Much of the protoplanet's mantle. Debris from the collision formed a ring around Earth. This debris coalesced to form the moon. The Hadean Eon Volcanic outgassing created a deadly atmosphere. N2, NH3, CH4, H2O, CO, CO2, and SO42- were components. This atmosphere had a greater density that today's. Meteorite impacts were abundant between 4.0 and 3.9 Ga. This would have destroyed early formed crust. Oldest evidence of crust is 4.03 Ga. Early formed crust was bombarded by meteorites. Know Era, Period, and Epoch
Hadean The Archean Eon Volume of continental crust increased dramatically. 85% of modern continental area present by end of Archean. Signals full development of plate tectonic processes. The Archean Eon Archean cratons consist of 5 principal rock types. Gneiss Hi-grade metamorphics from Archean collisions. Greenstone Metamorphosed fragments of mafic rocks. Granite Magmas from partial melting of the crust. Graywacke Sedimentary debris derived from arcs. Chert Silica precipitated in the deep sea. The Archean Eon Life first appeared during the Archean. Evidence? Biomarker molecules. Isotopic signatures. Preserved fossil cells. Clear evidence of life in rocks dated to 3.5 Ga. Life may have started earlier. Oldest undisputed bacteria fossils ~ 3.2 Ga. 5 Know Era, Period, and Epoch Hadean The Proterozoic Eon New continental crust formed, but at slower rates. 90% of Earth's continental crust by the middle Proterozoic. Continents cooled and strengthened to become cratons. Redbeds (red from Fe-oxides) don't appear before 2.2 Ga Atmospheric oxygen (O2) skyrocketed 2.4 to 2.2 Ga. 5 Know Era, Period, and Epoch Hadean The Phanerozoic Eon Phaneros = visible; zoic = life. The most recent 542 Ma of Earth history. Began with appearance of diverse hard-shelled organisms. Hardshells vastly increased fossil preservation. Made possible a more complete archive of life on Earth. Know Era, Period, and Epoch
5 Hadean The Early Paleozoic Cambrian and Ordovician life evolution: Earliest Cambrian Hard shells appeared for the 1st time. Massive diversification followed: the "Cambrian explosion." Reflects the evolution of a complex ecosystem. Plankton, deposit feeders, giant predators. The Early Paleozoic Cambrian and Ordovician life evolution: Life during the Ordovician included several "firsts." The 1st vertebrates jawless fish (agnathans). The 1st crinoids (flower-like echinoderms). The 1st green algae and primitive land plants. The end of the Ordovician witnessed a mass extinction. The seas roiled with life, but there was no life on land yet. Know Era, Period, and Epoch
5 5 Hadean The Middle Paleozoic Silurian and Devonian life evolution: Fish rapidly evolved and proliferated. Jawed fish. Boney fish. The 1st land animals followed plants. Scorpions. Spiders. Insects. Crustaceans. 5 Know Era, Period, and Epoch 5 Hadean The Late Paleozoic Carboniferous and Permian paleogeography: Pangaea was a massive supercontinent. The interior was a vast desert far from ocean moisture. Large deposits of red (oxidized) dune and fluvial sandstones. Large accumulations of evaporites. Thick coals formed equatorially. Ice sheets spread across Gondwana The Late Paleozoic Carboniferous and Permian life evolution: Life continued to evolve. Dense tropical wetlands hosted vegetation and giant insects. Amphibians diversified. Reptiles appeared for the 1st time. The amniote egg permitted reproduction away from water. Reptiles populated previously inhospitable environments. The Late Paleozoic Carboniferous and Permian life evolution: The Paleozoic ended with the Permian extinction. 90% of all marine species disappeared. Some evidence links the extinction to a bolide impact. 5 Know Era, Period, and Epoch 5 Hadean Early and Mid-Mesozoic Era Triassic and Jurassic paleogeography: The supercontinent Pangaea lasted 100 million years. Pangaea began to rift late Triassic to early Jurassic. Rifting started in the North Atlantic. A thin narrow ocean had opened by the late Jurassic. This basin accumulated thick evaporite deposits. Early and Mid-Mesozoic Era Triassic and Jurassic paleogeography: Western North America was an active margin. Subduction created island arcs. Collision added arcs and microcontinents to N. America. Sonoma orogeny Perm Tri. Nevadan orogeny Late Jur. Early and Mid-Mesozoic Era Triassic and Jurassic life evolution: By end of the Triassic the first true dinosaurs appeared. Dinosaurs differ from other reptiles in significant ways. Legs are positioned beneath their bodies. They bear evidence of warm-bloodedness. By the end Jurassic giant sauropods were abundant. 5 5 Know Era, Period, and Epoch Hadean The Late Mesozoic Cretaceous paleogeography: Breakup of Pangaea continued through the Cretaceous. South America separated from Africa. Antarctica separated from Australia. India broke from Gondwana and raced toward Asia. Passive margins developed along the Atlantic. The Late Mesozoic Cretaceous paleogeography: In western North America the Sierran continental arc grew. Arc volcanoes have long eroded away. Roots of the arc are exposed as the Sierra Nevada batholith. An accretionary prism grew on the overriding plate. These sediments form the present-day Coast Range. The Late Mesozoic The late Mesozoic was a time of unusual volcanic activity. Attributed to unusually large hot spots. Late Mesozoic volcanism influenced the climate. Volcanic CO2 (8x modern values) warmed the atmosphere. Melting ice caps contributing to sea level rise. The Late Mesozoic Volcanic CO2 (8x modern values) warmed atmosphere The K-T (CretaceousTertiary) boundary event. There is abundant evidence of catastrophic change. Instantaneous global change in fossil assemblages. Sudden mass extinction of most species on Earth. The dinosaurs that had ruled the planet for 150 Ma vanished. 90% of plankton disappeared. 75% of plant species vanished. 5 Know Era, Period, and Epoch Hadean The Cenozoic Era Cenozoic (65 Ma present) The most recent history Cenozoic paleogeography: Fragments of Gondwana collided with Europe and Asia. Closed the Tethys Ocean. Deformed and uplifted the Alpine-Himalayan chain. The Cenozoic Era Cenozoic paleogeography: The large Farallon plate was almost completely consumed. A transform fault replaced part of the Farallon trench 40 Ma. The San Andreas/Queen Charlotte fault system formed. The Juan de Fuca plate remains as a Farallon remnant. The Cenozoic Era Cenozoic life evolution: After the K-T boundary, plant life recovered. Forests of angiosperms and gymnosperms reappeared. The 1st grasses appeared in the mid-Cenozoic. Dinosaur descendants (birds) diversified and spread. 5 Know Era, Period, and Epoch
5 Hadean The Cenozoic Era Cenozoic paleogeography: The Quaternary Period (2 Ma present): Cool climate. Pleistocene ice ages Continental scale glaciation. Glaciers have advanced and retreated at least 20 times. Modern landscapes sculpted by glacial erosion/deposition. Climate warmed 11 Ka; Earth is currently in an interglacial time interval. The Cenozoic Era Cenozoic life evolution: What sparked the evolution of the genus Homo? Climate changes that led to the spread of grasslands? Permitted departure from the trees. Life on the ground allowed more time for infant development. This permitted growth of larger brains. Homo erectus appeared ~ 1.6 Ma. Made stone axes. Homo sapiens appeared ~ 500 Ka. Modern humans appeared ~150 Ka. Many giant mammals died off 10 Ka. Climatic change? Hunting pressure by humans? Human Population Growth "Anyone who believes that exponential growth can go on forever in a finite world is either a madman or an economist." -Kenneth Boulding The Geologic Time Scale ...
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This note was uploaded on 02/22/2012 for the course CE 70 taught by Professor Johnson during the Fall '07 term at Berkeley.
- Fall '07