Geo-101 Week 2 Notes
Geo-101 Week 2 Notes GEO 101
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This 8 page Class Notes was uploaded by godfreytorrance on Monday August 29, 2016. The Class Notes belongs to GEO 101 at University of Alabama - Tuscaloosa taught by Prof. Yong Zhang in Fall 2016. Since its upload, it has received 13 views. For similar materials see Intro Geology in Geology at University of Alabama - Tuscaloosa.
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Date Created: 08/29/16
Geo101|Chapter 11 a Biography of the Earth 1 Geological Time Table Geologists separate age of the Earth in Eons. i. Hadean ii. Archean iii. Proterozoic iv. Phanerozoic Eras i. Paleozoic ii. Mesozoic iii. Cenozoic Periods i. Cambian ii. Ordovician iii. Silurian iv. Devonian v. Carboniferous vi. Permian vii. Triassic viii. Jurassic ix. Cretaceous x. Paleogene xi. Tertiary xii. Neogene xiii. Quaternary Epoch i. Paleocene ii. Eocene iii. Oligocene iv. Miocene v. Pliocene vi. Pleistocene vii. Holocene Hadean Eon: Before the rock record Archean Eon: Birth of the continents and life Proterozoic Eon: The Earth in Transition Paleozoic Era: Continents reassemble, and life gets complex Mesozoic Era: Dinosaurs rule the Earth Cenozoic Era: The modern world comes to be. 2 . Hadean Eon Time Interval 4.573.85 Ga (Billion yrs) The planet was so hot that its surface was a magma ocean Underwent intense meteor bombardment A rock record of this Eon does not exist, because the Earth’s surface may have been partially molten and was pulverized by meteorites. Internal Differentiation: i. Dense iron alloy melted and sank downward (due to gravity) to form the Earth’s core, leaving lessdense mantle (ultramafic rock) behind 2 ii. Formation of the moon iii. Formation of a toxic atmosphere. 3. Archean Eon: Birth of the Continents and Life In this Eon (3.852.5Ga), the first continent crust (that still remains) formed, and the first rock record formed. The atmosphere changed: CO2 is the major component; little oxygen; so the air was unbreathable. First life: (archaea and bacteria) appeared. 4. Proterozoic Eon: The Earth in Transition Time interval: 2.50.54 Ga Cratons formed Craton: a longlived block of durable continental crust (commonly found in the stable interior of a continent) All Cratons that exist today formed in the Proterozoic Eon Cratons formed then sutured together to for Supercontinents Multicellular organisms appeared The atmosphere began to accumulate significant amounts of oxygen This is called the great oxygenation event. With the appearance of photosynthetic organisms, oxygen began to enter the atmosphere. Snowball Earth: At the end of the Proterozoic Eon, radical climate shifts occurred on Earth. Glacial covered all land, and the entire ocean surface froze. 5. Paleozoic Era: Continents Reassemble, and Life Gets Complex The early Paleozoic Era: CambrianOrdovician Periods (542444 Ma) 3 Paleogeography i. The supercontinent Pannotia broke up, yielding smaller continents. ii. Epicontinental Sea – is a shallow sea overlying a continent. Life Evolution Cambrian Explosion of Life – The remarkable diversification of life occurring at the beginning of the Cambrian Period Mass extinction at the end of Ordovician Period due to Ice Age. The Middle Paleozoic Era: SilurianDevonian Periods (444359 Ma) Paleogeography Continents continue to reassemble Examples: Acadian Orogeny & Caledonian Orogeny Orogeny – A mountain building event. Land plants and insects begin to appear. The late Paleozoic Era: CarboniferousPermian Periods (359251 Ma) Continents coalesced to form another supercontinent: Pangea. 6. Mesozoic Era: Dinosaurs Pangea broke apart, and the Atlantic Ocean formed. Convergent boundary dominated along the western North America During the Cretaceous periods, the continents flooded. The Earth’s climate continued to shift to warmer conditions, and sea level rose significantly. Dinosaurs reached their peak of success at this time. 4 Flowering plants and modern fish appeared. KT Boundary Event i. The mass extinction that happened at the end of the cretaceous period (65 million years ago) possibly due to the collision of an asteroid with Earth. ii. K stands for Cretaceous and T for Tertiary – Cause dinosaur extinction. 7 Cenozoic Era – Modern World Comes to Be Paleogeography During the Cenozoic i. The mountain belts of today rose ii. Modern plate boundaries became established iii. Mammals diversified. Recently, ice age glaciers covered large areas. Life Evolution i. Various kinds of mammals filled niches left vacant by the dinosaurs. ii. The human genus, homo, appeared and evolved through the Pleistocene Ice Age. 8 Geologic Time Geologic Time – The span of time since the formation of the Earth. The principle of uniformitarianism – the present is key to the past. Or in other words, according to this principle, physical processes (that operate in the modern world) also operated in the past, at roughly the same rate. Relative Age – The age of one geologic feature with respect to another. 5 Numerical Age – The age of one geologic feature given in years. It is also called the “Absolute Age.” 9 How to Define Relative Age Principle of Uniformitarianism – Physical processes (we observe operating today) also operated in the past, at roughly the same rates. i. The process that formed cracks today also formed the mudcracks preserved in the ancient rocks Original Horizontality – Layers of sediment, when first deposited, are fairly horizontal, because sediments accumulate on surface of low relief in a gravitational field. Superposition – In a sequence of sedimentary rock layers, each layer must be younger than the one below. This is because a layer of sediment cannot accumulate unless there is already a substrate where it can collect. Lateral Continuity – Sediments generally accumulate in continuous sheets within a given region i. Layers can be continuous over broad areas when the first deposited. Erosion may later remover part of a layer. Cross Cutting Relations – If one geologic feature cuts across another, the feature (that has been cut) is older so younger features cut older ones. Baked Contacts If an igneous intrusion “bakes” (metamorphoses) surrounding rocks, then the rock (that has been baked) must be older than intrusion. So the intrusion is younger. Inclusion – A rock containing an inclusion (fragment of another rock) must be younger than the inclusion so the inclusion is older than the host rock. 6 Fossil Succession – The assemblage of fossils in strata changes from base to top of a sequence. i. Or in other words, fossil species occur in a predictable order ii. For example, a bed containing fossil F is younger than a bed containing Fossil A iii. Once a species becomes extinct, it never reappears. Unconformities – A stratum (plural: strata) 1 Stratigraphic Formations a. A given succession of strata that can be traced over a fairly broad region. b. Stratigraphic Column – Shows the succession of strata in the region. 2 Stratigraphic Correlations a. The process of determining the stratigraphic relationship between the strata at one location and the strata at another is called “correlation.” b. Method 1: Lithologic Correlation – Geologists typically correlate formations between nearby regions based on similarities in rock type. c. Method 2: Fossil Correlation – sometimes we rely on fossils to define the relative ages of sedimentary units. 3 Geologic Column a. Geologic column is a composite stratigraphic chart that represents the entirety of the Earth’s history. b. The column is divided into segments. The largest subdivisions break Earth history into four eons: the hadean, Archean, Proterozoic, and Phanerozoic eons. 7 c. The Phanerozoic eon is subdivided into three eras. We further divide each era into periods and each period into epochs. d. The Hadean Eon is not shown because rocks do not preserve a record of this time interval (i.e., rocks were formed after the hadean eon.) 4. How to Determine Numerical Age a. Isotopic Dating: specifies numerical ages in years b. Geochronology: The overall study of numerical ages c. Radioactive Decay: Radioactive isotopes decay at a constant rate characterize by a known halflife. d. What does an isotope date mean? i. The isotopic age of a mineral specifies the time at which the mineral cooled below a closure temperature. ii. Closure Temperature: The temperature below which isotopes are no longer free to move iii. If the temperature is higher than the closure temperature, then isotopes can move freely and therefore change the isotope ratio, resulting in a meaningless isotopic date. e. Can we isotopically date a sedimentary rock directly? i. No. f. We can use isotopic dating to determine when an igneous rock solidified and when a metamorphic rock cooled. g. To date sedimentary strata, we must examine crosscutting relations with dated igneous or metamorphic rock. 8
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