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EESA06H3 (234)
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Chapter 19&20

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Environmental Science
Nick Eyles

Chapter 19: Time and geology What is Uniformitarianism? - Uniformitarianism – principle that geologic processes operating at the present are the same processes that operated in the past. “The present is the key to the past” (uniform rate) - Actualism – physical laws are independent of time and location (same meaning as uniformitar.) Geological Time Scale – a sort of calander to which events and rock units can be referred - Earth was only 6,000 years old – biblical chronology, Noah’s flood - features we observed in rocks and landscapes where supernaturally and catastrophically James Hutton – “father of modern day geology” Charles Lyell – Principles of Geology book geological features could be explained by present-day processes How can the Sequence of Past Geological events be determined? Numerical Age (absolute age) – age given in years or some other unit of time Relative Time – the sequence in which events took place (not measured in time units) - ex: Grand Canyon can be analyzed in 4 parts: horizontal layers of rock inclined layers rock underlying the inclined layers (plutonic + metamorphic rock) the canyon itself, carved into these rocks Principles Used to Determine Relative Age - Contacts – boundary surface between two different rock types or ages of rocks - Formations – bodies of rock of considerable thickness with recognizable characteristics that make each distinguishable from adjacent rock units, named after local towns or landmarks Stratigraphy – subdiscipline of geology, that uses interrelationships between layered rock or sediment to interpret the history of an area or region uses 4 principles: 1) Original Horizontality – the disposition of most water-laid sediment in horizontal or near horizontal layers that are essentially parallel to the Earth’s surface 2) Superposition – a principle or law stating that within a sequence of undisturbed sedimentary rocks, the oldest layers are on the bottom and the youngest on the top 3) Lateral Continuity – principle that states that an original sedimentary layer extends laterally until it tapers or things at its edges 4) Cross-Cutting Relationships – a principle or law stating that a disrupted pattern is older than the cause of disruption Other Time Relationships: - Inclusion – fragments included in a host rock are older than the host rock Unconformities - a surface that represents a break in the geological record, with the rock unit immediately above it being considerable younger than the rock beneath it 1) Disconformity – a surface that represents missing rock strata, but beds above and below that surface are parallel to one another 2) Angular Unconformity – an unconformity in which younger strata overlie an erosion surface on tilted or folded layered rock, implies the following sequence of events from oldest to youngest: Deposition and lithification of sedimentary rock Uplift accompanied by folding or tilting of layers Erosion Renewed deposition on top of eroded surface 3) Nonconformity – an unconformity in which an erosion surface on plutonic or metamorphic rock has been covered by younger sedimentary or volcanic rock Crystallization of igneous or metamorphic rock at depth Erosion of a least several kilometers of overlying rock (the great amount of erosion further implies considerable uplift of this portion of Earth’s crust) Deposition of new sediment, thick eventually becomes sedimentary rock, on the ancient erosion surface How can rock units be traced from one area to another? - Correlation – determining time equivalency of rock units, rock units may be correlated within a region, a continent, and even between countries Physical Continuity – being able to physically (visually) follow a rock unity between two places Similarity of Rock Types – same sequence of rocks - Tillites – glacially deposited sedimentary rock - Key bed – a very distinctive layer Correlation by Fossils - Paleontologist – specializes in the study of fossils - Faunal succession (William Smith) – a principle or law stating that fossil species succeed one another in a definite and recognizable order; in general, fossils in progressively older rock show increasingly greater differences from species living at present - Index fossil – a fossil from a very short-lived, geographically widespread species known to exists during a specific period of geological time - Fossil assemblage – various different species of fossils in a rock layer How do we use Relative Dating to Understand Geological Time? The Standard Geologic Time Scale – a worldwide relative time scale based on fossil assemblages - Relative time scale: 3 eras which are subdivided into periods which are subdivided into epochs - Precambrian – the vast amount of time that preceded the Paleozoic Era - Paleozoic Era (‘old life’)– began with the appearance of complex life, as indicated by fossils - Mesozoic Era (“middle life’) – dinosaurs became the dominant animals - Cenozoic Era (‘new life’) – Quaternary Period – the youngest geologic period, includes present time Pleistocene Epoch – an epoch of the Quaternary Period characterized by several glacial ages Holocene Epoch (Recent) – How can we determine the absolute age of rocks? - Oldest rock from northwest Canada = 4.03 billion years old - Oldest known mineral, zircon, from Austrailia = 4.4 billion years old Isotopic Dating – determining the age of a rock or mineral thru its radioactive elements and decay products (previously and somewhat inaccurately called radiometric or radioactive dating) - Geochronologists – specialize in isotopic dating - Isotopes – atoms (of the same element) that have different numbers of neutrons but the same number of protons (40K becomes 40Ar, 238U decays to 206U=206Pb) - Radioactive decay – spontaneous nuclear disintegration of certain isotopes with unstable nuclei Alpha emission – ejection of two protons and two neutrons from a nucleus Beta emission – release of an electron from a nucleus Electron capture – proton in the nucleus captures an orbiting electron 19.1 Highlights of the Evolution of Life through Time - Oldest fossils are prokaryotes (lack nucleus) = 3.5 billion years ago - Eukaryotes (single-celled, have nucleus, reproduce sexually) = 1.4 billion years ago - Jellyfish + worms (multicellular) = 700-550 million years ago - Ediacaran biota (soft-bodied animals) = 565-543 m.y. ago – found at Mistaken Point NFLD - Paleozoic – greatest mass extinction ever to occur 95% species died, mostly ocean species - Mesozoic – dinosaurs and mammals, second largest extinction 75% of species died - Cenozoic – age of mammals, hominids 19.2 Demise of the Dinosaurs – Was it Extraterrestrial? - Walter Alvarez and Luis Alvarez – hypothesized that extinction was caused by large asteroid - Marked by (Cretaceous and Tetiary Period) K-T boundary – has abundant Iridium content - Chicxulub crater near the coast of Mexico’s Yucatan peninsula What is Radiocarbon Dating? - Ratio of 12C and14C in organic remains determine the time elapsed since death of organism Cosmogenic Isotope Dating (surface exposure dating) – uses the effects of constant bombardment by neutron radiation coming from deep space (cosmogenic) of material at Earth’s surface 19.3 Radon, A Radioactive Health Hazard - Radon – a odorless, colorless gas, can caused lung cancer, product of 238U decay to 206Pb, has half life of only 3.8 days - where bedrock is granite, gneiss, limestone, black shale, or phosphate rock – high uranium - Health Canada says exposure should only be 800 Becquerel per cubic meter - Prevent – good air circulation thru house, basements made airtight 19.4 Calculating the age of rock When can Isotopic Dating Techniques Be Used? Isotopic age to be accurate - Isotopic system has remained closed – rock collected must show no signs of weathering or hydrothermal alteration - There were no daughter isotopes in the system at the time of closure or make corrections for probable amounts before “clock” was set - Must be sufficient parent and daughter atoms to be measured by the mass spectrometer How Reliable is Isotopic Dating? - Confirm with relative age dating Combining Relative and Numerical Ages - Precambrian Eon – the largest unit of geological time Prearchean (Hadean) Eon – Archean Eon - Proterozoic Eon (‘beginning life’)– Phanerozoic Eon (‘visible life’) – Paleozoic Era, Mesozoic Era, Cenozoic Era How Old Is the Earth? - James Ussher 1625 – Bible to determine earth’s age - Lord Kelvin 1866 – rate at which Earth loses heat, forgot about radioactivity - Earth is now regarded as between 4.5 and 4.6 billion years old – meteorites and lunar rock Comprehending Geologic Time 19.5 Mapping Time and Terrain - Geographic Information Systems and digital mapping technology Chapter 20: Geological History of Canada Canada: A Young Nation, But an Old Country - Acasta Gneiss (4 Ma)- North West Territories, which forms part of the Slave Province of the Canadian Shield - North America was essentially complete by 65 million years ago - Last ice sheet – 12,000 years ago left southern parts of country, 6,000 years ago finally melted in Labrador - Baffin Island – remnants of this vast ice sheet still survive on Baffin Island as Penny and Barnes Ice Caps What are the main geological “Building Blocks” of North America? Geologic Jigsaw - Craton – the ancient core of North America, composed of a complex assemblage of several distinct geologic provinces - Geologic provinces – broad regions of similar rocks, with characteristics that differ significantly from rock types present in adjacent areas Sub-Provinces – fault-bounded units containing similar rocks types, structures and mineral deposits - Terranes – provinces + subprovinces, discrete fragments of oceanic or continental material that have been added to a craton at an active margin by accretion (material is added to a landmass) The North American Craton versus the Canadian Shield - North American Craton – a large, continent-sized block of distinct geology aking up the basement of much of North America (and Greenland) - Canadian Shield – exposed part of the craton, and consists of a gently undulating surface that rises inconspicuously, almost like an arch, in its center (peneplain) consists predominantly of very old, Archean and Proterozoic rocks (age 1-4 Ga years old, lack fossils) - Peneplain – “almost plain”, caused by erosion and beveling Margins of Shield are buried below younger sedimentary rocks, ancient peneplain surface now forms an unconformity between the craton below and younger rocks above Grand Canyon in Arizona –unconformity separates metamorphic rocks of the craton from overlying Paleozoic cover rocks - Cover strata – younger sedimentary strata that bury the more ancient craton around its margins Deposited when the outer margins of the craton were depressed and flooded by shallow seas – occurred during orogenies , collision of other landmasses with the craton Cover rocks – thicker than 10km The Geologic Jigsaw of the North American Craton - Geological provinces (Sir William Logan) – areas of the shield with distinctive geological characteristics 20.1 The Sudbury Impact Structure: Collision of an Ancient Meteorite - Oval shape crater – squeezed by Penokean Orogeny, depth of 35km, rich in nickel, copper and platinum ores - Shatter Cones – V-shaped cones that form when rocks are struck violently during a meteorite impact event - Breccia (Pseudotachylite Glass) – formed by disintegration and mixing of rock when hit by a large meteorite and broken into angular fragments Deconstructing North America: 1) The original NA continent, Arctica which started to form about 2.5 billion years ago from smaller continents and was completed by about 1.9 billion years ago when old Archean cratons (ex: Slave, Nain provinces) were welded together by the Trans-Hudson Orogen and others 2) Added to the North American continent during the formation of Nena about 1.8 billion years ago after the Penokean Orogeny 3) Added During the Formation of Rodinia about 1.3 billion years ago during the Grenville Orogeny 4) Added during the formation of Pangea about 600 million-300 million years ago – Appalachian Mt 5) Added after the breakup of Pangea about 250 million years ago – Corillera How Did the North American Continent Evolve? (5) Stages in the Evolution of the North American Continent - Orogens – consists of crushed and deformed rocks that represent the remains of mountain belts or volcanic arcs formed during collision (ex: Trongat and Wopmay) - Wilson cycle – process of repeated continental aggradation and breakup forms supercontinents - Supercontinents – a giant co
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