EOSC 326 Module A.pdf

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University of British Columbia
Earth and Ocean Sciences
EOSC 326
Michael Wheeler

MODULE A: Geological Principles 1. Geological Time a. Age of the Earth James Ussher (1581-1665) - Archbishop of Armagh - one of earliest and most influential figures in interpretation of geological time - published a chronology of Earth's history using all dates mentioned in Bible to establish a timeline -established first day of creation was Oct 22 4004BC, which would make Earth a little over 6000 years old George Louis De Buffon (1707-1788) -believed Earth to have been initially hot molten mass -heated iron spheres (thought was reasonable model for structure of planet) and calculated time they took to cool -using this method Buffon believed Earth was 75,000 years old John Joly -published paper in 1899 in which he estimated Earth's oceans (which he believed to be same age as the planet) to be about 90 million years old -calculated this by estimating how long it would take for the oceans to reach their current salinity (from an original fresh water state) as salt is added via erosion of minerals in the rocks. -radioactive decay in minerals is the technique which provided us with the current age of the Earth at 4.6 Billion yrs. b. Deep Time Deep time: concept of trying to understand the evolution of the Earth and its biological systems 2. Geological Concepts and Terminology Mineral: a naturally occurring crystalline solid with a characteristic chemical composition, a highly ordered atomic structure, and specifi c physical properties Rock: an aggregate of minerals. Rocks fall into a basic 3-fold classifi cation: 1. Igneous 2. Metamorphic 3. Sedimentary A. Igneous Rocks Igneous rocks: (derived from latin "ignis" meaning fi re) formed by the cooling of magma or lava. Generally composed of interlocking crystals of varying sizes -if magma cools within Earth's crust it is referred to as an intrusive igneous rock. As they cool slowly they often develop large crystals -if magma escapes via volcanic activity and forms a lava the igneous rock is called extrusive igneous rock. These rocks cool quickly, so crystals that form are very small and often invisible to human eye B. Metamorphic Rocks Metamorphic rocks: form as a result of the transformation of an existing rock via heat, pressure and/or the action of fluids. - commonly occur deep in Earth's crust C. Sedimentary Rocks Sedimentary rocks: form in response to particular environmental conditions and as such, provide clues to Earth's past including climate, ancient geography and life forms. -form via sedimentation of materials at the Earth's surface and within water bodies, commonly in layers called strata or sedimentary beds. Diagensis: transformation of sediments into a sedimentary rock via a collective variety of chemical, physical and sometimes biological processes. D. Clastic Sedimentary Rocks Most sedimentary rocks are produced by the erosion of pre-existing rocks producing fragments or grains that are transported and deposited at various distances from the site of erosion --> these rocks are called clastic sedimentary rocks -Clastic sedimentary rocks provide a simple tool that can be used to determine the source, origin and length of transport of a particular sediment prior to it becoming a sedimentary rock -If a sediment is deposited close to rocks from which it originally eroded, it will have a number of characteristics that will uniquely identify it as such: - large number of coarse/angular grains and clasts (greater than 4mm), higher proportion of unstable minerals and fragments of rocks, exhibit poor sorting (variation in clast/grain sizes) -sediment with characteristics as described above is called an immature sediment -mature sediment: undergone extensive transport, fi ne grained, composed mostly of well-rounded and well-sorted quartz grains E. Calcium Carbonate calcium carbonate: precipitated sediments by various creatures including corals and mollusks to form thick deposits of limestone -microplankton are also calcium carbonate producers, such as cocclithophores. During Creatceous, warm shallow oceans covered much of Earth's continents providing perfect conditions for proliferation of cocclithophores and ultimately the generation of vast thicknesses of fi ne grained limestone F. Evaporites Evaporites: intense evaporation of water precipitate sediments in the form of salt crystals -can form a number of ways including: inland sea or part of ocean with restricted contact to wider ocean (occur in arid areas with limited freshwater input) 3. Stratigraphy stratigraphy: study of how rock layers are arranged. a. Principle of Superposition: -states that in layered strata, the oldest layer will be at the bottom of the exposed strata and the youngest at the top b. Principle of Original Horizontality: sediments are deposited horizontally, after transformed into rock possible for strata to become tilted by tectonic movements c. Principle of lateral Continuity: stratum of sedimentary rock will continue in all directions until it thins, grades into another type of sediment or comes against the edge of the depression into which it is being deposited. d. Cross-cutting relationships: relationships between stratigraphic units indicated that the Earth was very old. f. Unconformities: represent periods of non-deposition of sediment or active erosion of strata Several types of unconformity are recognized: 1. Disconformity: exists where the layers above and below an erosional boundary have the same orientation 2. Nonconformity: develops where sediments are deposited on top of an eroded surface of igneous or metamorphic rocks 3. Paraconformity: strata on either side of unconformity are parallel, there is little apparent erosion 4. angular unconformity: strata is deposited on tilted and eroded layers h. Relative Dating: working out the history of a geological section inherent errors with this estimate: -the rate of accumulation of sediment might not be constant -period of non-deposition or active erosion may have occurred -during diagensis, sediments are compressed and compacted so that the vertical extent of sedimentary rocks may not represent the original vertical extent of sediment i. Uniformitarianism: basis for the understanding of the Earth and the science of modern geology. Actualism: acknowledges that although most geological processes are very slow, some geological processes can cause changes that are relatively geologically instantaneous LESSON 2: Sediments and Geological Time 1. Sedimentary Facies sedimentary environments: areas where sediments are being deposited a. Near Shore Sedimentary Environment The coarsest (most dense) portion of the sediment from the river is deposited close to the shoreline where the water is most turbulent. The finer (least dense) material stays in suspension until it settles out of the water where conditions are less turbulent -beyond the area where all the material from the river has been deposited, calcium carbonate secreting organisms provide the main sediment type: carbonate mud. This accounts for the changing lateral character of the sediments in this environment from coarse sandy material close to shore line passing into mud/clay and ultimately carbonate mud further off shore. facies: environmentally controlled sediment differences -refers to all the characteristics that can be used to define a particular sedimentary unit: sediment type (eg. sandstone,mudstone, limestone), presence of any sedimentary structures (ex. ripples, cross bedding), and sometimes fossil content -facies changes are gradual b. The temporal - Sea Level Component over time, the distribution of sedimentary facies is controlled by changes in sea level. -sea level has changed globally many times and can also change locally. Scenario 1: No sea Level Change R
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