Lecture 21

Lecture 21 - Observe the orientation of rocks at Siccar...

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Unformatted text preview: Observe the orientation of rocks at Siccar Point, Scotland ayers l Red Unconformity Vertical, gray layers 09.00.a1 Observe the layers in these two photographs, which Observe show the same sequence of rocks. What is different and what do you think happened? and Most sediment is deposited Most in horizontal layers in If layers are not horizontal, If something has happened (deformation) (deformation) 09.01.a Younger Units Deposited on Older Units Tan sediment deposited Tan over older rock over Red layers deposited Red over tan over Third layer is Third youngest and is on top is 09.01.a How Does Radioactive Decay Occur? 09.02.a Before decay, Before unstable parent atoms parent Example Example for 1000 atoms atoms Half the parent atoms Half decayed to daughter atoms (time = half life) atoms Before Any Decay After a second half After life, only ¼ parent atoms remain atoms After Two Half-Lives 250 750 After One Half-Life 500 500 Atoms of Parent Atoms of Daughter 1,000 0 What Can Isotopic Ages Tell Us? 09.02.c Age of eruption Age of solidification Age of meta. event When rock cooled Age of source Age of sediment of Age of recent Age sediment sediment Common Radioactive Decay Series Potassium (K) Potassium decays to argon (Ar) decays Rubidium (Rb) decays Rubidium to strontium (Sr) to Dating a rock by several methods Carbon-14 (C) decays Carbon-14 to nitrogen (N) to 09.02.c Thorium (Th) and uranium Thorium (U) decay to isotopes of lead (Pb) lead How a Typical Landscape Forms Preexisting rock Preexisting covered by sea covered Sea deposits layer Sea of sediment of Environment changes, Environment depositing more rock depositing Deposition stops Area eroded by Area rivers, etc. rivers, Continued erosion 09.03.a Observe this photograph and consider the Observe sequence of events that likely occurred sequence 09.03.a1 Observe this photograph and consider the Observe sequence of events that likely occurred sequence 09.03.a2 Determining Ages of Landscapes 09.03.b Landscape surface Landscape younger than rocks on which it is carved which This photograph shows This a dark basalt flow in a valley, next to a cliff of tan sandstone. What can you say about the age of the landscape? age Landscape surface older Landscape than rocks deposited on top than Other Indications of Landscape Age 09.03.c A surface with surface well-developed soil is older than one with less soil than In this scene, consider In relative ages of different levels in the landscape. If you were there, how could you test your interpretation? interpretation? Most river Most terraces formed before erosion down to present level erosion What Does an Unconformity Represent? Limestone folded and Limestone eroded eroded Gray limestone Gray deposited under water water Conglomerate Conglomerate deposited on top of eroded surface forming an unconformity unconformity Unconformity 09.04.a Rocks above unconformity Unconformity Rocks be unconf low ormity Identify the angular Identify unconformity in each photograph photograph 09.04.a ove ocks ab R ity conform un Rocks above unconf ormity Unc onfo rmity rmity fo ncon U Rocks below unconformit y Rocks below Rocks unconformity unconformity How Does a Nonconformity Form? 09.04.b Noncon formity Nonlayered Nonlayered rock uplifted and eroded rock Observe the Observe nonconformity drawn on this photograph photograph Erosion Erosion surface buried by sediment surface Sandstone Granite How Do Disconformities Form? 09.04.c Weathering erodes Weathering surface surface Deposition of Deposition horizontal layers horizontal Disconfo rmity Eroded surface Eroded covered by later sediment covered Observe this photograph and identify two disconformities 09.04.c Mississippian Limestone Mississippian Disconformity 2 Devonian Formation Cambrian Limestone Disconformity 1 Observe the disconformity in this photograph 09.04.c Disconformity Mississippian Limestone Devonian Formation How Are Fossils Preserved? Shells/hard parts Bones Replacement Cast or mold Amber Thin carbon film Impressions Constructed feature 09.05.a Traces of Creatures in Rock Record Footprints What are some aspects of a What creature you could infer from a sequence of footprints? sequence Burrows of worms, etc. How would you try to figure How out what kind of creature made a particular kind of burrow? burrow? 09.05.b How Fossils Vary with Age Mammals and Mammals grasses grasses Dinosaurs Dinosaurs and flowering plants plants Crinoids, coral, Crinoids, clams, certain fish, plants, insects, and amphibians amphibians Simple Simple creatures and fossils, such as stromatolites stromatolites 09.06.a Geologic Time Scale Boundary based Boundary on mass extinction (dinos and others) Boundary based Boundary on major mass extinction called the Great Dying Great Boundary based Boundary on widespread appearance of hard-shelled organisms organisms Older subdivisions of Cenozoic of 09.08.a1 ...
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