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21.4 – Key Beds, Absolute Dating, and FOSSILS Key Beds Definition: Rock or sediment in history, and cover a that can be traced back to a very area. Absolute Age Dating Four ways: Radiometric Dating (review) Certain elements have radioactive properties. That is, they lose atomic parts from their We call this losing of parts When they lose , the element changes to a whole element When they lose , they become a different of the same element, with a new atomic mass. The radioactive decay happens at a rate So we can use these to determine how long ago a rock layer was formed. Radiometric Dating (more review) We use half-life calculations, given the amount of parent material and daughter material in a sample. A is the amount of time it takes for the element to decay. Absolute Dating-Tree Rings tree and seasonal events = the science of studying to determine the age of the Absolute Dating-Ice Cores – created by drilling hollow tube through ice up to several kilometers thick Drilling down through ice gives a record of events and events. Ice shows seasonal changes, like tree rings do. ice has bubbles and crystals than winter ice does. Absolute Dating-Varves : bands of alternating light and dark-colored of sand, clay and silt Rates of sedimentary deposits vary by (as with ice cores and tree rings) Help study melting patterns The Fossil Record and Life The record provides evidence about the history of life on Earth. The fossil record also shows that different groups of have changed over time. is the gradual change in species over long periods of time. When geologists find fossils in rocks, they know that the rocks are about the age as the fossils. Thus, they can that the same fossils found elsewhere are also of the same age. Radiolarians Microfossils organisms with hard shells that have populated the oceans since the Period Used by petroleum geologists to determine the age of rocks that might produce Original Preservation The picture on the preceding slide is from the La Brea Tar Pit in California. The soft parts of a mammoth were preserved in the aforementioned Pit. : PLANT AND ANIMAL REMAINS THAT HAVE BEEN ALTERED VERY LITTLE SINCE THE ORGANISM’S DEATH (USUALLY TISSUE DECAYS BUT IN ORIGINAL PRESERVATION IT DOES ) Original Preservation Example The insect is completely (hard and soft parts) in amber. Altered Hard Parts The soft portion decays away quickly and the hard portion (bones, shells, cell walls) can become fossils in one of ways: 1) : a) pores in hard parts are filled in with minerals from groundwater b) groundwater comes into contact with original hard parts mineral and replaces the material with a different mineral 2) : Original mineral retains the same chemical formula although takes on a crystalline structure for greater long term stability Molds/Casts : impression left behind in the sediment where a shell once was : filled in mold Trace Fossils Provide evidence of how an organism , and obtained Examples: worm trails, footprints, tunneling burrows, gastroliths (rocks in dinosaur stomachs- left) and coprolites (fossilized feces- right)