Geologic time the present is the key to the past

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Unifor m itarianism “The Present is the Key to the Past” Laws of nature DON’T change with time Past events explained & estimated by moder n processes Uniformitarianism Original Horizontality sedimentary rocks form in horiz ontal layers parallel to Earth’s surface and will remain that way unless disturbed SILT SAND CLAY SILT CLAY Horizontal Layers of the Grand Canyon • Relative Time – determination of the age of a rock or geologic event by comparing with other rocks/geologic events Can you infer who is older? He is Ways to Determine Relative Time………? Coming Right Up!! Law of Superposition In undisturbedsedimentary sedimentary rock or lava flowsflows: In undisturbed rock or lava Top = youngest layer Bottom = oldest layer Law of Cross-Cutting Relationships Any feature that cuts across a rock or layer of sediment is y ounger than the rock or layer it cuts Example of CrossCutting Intrusion Intrusion- igneous - igneousrock rockthat thatforms formswhen whenmagma magma squeezes squeezesbetween betweenexisting existingrock rockand andhardens hardens Examples of Cross-Cutting Example of Cross-cutting Extrusion - igneous rock that forms when lava flows on Earth’s surface and hardens Key Bed - single rock layer that is recognizable, widespread, and exists for a short time Correlation - matching rock layers from one area to another This can be done in several ways: “Walking the outcrop” Matching similar rock features Index fossils Correlating or matching index fossils from three outcrops Correlate the rock sequences to make a single rock column for the geologic history of the region (oldest on bottom) Youngest Oldest No place on the earth has all the rocks formed through time So geologists use correlation to make a single rock column that shows the entire history of the earth • Correlation means “to match” • Geologists match all of the following: – – – – rock types rock sequences ash layers from volcanic eruptions index fossils Page & of the Reference Tables Each black vertical bar represent the existence of a group of organisms Extinction First appearance MASS OF UNKNOWN (G) DECAY TIME (YEARS) MASS OF C14 (G) DECAY TIME (YEARS) Isotopes - different forms of the same element - normal # of protons - more than normal # of neutrons Normal Hydrogen -1 proton - neutrons - electron Isotope of Hydrogen - proton - neutron - electron Radioactive Isotope of Hydrogen proton neutrons electron Radioactive Hydrogen H-3 [...]... atoms, maybe cancer-causing Radioactive isotope - an isotope that undergoes radioactive decay - used to determine absolute age Examples: Uranium 238 and Carbon 14 Radioactive Decay radioactive isotopes give off radioactive particles until they become stable isotopes (new elements) Example: Uranium - 238 Isotope Radioactive Radioactive Decay Lead-206 Isotope Stable Amount of time it takes 1/2 the atoms... follow the law of cross cutting relationships Law of Included Fragments Sediments are older than the cement and rock they are part of The pebble is older than the conglomerate rock it is found in However, there are exceptions to these laws…… Exceptions to Law of Superposition Rock layers can be overturned, older layers pushed on top of younger layers THEREFORE, geologists use these clues to find the. .. atoms in a given sample to go through radioactive decay is called it’s Half - Life Let’s look at the half - life of Uranium 238 U238 - Radioactive Pb206 - Stable decay product 1/4 1:0 1/8 1/2 : 1/2 1/32 ??? 1/16 7/8 3/4 15/16 31/32 Since the Half - Life of Every Isotope is … 1 2 3 Unique Always the same time Unaffected by: Mass, Volume, Temperature & Pressure We can use it to find: TH E ABSOLUTE AGE... of Unconformity • Absolute Time – finding an exact date for rocks or geologic events in YBP (years before present) Born 2004 or 1 YBP Born 1922 or 83 YBP How do geologists measure Absolute Time? 1 Radioactive Decay 2 Biological Clocks - Tree rings, coral growth rings, glacial lake layers of sediment Before you measure ABSOLUTE TIME you must understand the following terms… ISOTOPES - alternate forms of... these clues to find the original position of rock layers… Exceptions to Law of Superposition Graded Bedding - coarse, heavy particles - b Exceptions to Law of Superposition Ripple-Marks - tops of ripple marks poin Exceptions to Law of Superposition Cross-Beds - curved at bottom layer, cut o Unconformity Gaps or missing layers in the rock record du Angular unconformit y Formation of an Unconformity... the Half - Life of Every Isotope is … 1 2 3 Unique Always the same time Unaffected by: Mass, Volume, Temperature & Pressure We can use it to find: TH E ABSOLUTE AGE OF ROCKS!!! Half-Lives of Special Isotopes Uranium 238 Lead 206 4.5 x 109 years (4,500,000,000 years) Carbon 14 Nitrogen 14 5.7 x 103 years (5,700 years)
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