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Earth’ s History Unit 6 A. Order of Geologic Events When studying the history of Earth, we must be able to place items or events in a particular order so we can determine which event happened first, which happened next, and so on. We order events in two ways: 1. Relative Dating: Determining whether one event or object is or younger than another. Example: You are younger than your Parents, but older than your baby brother or sister. 2. Absolute Dating: Determining exactly how old an object is or how long ago it occurred. Example: Your teacher is 29 years old. To determine an order of events using relative ages, we use the following four principles: 1. Superposition __________________ Layers of rock on the bottom are older than the rock layers above them, provided they have not been overturned. Youngest Rock Layer Oldest Rock Layer Cross-cutting relationships 2. __________________________ Rock layers are older than any faulting are younger event that cuts across them. Example: Folding& ________________________ than the surrounding rock. Igneous intrusions are also younger ________________________________________________________. The sedimentary rocks are older than the igneous intrusion. Note the contact metamorphism. 3. ____________________________________: Uniformitarianism Processes that occur now (erosion, deposition, plate movement, etc.) occur the same way now as they always have throughout geologic history. Original Horizontality 4. ____________________________Sediments that form sedimentary rocks are usually deposited in flat layers. If we see rock layers that are not flat and level, we can assume that they were changed (tilted, folded or faulted) after they formed. #1 1.___________________________________ 2.___________________________________ 3.___________________________________ 4.___________________________________ 5.___________________________________ #2 1___________________________________ 2___________________________________ 3___________________________________ 4___________________________________ #3 1___________________________________ 2___________________________________ 3___________________________________ 4___________________________________ #4 B. Matching Rock Layers When studying the history of Earth, we must also be able to compare rock layers found in one place to those found in another. and matching rock strata (layers) in different Correlation: Comparing _______________________________________________ locations to see if they formed at the same time or under similar _________________________________________________________ conditions. _________________________________________________________ Correlation of a single rock layer can often be difficult. Often, it is better to try to correlate a sequence of rock strata. This is often done by a technique called “walking the outcrop”: __________________________ following along an outcrop from one location to another __________________________________________________________ Walking the outcrop can be limited because the rock strata may be covered by sediment or other rock. Also, part of the rock strata may be missing from a location. Unconformity: _____________________________________________ A gap in the rock record due to rock having been eroded away or never been formed __________________________________________________________ There are two other methods used to correlate rock layers; index fossils and volcanic eruptions. Index Fossils: _______________________________________________ Fossils used to correlate rock layers. If two rock layers have the same index fossils, they are of the same age. __________________________________________________________ To be a good index fossil, a fossil must: Exist over a wide-spread geographical area a. __________________________________________________ Exist for a relatively brief period of time. b. __________________________________________________ Usually form rapid depositional events. Chemistry Volcanic Eruptions: _________________________________________ of ash allows precise correlation and placement in time. __________________________________________________________ C. Absolute Age of Rocks How can the absolute age of rock strata be determined? Some rocks contain elements that are unstable. Elements that are radioactive experience Radioactive Decay: __________________________________________ break-down of their nucleus to form new elements. __________________________________________________________ Uranium 238 Radioactive Decay Lead 206 + Energy As the uranium decays, it does so in a very predictable manner and at a predictable rate. Half-life: __________________________________________________ The amount of time required for half of the original element (parent element) to decay to its final element (daughter element). __________________________________________________________ As each half-life is reached, __________ one half of the remaining radioactive element has decayed. Notice that you will never reach a point where all of the radioactive element has decayed. However, there will come a the amounts remaining are so small they can no longer point where: _____________________________________________ be measured _____________. Radioactive Decay Animation a fraction of a second or on the order of Half-lives may be as short as ___________________ millions or billions of years _________________________. The front cover of your Earth Science Reference Table has radioactive decay data for several radioactive elements or isotopes. A radioactive isotope with a appropriate half life must be used to correctly measure age. If an item that is only several thousand years old was dated using an isotope with a half life of 5 billion years, not enough radioactive decay would have occurred to be measurable. If a very old item was dated using an isotope with a short half life, then not enough radioactive element would be remaining to measure. Radioactive Dating Example QUESTION: A sample is found to contain 100 grams total of the radioactive element Carbon-14 and its daughter element Nitrogen-14. If 25 grams of that total is C14 and 75 grams is N14, how old is the sample? ANSWER: First, determine the number of half lives this sample has undergone. 50 50 grams of N14. •After one half life, there is _____grams of C14 & _____ 25 •After two half lives, there is ____grams of C14 & _____ 75 grams of N14. 5,700 years (see your ESRT) and there So, if the half life of C14 is _______ have been ____ 2 half lives, then _________ 5,700 X _______ 2 = __________ 11,400 years old! D. Absolute Age of Rocks Geologic time is divided into units based upon: Dominant life forms on Earth ___________________________________________________ Mass extinctions ___________________________________________________ Major tectonic or orogenic (mountain building) events ___________________________________________________ time The units are not based directly on _______! Eons When dividing Earth history, begin with the largest units _______. They Eras they are divided into _______; Periods and they are are divided into _____; Epochs divided into _______. NOTE: Compared to the overall geologic time scale (about 4.6 billion insignificant years), human existence is considered ________________ Evidence found in rock strata and in their fossils allow scientists to infer what life and the environment were like at that particular time and in that particular location. For example, coral fossils found in limestone would warm shallow, ocean water indicate what type of environment? _____________________________ Of all the species on Earth, over __________ 99% of them have become extinct since life was first recorded in the rock record. evolved Most scientists believe the fossil record indicates life forms have ______ (changed) through geologic time, probably to adapt to changes in their environment. The Theory of Evolution This theory, know as _________________________, is not a change from simple life like one-celled organisms to complex like humans. It is an increase in the variety of life forms.