1 Earth’ s History Unit 6

2 Vocabulary List 1.Relative Dating 2.Absolute Dating 3.Superposition 4.Cross-cutting relationships 5.Uniformitarianism 6.Original Horizontality 7.Correlation 8.Unconformity 9.Index Fossils 10.Radioactive Decay

3 List of Focus Questions DateFocus ____What is relative dating? What principles do we follow so we can put events into order? ____What is correlation? What two things do we use to correlate rock layers? ____What is absolute dating? What is C 14 used to date?

4 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:____________________________ ___________________________________________ 2.Absolute Dating: __________________________ ___________________________________________

5 To determine an order of events using relative ages, we use the following four principles: 1.__________________ Layers of rock on the bottom are older than the rock layers above them, provided they have not been overturned. 2.__________________________ Rock layers are older than any event that cuts across them. Example: _________________________________ _________________________________________. Oldest Rock Layer Youngest Rock Layer The sedimentary rocks are older than the igneous intrusion. Note the contact metamorphism.

6 3. ____________________________________: Processes that occur now (erosion, deposition, plate movement, etc.) occur the same way now as they always have throughout geologic history. 3.____________________________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.

7

8

9

10

11 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. Correlation:__________________________________ ___________________________________________ 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”: ___________ ___________________________________________ ___________________________________________

12 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: _____________________________ ___________________________________________

13 Volcanic Eruptions: _________________________ ___________________________________________ To be a good index fossil, a fossil must: a. __________________________________ b. __________________________________ There are two other methods used to correlate rock layers; index fossils and volcanic eruptions. Index Fossils: _______________________________ __________________________________________

14 C. Absolute Age of Rocks How can the absolute age of rock strata be determined? Some rocks contain elements that are unstable. Radioactive Decay: __________________________ ___________________________________________ Radioactive Decay Uranium 238 Lead Energy As the uranium decays, it does so in a very predictable manner and at a predictable rate.

15 Half-life: ___________________________________ ___________________________________________

16 As each half-life is reached, __________ 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 point where: ___________________________ __________________________. Half-lives may be as short as ___________________ or on the order of _________________________. 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.

17 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 C 14 and 75 grams is N 14, how old is the sample? ANSWER: First, determine the number of half lives this sample has undergone. After one half life, there is _____grams of C 14 & _____ grams of N 14. After two half lives, there is ____grams of C 14 & _____ grams of N 14. So, if the half life of C 14 is _______ years (see your ESRT) and there have been ____ half lives, then _________ X _______ = __________ years old!

18 D. Absolute Age of Rocks Geologic time is divided into units based upon: ___________________________________________ The units are not based directly on _______! When dividing Earth history, begin with the largest units _______. They are divided into _____; they are divided into _______; and they are divided into _______. NOTE: Compared to the overall geologic time scale (about 4.6 billion years), human existence is considered ________________

19 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 indicate what type of environment? _____________________________ Of all the species on Earth, __________ of them have become extinct since life was first recorded in the rock record. Most scientists believe the fossil record indicates life forms have ______ (changed) through geologic time, probably to adapt to changes in their environment. 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.