Chapter 8: The Rock Record

Transcription

1 Chapter 8: The Rock Record

2 Vocabulary 1. Uniformitarianism 2. Relative age 3. Strata 4. Beds 5. Law of superposition 6. Unconformity 7. Law of crosscutting relationships 8. Principle of original horizontality 9. Graded bedding 10. Cross-beds 11. Ripple marks 12. Fault 13. Intrusion 14. Absolute age 15. Varves 16. Radiometric dating 17. Radioactive isotopes 18. Parent isotope 19. Daughter isotope 20. Half-life 21. Carbon Fossils 23. Paleontology 24. Trace fossil 25. Amber 26. Petrification 27. Molds 28. Coprolite 29. Gastroliths 30. Index fossil

3 I. Determining Relative Age A. Uniformitarianism 1. Principle created by James Hutton, a Scottish farmer in the 1700s a. Observed geologic processes occurring on his farm & concluded that the same forces that changed his farm must have changed Earth in the past 2. Definition: current geologic processes (volcanism, erosion, earthquakes, etc.) are the same processes that changed Earth s surface in the past a. Only thing that changes is the rate at which processes occur b. Brought up questions about Earth s age

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5 B. Relative Age 1. Definition: the age of an object in relation to the ages of other objects; how old something is in comparison to something else; NOT a number 2. Looking at rock layers & determining the order in which they were deposited can give us information about Earth s past 3. One way to learn about Earth s past is to determine the order in which rock layers & other rock structures formed i. Strata (layers of rock) can show the order in which events occurred

6 4. There are a few principles that can be used to determine the order in which rock layers formed. C. Law of Superposition 1. Definition: a sedimentary rock layer is older than the rocks above it and younger than the layers below it if rock layers are not disturbed 2. Sedimentary rocks are formed when new sediments (sand, silt, mud, etc.) are deposited on top of preexisting layers of sediment 3. Sediments accumulate & harden into layers called beds (boundaries between beds are bedding planes)

7 D. Principle of Original Horizontality 1. Sedimentary rocks left undisturbed will form & remain in horizontal layers (gravity!) 2. Sedimentary rocks that are not horizontal were most likely affected by tectonic activity/crustal movement after the layers were deposited a. Sometimes tectonic forces push older layers on top of younger ones or overturn a group of rock layers there are ways to determine if this happened & what the original layer positions were

8 3. Graded Bedding a. Usually, the largest particles of sediment (biggest rocks) are located at the bottom, and smaller particles are on top of those b. Graded bedding is the arrangement of layers in which coarse and heavy particles are in the bottom layers c. If larger particles are located in the top layers, the layers may have been overturned by tectonic forces 4. Cross-beds a. Scientists can also look at shape of the bedding planes. b. When sand is deposited, sandy sediment forms beds at an angle to the bedding plane, called cross-beds Studying the shape of the cross-beds can help determine the original position of the layers

9 5. Ripple Marks a. Definition: small waves that form on surface of sand due to wind or water; when sand solidifies into sandstone, marks can be preserved b. Usually, the crests (tops) of ripple marks point upwards; if not, the rock layers have been disturbed

10 E. Unconformities 1. Definition: a time gap in the geologic record, most likely due to rock layers being eroded or a time period in which sediment is not being deposited 2. Three types: a. Nonconformity: horizontal layers on top of unstratified rock b. Angular unconformity: horizontal layers on top of tilted layers c. Disconformity: horizontal layers deposited on top of older horizontal layers with an uneven surface

11 II. Determining Absolute Age A. Definition: the numeric age of an object or event, often stated in years before the present, as established by an absolute-dating process, such as radiometric dating 1. Scientists use a variety of ways to determine absolute age of a rock formation. Some methods involve geologic processes that can be observed and measured over time; others involve the chemical composition of certain materials in rock

12 B. Absolute Dating Methods 1. Rates of Erosion: only practical for geologic features that formed within the past 10,000 to 20,000 years 2. Rates of Deposition: by using data collected over an extended period of time, geologists can estimate the average rates of deposition for common sedimentary rocks. 1. Not always accurate because not all sediment is deposited at an average rate this usually only provides an estimate of absolute age

13 3. Varve Count: some sedimentary deposits show definite annual layers, called varves. a. Definition: a pair of sedimentary layers (one coarse, one fine) that is deposited in an annual cycle b. The varves can be counted much like tree rings to determine the age of the sedimentary deposit c. Thick layer is generally light-colored & sandy, deposited in summer d. Thin layer is generally darker & clay-like, deposited in winter

14 4. Radiometric Dating a. Definition: a method of determining the absolute age of an object by comparing the relative percentages of a radioactive (parent) isotope and a stable (daughter) isotope b. Rocks generally contain small amounts of radioactive material that can act as natural clocks c. Atoms of the same element that have different numbers of neutrons are called isotopes d. Radioactive isotopes have nuclei that emit particles and energy at a constant rate regardless of surrounding conditions; this rate & the breakdown of the atoms can be used to determine absolute age

15 e. Half-Life i. Definition: time required for half of a sample of a radioactive isotope to break down by radioactive decay to form a daughter (stable) isotope ii. The time required for half of any amount of a specific radioactive isotope to decay is always the same & can be determined for any isotope. iii. By comparing the amounts of parent and daughter isotopes in a rock sample, scientists can determine the age of the sample. iv. The greater the percentage of daughter isotopes present in the sample, the older the rock is.

16 f. Useful Radioactive Isotopes i. Uranium-238 ( 238 U) is an isotope of uranium that has an extremely long half-life; most useful for dating geologic samples that are more than 10 million years old (half-life: ii. iii. iv. 704 million years) Potassium-40 ( 40 K) has a half-life of 1.25 billion years; used to date rocks that are between 50,000 and 4.6 billion years old (half-life: 1.25 billion years) Rubidium-87 has a half-life of about 49 billion years; used to verify the age of rocks previously dated by using 40 K (48.1 billion years) Long half-lives are useful for dating older rock; shorter half-lives are useful for dating younger rock

17 g. Carbon Dating i. Younger rock layers may be dated indirectly by dating organic material found within the rock. ii. Organic remains, such as wood, bones, and shells that are less than 70,000 years old can be determined by using a method known as carbon-14 dating, or radiocarbon dating. iii. All living organisms have both the carbon-12 and carbon- 14 isotope.

18 iv. To find the age of a sample of organic material, scientists first determine the ratio of 14 C to 12 C. v. Then they compare that ratio with the ratio of 14 C to 12 C known to exist in a living organism. vi. While organisms are alive, the ratio of 14 C to 12 C remains relatively constant. Once a plant or animal dies, the ratio begins to change the amount of 14 C decreases steadily. vii. Scientists can determine the age from the difference between the ratios of 14 C to 12 C in the dead organism.

19 III. The Fossil Record A. Fossils: trace or remains of an organism that lived long ago, commonly preserved in sedimentary rock 1. Paleontology is the scientific study of fossils 2. Can provide info on relative & absolute age of rocks, clues to past geologic events & climates, and evolution

20 B. Fossilization Processes 1. Fossils can only be formed from dead organisms that are buried quickly or protected from decay; only hard parts are fossilized (bones, teeth, shells, wood, etc.) 2. Mummification a. Found in dry locations place where decay-causing bacteria can t survive b. Some ancient civilizations mummified their dead by extracting the body s internal organs and then wrapping the body in prepared strips of cloth

21 3. Amber: hardened tree sap a. Insects become trapped in the sticky sap and are preserved when the sap hardens b. Delicate features are usually preserved (legs, antennae) 4. Tar Seeps : thick petroleum on Earth s surface covered by water a. Animals that come to drink the water can become trapped in the sticky tar; the remains of the trapped animals are covered by the tar and preserved.

22 5. Freezing a. Frozen soil & ice can protect and preserve organisms because most decay-causing bacteria cannot survive freezing temperatures 6. Petrification a. Occurs when minerals (usually as a part of groundwater) seep through layers of sediment & replace the organic content of a dead organism. b. Once water evaporates, the minerals, now in a near-perfect replica of the organism, are all that remains c. Common petrifying minerals: silica, calcite, pyrite

23 C. Types of Fossils 1. Trace fossil: fossilized mark that formed in sedimentary rock by the movement of an animal a. May or may not contain parts of the original organism b. Can provide info about prehistoric life and the animal s appearance & activities 2. Carbon films: carbonized residues of leaves, stems, flowers, & fish made in soft mud or clay a. When original organic material partially decays, it leaves behind a carbon-rich film that displays the surface features of the organism

24 3. Molds and Casts a. When a shell is buried in sediment, it will eventually decay & leave an empty space called a mold b. When sand or mud fills a mold and hardens, a cast forms: replica of the original organism 4. Coprolites: fossilized dung or waste materials from ancient animals a. Can be cut into thin sections & microscopically observed to gain information on the feeding habits of ancient animals, such as dinosaurs b. Animals that come to drink the water can become trapped in the sticky tar; the remains of the trapped animals are covered by the tar and preserved.

25 5. Gastroliths: stones from digestive systems of dinosaurs a. Some dinosaurs had stones in their digestive systems to help grind their food, which often survive as fossils b. Have a smooth, polished surfaces & are often found near dinosaur remains 6. Index Fossils: used to establish age of rock layers because it is unique, common, & only existed for a short period of geologic time a. Can be used to determine relative & absolute ages of rock layers