Rock Record

1. Rock Record Spring 2013

2. Determining Relative Age • Geologists estimate that the earth is 4.6 billion years old • James Hutton • Principle of uniformitarianism- current geologic processes are the same processes that were at work in the past • This principle is one of the foundations of geology.

3. Determining Relative Age • Relative age- indicates that one layer is older or younger than another layer based upon its position • Scientists can determine the order in which rock layers were formed • Commonly done in sedimentary rocks

4. Determining Relative Age • Law of Superposition • States that an underformed sedimentary rock layer is older than the layers above it and younger than the layers below it. • Unconformities- this is when layers of rock are not in their original position • Nonconformity- stratified rock lies on stratified rock • Angular Unconformity- boundary between the tilted layers and the horizontal layer • Disconformity- boundary between the older eroded surface, and the younger overlying layers is nearly horizontal

5. Determining Relative Age • Crosscutting Relationships • Law of crosscutting relationships • States that a fault or an intrusion is always younger than the rock layers it cuts through

6. Determining Absolute Age • Rates of Erosion & Deposition • Studying rates of erosion allows scientists to determine how old a feature is • Ex: Niagra Falls– the banks of Niagra Falls are eroding at about 1.3 m/year so it is estimated that it is 9000 years old • This allow good approximations only for features10,000 to 20,000 years old

7. Determining Absolute Age • Rates of Erosion & Deposition • The rate of sediment deposition can be calculated using data collected over a long period of time. • On average 30 cm of sedimentary rock are deposited over a period of 1000 years

8. Determining Absolute Age • Varve Count • Varve- an annual layer of sedimentary deposit on a lake bed • Consist of light colored and a dark colored band • A coarse summer layer & a fine winter layer form one varve • Usually formed in glacial lakes

9. Determining Absolute Age • Radioactive Decay • Radioactive isotopes have nuclei that emit particles and energy at a constant rate. • The natural radioactivity found in rocks gives scientists an accurate way to find the absolute age of rocks. • As an atom emits particles and energy, the atom changes into a different isotope of the same element or an isotope of a different element. • Scientists measure the concentrations of the original radioactive isotope & the newly created isotopes & then compare the proportion of the two to determine the age of the rock.

10. Determining Absolute Age • Half-Life • The time it takes for half the mass of a given amount of a radioactive element to decay into its daughter elements • Ex: Uranium-238 decays into Lead-206: uranium is the parent element, lead is the daughter element

11. Determining Absolute Age • Carbon Dating • Carbon dating is used when rock samples are recently formed • Rocks contain carbon-14 • Carbon-14 decays into carbon-12

12. Fossil Record • Paleontologists- scientists that study fossils • Fossils- remains or traces of animals or plants from a previous geologic time. • Provide clues about events, climates, & evolution • Evolution- change of living things over time. • Paleontology- study of fossils • Fossils are almost always found in sedimentary rock

13. Fossil Record • Kinds of Fossils • Bones • teeth • Shells • Rarely an entire organism • Replicas • Imprints

14. Fossil Record • Preservation of Organisms • Only dead organisms that are buried quickly or protected from decay can become fossils. • Mummification- drying • Mummified organisms are often found in desert caves and buried beneath desert sands • Amber- hardened try sap • Insects are preserved this way when they get trapped in the sap and it hardens

15. Fossil Record • Preservation of Organisms cont… • Tar Seeps – formed by thick petroleum oozing to the earth’s surface • Animals become trapped in the sticky tar and tar eventually covers them preserving them • Freezing • Low temperatures of frozen soil & ice protect & preserve animals because most bacteria cannot survive freezing temperatures

16. Fossil Record • Petrification • Petrification- mineral solutions remove original organic materials and replace them with new materials • Often results in a near perfect replica of the original organism

17. Fossil Record • Traces of Organisms • Trace fossils- tracks, footprints, borings, & burrows • Formed when sand or silt fills the hollowed out area and hardens • Imprints, Molds, & Casts • Imprints are usually of leaves, stems, flowers, & fish and are formed from soft mud or clay that has hardened • Molds empty cavities left by the shells of snails & parts of trees ; retains the surface markings & shape of the original organism • Casts form when a mold fills with mud or sand; it is a replica of the original organism

18. Fossil Record • Trace Organisms cont… • Coprolites • coprolites- fossilized dung or waste materials from ancient animals • Gastroliths • Gastroliths- stones that were found in the digestive systems of dinosaurs to help grind their food

19. Fossil Record • Interpreting the Fossil Record • Provide clues to the relative ages of rocks • Reveal environmental changes & how they have affected organisms through the geologic history of the earth

20. Fossil Record • Index Fossils • Fossils found exclusively in rock layers of a particular geologic age • To be considered an index fossil… • Must be present in rocks scattered over a wide area of the earth’s surface • Index fossils must have features that clearly distinguish them from all other fossils • Organisms from which the index fossil formed must have lived during a relatively short span of geologic time • Must occur in fairly large numbers within the rock layers

21. Fossil Record • Fossil Clues to the Past • Fossil tells us about different climate and environmental changes that occurred in the past. • Ex: tropic plant and animal fossils have been found in both Canada and the Arctic