Chapter 8 The Rock Record

Section 1:Determining Relative Age uniformitarianism - a principle that geologic processes that occurred in the past can be explained by current geologic processes Geologists estimate that Earth is about 4.6 billion years old, (an idea that was first proposed by James Hutton in the 18th century)

C. relative age the age of an object in relation to the ages of other objects 1. Layers of rock, called strata 2. law of superposition - a sedimentary rock layer is older than the layers above it and younger than the layers below it if the layers are not disturbed

D. unconformity - a break in the geologic record created when rock layers are eroded or when sediment is not deposited for a long period of time – 3 types! 1. A nonconformity is an unconformity in which stratified rock rests upon unstratified rock. 2. An angular unconformity is the boundary between a set of tilted layers and a set of horizontal layers. 3. A disconformity is the boundary between horizontal layers of old rock and younger, overlying layers that are deposited on an eroded surface

E. law of crosscutting relationships the principle that a fault or body of rock is younger than any other body of rock that it cuts through

Section 2: Determining Absolute age Absolute Dating Methods 1. absolute age - 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 2. scientists estimate absolute age is to study rates of erosion

3. Scientists can also estimate absolute 3. Scientists can also estimate absolute age by calculating the rate of sediment deposition 4. Varve Count a. varve - a banded layer of sand and silt that is deposited annually in a lake, especially near ice sheets or glaciers b. varves can be counted much like tree rings to determine the age of the sedimentary deposit.

B. Radiometric Dating 1. radiometric dating - 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. 2. half-life - the time required for half of a sample of a radioactive isotope to break down by radioactive decay to form a daughter isotope.

3. The greater the percentage of daughter 3. The greater the percentage of daughter isotopes present in the sample, the older the rock is. 4. Radioactive Isotopes a. Uranium-238, or 238U, an isotope of uranium , extremely long half-life, most useful for dating geologic samples that are more than 10 million years old. b. Potassium-40, or 40K, has a half-life of 1.25 billion years, used to date rock that are between 50,000 and 4.6 billion years old. c. Rubidium-87 has a half-life of about 49 billion years, used to verify the age of rocks previously dated by using 40K.

5. Carbon Dating a. All living organisms have both the carbon- 12 and carbon-14 isotope. b. To find the age of a sample of organic material, scientists compare the ratio of 14C to 12C and then compare this with the ratio of 14C to 12 C known to exist in a living organism. c. Once a plant or animal dies, the ratio begins to change, and scientist can determine the age from the difference between the ratios of 14C to 12C in the dead organism.

Section 3: The Fossil Record Interpreting the Fossil Record 1. fossils the trace or remains of an organism that lived long ago, most commonly preserved in sedimentary rock 2. paleontology the scientific study of fossils

B. Fossilization 1. Mummification Mummified remains are often found in very dry places, because most bacteria which cause decay cannot survive in these places 2. Amber Hardened tree sap is called amber. Insects become trapped in the sticky sap and are preserved when the sap hardens

3. Tar Seeps a. When thick petroleum oozes to Earth’s surface, the petroleum forms a tar seep. b. commonly covered by water. c. The remains of the trapped animals are covered by the tar and preserved. 4. Freezing Because most bacteria cannot survive freezing temperatures, organisms that are buried in frozen soil or ice do not decay.

5. Petrification The substitution of minerals for organic material , results in the formation of a nearly perfect mineral replica of the original organism. C. Types of Fossils 1. trace fossil - a fossilized mark that formed in sedimentary rock by the movement of an animal on or within soft sediment

2. Imprints - Carbonized imprints of 2. Imprints - Carbonized imprints of leaves, stems, flowers, and fish made in soft mud or clay have been found preserved in sedimentary rock.

Five types of fossils: Insect preserved in amber Petrified wood (permineralization) (C) Cast and mold of a clam shell (D) Pyritized ammonite (E) Compression fossil of a fern.