Chapter 21 Fossils the Rock Records Produced by

Chapter 21 Fossils & the Rock Records Produced by S. Koziol 11 -4 -2014

21. 1 The Geological Time Scale Objectives • Describe the geological time scale • Distinguish among the following geological time scale divisions: eons, era, period, and epochs

Geological time scale The geological time scale divides Earth’s history into units from its origin to the present. Remember scale issues….

Geologic time in order from shortest to longest epoch, period, era, eon

Geologic time – divisions? Periods are defined by the abundance or extinction of life-forms.

Geological time (continued) On the geologic time scale, the smallest unit of time is called an epoch. We are currently in the Holocene epoch of the Quaternary period. Greater collection of fossils has been found and paleontologists have more detailed knowledge of the events that occurred during those times.

Geological Time – most recent The most recent period is the Quaternary The Anthropocene is an informal geologic chronological term for the proposed epoch that began when human activities had a significant global impact on the Earth's ecosystems.

Geological Time - longer The Archean and Proterozoic are examples of eons.

Eras vs Periods Both are units of time on the geologic time scale. Eras Periods Eras are longer spans of time, measured in hundreds of millions to billions of years. They are defined by differences in life-forms found in rocks. Periods are usually measured in terms of tens of millions of years to hundreds of millions of years. They are defined by the life-forms that were abundant or became extinct during the time in which specific rocks were deposited.

Geological Time Scale (Blank)

21. 2 Relative-age Dating of Rocks Objectives • Apply the principle for determining relative age to interpret rock sequences. • Describe an unconformity and how it is formed within the rock record.

Laws of Superposition Student guided , remember the foam boards

Undisturbed rock sequence The oldest rock layer in an undisturbed rock sequence occurs at the bottom of the sequence. Which rock layer was laid down 1 st? Which book was laid down 1 st?

The principle of superposition states that, in an undisturbed sequence, the oldest rocks are at the bottom of the sequence and successive layers are younger than those below them.

Cross-cutting relationship You can use the principle of cross-cutting relationships to infer that a fault or an intrusion is younger than the rock it cuts across.

Correlation The matching of rock layers from one geographic area with those of another area is known as correlation.

Uniformitarianism The principle of uniformitarianism states that the processes occurring today have been occurring on Earth since it formed. However, the rate, intensity, and scale with which these processes occur have changed.

Unconformity When part of the rock record is destroyed, the erosional gap that forms is an unconformity.

Angular unconformity The gap in the rock record that occurs between folded or uplifted rock layers and a sedimentary rock layer on top of them is called an angular unconformity.

Nonconformity A buried erosional surface between a nonsedimentary rock and a sedimentary rock is called a nonconformity. A nonconformity in the Wind River gorge of Wyoming.

21. 3 Absolute-age Dating of Rocks Objectives • Explain the several different methods used by scientist to determine absolute age. • Describe how objects are dated by the use of selected radioactive elements. • Explain how annual tree rings and glacial varves are used to date geological events.

Dating techniques Relative-age dating places the ages of rocks and the events that formed them in order, without exact dates. This is done by comparing one event with another or one rock with another rock. Absolute-age dating In contrast, absolute-age dating determines the actual age of a rock, a fossil, or an object. This is done through radiometric dating, a process that determines the ratio of parent material to daughter product in a given sample of rock or fossil.

Half-life The amount of time it takes for one-half of the original amount of an isotope to decay is known as its half-life.

Radiometric dating is used to determine the absolute age of a rock. This is done using the idea of half-life. Parent Daughter t 1/2 Useful Range 238 U 206 Pb 4. 5 b. y 235 U 207 Pb 710 m. y 232 Th 208 Pb 14 b. y 40 K 40 Ar & 40 Ca 1. 3 b. y >10, 000 years 87 Rb 87 Sr 47 b. y >10 million years 14 C 14 N 5, 730 y 100 - 70, 000 years Type of Material >10 million years Igneous Rocks and Minerals Organic Material

Key Bed A key bed contains distinctive material that geologists can easily recognize in the rock record and use as a time marker.

Dendochronology The science of dendochronology uses the annual growth of tree rings to date events and environmental changes.

Permineralization In the process of permineralization, pore spaces within an organism’s shell are filled in with mineral substances.

21. 4 Remains of organisms in the rock records Objectives • Define fossil. • Explain several methods by which fossils can be preserved. • Describe the characteristics of an index fossil. • Discuss how fossils can be used to interpret Earth’s past physical and environmental history.

Amber Fossil insects can be found imbedded in amber, the hardened sap of prehistoric trees.

Mold When the original parts of an organism in a sedimentary rock are weathered and eroded, a hollowed-out impression called a mold forms.

Casts Natural casts can be formed when minerals are deposited within the mold.

Fossils Molds, casts, coprolites, and petrified wood are all example of fossils

Original Preservation (rare) Mummified animals found in dry caves can be examples of fossils with original preservation. Discovered in 1977, this six-to-eight-month-old woolly mammoth baby named Dima is pictured in situ near Kirgiljach River in northeast Siberia. 5, 300 year old Iceman discovered in Austrian Alps, 1991

Fossils are the remains or evidence of once-living plants or animals. They provide clues about Earth’s past environmental conditions and evolutionary changes in organisms over time. They also help to correlate rock layers from one area to another.

Index fossils Geologists use index fossils to correlate rock layers over large geographic areas or to date a particular rock layer. To be useful, an index fossil must be easily recognized, abundant, and widely distributed geographically. It must also have lived over a short period of time.

Index fossils (continued) Index fossils are useful to geologists if the fossils have lived over a short period of time

Evolution The adaptation of life-forms to changes in the environment is known as evolution.

Evolution (continued) Evolution is the process by which organisms adapt to changes in their environments.

End of the Precambrian The end of the Precambrian is marked by the appearance of organisms with hard parts. Remember this is speaking to the end of the pre-Cambrian