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Chapter 16 Fossils and the Rock Record The Rock Record • Geologic Time Scale • Divisions of time based on types of fossils found preserved in the rock • Present: Holocene Epoch Geologic Time Scale • Eon: largest division; first 3 eons make up 90% of geologic time • Era: eons are divided into eras; paleo = old, meso = middle, ceno = recent, zoic = life • Period: some named for the geographic region where characteristic fossils were first described • Epoch: smallest divisions; present epoch is Holocene Relative Age Dating • Methods of putting events in order without a specific time scale • Uniformitarianism: idea developed by James Hutton, Scottish geologist, during the 1700s. • Geologic processes (weathering, erosion, deposition) that occur today have been occurring since Earth formed. Principles of Relative Age Dating 1. Original horizontality: sedimentary rocks are deposited in horizontal layers Principles of Relative Age Dating 2. Superposition: younger rocks are deposited on top of older rocks; oldest rocks are at the bottom. Principles of Relative Age Dating 3. Cross-cutting relationships: a fault or intrusion is younger than the rock it cuts across Principles of Relative Age Dating 4. Inclusions: embedded fragments of rocks (inclusions) are older than the rock layer that contains them. Gaps in the Rock Record • Unconformities: buried surfaces of erosion, three types – Disconformity: layers of sedimentary rock have been eroded and new layers deposited on top – Nonconformity: sedimentary rock deposited on top of igneous or metamorphic rock – Angular unconformity: horizontal layers are deposited on top of tilted layer Unconformities Correlation • Correlation: matching of unique rock outcrops or fossils in separate places Correlation using fossils Absolute Age Dating • Establishes an actual numeric age of rocks and other objects • Radiometric decay Radiometric Dating • Radioactive elements change into new elements at a constant rate. • The relative amounts of parent material and daughter material can be used to determine the age of rock and organic material. Other Absolute Age Dating Methods • Dendrochronology (tree rings): growth rings produced yearly (approximately 10,000 years) Other Absolute Dating Methods • Ice core samples: annual layers of snow deposits from glaciers Other Absolute Dating Methods • Varves: alternating dark and light sediments deposited seasonally (approximately 120,000 years recorded) Fossil Remains • Fossils: preserved remains or traces of once living organisms – Original preservation: frozen, dried, or trapped in amber or tar pits – Altered hard parts: shells, bones, or cell walls replaced with new minerals – Molds/casts: preserved impression of an organism – Trace fossils: footprints, gastroliths, coprolites Fossils