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