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Chapter 16
Fossils and the Rock Record
The Rock Record
• Geologic Time
• Divisions of
time based on
types of fossils
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
• Epoch: smallest divisions; present epoch is
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
Principles of Relative Age Dating
3. Cross-cutting relationships: a fault or
intrusion is younger than the rock it cuts
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
• 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
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