chapter 21-fossils and the rock record
TRANSCRIPT
Chapter
2121Fossils and the Rock Record
Rocks Record Earth History
21.1 The Rock Record (p. 590)
Geologic Time Scale: record of Earth’s history from its origin 4.6 billion years ago Rocks record geological events and
changing life forms of the past.
We have learned that Earth is much older than anyone had previously imagined and that its surface and interior have been changed by the same geological processes that continue today.
Geologic Time Scale
21.1 The Rock Record (p. 590)
A record of Earth’s history form its origin 4.6 billion years ago
Structure of the Time Scale
21.1 The Geologic Time Scale
Eons represent the greatest expanses of time. Eons are divided into eras. Each era is subdivided into periods. Finally, periods are divided into smaller units called epochs.
Precambrian era 3 earliest eons that make up 90% of geologic time Earth was formed and became
hospitable during this time Fewer and simpler lifeforms than at
present
The Geologic Time Scale
A Brief History of Geology
21.2 Relative Age Dating (p. 595)
Uniformitarianism geologic processes occurring today have been occurring since Earth formed
21.2 Relative Age Dating
Relative dating tells us the sequence in which events occurred, not how long ago they occurred (not numerical).
Law of Superposition• oldest rock is at the bottom layer and gets
younger as layer is closer to the top-see Grand Canyon example on p. 596
Ordering the Grand Canyon’s History
Relative Dating—Key Principles
21.2 Discovering Earth’s History
Principle of Original Horizontality• The principle of original horizontality means
that layers of sediment are generally deposited in a horizontal position.
Disturbed Rock Layers
21.2 Relative Age Dating
Cross-Cutting Relationships• intrusions are younger than the rock they cut
across -faults are a good example
Inclusions• When a layer is exposed due to erosion, the
sediment may move to a younger, neighboring area of rock and become a part of it.
Applying Cross-Cutting Relationships
Formation of Inclusions
21.2 Relative-Age Dating
Unconformities (photos on p. 598)
• Read paragraph on p. 598. Summarize unconformities in your own words using an example.
-types: disconformity, nonconformity, angularDisconformity: horizontal sedimentary rock overlies
another layer of horizontal sedimentary rockNonconformity: horizontal sedimentary rock
overlies a different rock type (easier to identify) Angular: indicates that during the pause in
deposition, a period of deformation (folding or tilting) and erosion occurred.
Formation of an Angular Conformity
A Record of Uplift, Erosion, and Deposition
Correlation of Rock Layers
21.2 Relative-Age Dating
Can use fossils to correlate rock formations in locations that are geographically distant
Correlation of Strata at Three Locations
Molds and Casts
• Mold: formed when sediment covers an dead organism and creates a hollowed-out depression in the material
• Cast: formed when a mold is filled with a material
• See p. 608 for photo
Types of Fossilization
Absolute-Age Dating
21.3 Absolute-Age Dating (p. 601)
Enables scientists to determine the numerical age of rocks and other objects.
Radioactivity
21.3 Dating with Radioactivity
Radioactivity the spontaneous decay of certain unstable atomic nuclei.
Radioactive decay the emission of radioactive particles and the resulting change into other isotopes Constant rate of decay, so can use to
determine the absolute age of a rock or object
Common Types of Radioactive Decay
Radiometric Dating and Half-Life
21.3 Dating with Radioactivity
Radiometric dating scientists date an object using radioactive isotopes
half-life the amount of time necessary for one-half of the nuclei in a sample to decay to a stable isotope
-used by scientists to determine absolute age
The Half-Life Decay Curve
Radiometric Dating
21.3 Dating with Radioactivity
Each radioactive isotope has been decaying at a constant rate since the formation of the rocks in which it occurs.
Radiometric dating is the procedure of calculating the absolute ages of rocks and minerals that contain radioactive isotopes.
Radiometric Dating As a radioactive isotope decays, atoms of
the daughter product are formed and accumulate.
21.3 Dating with Radioactivity
An accurate radiometric date can be obtained only if the mineral remained in a closed system during the entire period since its formation.
Radioactive Isotopes Frequently Used in Radiometric Dating
Dating with Carbon-14 Radiocarbon dating is the method for
determining age by comparing the amount of carbon-14 to the amount of carbon-12 in a sample.
21.3 Dating with Radioactivity
When an organism dies, the amount of carbon-14 it contains gradually decreases as it decays. By comparing the ratio of carbon-14 to carbon-12 in a sample, radiocarbon dates can be determined.
Importance of Radiometric Dating Radiometric dating has supported the ideas
of James Hutton, Charles Darwin, and others who inferred that geologic time must be immense.
21.3 Dating with Radioactivity
Other ways to determine absolute age (p. 604)• Dendochronology: tree rings: width of
rings depend on environmental conditions– If a lot of rain, ring is wider
Fossil Formation
21.4 Fossil Remains (p. 606)
Fossils remains or traces of prehistoric life. -important components of sediment and sedimentary rocks.
The type of fossil that is formed is determined by the conditions under which an organism died and how it was buried.
Evolution: change in species over time, can use fossils to determine
Fossil Formation (p. 607-608)
21.4 Fossil Remains
Original preservation very little alteration since death of organism
Altered Remains (most fossils are)• The remains of an organism are likely to be
changed over time.• mineral replacement: pore spaces filled with
groundwater, the original mineral is replaced by a different mineral from the water
• recrystallization: fossil subjected to high heat and temperature, changing its composition
Fossils and Correlation
21.4 Fossils: Evidence of Past Life
The principle of fossil succession states that fossil organisms succeed one another in a definite and determinable order. Therefore, any time period can be recognized by its fossil content.
Index fossils are widespread geographically, are limited to a short span of geologic time, and occur in large numbers.
Overlapping Ranges of Fossils
Fossil Formation
21.4 Fossils: Evidence of Past Life
Indirect Evidence• Trace fossils are indirect evidence of prehistoric
life.
Conditions Favoring Preservation• Two conditions are important for preservation:
rapid burial and the possession of hard parts.