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.