Geologic TimeGeologic Time

The Rock RecordThe Rock Record

• Rocks record geological events and changing life forms

• UniformitarianismUniformitarianism: (James Hutton) the forces and processes that we observe today have been at work for all of earth’s history

• Earth is a lot older than originally thought!

Relative DatingRelative Dating

• Tells us the sequence of events, not the actual time the events occurred

• Law of Superposition: Law of Superposition: in an undeformed sequence of sedimentary rocks, each bed is older than the one above it and younger than the one below it

• Original Horizontality: Original Horizontality: layers of sediment are deposited in a horizontal position

Relative DatingRelative Dating

• Cross-Cutting:Cross-Cutting: when faults cut through rock layers, or magma intrudes, the intrusion or fault is younger than the rock affected

• Inclusions: Inclusions: Pieces of one rock are contained within another; the rock containing inclusions is younger

Relative DatingRelative Dating

• Unconformities:Unconformities: a long period during which deposition stops, erosion removes previously formed rocks, then deposition resumes

• 3 Types: 3 Types: angular unconformity, discomformity, and nonconformity

• Correlation: Correlation: matching up rocks of similar age in different parts of the earth

Relative DatingRelative Dating

• Angular unconformity: an unconformity where horizontally parallel strata of sedimentary rock are deposited on tilted and eroded layers

• Nonconformity exist when the sedimentary rock lies above and was deposited on the pre-existing and eroded metamorphic or igneous rock

• Disconformity: unconformity between parallel layers of sedimentary rocks which represents a period of erosion or non-deposition.

A = Angular unconformity

B= Nonconformity

C=Disconformity

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Fossil RecordFossil Record

• Fossils are the remains or traces of prehistoric life

• Fossil type depends on conditions at death and how it was buried

• Fossils are typically found in sedimentary rock

• Unaltered RemainsUnaltered Remains: bones, teeth or shells of an animal remain unchanged

Fossil RecordFossil Record

• Altered Remains:Altered Remains: most likely event

• Petrified: “turned into stone”

• Molds: the structure is buried and dissolved. Reflects only shape and surface markings

• Casts: If the hollow spaces are filled with mineral matter

• Trace fossils: indirect evidence; tracks

Fossils and CorrelationFossils and Correlation

• Fossils succeed one another in a definite order; any time period can be recognized by the fossil content of the rock

• Index Fossils:Index Fossils: fossils used as time indicators

• Sometimes groups of fossils are used as the index

• Also used to interpret the environment

Fossils and CorrelationFossils and Correlation

• Conditions favoring preservation:Conditions favoring preservation:– Rapid burialRapid burial

– Possession of hard body parts Possession of hard body parts (skeleton)(skeleton)

Types of Fossilization

Overlapping Ranges of Fossils

Dating with Radioactivity

Basic Atomic Structures• Atomic number is the number of protons in

the atom’s nucleus

• Mass number is the number of protons plus the number of neutrons in an atom’s nucleus.

• Isotopes are atoms of the same element containing a different number of neutrons

Dating with Radioactivity

Radioactivity Radioactivity is the spontaneous decay of

certain unstable atomic nuclei. Particles and energy are released from the

nucleus A more stable daughter is the result

Common Types of Radioactive Decay

Dating with Radioactivity

Half-Life A half-life is the amount of time necessary

for one-half of the nuclei in a sample to decay to a stable isotope.

The Half-Life Decay Curve

Dating with Radioactivity

Radiometric Dating

• 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

Dating with Radioactivity

• As a radioactive isotope decays, atoms of the daughter product are formed and accumulate

• An accurate radiometric date can be obtained only if the mineral remained in a closed system since its formation.

Radioactive Isotopes Frequently Used in Radiometric Dating

Dating with Radioactivity

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

• 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

The Geologic Time Scale

Structure of the Time Scale

• Based on their interpretations of the rock record, geologists have divided Earth’s 4.56-billion-year history into units that represent specific amounts of time.

• Taken together, these time spans make up the geologic time scale

The Geologic Time Scale

Structure of the 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

The Geologic Time Scale

Structure of the Time Scale

There are three eras within the Phanerozoic eon:

• Paleozoic, which means “ancient life,”

• Mesozoic, which means “middle life,”

• Cenozoic, which means “recent life.”

The Geologic Time Scale

• Each period within an era is characterized by somewhat less profound changes in life forms as compared with the changes that occur during an era

• The periods of the Cenozoic era are divided into still smaller units called epochs, during which even less profound changes in life forms occur.

The Geologic Time Scale

Precambrian Time

• During Precambrian time, there were fewer life forms. These life forms are more difficult to identify and the rocks have been disturbed often

The Geologic Time Scale