Geologic Time

The Rock Record Rocks record geological events and changing life forms Uniformitarianism: (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 Dating Tells us the sequence of events, not the actual time the events occurred 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: layers of sediment are deposited in a horizontal position

Relative Dating Cross-Cutting: when faults cut through rock layers, or magma intrudes, the intrusion or fault is younger than the rock affected Inclusions: Pieces of one rock are contained within another; the rock containing inclusions is younger

Relative Dating Unconformities: a long period during which deposition stops, erosion removes previously formed rocks, then deposition resumes 3 Types: angular unconformity, discomformity, and nonconformity Correlation: matching up rocks of similar age in different parts of the earth

Relative 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

Correlation

Fossil 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 Remains: bones, teeth or shells of an animal remain unchanged

Fossil Record 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 Correlation Fossils succeed one another in a definite order; any time period can be recognized by the fossil content of the rock Index Fossils: fossils used as time indicators Sometimes groups of fossils are used as the index Also used to interpret the environment

Fossils and Correlation Conditions favoring preservation: Rapid burial Possession of hard body parts (skeleton)

Types of Fossilization Makes no sense without caption in book

Overlapping Ranges of Fossils Makes no sense without caption in book

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 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 Makes no sense without caption in book

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 Makes no sense without caption in book

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 Makes no sense without caption in book

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 Makes no sense without caption in book