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Geological Time. By: Breann Diehl. How do geologists deal with time?. Relative age - geologists can tell if something is younger or older than something else Absolute age – geologists can assign a number to the amount of time that has passed. Stratigraphy.

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geological time

Geological Time

By: Breann Diehl

how do geologists deal with time
How do geologists deal with time?
  • Relative age - geologists can tell if something is younger or older than something else
  • Absolute age – geologists can assign a number to the amount of time that has passed.
stratigraphy
Stratigraphy
  • It is the study of strata (layers) in the Earth’s crust
  • There are two laws of Stratigraphy: Original Horizontality and Stratigraphic Superposition
  • Original Horizontality: strata deposited in layers that are horizontal
  • Stratigraphic Superposition: sequence of layers are deposited as older layers on the bottom and the younger ones on top
breaks in stratigraphic record
Breaks in Stratigraphic Record
  • The Earth’s crust is changing continuously due to uplift, subsidence, and deformation
  • Erosion or deposition of sediment is occuring at any given time
  • Hiatus – when there is a break in the stratigraphic record due to erosion removing previously deposited sediment or if sediment is not being deposited
three types of unconformity
Three Types of Unconformity
  • Unconformity is a surface of erosion or non-deposition
  • Angular Unconformity-angular layers in the strata that originally were deposited horizontally
  • Disconformity- recognized by correlating one area to another and in one, a layer is missing
  • Nonconformity- occurs where rocks that formed deep in the earth are overlain by sedimentary rocks formed at the earth’s surface
stratigraphic classification
Stratigraphic Classification
  • Rock Stratigraphic Units – Distinctive bodies of rocks that differ from the rocks above and below . This one is based on physical characteristics, and the unit is formation
  • Time Stratigraphic Units – bodies of rocks that were deposited during the same geologic time interval. This is based on the time which the material was formed, and the unit is a system.
correlation of rock units
Correlation of Rock Units
  • In order for rocks to be correlated over large areas, they must be determined as being equivalent
  • Determination of equivalence is based on: relative age and physical criteria
absolute geological time
Absolute Geological Time
  • In history there have been many attempts to establish the a measure of the absolute geological time
  • 1. In 1889, the age of the Earth was determined on how long it would take the oceans to obtain their present salt content. The estimate for Earth was 90 million years
  • 2. The age of the Earth was determined from the time required to cool from an initially molten state. The estimate was about 100 million years
  • 3. Finally in 1896, radioactivity was discovered as well as the fact that radioactive decay occurs at a constant rate
radiometric dating
Radiometric Dating
  • This relies on the fact that there are isotopes:
  • Radioactive Isotopes – “parent isotopes” that decay at a constant
  • Radiogenic Isotopes – are formed by the decay therefore are called “daughter isotopes”
  • Half-Life: the rate at which radioactive isotopes decay
radiocarbon dating c14
Radiocarbon Dating (C14)
  • This one is different than most other methods because it can’t be used to directly date rocks, but can be used to date organic material produced by once living organisms.
  • Measuring the amount of C14 in a dead material allows us to determine the elapsed time since the organism died
  • Only used to date materials younger than about 70,000 years
clues to the age of the earth
Clues to the Age of the Earth

Meteorites: Ages of most primitive meteorites are around 4.6 billion years

Moon Rocks: Ages of the moon rocks are all within a range between 4.0-4.6 billion years

Conclusion: The solar system and Earth must be at least 4.6 billion years old

historical perspective
Historical Perspective
  • The first people who needed to understand the geological relationships of rocks were miners
  • In 1669, Nicolaus Steno described two types of geologic principles: sedimentary rocks are laid down horizontally and younger rocks are deposited on top of older ones
  • In 1795, James Hutton’s concept that the natural geologic processes were uniform in frequency and magnitude, known as “principle of uniformitarianism”
  • In 1815, William Smith (a surveyor, canal builder, and amateur geologist) demonstrated the principle of faunal succession, which says that fossils found in rocks are in a very definite order.
divisions of geological time
Divisions of Geological Time

There are two main division of geological time: Precambrian and Phanerozoic

These are the two eons.

precambrian
Precambrian
  • This eon makes up about 90% of the history of the Earth
  • This was the time described to refer to the period of the Earth before the formation of rocks with recognized fossils
  • During this time the Earth formed, life arose, the first tectonic plates were formed and began to move, eukaryotic cells evolved, and the atmosphere became rich with oxygen
  • Three divisions of Precambrian are the Hadean, the Archean, and the Proterozoic Eras
hadean era 4 6 billion years
Hadean Era 4.6 billion years
  • The formation of Earth from dust and gas that was orbiting the sun
  • The Earth changed from liquid to solid
  • There was oceans of liquid rock, volcanoes blasting all over the place, and boiling sulfur
  • Meteors and asteroids fell constantly from the solar system
  • The air was hot, thick and made up of carbon dioxide, water vapor, sulfur, and nitrogen
  • No rocks are found in this Era
archean era 3 8 2 5 billion years
Archean Era 3.8-2.5 billion years
  • Earth has cooled down, oceans formed, most of the carbon dioxide is gone and atmosphere is mostly filled with nitrogen, and the sky has normal clouds and rain
  • Lava cooled to form the ocean floor, but the interior of Earth is hot and active with volcanoes.
  • The volcanoes have formed islands and are the only land surface at this time
  • Asteroids and meteorites are mostly gone
  • Appearance of blue-green algae, which are single celled bacteria, and the oldest fossils are found in the Archean rocks
proterozoic era 2 5b 543m years
Proterozoic Era 2.5b-543m years
  • There are two supercontinents
  • Earths surface has cooled more and fewer volcanoes than in the Archean
  • Even though the Earth’s surface is moving fast the cores of the continents are quite large and stable
  • The only life is found in the ocean, but single celled organisms appeared with a nucleus, and towards the end of this era there are multi-celled creatures
  • Atmosphere is about the same, but oxygen is starting to appear in the air from the blue-green algae
  • Earth is very cold and has huge glaciers
phanerozoic eon
Phanerozoic Eon
  • Represents the time during which the majority of macroscopic organisms, algae, fungal, plant, and animals lived
  • The appearance of animals with external skeletons and internal skeletons later on
  • Three subdivisions: Paleozoic, Mesozoic, and Cenozoic
paleozoic era 543 248 million years
Paleozoic Era 543-248 million years
  • There was an explosion in the diversity of plant and animal life from the multi-celled animals
  • However, the largest mass extinction in history occurred which wiped out about 90% of the marine life
  • Animals, plants, and fungi colonized the land while the insects ruled the air
  • Six major continental land masses, but moved further apart in this era
  • Important rocks of this era are limestone and coal
mesozoic era 248 65 million years
Mesozoic Era 248-65 million years
  • It’s divided into three time periods: Triassic, Jurassic, and Cretaceous
  • Appearance of dinosaurs but, except for the birds, became extinct by the end of the Cretaceous period
  • New types of terrestrial vegetation occurred. Older ones like ferns were seen, then came the gymnosperms, and finally the angiosperms
cenozoic era 65 million years to the present
Cenozoic Era 65 Million Years to the Present
  • Two time periods: Tertiary and Quarternary
  • Sometimes called the “Age of the Mammals”: whales, saber-tooth cats, elephants, and giant sloths, and finally humans
  • The Earth is basically what we see today: continents where they are now, Rocky and Andes Mountains, the climate is about the same
  • Humans have become the dominant terrestrial life form.
acknowledgements
Acknowledgements
  • www.tulane.edu/~sanelson/geol111/geotime.htm
  • www.ucmp.berkeley.edu/exhibit/histgeoscale.html
  • www.ucmp.berkeley.edu/precambrian/precambrian.html
  • www.cotf.edu/ete/modules/msese/earthsysflr/cambrian.html
  • www.ucmp.berkeley.edu/exhibit/histgeoscale.html
  • www.ucmp.berkeley.edu/help/timeform.html