Geological Time

1. Geological Time By: Breann Diehl

2. 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.

3. 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

4. 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

5. 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

6. 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.

7. 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

8. 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

9. 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

10. 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

11. 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

12. 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.

13. Divisions of Geological Time There are two main division of geological time: Precambrian and Phanerozoic These are the two eons.

14. 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

15. 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

16. 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

17. 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

18. 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

19. 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

20. 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

21. 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.

22. 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