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Geologic Time Notes

by: Alex Casale

Geologic Time Notes 80203 - GEOL 1010-003

Alex Casale
GPA 3.6
Physical Geology
Mine Dogan

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Geologic Time Notes from Lectures 1-4
Physical Geology
Mine Dogan
Class Notes
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This 9 page Class Notes was uploaded by Alex Casale on Sunday October 18, 2015. The Class Notes belongs to 80203 - GEOL 1010-003 at Clemson University taught by Mine Dogan in Summer 2015. Since its upload, it has received 46 views. For similar materials see Physical Geology in Geology at Clemson University.

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Date Created: 10/18/15
Geologic Time 101815 148 PM Creating a Time Scale Relative Dating Principals The importance of a time scale 0 Rocks record geologic and evolutionary changes throughout Earth s history 0 Without a time perspective these events have very little meaning 0 Numerical and Relative Dates 0 Numerical dates specify the number of years that have passed since an event occurred Example limestone is 250 million years old 0 Relative dates place rocks in a sequence of formation put things in a time order and look at things relative to each other The Hermit Shale is older than the Coconino Sandstone Principals for establishing relative dates 0 1 Superposition 2 Original Horizontality 3 Lateral Continuity 4 CrossCutting Relationship 5 o Inclusions 6 Unconformities SUPERPOSITION In an undeformed sequence of sedimentary rocks each bed is older than the one above and younger than the one below 0 This principle also applies to surface features like lava flows and beds of ash ORIGINAL HORIZONTAL POSITION Layers of sediment are generally deposited in a horizontal position 0 Rock layers that are flat have not been disturbed Curved or any other shape besides horizontal they have been through a chronic event LATERAL CONTINUITY 0 Beds originate as continuous layers that extend in all directions until they eventually thin out or grade into a different sediment type Allows us to infer that the layers were originally continuous across the canyon CROSS CUTTING RELATIONSHIPS Younger features cut across older features INCLUSIONS Fragments of one rock unit that are enclosed within another rock unit 0 The rock containing the inclusion is younger UNCONFORMITIES Layers of rock that have been deposited without interruption completely horizontal are called conformable layers 0 An unconformity is a break in the rock record produced by nondepostion and erosion of rock units 0 3 Basic Types Assume that horizontally positioned sedimentary layers 0 1 Angular Unconformity Tilted rocks are overlain by flatlying rocks 0 2 Disconformity Sedimentary strata on either side of the unconformity are parallel o 3 Nonconformity Sedimentary strata overlay metamorphic or igneous rocks 0 All 3 types of unconformities can be found in the Grand Canyon Geologic Time 101815 148 PM Foss s Traces or remains of prehistoric life preserved in rock 0 Paleontology is the study of fossils Knowing the nature of life that existed at a particular time helps researchers understand past environmental conditions 0 Hard materials from animals are easier to preserve and identify 0 Group fossils under different themes Types of Fossils 0 Permineralization o Mineralrich groundwater flows through porous tissue and precipitates minerals o Petrified Wood Molds and Casts o A mold is created when a shell is buried and then dissolved by underground water 0 A cast is created when the hollow spaces of mold are filled Carbonization and Impressions 0 Carbonization happens when an organism is buried followed by compression which squeezes out gases and liquids leaving a thin film of carbon 0 Effective at preserving leaves and delicate animals 0 Impressions remain in the rock when the carbon film is lost 0 Amber 0 The hardened resin of ancient trees 0 Effective at preserving insects Trace Fossils 0 Indirect evidence of prehistoric life 0 Includes tracks burrow coprolites and gastroliths Conditions Favoring Preservation Most organisms are not preserved 0 Rapid burial and the possession of hard parts increases the chances of preservation Correlation of Rock Layers 0 Correlation involves matching of rocks similar ages from different regions 0 Correlation provides a more comprehensive view of the rock record 0 Correlation with limited areas 0 Often accomplished by noting the position of the bed in a sequence of strata o Involves matching of rocks of similar ages from different regions 0 To correlate over larger areas fossils are needed Correlation of Rock Layers and Fossils Principle of fossil succession o The principle of fossil succession states that fossils are arranged according to their age 0 Example age of trilobites fishes reptiles mammals Index Fossils and Fossil Assemblages 0 Index fossils are widespread geographically and limited to a short period of geologic time o A fossil assemblage is a group of fossils used to determine a rock s age 0 Environmental Indicators Fossils can be used to infer information about past environments Example shells of organisms can be used to infer positions of ancient shorelines and seawater temperatures Geologic Time 101815 148 PM Dating with Radioactivity Reviewing Basic Atomic Structure 0 The nucleus contains protons and neutrons o Protons are positively charged particles with mass 0 Neutrons are a combination of a proton and electron and have a neutral charge 0 Both fill up the mass of the nucleus Electrons are negatively charged particles that orbit the nucleus 0 The atomic number is the number of protons in the nucleus 0 Isotopes have the same number of protons and different number of neutrons Radioactivity The spontaneous decay in the structure of an atoms nucleus Types of Radioactive Decay 0 Alpha Emission 0 an alpha particle 2 protons and 2 neutrons are ejected from the atom 0 Mass number is reduced by 4 and the atomic number is lowered by 2 0 Beta Emission o A beta particle an electron is ejected from the nucleus of the atom o The mass number remains unchanged and the atomic number is increased by 1 0 Electron Capture 0 An electron is captured into the nucleus 0 The mass number remains unchanged and the atomic number is decreased by 1 Radiometric Dating 0 Uses the decay of isotopes in rocks to calculate the age of that rock 0 HalfLife 0 A halflife is the amount of time required for half of the radioactive isotope to decay o Radioactive parent isotopes decay to stable daughter isotopes 0 When the ratio of parent to daughter is 11 one halflife has passed When 2 half lives have passed the ratio is 13 Radioactive Dating Principles 0 Uses various isotopes A complex process 0 Determining the quantities of parent and daughter isotopes must be precise 0 Some radioactive materials do not decay directly into stable daughter isotopes With each passing halflife 50 of the remaining parent decays to daughter atoms 0 As the parent atoms decrease the daughter atoms increase 0 Several naturally occurring radioactive isotopes are useful for dating rocks 0 PotassiumArgon 0 Has a half life of 131 billion years 0 Can date rocks as young as 100000 years 0 Potassium40 40K decays to Argon40 40Ar and Calcium 40 40Ca o 40Ar is a gas and only present in rocks as the daughter product of the decay of 40 0 Example uranium238 has 14 steps to ultimately decay to the stable daughter lead206 0 Sources of Error 0 The system must be closed 0 No external addition or loss of parent or daughter isotopes 0 Fresh unweathered rocks are ideal to use for radiometric daUng Earths Oldest Rocks 0 Oldest rocks are found on the continent o All continents have rocks exceeding 35 billion years old 0 Confirms that geologic time is immense Dating with Carbon14 Radiocarbon dating uses the radioactive isotope carbon14 to date geologically recent events 0 The half life of carbon14 is 5730 years 0 Can be used to date events as old as 70000 years 0 14C is produced in the upper atmosphere from cosmicray bombardment 0 14C is absorbed by plants through photosynthesis 0 14C only useful in dating organic matter 0 All organisms contain a small amount of 14C Geologic Time 101815 148 PM The Geologic Time Scale The geologic time scale encompasses all of Earth history 0 Subdivides geologic history into units 0 Originally created using relative dates 0 Structure of the geologic time scale O 0 00000 O O 0 An eon represents the greatest expanse of time The Phanerozoic eon visible lifequot is the most recent eon which began about 542 million years ago Eons are divided into eras The Phanerozoic eon is divided into three eras 1 Paleozoic era Ancient lifequot 2 Mesozoic era Middle Lifequot 3 Cenozoic Era recent lifequot Each Phanerozoic is divided into periods The Paleozoic era has 7 periods The Mesozoic and Cenozoic eras have 3 periods Each period is divided into epochs Except for the 7 most recent epochs into the Cenozoic most epochs are termed early middle and late Precambrian time before the life Simple life forms that lacked a hard part First abundant fossil evidence does not appear until the beginning of Cambrian period Many Precambrian rocks are highly deformed metamorphic rocks Most detail in the geologic time scale is in the Phanerozoic eon The 4 billion years prior to the Cambrian period are divided into two eons and often collectively refereed to as the Precambrian Proterozoic before life Achaean ancient Less is known about earth the further you go back in time Terminology and the geologic time scale O Precambrian is an informal name for the eons before the Phanerozoic o Hadean refers to the earliest interval of earths history 0 Geologic time scale is continuously updated Sedimentary Rocks can rarely be dated directly by radiometric means due to the lack of igneous material in them or weathering processes Geologist must rely on igneous rocks in the strata Radiometric dating determines the age of the igneous rocks Relative dating techniques assign date ranges to sedimentary rocks Very precise process


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