CHAPTER 8 DETERMINING EARTH S AGE RELATIVE AND ABSOLUTE ROCK AGES Loulousis
Bellringer What are 5 visual clues that help you determine if someone is older or younger than you? Color of hair Wrinkles in skin Height General demeanor Style of dress
8.1 Determining Relative Age Objectives Explain the principle of uniformitarianism and compare the three types Explain how the law of superposition can be used to determine relative age of rocks. Apply the law of crosscutting relationships to determine the relative age of rocks. Explain the importance of the law of original horizontality
Uniformitarianism a principle that states geologic processes that occurred in the past can be explained by current geologic processes James Hutton (18 th century) theorized that the same forces that change Earth s surface now, such as volcanism and erosion, are the same forces that were at work in the past.
Earths Age Until Hutton scientists thought Earth was only 6,000 years old, and that all Earth s geologic features had formed at the same time. (its actually 4.6 billion yrs. old) Hutton argued that the changes on his farm operated very slowly and must have taken millions of years Hutton s ideas about uniformitarianism encouraged other scientists to learn more about Earth s history.
Stratigraphy Stratigraphy- The branch of geology that seeks to understand the geometric relationships between different rock layers (called strata), and to interpret the history represented by these rock layers.
Stratigraphy Geologists try to determine the order in which events have happened during Earth s history They rely on rocks and fossils to help them in their investigation Relative dating Process of determining whether an event or object is older or younger than other events or objects
Relative Age the age of an object in relation to the ages of other objects indicates one layer is older or younger than another but does not indicate the rock s actual age in years
Stop and Think How does sedimentary rock form? Sedimentary rocks form when new sediments are deposited on top of old layers of sediment Scientists study the layers in sedimentary rocks to determine relative age
1. Law of Superposition Layers of sedimentary rock, such as the ones shown below, are stacked like pancakes As you move from top to bottom in layers of sedimentary rock, the lower layers are older Law of Superposition -states that younger rocks lie above older rocks, if layers have not been disturbed Stratum (strata) -A horizontal layer of material, especially one of several parallel layers arranged one on top of another.
Example of Relative Age Dating and Correlation
2. Law of Original Horizontality Scientist know that sedimentary rock generally forms in horizontal layers Original Horizontality Law states that sedimentary rocks left undisturbed will remain in horizontal layers Not all rock sequences are arranged with oldest layers on the bottom and youngest layers on top Some rock sequences have been disturbed by forces within Earth These forces can: Push other rocks into a sequence Tilt or fold rock layers Break sequences into moveable parts
The Geologic Column Geologic column- An ideal sequence of rock layers that contains all the known fossils and rock formations on Earth, arranged from oldest to youngest Use column to: Interpret rock sequences Identify the layers in puzzling rock sequences
Disturbed Rock Layers Geologists often find features that cut across existing layers of rock They assign relative ages to the features and layers The features must be younger than the rock layers because the rock layers had to be present before the features could cut across them
Four Types of Rock Disturbances 1. A fault Break in Earth s crust along which blocks of crust slide relative to one another 2. An intrusion Molten rock from Earth s interior that squeezes into existing rock and cools 3.Folding Occurs when rock layers bend and buckle from Earth s internal forces 4. Tilting Occurs when internal forces in Earth slant rock layers
Clues for Original Horizontality 1.Graded-bedding: size of particles in the layers 2. Bedding plane shape: Scientists can study the shape of the cross-beds to determine the original position of the layers (beds are called cross-beds) 3. ripple marks: small waves that form on the surface of sand due to the action of water or wind If undisturbed crests of the ripple marks point upward
Unconformities a break in the geologic record created when rock layers are eroded or when sediment is not deposited (nondeposition) for a long period of time The missing layers create a break in the geologic record, called an unconformity. Like missing pages in book Types of Unconformities Disconformities Nonconformities Angular unconformities
Types of Unconformities 1.Disconformities exist where part of a sequence of parallel rock layer is missing
Types of Unconformities 2. Nonconformities exist where sedimentary rock layers lie on top of an eroded surface of nonlayered igneous or metamorphic rock 3. Angular unconformities exist between horizontal rock layers and rock layers that tilted or folded
Law of Crosscutting Relationships a fault or body of rock is younger than any other body of rock that it cuts through List strata, the intrusion, and fault in order from oldest to youngest based on relative ages.
Applying Law of Crosscutting and Superposition A conglomerate B Shale C Basalt D Limestone E- Sandstone F -fault 1. Place the geologic features in order from oldest to youngest
A Sandstone B Sandstone C- Shale F Faulting G Magma instrusion forms Granite L- Limestone
Bellringer 1. Explain why it is important for scientists to be able to determine the relative age of rocks. 2. State the principle of uniformitarianism in your own words.
8.2 Determining Absolute Age Objectives Summarize the limitations of using the rates of erosion and deposition to determine the absolute age of rock formations. Explain how varves are used to determine absolute age Explain how the process of radioactive decay can be used to determine the absolute age of rocks. Identify four types of radioactive decay
Absolute Age Absolute dating- any method of measuring the age of an event or object in years absolute age -the numeric age of an object or event, often stated in years Scientists use a variety of ways to determine absolute age Varves Radioactive decay Erosion rate and deposition rate are limited to short time frame (10,000 years) and not always accurate because rates can change over time
Varves Varve- banded layer of sand and silt deposited annually in a lake Like counting tree rings Usually in glacier lakes Summer snow/ice melts quickly and in winter freezes quickly making a permanent layer of sediment
Radioactive Decay Radiometric dating - determining the absolute age of a sample, based on the ratio of parent to daughter material If you know rate of decay for element in a rock you can figure out the absolute age of rock To determine the absolute ages of fossils and rocks scientists analyze decay of radioactive element isotopes Isotopes- atoms of the same element that have the same number of protons but different number of neutrons
Radioactive Decay Most isotopes are stable, meaning they stay in their original form Other isotopes are unstable and are called radioactive Radioactive decay- radioactive isotopes tend to break down into stable isotopes of the same or other elements
How does it work? decay Radioactive decay occurs at a steady rate so scientists can use the relative amounts of stable and unstable isotopes present in an object to determine the objects age Parent isotope the unstable radioactive isotope Daughter isotope stable isotope produced by the radioactive decay of the parent isotope
Rate of radioactive decay is constant So scientists can compare the amount of parent material with the amount of daughter material to date rock The more daughter material there is the older the rock
Half-life the time needed for half of a sample of radioactive substance to undergo radioactive decay After every half-life, the amount of parent material decreases by one-half
Four Radiometric-dating techniques 1. Potassium-Argon Method Potassium-40 has a half-life of 1.3 billion years, argon is daughter material Used to date rock older than 100,000 years 2. Uranium-Lead Method Uranium-238 has half-life of 4.5 billion years, daughter material is lead- 206 Used to date rocks more than 10 million years old
3. Rubidium-strontium Method Rubidium-87 forms daughter material of strontium-87 Half-life is 49 billion years, used on rocks older than 10million years Carbon-14 method Plants and animals contain constant carbon-14 which decreases when dead Half-life is 5,730 years Used on things that lived within the last 50,000 years
8.3 Fossils Objectives Identify 5 major types of fossils and explain how they form Describe what an index fossil is and why geologist can use them in absolute dating
8.3 Fossil Record Fossil- preserved remains of a once-living organism (usually in sedimentary rock) Fossil information Clues about organisms (body, lifestyle, evolution..) Evidence about how Earth s surface has changed Understand past environments were like
Types of Fossils 5 Main Types Molds and Casts Petrified Fossils Carbon Films Trace Fossils Preserved Remains Other Types Mummification Coprolites- fossilized dung or waste
Molds and Casts Mold forms when hard parts of an organism are buried in sediment Hard parts completely dissolve and leave behind a hollow area with organisms shape Cast forms as the result of a mold Water with dissolved minerals and sediment fills the mold s empty space making a cast A cast is the opposite of its mold
Petrified Fossils Petrified means turning into stone Form when minerals replace all or part of an organism Water (full of dissolved minerals) seeps through sediment to dead organism and evaporates leaving hardened minerals behind
Carbon Films All living things have carbon Eventually after decay all that s left of an organism is carbon The thin layer of carbon left behind can show an organism s delicate parts like leaves on a plant
Trace Fossils Show the activities of organisms When animal makes footprint in sand or must and over time sediment buries it becoming solid rock
Index Fossils Trilobite-lived from lower Cambrian Period to Permian Period Fossil used to establish the age of rock layers because it is distinct, abundant, and widespread Only existed for a short span of geologic time Can help determine relative age of rock and estimate absolute age Because lived short geologic time, the rock layer in which an index fossil is found can be dated accurately
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