2Activator: Student Line Up What is the exact position of _______ in the line?What is an estimate of _______ in the line?What is the difference between exact and estimate?
3Absolute vs. Relative Age (Draw a T Chart in your Notebook) Determining the age of an object in relation to other objects“Estimate”Index Fossils, Rock layersExample: A friend tells you she has 2 brothers – one that’s older than her and one that’s youngerAbsolute AgeDetermining the actual age in years of an object“Exact”Radiometric dating, half life, radioactive decayExample: I have one brother who is 8 years old and one who is 17 years old
4Absolute = exact Relative = estimate Mnemonic Memory Help When you hear these two terms, think…Absolute = exactRelative = estimate
5How old is old?Scientists use several strategies to determine age of rocks and fossils.Absolute Age: actual age of an object or how long ago an event occurred by using radioactive dating.Radioactive Dating: measures age by comparing the amount of radioactive element isotope present to how much is normally present in that materialRelative Age: age of an object compared to another- “I am older than you”
6GEOLOGIC DATING: ABSOLUTE AGE DETERMINATION Radioactivity was first discovered by Henri Becquerel in 1896 and Polish-French chemist Marie Curie discovered that radioactivity produced new elements (radioactive decay).Ernest Rutherford first formulated the law of radioactive decay and was the first person to determine the age of a rock using radioactive decay methods.Marie CurieErnest Rutherford
7GEOLOGIC DATING: ABSOLUTE AGE DETERMINATION The number of protons (the atomic number) is fixed for any element and is unique for each element but the number of neutrons in atoms of different elements can vary. Atoms of an element having different numbers of neutrons are referred to as the isotopes (of that element).Figure 3.3: Schematic representation of the isotopes of carbon. Carbon has an atomic number of 6 and an atomic mass number of 12, 13, or 14, depending on the number of neutrons in its nucleus.M&W4 Fig. 3.3; M&W5 Fig. 3.47
8GEOLOGIC DATING: ABSOLUTE AGE DETERMINATION Radioactive decay occurs when an isotope of one element is transformed into a different element by changes in the nucleus. There are three different decay mechanisms:“Parent”“Daughter”Figure 17.18: Three types of radioactive decay. (a) Alpha decay, in which an unstable parent nucleus emits 2 protons and 2 neutrons. (b) Beta decay, in which an electron is emitted from the nucleus. (c) Electron capture, in which a proton captures an electron and is thereby converted to a neutron.M&W4 Fig ; M&W5 Fig
9How can we tell age based on the number of parent isotopes? Radioactive isotopes “decay” at a particular rate. We express this rate as the “HALF-LIFE”, which is the time it takes for HALF of the parent isotopes to decay.
10Absolute Age Half life; the amount of time (years) it takes for 1/2 of the radioactive isotope to decay by 1/2, indicates the age of the object.
11So… Why Do We Care? Relative and Absolute Dating tell us… The age of the Earth as well as its historyWhen the dinosaurs roamed the earthHow the Earth was and where it is goingPossibly Skip this slide?Need Transition
12Which layer do you think is the oldest and youngest? Law of Superposition Says: “ The rocks at the bottom are OLDER than the rocks above”
13The Position of Rock Layers It can be difficult to determine a rocks absolute age. So… scientists use the law of superposition.According to the law of superposition, in horizontal sedimentary rock layers the oldest layer is at the bottom. Each higher layer is younger than the layers below it.
14GEOLOGIC TIME SCALEa series of time intervals that divides Earth’s historyEach layer of rock represents specific interval of timeIndex fossils help determine specific periodTime periods divided by specific events like mass extinctions
15ROCKS TELL A STORY Rocks can tell where they were made and when Sedimentary rocks can have fossils in themRocks can tell when mass extinctions happenedSTUDY AND COMPARISON OF ROCK LAYERS OR STRATA IN THE 19TH CENTURY LED SCIENTISTS TO BELIEVE THAT A CORRELATION EXISTS BETWEEN PLACE AND TYPE OF ROCK
16LAW OF SUPERPOSITIONFor undisturbed rocks, the oldest layer is on the bottom and the youngest is on topSupai is oldest – WHY?
17PALEONTOLOGY the study of fossils remains of ancient life Body fossils vs. trace fossilsBody = remain of organism, like bones;Trace = evidence of organism, like footprintsScientific datingAbsolute dating (gives age in years)uses radiometric / radioactive dating (isotopes)Relative dating (gives age before, after, during)uses observation of rock layersRadiometric dating: use the natural radioactivity of certain elements found in rocks to help determine their absolute age- the use of half-lifes to determine the absolute age of a sample.In radioactive dating, scientists calculate the age of a sample based on the remaining radioactive isotopes.Radioactive elments decay into nonradioactive elements at a steady rate which is measured in a unit called half-life.
18FOSSILSTraces and preserved remains of ancient life found within rock layersFossils show:BiodiversityHow species have changed over timeCorrelation between rock layers from around the worldRelative ages to particular strataEvidence for the geological time scaleTraces are footprints, droppings, or any other type of evidnece an organism might leave behindHow fossils form:Dead organisms are buried by layers of sediment, which forms new rock. Then the preserved remains may be discovered and studied.
19SCIENTIFIC DATINGAbsolute Dating: numerical dating to give rocks an actual date or date range, in number of yearsRelative Dating: compare how old something is in comparison to something else;used to put rocks and geological events in correct chronological orderHOW?Use sedimentary rocksUse fossilsStudy strata
20INDEX FOSSILFossil that defines and identifies geologic periods; often in only one layer of rockEasily recognizableExists over a short geologic time range (found only in a few layers of rock)Wide distribution (geographic range is world-wide)
21Ex/ INDEX FOSSIL: AMMONITE Ammonite fossilsare found worldwide,existed for only a very specific period of timeThis means ammonites are found in very specific layers of rock;Once we know the ammonites, then we can determine the age of any fossil found next an ammonite fossil.Traces are footprints, droppings, or any other type of evidnece an organism might leave behindHow fossils form:Dead organisms are buried by layers of sediment, which forms new rock. Then the preserved remains may be discovered and studied.
22Let’s Practice Together! Which rock layer is the oldest?Which rock layer is the youngest?Which is older, layer M or layer R? How do you knowPut them in order from oldest to youngestWhich is younger R or T? How do you know?
23Think*Write*Pair*Share Is dating rocks using the law of superposition a form of relative dating or absolute dating? Explain your answer
24Remember… Law of Superposition is a form of relative dating Relative dating is based on the rocks that are surrounding it, NOT the exact ageWe know which is older but we do not know HOW old exactly
25TICKET OUT!Which rock is younger, limestone or basalt? How do you know?Which of the labeled rocks is oldest? How do you know?Which of the labeled rocks is youngest? How do you know?Name the rocks in order from oldest to youngest.
26Other Clues to Relative Age Clues From Igneous RockLava that cools at the surface is called an extrusion. Rock below an extrusion is always older.Magma that cools beneath the surface is called an intrusion. An intrusion is always younger than the rock layers around an beneath it.
27Other Clues to Relative Age Faults (a break in the rock) are always younger than the rock it cuts through!Unconformities: An unconformity is a gap in the geological record that can occur when erosion wears away rock layers and other rock layers form on top of the eroded surface.
28Fossils Fossils are usually found in sedimentary rocks. When an organism dies, its soft parts often decay quickly leaving only the hard parts to fossilize.Ex. Bones, Shells, Teeth, or Seeds
29Using Fossils to Date Rocks! Scientists use index fossils to match rock layers.An index fossil must be widely distributed and represent a type of organism that existed only briefly.They are useful because they tell the relative ages of the rock layers they are found in.
30Fossils give us clues about. . . How old a rock is.What life existed in the past.When big events happened.Climate in the past.
31Fossils and Climate (write) Climate=average conditions like temperature, humidity, wind, and rainfall.Fossils show us how climate changes over time.
32Ice Cores (write) Ice forms layers similar to that of rock Also follow law of superpositionContain elements that help describe climate of EarthScientists drill down and pull ice cores upAn ice core from Antarctica 3km long showed 740,000 years of history and 8 ice ages!!!