Presentation on theme: "Geologic Time and Earth History"— Presentation transcript:
1Geologic Time and Earth History In this segment, we will learn how geologists:determine the relative ages of rock units,determine the divisions of the geologic time scalehow radiometric techniques can be used to date some rocks.Dr. R. B. Schultz
2Formation of the Earth National Geographic Video How did the Earth form?Formation of the Earth National Geographic Video
3How do geologists determine how old rocks are? Relative dating - determine whether the rock is older or younger than other rocks2. Radiometric dating -- use radiometric dating techniques to determine how long ago the rock formed in the exact number of years*Not all rocks can be dated absolutely,so combinations of techniques are used.
7Interpreting the Rock Record: James Hutton, late 1700s – (considered to be "Father of Geology")realized that most sedimentary layers were deposited from gradual, day-to-day processes.Determined that it took a long time to form these rocks. This was far different from what others believed prior to this time.Principle of Uniformitarianism:The forces and processes that we observe today have been at work for a very long time. Examples: plate tectonics volcanismmountain building earthquakessedimentation
8Principles associated with Relative Dating Principle of SuperpositionRock layer above is younger than the ones below it.(Oldest on bottom, youngest on top)May not apply to rocks that have been folded.can get turned upside-down
9Principles associated with Relative Dating Principle of Original HorizontalitySedimentary layers are deposited in approximately horizontal sheets.If layers are folded, deformation must have occurred after rocks formed.Age of folding is younger than youngest deformed rock unit.
11Principles associated with Relative Dating Principle of Crosscutting RelationshipsAny feature (e.g. fault or intrusion) that cuts across rocks is younger than the youngest rock that is cut.
12Illustration of Relative Age Principles Cross Cutting RelationsOriginal HorizontalitySuperposition
13Correlation:determining that rocks are the same formation (may mean rocks are the same age)ABCBCA
14Principles associated with Relative Dating UnconformitiesSurfaces in rock that represent periods of erosion or non-deposition.In other words, time has been left out of the physical geologic rock record.
16Geologic Time Scale Developed in 1800s from relative dating of rocks More recently, radiometric techniques have allowed us to determine ages of units in years before present.Many of the names relate back to localities in England (Ex: Devonian from Devonshire)Divisions of Geologic Time Scale:Eons Eras:Paleozoic -- Mesozoic -- CenozoicOldest > YoungestPeriods of the Phanerozoic: Paleozoic EraPermian (youngest)Pennsylvanian together with Mississippian are called "Carboniferous" in Great BritainEpochs of Tertiary and QuaternaryPaleoceneEoceneOligoceneMiocenePliocenePleistocene
17The Geologic Time Scale Most recent “Ice Age”“Humans” arriveMajor Mass ExtinctionAge of DinosaursMajor Mass ExtinctionAge of Coal FormationAge of FishesFirst multi-celled organismsThe Geologic Time ScaleOrigin of the Earth 4.55 Billion years
18Earth during the Silurian (430+ million years ago)
19Devonian ~410 million years agoMississippian ~330 million years agoPermian ~250 million years ago
20Triassic ~200 million years agoJurassic ~190 million years agoCretaceous ~100 million years agoEarly Cenozoic ~50 million years ago
22ActivityOpen your books to pg. 372: “Quick Lab – Relative Dating” Directions: Each group needs one piece of loose leaf to complete the Quick Lab questions together. Be ready to describe your reasoning!
23Quick Lab: Relative Dating Answers #1. 8 of , 6 of , 3 of , 7 of , 10 of ,5 of , Ace of , 4 of , 9 of#2. All the cards, except the 2 of , could be put in sequence.#3. An intrusion would be like slipping a card in from the side.
24AssignmentTurn to page 341.Each person needs to complete: #1-8 on a new sheet of loose leaf paper.Due tomorrow.
25For Review: What is relative geologic dating? Why is it important? What is the second way geologists can date fossils and rocks?
27Radiometric Dating: Radioactivity RadioactivityWhen nuclei of an atom are unstable, they spontaneously break apart (decay) during this process.
28Rate of Radioactive Decay Radioisotopes decay at a constant rate.Rate of decay is measured by half-lives.Half-life -- time it takes for one-half of the radioactive material to decay.
29Radiometric Dating: Radiometric Age Dating Radioisotopes are trapped in mineralsRadioisotopes decay through time, and stable isotopes are formed.Common isotopes used in age datingU-Pb -- half-life of U-238 is 4.5 b.y.K-Ar -- half-life of K-40 is 1.3 b.y.Carbon half-life of C-14 is 5730 yrs
30Radiometric Dating Example By using the appropriate radioactive isotope (knowing its halflife time), and measuring the amount of the isotope in the rock,we can deduce how long it has taken to decay down to thepresent amount in the rock.Example: A rock has ½ of the original carbon-14 material in it.The half-life of carbon 14 is 5730 yearsThe rock must have decayed (lost) 50% of its original carbon-14 material and is now 5730 years old.In a period of 5730 years from now, the rock will contain 25% of its original carbon 14 material.Theoretically, there will always be some trace of carbon-14 present in the rock…it will never decay totally.
31Radiometric Dating Example Click to Run Animation
32AssignmentTurn to page 351.Each person needs to complete: #1-6 on a new sheet of loose leaf paper.Due tomorrow.