Transcript 1 half-life
Science is the investigation of ideas. Scientific ideas change as new evidence is discovered. Scientific models and theories evolve and change. One notable changewas the change from Catastrophism to the principle of Uniformitarianism. Reference: New text, pages 5 - 6 Catastrophists thought that Earth’s physical features (mountains, canyons, and almost all landforms) formed by sudden spectacular events (catastrophes) produced by unknowable causes that no longer operate and can not be explained by nature. ( James Ussher) A relatively young aged Earth. This is not to say that catastrophic events do not occur today , tsunamis, earthquakes, volcanic eruptions, etc. Uniformitarianism.- This idea was first recognized by a Scottish Geologist named James Hutton. Hutton came to the conclusion that, “ the present is the key to the past.” and “that, the physical, chemical, biological laws that operate today to shape Earth also operated in the past.” This statement included two concepts; 1) the geologic processes at work today were also active in the past. 2) the present physical features of Earth were formed by these same processes, at work over long periods of time. The ages of geological events can be determined in two different ways. 1) Relative Dating Reference: 2) Absolute Dating (Tarbuck and Lutgens Text) Relative Dating: pages 7-8 & 218-228 Absolute Dating: pages 228 - 235 Places events in a sequence of events of formation, but does not identify their actual date of occurrence. Can’t tell us how long ago something happened, only that it followed one event and preceded another. Relative dating techniques include; 1) Principle (Law) of Superposition 2) Principle of Original Horizontality 3) Principle of Cross-Cutting Relationships 4) Principle of Inclusions 5) Fossil Succession ( index fossils) 6) Unconformities 7) Folding and Faulting 8) Contact metamorphism Law of Superposition in any undisturbed sequence of sedimentary rocks, a sedimentary layer is older than the layers above it and younger than the layers below it. - Principal of Original Horizontality - states that most layers of sediment are deposited in a horizontal position. If rock layers are folded or inclined, then the layers must have been moved into that position by crustal disturbances. Law of Crosscutting Relationships - states that an igneous rock or geologic feature is younger than the rocks it has intruded, or cuts across. Two examples of cross-cutting in this diagram: Fault cuts rock units A, B, C, D, & dike. Igneous Dike cuts rock units A, B, & C. Law of Included Fragments - states that pieces of one rock found in another rock must be older than the rock in which they are found. Rock fragments from rock unit “D” is included in layer “E” above it. Fossil Succession William Smith, an English surveyor and engineer: In a stratagraphic sequence, different species of fossil organisms appear in a definite order; once a fossil species disappears in a sequence of strata, it never reappears higher in the sequence. Unconformities are buried eroded surfaces which represent gaps in geologic time. Three types of unconformities include; 1) Angular Unconformity 2) Disconformity 3) Nonconformity Compressional forces cause sedimentary layers to fold as the layers uplifted. The folded layers are eroded and sinks where sediment is deposited over the erosional surface. Rock layers below the erosional surface are inclined at an angle to the erosional surface and layers above. STEP 1 STEP 2 STEP 3 STEPS 4-6 Rock layers above and below the erosional surface are parallel. Sinking The eroded surface of metamorphic or plutonic igneous rocks are buried by younger sedimentary layers. sinking 3 types of unconformities are seen These two features will be studied in more detail layer in the course. Folding is when sedimentary rocks are compressed and bend into a wave-like pattern. Fault is a crack in the earth’s surface along which movement occurs. When molten rock comes into contact with older rock the heat causes a kind of baking that changes the original rock. Contact metamorphism Correlation- the matching up of rocks from different areas based upon their fossils, rock type, color, and/or texture. Fossil Correlation Rock Symbols Contact metamorphism xxxxxxx Granite Basalt Arrange the letters in order from oldest to youngest. Identifies the actual date of an event. Example, the extinction of the dinosaurs about 66 million years ago. Absolute dating methods include; 1) Tree Rings - The age of a tree is found by counting the total number of rings. 2) Varves - any sediment layer that shows a yearly cycle. Varves are often seen in glacial lakes dating back to the ice age. 3) Radiometric Dating - calculating absolute ages of rocks and minerals that contain radioactive isotopes. Radioactive Dating A radioactive sample is the Parent Material and the decayed product is called the Daughter Material. When both are added together it equals 100%. Several different dating methods can be used to find the age of different rocks. Some of these dating methods and corresponding half-lives ; 1) Uranium-238 decays to Lead-206 ! 4.5 Billion Years 2) Uranium-235 decays to Lead-207 ! 713 Million Years 3) Potassium-40 decays to Argon-40 ! 1.31 Billion Years 4) Carbon-14 decays to Nitrogen-14 ! 5730 years 5) Rubidium-87 decays to Strontium-87 ! 47 Billion Years Calculating Absolute Age with Radioactivity Radioactive elements (Isotopes) • Elements which are unstable in nature and give off radiation as they undergo radioactive decay to become stable. • Three types of radiation can be given off during radioactive decay: 1) Alpha, 2) Beta, and 3) Gamma Particles. • Radioactive elements decay at constant rates and are thought to start decaying as soon as the rock has formed. •The rate at which a radioactive element decays is called its half-life. Radioactivity Problem These questions could make reference to the radioactive parent isotope in; • Fraction Form (ex. 1/16th) • Percent Form (ex. 25%) • Amount in Grams (360 grams) Question: Calculate the age of a rock using the K - 40 Ar – 40 dating method (which has a half – life of 1.3 billion years), if you know that 12.5% of the parent material now remains in the rock sample. Information Given in Problem: Half-life of radioactive sample 1.3 Billion Years Parent material remaining 12.5% The key to solving radioactive problems is that the number of halflives (represented by “N”) must be found. To find the number of halflives (N) that passed when 12.5% of the radioactive sample remains we can use a chart and follow the following steps: Note: 0 1 2 100% 50% 25% 3 12.5% After 3 half-lives 12.5% of radioactive sample remains. Thus, “N” = 3 The original amount before any radioactive material decayed was 100%. This is represented in the chart as zero half-lives. Find how many half-lives the radioactive sample has to go through so that 12.5% remains. 1 half-life 2 Half-lives 3 Half-lives To calculate the Age of the radioactive sample, use the following formula; Age = “N” x # of years per half-life Age = 3 x 1.3 billion years Age = 3.9 billion years Problem Type #1: Fraction of parent material remaining Given the half-life of U-235 is 0.7 billion years, determine the age of a sample of U-235 if 1/16 of the starting material remains. Given: Half-life = 0.7 billion years Fraction of parent (U-235) remaining = 1/16 You must first find out how many half-lives have passed if 1/16 of the parent (U-235) remains. Problem Type #1: Fraction of parent material remaining Given the half-life of U-235 is 0.7 billion years, determine the age of a sample of U-235 if 1/16 of the starting material remains. Given: Half-life = 0.7 billion years Fraction of parent (U-235) remaining = 1/16 You must first find out how many half-lives have passed if 1/16 of the parent (U-235) remains. Age = # of Half-lives x Time for 1 Half-life Age = ( 4 ) (0.7 Billion years) Age = 2.8 Billion years Problem Type #2: Mass of parent material remaining 1200 g of a radioactive element has decayed to produce 150 g of the element. If the half-life of the mineral is 0.40 billion years, what is the age of the sample? Given: 1200 grams decays to 150 grams & Half-life = 0.4 Billion years You must first find out how many half-lives have passed when 1200 grams decays to form 150 grams Problem Type #2: Mass of parent material remaining 1200 g of a radioactive element has decayed to produce 150 g of the element. If the half-life of the mineral is 0.40 billion years, what is the age of the sample? Given: 1200 grams decays to 150 grams & Half-life = 0.4 Billion years 1200 g 600 g 300 g 150 g Age = # of Half-lives x Time for 1 Half-life 3 Half lives Age = ( 3 ) (0.4 Billion years) Age = 1.2 Billion years Problem Type #3: Decay Graph Element X has a half-life of 250,000 years. Suppose that 256 g of element X were initially present in a sample of rock. (i) Construct a half-life decay graph to illustrate the decay process for 5 half-life periods. (ii) How many grams of element X will remain after one million years have expired? Information Given: Half-life = 250,000 years Mass of “X” = 256 grams (Initial amount of radioactive element) Problem Type #3: Decay Graph (i) Construct a half-life decay graph to illustrate the decay process for 5 half-life periods. Problem Type #3: Decay Graph (i) Construct a half-life decay graph to illustrate the decay process for 5 half-life periods. Problem Type #3: Decay Graph (ii) How many grams of element X will remain after one million years have expired? You must first find out how many half-lives can pass in 1 million years. # Half-Lives # Half-Lives # Half-Lives = Total time Time 1 Half-Life = 1,000,000 yrs 250,000 yrs = 4 Answer: 16 grams will remain after 1 million years. Metamorphism resets the radioactive clock. Addition( hydrothermal fluids) or loss(leaching) of parent or daughter give false ages. Sedimentary rocks are formed from previously existing weathered and eroded rocks therefore gives different ages for different parts of the sample. Certain parent isotopes are only appropriate under certain Conditions, C-14 only on once living organisms. Some Isotopes have to long or short half-lives. What is a fossil? Fossils are the remains or traces of organisms found in sedimentary rocks. What conditions are necessary for fossils to form? 1) Rapid burial If the organism is quickly buried by sediment it is protected from being eaten by scavengers or is decomposed by bacteria. 2) Presence of hard body parts – fossils of organisms that contained hard parts are abundant in the fossil record, but only rare traces of soft tissue organisms are seen as fossils. 3) Low oxygen environment - in a low oxygen environment there is less bacteria and decomposition is slower. What is the importance of fossils to geologist? And what does a fossil indicate? 1) Fossils indicate the age of sedimentary rocks. Approximate age of the rock can be determined if we know when a life form existed on Earth. 2) Fossils indicate the environments in which rocks formed. Example, fossils of coral indicate a warm tropical environment. 3) Fossils are used to correlate (match up) rocks. What is the importance of fossils to geologist? And what does a fossil indicate? 4) Fossils provide the basis by which the subdivisions of the Geologic Timescale are divided. Division of the Geologic timescale is marked by some significant event in the evolution of Earth. Example, extinctions marked the end of two eras; the extinction of trilobites marked the end of the Paleozoic Era the extinction of dinosaurs marked the end of the Mesozoic Era. 5) Fossils can also indicate evolutionary pathways. Fossil evidence show the progression (evolution) of life forms with time. Fossils are preserved in the rock record in several ways; 1) 2) Petrification Carbonization 3) Mold and Cast 4) Preservation Ice, Mummification, and Amber 5) Traces Tracks, Burrows, and Coprolites. occurs when the small internal cavities and pores of the original structure are filled with precipitated mineral matter. occurs when cell walls and solid material are removed and replaced by mineral material carried by underground water. sometimes internal details and structures are retained. occurs when fine sediment encloses delicate matter such as leaves in a oxygen poor environment. As time passes, pressure squeezes out the liquid and gaseous components of the organism leaving behind a thin residue of carbon. often preserve a replica of a plant or animal in sedimentary rocks. an organism is buried in sediment and then dissolved by underground water leaving a hollow depression or an impression, called a mold. The mold shows only the original shape and surface markings of the organism; it does not reveal the internal structure. When minerals or sediment fills the hollow depression or impression it forms a cast. Original remains can be preserved in ice or in amber (tree sap). Both ice and amber protects the organism from decay (oxygen free environment) and from pressures that would crush the organisms. The entire animal has been preserved, even the soft parts which usually decay and disappear. Examples: (1) Woolly Mammoths preserved in ice in Alaska and Siberia. (2) Insects preserved in tree sap (amber). Cane in Jurassic Park. show traces left in the rock by an animal, such as; 1) Tracks - animal footprints made in soft sediment that latter formed solid sedimentary rock. 2) Burrows - animal trails made in soft sediment that latter formed solid sedimentary rock. 3) Coprolites - Fossil dung (feces) and stomach contents. Fossils of forms of life which existed during limited periods of geologic time and thus are used as guides to the age of the rocks in which they are preserved. Example: Paradoxides Trilobite were Cambrian life forms. Conditions for a good index fossil: 1. Fossils found over a wide area of the earth’s surface. 2. Species of organism must have been short lived geologically speaking. Precambrian Little direct evidence of fossils, due to lack of hard body parts. Fossil evidence include; algae, bacteria, and traces of soft body organisms. Paleozoic Era -- “Age of the Invertebrate”. Invertebrates evolved into vertebrates; First land plants evolved in the Silurian. Abundance of fishes in the Devonian which is known as the “age of the fishes”. Lung fish evolved into amphibians throughout the the Mississippian and Pennsylvanian. Amphibians evolved into reptiles in the Permian and reptiles are known as the first true land dwellers. Hard shelled eggs made this possible. Mass extinctions of invertebrates including trilobites and numerous other marine species occurred at the end of the Paleozoic Era. Mesozoic Era -- “Age of the Reptiles” Dinosaurs became dominant. First birds are seen during this time The end of the Mesozoic Era was marked by mass extinctions of reptiles including dinosaurs and numerous other species. Cenozoic Era -- “Age of the Mammals”. Mammals evolve and dominate during this time. Flowering plants are the dominant land plant. Some mammals became extinct during the late Cenozoic (11,000 years ago). These include the mastodon, mammoth, saber-tooth cat, large ground sloth, giant bison and others. Single celled-Invertebrates – Fish - First land plants – Amphibians – Reptiles – Birds – Flowering Plants -Mammals Summers I Fly Fish And Ride Bikes For Months Reference: Tarbuck & Lutgens Pages 10 & 237 Scientist and their contribution to the Geologic Time Scale. Nicolaus Steno Principle of Original Horizontality. Principle of Superposition. James Hutton and Charles Lyell Principle of Uniformitarianism William Smith Principle of Faunal Succession What do the divisions of the geologic time scale signify? Eons Eras Divisions of Geologic Time Eon, Largest span of time Era, Period, Epoch Smallest span of time MESOZOIC PROTEROZOIC P H A N E R O Z O I C C E N O Z O I C P A L E O Z O I C Names of the eons Phanerozoic (“visible life”)—The most recent eon, began about 540 million years ago Proterozoic Archean Hadean—The oldest eon Era—Subdivision of an eon Eras of the Phanerozoic eon Cenozoic (“recent life”) Mesozoic (“middle life”) Paleozoic (“ancient life”) Eras are subdivided into periods Periods are subdivided into epochs Precambrian time Nearly 4 billion years prior to the Cambrian period. Not divided into smaller time units because the events are not known in great detail, due to age, erosion and lack of fossil life forms. First abundant fossil evidence does not appear until the beginning of the Cambrian Mass extinctions are episodes in geologic history where mass amounts of organisms (species) are killed off. Two major periods of extinction is recognized in Earth’s history: 1) Permian – Triassic Boundary (End of Paleozoic) 2) Cretaceous – Tertiary Boundary (End of Mesozoic) Reference: Tarbuck and Lutgens Pages 298 & 304 The most widely accepted hypothesis for the extinction at the end of the Paleozoic Era, is the plate tectonic assembly of Pangaea and the loss of habitat. The most widely accepted hypothesis for the extinction at the end of the Mesozoic Era, is the impact of a great meteorite and the corresponding disruption of climate. Other possible explanations; 1) 2) 3) 4) falls in sea levels, climatic changes, prolonged volcanic eruptions, and, periods of lack of oxygen in oceans.