Lecture Outline Friday Feb. 21 Wednesday Feb. 28, 2018

Transcription

1 Lecture Outline Friday Feb. 21 Wednesday Feb. 28, 2018 Questions? Key Points for today What are the 5 relative age dating principles and how do you apply the principles to determine a geologic history for an area? Chapter 10 - Geologic Time Telling time geologically Earth s history is One of the Goal of geology is to unraveling Earth history Earth s geologic clock How do we tell time Absolute dating - determining event s Relative dating - putting rocks/events in Relative Dating: Trying to put rocks in correct order of age without really knowing how old they are. We do this by looking at the physical relationships between the rock units based on their stratigraphy. Stratigraphy- is a series of rock layers in a region that has similar physical properties, may contain the same fossils and can be mapped as a unit. Principles of Relative Dating 1) Principle of original horizontality: If rocks do not lie in horizontal beds, they have been disturbed by some kind of tectonic force. Exception: Crossbeds 2) Principle of superposition: In an undisturbed sequence of rocks, 3) Faunal Succession Fossils as timepieces Use of index fossils to correlate rocks (formations) from different locations Formation is a series of rock layers in a region that has similar physical properties, may contain the same fossils and can be mapped as a unit 4) Unconformities:. (deposition of rocks isn't continuous forever) 1. = an unconformity in which the rocks above and below the unconformity are parallel. 2. Angular Unconformity: (in your own words) 3. : sedimentary rocks in contact with crystalline igneous or metamorphic rocks. (dissimilar rock types in contact with each other)

2 sedimentary metamorphic igneous 5) Cross-Cutting Relations: Dike cross-cuts the pre-existing rock layers. Therefore, it is than the rock layers it cuts across. Fault cross-cuts all rock layers. Therefore, the fault is than the rock layers it cuts across. (Block diagram example showing how to handle the relative dating principles) Please review Figure 10.2 in your text to see how to apply the relative dating principles to construct the geologic history of an area. For additional help, review the Relative Age Dating help document posted on the lecture web page. Key Points for today How are isotopes used for absolute age dating? How do you set up a problem and do a simple calculation? How are fossils preserved and how are index fossils used? Geologic time 4.6 billion years in the making. What are a few of the important time breaks and why they are important? Absolute Dating Absolute dating provides ages in years. Radiometric Dating- uses to determine absolute age. The radioactive isotope, the parent isotope, evolves into a decay product, the daughter isotope, at a certain rate. Isotopes: Atoms with identical # of protons, but different # of neutrons. Radiometric Dating The ratio of parent : daughter determines the age of the rock. Half-life of a radioactive element is the time it takes for to decay into daughter product. Not all radioactive elements decay at the same rate, each isotope has a different rate.

3 There are several isotope systems that can be used date rocks and other material (do not memorize, used only for explanation purposes) Parent daughter half life P dating range minerals that can be dated Uranium238 Lead b.y. 10m.y.-46b.y. zircon and apatite Potassium40 Argon b.y. 50, b.y. muscoviete, biotite, hornblende Carbon14 Nitrogen yr ,000yr wood, charcoal, water containing dissolved CO2 What we are actually dating is a mineral that contain the isotope of interest. What kinds of rocks can be dated with radioactive methods? - In rocks, the clock begins when the molten rock cools dates (formation) o No parent or daughter atoms added or removed once started the radioactive decay o No resetting with metamorphism (or get time of metamorphism) - For rocks radiometric dating gives the age of all the grains which weathered to form the sediments in the sedimentary rocks - For metamorphic rocks radiometric dating gives the age of rather than the initial age of crystallization (formation) Example: You find a pluton with a radioactive element with a known half-life of 100,000 years. You collect samples and send them to WSU for analysis. Your results tell you that 25% of the parent isotope remains. How old is the intrusion? Hint: First determine the # of half lives passed. Number of passed half lives x known half life = age of sample. Answer: Parent Daughter Half-life half-lives x 100,000 years = years Example: You collect a sample of Granite and it contains biotite mineral crystals so it can be dated using K-40 to Ar-40 decay pair. If there are 1 K-40 atoms for every 3 Ar-40 atoms in the sample how old is the Granite. The half-life of K-40 is 1.3 billion years. Answer: Parent (Ar-40) Daughter (K-40) Half-life half-lives x 1.3 by = by Carbon 14: Where does it come from and how do plants and animals take it in? Reasons why we don't use C-14 in rocks: The half-life is only 5730 years, so you can't date anything older than about 70,000 years.

4 Carbon-14 can be used to determine: - Ages of recent lava flows - Ages of recent ash + pumice falls - Ages of recent landslides, lahars Example: You find a piece of charcoal in an ash bed from a volcanic eruption with 12.5% of the amount of C-14 as it had originally. Approximately how old is this piece of wood (and the volcanic eruption that contains it)? Answer: Parent (C-14) Daughter (N-14) Half-life half-lives x 5730 years = years Review Application of relative age dating principles Youngest Oldest Geologic Time Scale Can be though of as a relative-age calendar of the Earth s geologic history A key component to constructing the geologic timescale is the correlation of fossils between outcrops. How do we get fossils? Usually is required Examples: - Soft mud - Flood deposits - Volcanic ash preserve the best (hard parts) - soft body parts don t preserve well - hard parts usually undergoes. Petrified Wood: Plant cells have been replaced by quartz (SiO2) {many are buried in volcanic landslides or eruptions} Most bones have been replaced by minerals

5 Trace Fossils: Evidence that the organism was there, although the organism is gone. Casts + molds of footprints Burrows- The burrowing animals are gone, but their burrows remain. Index Fossil : - used to identify the relative age of a rock unit - used in stratigraphic correlation of rock units from one location to another What makes a good index fossil: - Unique and easy to identify - Lived for a (preferably only thousands to hundreds of thousands of years) - Geologic Time Scale Can be though of as a relative-age calendar of the Earth s geologic history 1. Time is divided into Eons, Eras, Periods, and Epochs 2. The time between each division is NOT the same. Geologic Time Scale was constructed by: 1. Determine the relative ages of sedimentary rocks by simple rule of superposition (relative age dating principles) and by local and global fossil record. 2. Can use deformation and angular unconformities to date tectonic episodes in relationship to the rock sequence. 3. Can use cross-cutting relationships to establish relative ages of igneous bodies or faults cutting through the sedimentary rock. The geologic time scale is a worldwide effort by geoscientists spanning over 150 years. In the last 50+ years, absolute age dating methods are used to refine the time scale. The time scale is still being refined and updated. For a quiz or exam you need to know the Eon and Era designation. Eons Phanerozoic - last 543 million years Eras Cenozoic - recent life 0-65 mya (mammals, humans) Mesozoic - age of middle life mya (dinosaurs, 1st small mammals) Paleozoic - ancient life mya (fish, trilobites, clams, corals, ferns ) Precambrian - from birth of Earth (4.6by) up to before complex life forms developed (greater than 543 m.y.) (algae, bacteria, some fossils without shells like jellyfish)

6 A very brief history of life on Earth The rest of the lecture period addresses the history of life on Earth. The bulk of the material presented is not in the text book and you are only responsible for the information that answers the following questions. Questions: Approximately, when was the earliest evidence indicating life existed on Earth? What form did early life take? Why is the time break between Proterozoic (Precambrian) and the Phanerazoic (Paleozoic Era) placed at 543mya? What changes are noted in the fossils that exist during this break? At the end of what Era were all landmasses united to form Pangea? What event marks the Paleozoic Mesozoic boundary? What happened? What Era did the dinosaurs flourish? What event marks the Mesozoic Cenozoic boundary? What happened? What was the name of the impact crater in the Yucatan Peninsula, Mexico? The presence of what element supports an impact hypothesis? The ancestors to modern man, Homo Sapiens Sapiens, first appeared approximately how many years ago?