2The geologic time scale is based upon rock and fossil evidence The geologic time scale is based upon rock and fossil evidence. It is broken into the following divisions from largest to smallest: eon, era, period, and epoch. An interactive timeline shows how long geologic time really is.
3Charles Darwin’s Theory of Evolution describes the process of change that produces new life forms over time.
4Evolution is driven by the theory of natural selection, the idea that the organisms that survive to produce offspring are those that have inherited the most favorable traits for surviving in a particular environment.
5These cactus plants show convergent evolution These cactus plants show convergent evolution. The one on the left is from Arizona, the one on the right is from Africa.
6A fossil is evidence of earlier life that has been preserved in rock.
7A fossil can be original remains (not very common), replaced remains, molds/casts, trace fossils, and carbonaceous films.
9Relative Dating is the process of placing geologic events in the process in which they occurred.
10The Grand Canyon is one of the best places for observing relative time.
11There are certain principles for dating rock layers.
12The principle of superposition states that the oldest rock layer will be at the bottom and the youngest at the top. Which picture shows the oldest rocks?
13The principle of original horizontality says that sedimentary rocks are first deposited horizontally.
14Rocks can be folded, tilted, or faulted after they are deposited horizontally.
15Rocks can also be intruded by magma which later cools to form rock.
16The Principle of Cross Cutting Relationships says that an igneous intrusion is always younger than the rock it has intruded or cut across.
17Number this diagram from oldest to youngest Number this diagram from oldest to youngest. 1 is always oldest (the first to form).
18Observe this animation to help you visualize geologic processes Observe this animation to help you visualize geologic processes. Notice the unconformity, a gap in the sedimentary rock record.
19An angular unconformity occurs when younger, flat strata are deposited on top of the older strata that have been tilted at an angle.
20A disconformity occurs when younger, flat strata are deposited on top of the older flat strata. The older flat strata is uplifted and eroded. The layers are then re-submerged under water and a second instance of deposition occurs on top of the unconformity.
21A nonconformity occurs when sedimentary layers are deposited on top of igneous or metamorphic rock.
22Correlation is the matching of rock layers from one area to another Correlation is the matching of rock layers from one area to another. Correlated strata will have the same age.
23Methods of Correlation Walking the outcropMatching Rock CharacteristicsUsing Index FossilsClimate indicating fossilsMatching Key BedsStratigraphic Matching
24Index Fossils are useful to date layers because each layer contains fossils unlike those in the layer above or below.
32Relative dating helps us to figure out the relative ages of rocks, but it does not help us to figure out the absolute age of the rocks. So, what do we do?
33We use Absolute Dating to put numbers on our dates.
34Trees can be used to record time Trees can be used to record time. We can use tree ring history (dendrochronology) to put an actual date on a historic occurrence and to gauge past climates.
35Varves, annual deposits of sediments, can be used for geologic dating purposes. Sediments deposited in glacial lakes vary with the seasons. Thick, light colored, sandy layers are deposited in spring and summer when runoff of water from a glacier is greater. Thin, dark colored, clay layers are deposited in winter. Counting annual glacial varves can help us to date items up to 15,000 years old.
36Absolute dating can also be based upon the concept of radioactivity of chemical isotopes. This is also known as radiometric dating.
37Radioactive decay is based upon the conversion from one isotope to another, but what is an isotope?
38Remember from chemistry that an isotope is any element with more neutrons than protons.
39Radioactive isotopes are unstable and want to achieve stability Radioactive isotopes are unstable and want to achieve stability. To do this, they give off radiation until they are stable. This usually involves changing from one element to another.
40The parent isotope is the original element and is unstable The parent isotope is the original element and is unstable. The daughter isotope is the product of decay and is stable.
41There are three types of radioactive decay Alpha Decay: Atomic # decreases by 2 and atomic mass decreases by 4 because 2 protons and 2 neutrons are expelled from the nucleus of the atom. Remember that the proton determines the atomic # and protons + neutrons determines the atomic mass.
42There are three types of radioactive decay Beta Decay: Atomic # increases by 1 and atomic mass does not change because an electron is expelled from a neutron, thereby turning the neutron into a proton. Remember that the proton determines the atomic # of an atom.
43There are three types of radioactive decay Electron Capture: Atomic # decreases by 1 and atomic mass does not change because an electron is added to a proton, causing a neutron to form. Remember that the proton determines the atomic # of an atom.
44Radioactive decay will continue until the resulting atom is no longer radioactive. In other words, a stable isotope is formed.
45Half life is the time it takes for half of the atoms of unstable parent isotope to decay to stable daughter isotope.
47In Radiometric Dating, scientists use radioactivity and half-lives of elements to measure absolute time.
48Scientists measure the amounts of parent and daughter isotope in a rock to find its age.
49Isotopes Used in Radiometric Dating. Parent IsotopeDecay SystemDaughter IsotopeHalf-Life (years)Effective Range (years)Possible Materials for datingCarbon-14BetaNitrogen-145730100-70,000Once-living matter (wood, charcoal, bone)Uranium-238Alpha and BetaLead-2064.5 Billion> 10 MillionUranium-bearing minerals (zircon)Rubidium-87Strontium-8747 BillionMicas, feldspars, metamorphic rocksPotassium-40Beta captureArgon-401.3 Billion> 50,000Micas, amphiboles, feldspars, volc. rocks
50Example 1: If a scientist finds 100% parent isotope in a rock, how old is the rock if the half life of the parent isotope is 2 million years? Look at the following graph, find where the line intersects 100%, then read the value for the number of half lives on the x axis.
54Example 2: If a scientist finds 25% parent isotope in a rock, how old is the rock if the half life of the parent isotope is 4 million years? Look at the following graph, find where the line intersects 25%, then read the value for the number of half lives on the x axis.