HISTORICAL GEOLOGY Relative & Absolute age, fossils and geologic time
Historical Geology A. Measuring Time 1. Relative Time (Relative Age) Measurement of time using comparison. No numbers used; uses words like younger and older. a. Example Joe is older than Mike 2. Absolute Time (Absolute Age) Measurement of time using actual numbers a. Example Mike is 15 years old
B. Measuring Earth s Age (Relative Time) 1. Many Paleontologists use rock layers a. Fossils are mainly found in sedimentary rock. b. The layers help tell the relative age of rocks c. The LAW OF SUPERPOSITION states that oldest layers of rock are on the bottom and younger layers are on the top.
Unconformity eroded rock surface pushed up from deeper within earth that is much older than the new rock layers above it. Glacial Till large buildup of rock pieces carried away in the bottom of a glacier Kettle Lake lake formed when a large block of glacial ice melts leaving a depression in the ground. Drumlins long, smooth, canoe shaped hills made of glacial till Peat substance made of decayed plant fibers, softest type of coal
2. Faults can be used to help determine the relative age of a rock layers and the order in which events happened. a. A rock layer had to be laid down before a fault can occur. b. Younger features cut across older features this is called CROSSCUTTING RELATIONSHIPS 3. A fault is a break or crack in part of Earth s crust. 4. UNCONFORMITY a break/gap in the rock record that separates series or layers of deposits. a. Layers that are exposed above sea level are often times eroded away with brand new layers being deposited above the erosion mark.
C. Determining Absolute Age 1. Radioactive Dating a. The breaking down of atoms to form different isotopes of the same element or a completely new element Carbon-14 Carbon-12 b. Energy is given off in the form of - Gamma Rays - Alpha Particles - Beta Particles
2. Half-Life a. The amount of time it takes for half of the original atom to decay 1. Uranium-238 = 4.5 billion years 2. Carbon-14 = 5,700 years 3. Oxygen = 19 seconds b. In the amount of time given (half-life time), half of the original substance will be gone.
c. Example: If we start with 20 grams of carbon- 14, how long will it take to break down to 5 grams Start of 0 years = 20 grams 5700 years = 10 grams 11, 400 years = 5 grams As you move down the left hand side of these equations, keep ADDING THE HALF-LIFE AMOUNT. As you move down the right hand side of these equations, divide by 2 (take half!)
Fossils Historical Geology continued A. Description - Any remains or evidence of a living thing B. Types 1. Shells 2. Bones 3. Pollen/Grains 4. Seeds 5. Imprints 6. Molds & Casts
C. Formation of Fossils 1. Preservation without change a. Frozen plants or animals/objects stuck in amber (pine tree sap/resin) or tar 2. Replacement by a mineral a. Material decays and is slowly replaced by a silicate (petrified wood)
3. Formation of a carbon film a. As material decays a carbon film is left behind 1. Used only with living things 2. Used with half-life 4. Formation of an imprint mold 5. Filling of an imprint cast D. Index Fossils special marker used by geologists to identify layers of rock that are separated far apart.
Geologic Time A. Divisions 1. Eras largest division of geologic time a. Pre-Cambrian *** can be broken down into eons*** b. Paleozoic c. Mesozoic d. Cenozoic
Geologic Time 2. Periods subdivision of an era a. Paleozoic Era age of the invertebrates 1. Cambrian Period 2. Ordovician Period 3. Silurian Period 4. Devonian Period 5. Carboniferous Period broken down into Pennsylvanian & Mississippian in North American only 6. Permian Period - ** The trilobite died out at the end of the Permian Period, marking the end of the Paleozoic Era
Geologic Time b. Mesozoic Era age of the reptiles/dinosaurs a. Triassic Period b. Jurassic Period c. Cretaceous Period ** The dinosaurs died out at the end of the Cretaceous Period, marking the end of the Mesozoic Era
Geologic Time c. Cenozoic Era age of the mammals 1. Tertiary Period 2. Quaternary Period ** Epoch division of geologic time that occurs only in the Cenozoic Era; breaks down periods into smaller chunks of time ***CENOZOIC ERA, QUATERNARY PERIOD ARE STILL GOING ON NOW!!!!