Transcript Chapter 12
Chapter 12 Geologic Time TIME Relative Time • Places events in sequence but does not give their actual date of an event Absolute Time • Identifies the actual date of an event Relative Time Law of Superposition In a sequence of undisturbed sedimentary rocks, the oldest rocks will be at the bottom of the sequence and the youngest will be at the top Law of Cross Cutting An igneous rock is younger than the rocks it has intruded, or cut across 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 Unconformity A place in the rock record where layers of rock are missing. Layers could have been deposited and then removed Geologic Timetable • A summary of the major events of Earth’s history preserved in the rock record. Fossils are an important part of that history. Eras, Periods, and Epochs • Era is the longest segment of geologic time • Periods divide Eras • Epochs divide Periods Different Eras in Earth History • • • • • Archean Proterozoic Paleozoic Mesozoic Cenozoic Archean Era • The oldest era • Began when Earth formed 4-5 billion yrs. Ago Proterozoic Era • Began 2.5 billion yrs. Ago • Rocks contain fossils of simple plants and worms that lived in the oceans • No evidence of life on land in Proterozic Era Paleozoic Era • Began 570 million yrs. ago • More abundant fossil record • Rocks contain fossils of both land and ocean plants and animals Mesozoic Era • Began 250 million yrs. Ago • Time of Dinosaurs Cenozoic Era • Youngest of the Eras (we are currently in it) • Began 65 million yrs. Ago • Includes events like the Ice Age and the presence of Humans Fossils • Any evidence of earlier life preserved in a rock. • Can be pones, shells, impressions of leaves Types of Fossils Original remains • Actual unchanged remains of the plant or animal are preserved. Ex. Whole woolly mammoths that have been found frozen Types of Fossils AMBER • Hardened resin • Example animals getting trapped in sap from trees (think Jurassic Park) Types of Fossils Replaced Remains • The soft parts of the original animal hae disappeared and the hard parts have been replaced by mineral material • EX: Petrified Wood Types of Fossils Mold • Hollow depression in the rock • Shows the original form of the object Cast • When new materials fill in the mold Trace Fossils • Are evidence of life other than the remains of a plant or animal • EX: footprints, burrows Index or Guide Fossils • Three Characteristics of an Index Fossil 1. 2. 3. – Easily recognizable Widespread Limited in the time that they existed Used to identify age of rocks Rock Correlation • The matching of rock layers from one area to another • EX: volcanic eruptions Measuring Absolute Time • Tree Rings • Varves • Radioactive Decay Tree Rings • Counting rings can provide time in years (each ring is one year) • Can also be used to tell what weather conditions were like Varves • Any sediment that shows a yearly cycle Radioactive Decay • Radioactive decay is when one atom changes into a new lighter element • Half Life – the rate at which a radio active element decays