Geologic Time MMSA Earth-Space Science -Time scale based upon the fossil record -The fossil record indicates a wide variety of life has existed on Earth. Most has become extinct. -Humans have been around for 0.2 millions of years. Law of Superposition: Oldest rock layers are at the bottom of a sequence of layers Geologic Principles Law of Original Horizontality: Rock layers were deposited in a horizontal orientation. If units are not horizontal, an event occurred subsequent to the deposition which caused the layers to fold or tilt Geologic Principles Law of Lateral Continuity: Assumes that deposits originally extended in all directions Geologic Principles Cross-Cutting Relationship: An intrusion or fault is younger than the rock it cuts across Geologic Principles Law of Superposition: Oldest rock layers are at the bottom of a sequence of layers Law of Original Horizontality: Rock layers were deposited in a horizontal orientation. If units are not horizontal, an event occurred subsequent to the deposition which caused the layers to fold or tilt Law of Lateral Continuity: Assumes that deposits originally extended in all directions Cross-Cutting Relationship: An intrusion or fault is younger than the rock it cuts across Unconformities: an erosional surface becomes buried by younger rocks. Results in a gap in the rock record. Inclusions: pieces of rock from the layer below. The inclusions are older than the rock they are in. Absolute Age Dating of Rocks -Relative age dating: Comparing one event with another over time -Absolute Age Dating: determining an objects actual age by means of radiometric dating, tree rings, etc Absolute Age Dating of Rocks Based on Radioactive Decay: the emission of radioactive particles resulting in the change of one element into another. Radioactive Parent element More stable Daughter element Half-Life Half-life: length of time it takes for one-half of the original amount of an element to decay. DO THIS NOW: Assume you have a 1600 gram radioactive sample in your hand. Every year, half of the radioactive material decays into a more stable element. Plot time on the x-axis, and radioactive mass on the y-axis. Your first coordinate is (0, 1600), your second coordinate is (1, 800). Complete this graph for up to 6 years. Connect your points. Exponential Decay! Its not linear!
