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Geologic Time & The Rock Record Chapter 8 Chapter 8 Introduction James Hutton (1726-1797). Proposed that most geologic processes happen very slowly. Geologists sort Earth’s history into a sequence of events. • Relative Age - Position in that sequence; older or • younger than nearby layers Absolute Age - Numerical age can be determined through analysis of the products of radioactive decay Reading The Layered Rocks Layered rocks contain clues about past environments at/near surface. Sequence and relative ages provide basis for reconstructing Earth’s history. The study of strata is called stratigraphy. Review of Sediment Layers Most sediment is laid down in the sea, in shallow waters, or in streams. Each new layer is laid down horizontally over older ones. Law of original horizontality - sediments are deposited in strata that are horizontal or nearly horizontal. Kinds of Unconformities Unconformity - break or gap in sequence. (3 kinds) 1. Angular unconformity- older strata deformed and cut off by erosion before the younger layers were deposited across them. 2. Disconformity - irregular surface of erosion between parallel strata; no tilting; hard to recognize, because the strata above and below are parallel. 3. Nonconformity - Strata overlie igneous or metamorphic rock. Label the Sections: A B C D Which is the… Angular Unconformity? Disconformity? Conformity? Nonconformity? Significance of Unconformities Evidence of former seafloors uplifted by tectonic forces and exposed to erosion. Later tectonic forces depress the surface. The surface, in turn, becomes a site of deposition of sediment. Stratigraphic Classification The basis of rock stratigraphy is the formation. • - a collection of similar strata that are sufficiently • different from adjacent groups - basis of physical properties they constitute a distinctive, recognizable unit that can be used for geologic mapping over a wide area. Geo Time and Correlation • • • Correlation – comparing different places to see if they are from the same geo time and processes Determining the relative ages of exposed areas Use standard geo time scale to establishing ages Index fossils are useful for this. If a distinctive index fossil is recognizable at an outcrop, a rapid and reliable means of correlation is available. Index fossil: short-lived but geographically widely distributed. The Geologic Column and the Geologic Time Scale 19th century – • geologists assembled a geologic column • composite column • containing, in chronological order, the succession of known strata, fitted together based of fossils or other evidence of relative age. The corresponding column of time is the geologic time scale. Radioactivity In 1896, the discovery of radioactivity provided the needed method to measure the age of the Earth accurately. Different kinds of atoms of an element that contain different numbers of neutrons are called isotopes. • Most Isotopes are stable and don’t change. Radioactive Isotopes A few isotopes, such as 14C, are radioactive. • instability within the • nucleus. transform spontaneously to a nucleus of a more stable isotope of a different chemical element. The process is called radioactive decay. Half-Lives of Isotopes Radioactive decay varies among isotopes. Radioactive decay is measured in half-lifes • amount of time needed for the number of parent atoms to be reduced by one half. Radiocarbon Dating 14C is useful for dating young samples. Half-life of 14C is short - 5730 yrs 14C continuously created in the atmosphere through bombardment of 14C by neutrons created by cosmic radiation. So, How Old Is The Earth? “Moon dust” brought back by astronauts, is 4.55 billion years old. The Earth was formed approximately 4.55 billion years ago. How Fossils Are Formed Millions found – but not many Most living things are quickly recycled upon death. Scavengers and bacteria usually consume all but bones and shells. Freezing (refrigeration) Best means of preservation of ancient materials. Rare - continually frozen from death til discovery. Mammoths and wooly rhinoceros found in ice from Alaska and Siberia. • • Specimens with flesh, skin, and hair intact have been found. suggest that they were flash frozen, with food still in the mouth and stomach. Drying (desiccation) Mummified bodies discovered in arid parts Soft tissues preserved if completely dried. Asphalt La Brea Tar Pits - 100 pits filled with sticky asphalt or tar. formed by crude oil seeping through fissures in the earth. lighter elements evaporated leaving thick sticky asphalt. pits are famous for Pleistocene fossils The fossils date between 10 and 40 thousand years old. Asphalt is an excellent preservative. Amber (Unaltered preservation) Insects, spiders, and even small lizards have been found, nearly perfectly preserved in amber. Over millions of years, sap with our fly inside is polymerized and hardened into amber. Carbonization (distillation) Plant leaves, and some soft body parts of fish, reptiles, and marine invertebrates decompose leaving behind only the carbon. This carbon creates an impression in the rock outlining the fossil, sometimes with great detail. Casts & Molds molds and casts of organisms which have dissolved or rotted away, leaving only a trace of their existence. Casts and molds are types of fossils where the physical characteristics of organisms have been impressed onto rocks. buried or trapped in mud, clay, or other materials which hardened around them - leaving molds of the organism. There are two types of molds: external and internal. Permineralization (Petrification) most common Minerals fill the cellular spaces and crystallize. Shape of the original plant or animal is preserved as rock. Sometimes the original material is dissolved away leaving the form and structure but none of the organic material remains.