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The Rock Record Chapter 8 The earth is approximately 4.6 billion years old. James Hutton and Uniformitarianism • 18th century Scottish physician and farmer farmer • Believed that studying the present is the key to understanding the past. • Principle of Uniformitarianism – the processes that shape the Earth today are the same as the processes that shaped the Earth in the past. – Processes such as volcanism, erosion, etc. – Hutton’s theory suggested that the Earth was much older than 6000 years because these processes happen very slowly. Geologic processes slowly change the Earth surfaces. Relative Age • Rock layers (or strata) show the sequence of events that took place in the past. • Scientists can determine the order in which the rock layers formed. • Once they know the order, they can determine a Relative Age for each layer. • Relative Age indicates that one layer is older or younger than another layer. – Relative Age does NOT indicate the age of the rock layer in years. – Although many rocks form layers, sedimentary rock layers are usually used to determine Relative Age. Law of Superposition • Sedimentary rocks form when new sediments are deposited on top of old layers of sediments. • As the sediments accumulate, they are compressed and harden into sedimentary rock layers called beds. • The boundary between two beds is called a bedding plane. • The Law of Superposition states that an undeformed (unchanged) sedimentary rock layer is older than the layers above it and younger than the layers below it. – Generally, the deepest layer is the oldest and the top layer is the most recent. – The deepest layer was laid down first and the top layer last. Principle of Original Horizontality • Sedimentary rocks generally form in horizontal layers. • The Principle of Original Horizontality states that if sedimentary rock layers are left undisturbed they will remain in horizontal layers. • Scientists assume that sedimentary rock layers that are not horizontal have been tilted or deformed by crustal movements that happen after the layers were formed. Horizontal strata Deformed strata (undeformed/unchanged) (changed) Unconformity • A break in the geologic record (missing strata) • Movements of Earth’s crust can lift up rock layers that were buried and erode them; then new layers are deposited on top. • The eroded rock forms a gap in the geologic record = Unconformity • All rocks beneath an Unconformity are older than the rock above the Unconformity. • Three types of unconformities The development of a Unconformity Nonconformity – The boundary between stratified sedimentary rock and unstratified igneous rock Angular unconformity – The boundary between a set of tilted sedimentary rock layers and a set of horizontal layers Disconformity – The boundary that forms between older eroded rock layers and new rock layers deposited on top Law of Crosscutting relationships • Rock layers may be disturbed by faults or intrusions that cut across the rock layers. • A fault is a break or crack in Earth’s crust along which rocks shift their position. • An intrusion is a mass of igneous rock that forms when magma is injected into rock and then cools and solidifies (vertical intrusions are called dikes and horizontal intrusions are called sills). • The Law of Crosscutting Relationships states that a fault or igneous intrusion is always younger than all the rocks layers it cuts through. • If the fault or intrusion cuts through an Unconformity, the fault or intrusion is younger than all the rocks it cuts through above or below the Unconformity. Determining Absolute Age • Determines the numeric age of rock layers. • Absolute dating methods include – Rates of erosion • By studying how long it takes for rock to erode, scientists can estimate the age. • Can be accurate for geologic features that are between 10,000 and 20,000 years old. • Not accurate for older features because the rates of erosion can change over time. – Rates of deposition • Studying how long it takes for sediments to deposit Radiometric Dating • Rocks generally contain small amounts of radioactive materials (isotopes). • Radioactive isotopes are unstable • Radioactive isotopes break down (decay) at a unique and consistent rate. – The Half-Life is the time it takes for ½ the mass of given amount of radioactive material to decay into its daughter isotope. • These decay rates can be used to measure absolute ages of rocks (radiometric dating). • Scientists measure the concentrations of the original radioactive isotope (parent isotope) to the newly formed more stable isotopes (daughter isotopes). Radioactive Isotopes & Half-Lives Parent Isotope Daughter Isotope Half - life Effective dating range Carbon -14 Radiocarbon dating Nitrogen -14 5.730 years > 70,000 years Potassium -40 Argon -40 1.25 billion years 50,000-4.6 billion years. Rubidium - 87 Strontium- 87 48.8 billion years 10 million to 4.6 billion years Uranium - 235 Lead – 207 704 million years 10 million to 4.6 billion years Uranium - 238 Lead- 206 4.5 billion years 10 million to 4.6 billion years Thorium - 232 Lead - 208 14 billion years > 200 million years Alpha Decay Mass unstable element Atomic # Mass stable element Atomic # Example: 136 Cs 55 4 + He 2 4 + He 2 Beta Decay Mass unstable element Atomic # Mass stable element Atomic # Example: 136 Cs 55 0 + e-1 0 + e-1 Radioactive decay of Uranium-238 Calculating Half-Life T½ Time (years) Unstable amt. (mg) Stable amt (mg) Unstable amt (%) Stable Amt. (%) Unstable amt. (fractions) Stable Amt. (fractions) 0 0 4000 0 100 0 1 0 1 2 3 4 5 6 The Fossil Record • The remains of plants and animals (organisms) that lived in the past. • Fossils are found in sedimentary rock • Paleontologists study fossils • Fossils can be used for: – Relative and Absolute Dating – Understanding past geologic events – Provides information about the geologic history of Earth – Recreating past climates – Provides clues to environmental changes that have occurred in Earth’s past. – Tracking the evolution of organisms How Fossils can form Fossilization • Mummification – organisms dried out • Amber – hardened tree sap • Tar seeps- petroleum surface deposits trapped organisms • Freezing – frozen soil and ice preserved organisms • Petrification – mineral solutions in ground water replaced organic materials. (silica, calcite, pyrite) • Imprints – carbonized impressions of leaves, stems, flowers, fish made in soft mud or clay. • Molds and Casts – shells often leave a cavity which fills with sediment (mold) and a replica of the original organism (cast). • Coprolites – fossilized dung • Gastroliths – digestive stones from dinosaurs • Trace Fossils – evidence of past animal movements (footprints, tracks, borings, burrows). Index Fossils • Fossils can be used to determine the Relative Ages of rock layers that contain the fossils. • Index Fossils can be used to estimate Absolute Ages of specific rock layers that contain the fossils. • Index Fossils are fossils that only occur in rock layers of a particular age. – – – – – Index fossils must be present over a large area Index fossils must be unique Index fossils must have lived for short periods of time Index fossils must occur in large numbers Example: ammonites…180-206 million years ago Grey Fossil Site, Tennessee