The Rock Record Chapter 8 The earth is approximately 4.6 billion years old.

James Hutton and Uniformitarianism 18 th 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 - lifeEffective dating range Carbon -14 Radiocarbon dating Nitrogen years> 70,000 years Potassium -40Argon billion years50, billion years. Rubidium - 87Strontium billion years10 million to 4.6 billion years Uranium - 235Lead – million years10 million to 4.6 billion years Uranium - 238Lead billion years10 million to 4.6 billion years Thorium - 232Lead billion years> 200 million years

Alpha Decay Mass Mass 4 unstable element  stable element + He Atomic # Atomic # 2 Example: Cs  + He 55 2

Beta Decay Mass Mass 0 unstable element  stable element + e- Atomic # Atomic # -1 Example: Cs  + e

Radioactive decay of Uranium-238

Calculating Half-Life T ½Time (years) Unstable amt. (mg) Stable amt (mg) Unstable amt (%) Stable Amt. (%) Unstable amt. (fractions) Stable Amt. (fractions)

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… million years ago

Grey Fossil Site, Tennessee