Chapter 8 The Rock Record

Section 1 Earth’s Age Earth is thought to be 4.6 billion years old. James Hutton: 18th century Considered the Father of Geology Hutton’s principle of uniformitarianism is that current geologic processes are the same processed that occurred in the past

Ways of Dating the Earth Relative Age Strata: Layers of rock. Relative age indicates that one layer of rock is older or younger than another layer. Does not give an absolute age in years.

Law of Superposition Sedimentary rocks form when new sediments are deposited on top. They are compressed and harden into rock layers. Law of Superposition: In an un-deformed sedimentary rock layer, the layer is older than the layers above and younger than the ones below.

Principle of Original Horizontality Sedimentary rocks left undisturbed will remain in horizontal layers. We can assume that sed. Rock layers are that are not horizontal have been tilted or deformed by geologic forces

Other clues to the original positon Graded Bedding: Size of particles; will usually deposit large particles on bottom of deposit. Cross-Beds: Dunes are deposited with layers at an angle forming cross- beds when preserved. Ripple Marks: small waves the form because of the action of water or wind.

Unconformities A break in the geologic record. Shows that deposition stopped, rock may have been eroded away, and then deposition resumed.

Crosscutting Relationships When rock layers have been disturbed by faults or intrusions. Law of Cross Cutting Relationships: A fault or igneous intrusion is always younger than the rock layers it cuts through.

Put the order of events in order from oldest to youngest Put the order of events in order from oldest to youngest. Hint, E was deposited first…..

Determining Absolute Age Absolute Age: A numeric age Ways of determining: Rates of Erosion: Only accurate for geologic features that formed within the past 10,000 to 20,000 years. For older features, like the Grand Canyon that formed over millions of years, the method is less dependable since rates of erosion vary over time.

Determining Absolute Age Rates of Deposition: Also varies over time, so can only be used for estimates. Varve Count Varve: Like rings in a tree, varves are alternating layers of light colored band of coarse particles and dark band of fine particles. Usually deposited in glacial lakes where snow melt runoff form the course layers and in the winter, the clays settle out.

Determining Absolute Age Examples of Varves

Determining Absolute Age Radiometric Dating Using the radioactive isotopes naturally in rocks. Radioactive isotopes have nuclei that emit particles and energy at a constant rate regardless of surrounding conditions. As an atom emits particles, the atom changes into a different isotope of the same element.

Determining Absolute Age Radiometric Dating Using the radioactive isotopes naturally in rocks. Be measuring the concentration of the “parent isotope” and the newly formed “daughter isotope” and the known decay rate, the half life, the absolute age can be calculated. Half-life: The time it takes half of the mass of a given amount of radioactive isotope to decay into its daughter isotopes. The older the rock, the less parent is present and the more daughter is present.

Determining Absolute Age Radiometric Dating The key is to pick the isotope whose half life is closest to the estimated age of the sample. Carbon Dating If the sample is less than 70,000 years, it can be dated using Carbon-14.

Using the Fossil Record Fossils: The remains of animals or plants that lived in a previous geologic time. Paleontology: The study of fossils. By revealing the ways organisms changed through time, fossils provide clues to the environmental changes.

Fossilization Normally, dead plants and animals are scavenged by other animals or undergo decay, leaving no remains behind. Only remains buried quickly or protected from decay can become fossils. Usually only the hard parts (wood, bones, shells, teeth) are preserved.

Types of Fossilization Mummification: Very dry places where bacteria can’t survive. Amber: Hardened tree sap (amber). Insects become trapped. (Remember, Jurassic Park)

Types of Fossilization Tar Seeps: Thick petroleum oozes to the surface and are covered by water. Animals come to drink and become trapped. Tar covers and preserves.

Types of Fossilization Freezing: Low temperatures of frozen soil and ice protect and preserve organisms. (Woolly Mammoths) Petrification: Mineral solutions from groundwater replace the original organic materials. Commonly silica, calcite, and pyrite. (Petrified Wood)

Types of Fossilization Imprints: Carbonized imprints of leaves, stems, flowers, and fish made in soft mud or clay. When organism decays, carbon film left behind.

Types of Fossilization Molds and Cast: Shells often leave empty cavities (molds) within hardened sediments. Sand or mud fill in hold and hardens (cast), forming a replica.

Types of Fossilization Coprolites: Fossilized dung or waste materials from ancient animals. Have been able to examine and determine eating habits of dinosaurs. Gastroliths: Some dinosaurs has stones in their digestive tracts to grind food. (Much like modern birds.)

Types of Fossils Trace Fossils: Fossilized evidence of past organisms’ movements. (tracks, footprints, borings, and borrows)

Index Fossils Index Fossils: Fossils that occur only in rock layers of a particular geologic age. Must be present in rocks scattered over a large region. Must have features that clearly distinguish it from other fossils. The organisms must have been lived during a short span of time. Fossil must occur ion fairly large numbers.

Index Fossils and Absolute Age Because organisms that formed index fossils lived during short spans of time, the rock layer in which an index fossil was discovered can be dated. Example: Ammonites lived between 180 million and 206 million years ago. Rocks with ammonites would have been formed in this time.