Fossils, Geologic Time, Absolute & Relative Dating, and Natural Resources. Chapters 5 & 6

Transcription

1 Fossils, Geologic Time, Absolute & Relative Dating, and Natural Resources Chapters 5 & 6

2 How Do Fossils Form? Fossils are found in sedimentary rocks like sandstone, limestone, and shale In Virginia, most fossils are marine in origin. Our state fossil, Chesapecten jeffersonius, is a large extinct species of scallop that dates to approximately 4.5 million years ago. It was the first fossil ever described in North America and is named after Thomas Jefferson, one of our founding fathers, and an amateur paleontologist. Special conditions need to exist for a fossil to form: 1. Must be protected from scavengers 2. Must be protected from bacteria or microorganisms that can speed up the decay process 3. Must have hard parts like teeth or bones 4. Be near water so they can be buried fast

3 Can You Name That Fossil? 1 4 Petrified Cast 2 5 Carbonaceous Mold Amber 3 6 Trace and/or Mold

4 Geologic Time Scale All events in Earth s geologic history are recorded on the geologic time scale Divisions of geologic time are eons, eras, periods, and epochs Fossils from each major era (Paleozoic, Mesozoic, Cenozoic) are found in the rocks of Virginia. The end of each era is typically marked by a major extinction event. We are currently in the Cenozoic Era, the Quaternary Period, and the Recent Epoch. By using relative and absolute dating methods, we have been able to roughly estimate the age of the Earth to be about 4.6 billion years.

5 Virginia Fossils Paleozoic fossils: Cambrian, Ordovician, Silurian, and Devonian sedimentary rocks crop out in the Valley and Ridge and Appalachian Plateau provinces and preserve abundant marine fossils, indicating the presence of extensive, shallow seas. By the Carboniferous (i.e., 350 million years ago), the western part of the state was covered in lush, dense forests of scale trees (lycophytes), horsetails, and ferns. The accumulation of organic material in these large coastal swamps eventually produced Virginia s coal seams. Mesozoic fossils: In the Piedmont province, fossils of dinosaur footprints, freshwater fish, and insects are found in rift basin deposits of the Triassic. In the eastern part of the state, where the sea had not yet retreated, fossil oysters and belemnites have been recovered from a few Cretaceous outcrops. Cenozoic fossils: Dramatic changes in sea level occurred throughout the Cenozoic. The rise and fall of sea level is recorded in the richly fossiliferous rocks of Virginia s Coastal Plain. Amazing numbers of fossil clams, snails, and sand dollars can be found in these marine rocks, along with fossilized whale bones and shark teeth. Mastodon and mammoth fossils, while not common, can be found in Quaternary sediments that were deposited along rivers and lakes.

6 Relative Dating Placing events in the order in which they occurred. The Law of Superposition states that the oldest rocks are on the bottom of a sequence. The Law of Horizontality states that all rock layers are originally deposited horizontally. Then, they may be disturbed by faulting, folding, tilting, erosion, or intrusions. The Law of Cross-Cutting states that the intrusion is always younger than the surrounding rock.

7 Absolute Dating Placing a numerical age on an event. Radioactive isotopes are used to determine the ages of rocks and fossils. Radioactive isotopes (the parent) decay, or change, at a regular rate into elements that are stable (the daughter). The rate of decay of the parent is called its half-life. This is the time it takes for half of the parent to change into the daughter. Common radioactive isotopes are Carbon-14, Uranium-238, Strontium-90, and Potassium-40 Number of half-lives elapsed Fraction remaining Percentage remaining 0 1 / / / / / / / / n 1 / 2 n 100/(2 n )

8 Natural Resources Renewable resources are those that when used up, are replenished in a reasonable amount of time Examples of renewable resources are sunlight, water, and wood Nonrenewable resources are the fossil fuels that, when used, take millions of years to be replenished Fossil fuels consist of coal, petroleum, and natural gas

9 Virginia Resources Coal for energy Gravel and crushed stone for construction Silica for electronics Zirconium and titanium for advanced metallurgy (The scientific study and technology of extracting metals from ores, refining them for use, and creating alloys and useful objects from them) Limestone for making concrete