Geologic History Earth is very, very old
Earth s History The history of Earth and the ages of rocks can be investigated and understood by studying rocks and fossils. Evidence of ancient, often extinct life is preserved in many sedimentary rocks.
Fossils Fossils remains, imprints, or traces of prehistoric organisms 1. Fossils can form if the organism is quickly buried by sediments. 2. Organisms with hard parts are more likely to become fossils than organisms with soft parts.
Fossil evidence indicates that life forms have changed and become more complex over geologic time. Fossils
Relative Ages of Rocks Principle of superposition process of reading undisturbed rock layers 1. oldest rocks in the bottom layer 2. younger rocks in the top layers
Relative Ages of Rocks How old something is in comparison with something else is its relative age. 1. The age of undisturbed rocks can be determined by examining layer sequences. 2. The age of disturbed rocks may have to be determined by fossils or other clues
Relative Ages of Rocks The same rock layers can be found in different locations; fossils can be used to correlate those rock layers.
Absolute Ages of Rocks Absolute age age, in years, of a rock or other object; determined by properties of atoms
Absolute Ages of Rocks Unstable isotopes break down into other isotopes and particles in the process of radioactive decay.
Absolute Ages of Rocks Calculating the absolute age of a rock using the ratio of parent isotope to daughter product and the half-life of the parent is called radiometric dating. 1. Potassium-argon dating is used to date ancient rocks millions of years old. 2. Carbon-14 dating is used to date bones, wood, and charcoal up to 75,000 years old. 3. Earth is estimated to be about 4.6 billion years old; the oldest known rocks are about 3.96 billion years old.
Absolute Ages of Rocks Uniformitarianism Earth processes occurring today are similar to those which occurred in the past
The Geologic Time Scale Geologic time scale is a record of Earth s history from 4.6 billion years ago to the present Based upon rock and fossil records.
Eons An eon is the largest unit of time and is measured in billions of years. There are four Eons in Earth s History.
Eons 90% OF Earth s history falls in the Archean and Proterozoic Eons. This is referred to as Precambrian Time.
Eras An era is the next longest span of time. It is measured in hundreds of millions of years. Eras are defined by differences in life forms found in rocks.
Periods Periods are defined by the life forms that were abundant or became extinct at the time in which specific rocks were deposited. They are measured in tens of millions of years.
Epochs The fossil record of the Cenozoic Era is complete enough to allow further more precise divisions. This is the smallest unit of geological time. Not all periods have epochs.
In Virginia Fossils are found mainly in the Coastal Plain, Valley and Ridge, and Appalachian Plateau provinces. Most fossils in Virginia are of marine organisms, indicating that at one time Virginia may have been periodically covered by sea water. Paleozoic, Mesozoic, and Cenozoic fossils are found in Virginia.
In Virginia Virginia has 5 physiographic provinces with unique physical characteristics resulting from geological past.
Coastal Plain Flat area composed of young sediments over older crystalline base rocks. Layers of sediment produced by erosion of the Appalachian Mountains and Piedmont. Deposited when sea levels were higher.
Piedmont Area of rolling hills above ancient igneous rock and metamorphic rock. Igneous rocks are the roots of past volcanoes formed from an ancient episode of subduction prior to the formation of the Appalachian Mountains.
Blue Ridge High ridge separating the Piedmont from the Valley and Ridge. The billion year old igneous and metamorphic rocks of the Blue Ridge are the oldest in the state.
Valley and Ridge An area with long parallel ridges and valleys above ancient folded sedimentary rocks. Folding occurred during a collision between Africa and North America during the Paleozoic Era. This collision produced the Appalachian Mountains.
Appalachian Plateau Has a rugged, irregular topography above ancient, flat-lying sedimentary rock. This is where most of Virginia s coal resources come from.
Vocabulary Write down these vocabulary terms and define from notes of your text book. Quiz next class!!! Law of Superposition Relative Dating Absolute Dating Half-life Radiometric Dating Fossils Uniformitarianism Eons Eras Period Quarternary Period Epoch Precambrian Coastal Plain Piedmont Blue Ridge Valley and Ridge Appalachian Plateau