Formation of fossils and Geological time Scale

Formation of fossils Specific environmental conditions are necessary in order for fossils to form.

Fossils can form in several ways. Permineralization occurs when minerals carried by water are deposited around a hard structure.

A natural cast forms when flowing water removes all of the original tissue, leaving an impression.

Trace fossils record the activity of an organism.

Amber-preserved fossils are organisms that become trapped in tree resin that hardens after the tree is buried.

Preserved remains form when an entire organism becomes encased in material such as ice.

Specific conditions are needed for fossilization. Only a tiny percentage of living things became fossils.

Radiometric dating provides an accurate way to estimate the age of fossils. Relative dating estimates the time during which an organism lived. –It compares the placement of fossils in layers of rock. –Scientists infer the order in which species existed.

Radiometric dating uses decay of unstable isotopes. –Isotopes are atoms of an element that differ in their number of neutrons. neutrons protrons

–A half-life is the amount of time it takes for half of the isotope to decay. Radiometric dating uses decay of unstable isotopes. –Isotopes are atoms of an element that differ in their number of neutrons.

Five Radioactive Isotope Pairs Half-Life EffectiveMinerals and Isotopes of Parent Dating Range Rocks That Can ParentDaughter (Years) Be Dated Uranium 238Lead billion10 million toZircon 4.6 billionUraninite Uranium 235Lead million Thorium 232Lead billion48.8 billion Rubidium 87Strontium billion10 million to Muscovite Biotite Potassium feldspar Whole metamorphic or igneous rock Potassium 40Argon billion100,000 toGlauconite 4.6 billionMuscovite Biotite Hornblende Whole volcanic rock (Years) 4.6 billion

Radiocarbon and Tree- Ring Dating Methods Carbon-14 dating is based on the ratio of C-14 to C-12 sample. > Valid only for samples less than 70,000 years old. > Living things take in both isotopes of carbon. > When the organism dies, the "clock" starts. Carbon-14 dating is based on the ratio of C-14 to C-12 in an organic sample. > Valid only for samples less than 70,000 years old. > Living things take in both isotopes of carbon. > When the organism dies, the "clock" starts. Method can be validated by cross-checking with tree rings

Carbon 14 Cycle

The geologic time scale divides Earths history based on major past events.

Index fossils are another tool to determine the age of rock layers. Index fossils can provide the relative age of a rock layer. –existed only during specific spans of time –occurred in large geographic areas Index fossils include trilobites.

The geologic time scale organizes Earths history. The history of Earth is represented in the geologic time scale PRECAMBRIAN TIME Cyanobacteria This time span makes up the vast majority of Earths history. It includes the oldest known rocks and fossils, the origin of eukaryotes, and the oldest animal fossils.

Eras last tens to hundreds of millions of years. –consist of two or more periods –three eras: Cenozoic, Mesozoic, Paleozoic

Periods last tens of millions of years. –most commonly used units of time on time scale –associated with rock systems. Epochs last several million years.

The Geologic Time Scale Origin of the Earth 4.55 Billion years First multi- celled organisms Age of Fishes Age of Coal Formation Major Mass Extinction Age of Dinosaurs Major Mass Extinction Humans arrive Most recent Ice Age