SCALE OF GEOLOGIC TIME f 0301pg 29 Geologic

SCALE OF GEOLOGIC TIME

f 03_01_pg 29

Geologic Time Scale • International Commission on Stratigraphy (ICS) is the official organization that formally defines the Geologic Time Units. • On the Geologic Time Scale, boundaries between time units are placed where the Fossil Record reveals important changes in plants and animals. • http: //palaeos. com – Good Reference

Organization of Geologic Time

t 03_01_pg 30

Eons • Phanerozoic – Modern Life The following 3 eons were originally classified as eras but have recently been promoted to eons: • Proterozoic – Ancient Life • Archean – First Life Form • Hadean – No Life

Eras • Eras are geological time divisions based on the resemblance of living things to modern living things (Paleozoic Era = Old Life; Mesozoic Era = Middle Life, and so forth).

paleozoic

mesozoic

Cenozoic

Periods • Periods are subdivisions of Eras, based on many things, such as appearance of certain life forms, mountain building or orogenic episodes, volcanism, and so on, and are shorter than Eras.

Epochs • Subdivisions of Periods are called Epochs, and are based on similar but "smaller" episodes of mountain building or changes in forms of life present.

Development of Time Scale

Development of Time Scale • The Geologic Time Scale was developed through the independent work of several scientists using relative dating methods. • These scientists cross-correlated geologic time units based on fossil evidence. • This Time Scale continues to evolve as more information is collected.

Principles of Relative Dating f_pg 26

Absolute Dating “Radioactive Dating”

Early Attempts to Determine Earths Age • • • Biblical Calculation - 6, 000 Evolution of Fossils – 80 Million Sediment Deposition Rate: 1 Million to 1 Billion Ocean Salinity – 90 Million Cooling Rate – 24 to 40 Million Radioactive – 4. 6 Billion

Radoactive Dating – The Alpha, Beta, and Gamma of Decay – Radioactivity Lets Us Date Ancient Rocks with Confidence

t 03_03_pg 39

f 03_10_pg 38

The Principal Radioactive Timekeepers – – – Uranium-Lead Methods The Potassium-Argon Method The Rubidium-Strontium Method Carbon-14 Fission Track Dating Method

Rocks Used in Radioactive Dating

Why Igneous Rocks Are Best 1. Igneous rocks crystallize from magma, locking in the atoms of radioactive elements. 2. Igneous rocks minimize the loss of daughter atoms that are a result of the break-down of parent elements

Problems With Sedimentary Rocks • Mineral Grains become weathered affecting the concentration of daughter elements. Problems With Metamorphic Rocks • Changes in heat and pressure affects the concentration of daughter elements.

Carbon 14 Dating

Carbon-14 Dating • Compares the original C-12 concentration to the C-14 Concentration. Fission Track Dating Method • Analyzes the number of tracks that are produced from the break-down of uraniumbearing minerals.

THE END