CHAPTER THREE Time and Geology

Agenda Scale of Geologic Time Organization of Geologic Time Development of Time Scale Principles of Relative Dating Origin of the Phanerozoic Eon Periods Absolute Dating –Early Attempts –Radiometric Dating –Fission Tracking Application of Absolute Dating

SCALE OF GEOLOGIC TIME

f03_01_pg29

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

The Geologic Time Scale is generally divided on the basis of the earth's biotic composition, with the Phanerozoic Eon representing the period of Earth's history with advanced life forms, and the Pre Cambrian (Proterozoic, Archean and Hadean Eons) representing the period before advanced life.

Organization of Geologic Time

t03_01_pg30

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 Hadian – No Life

Eons Originally the Precambrian was divided into a more recent Proterozoic ("age of first life") a preceding Archeozoic ("first life") and an even earlier Azoic (lifeless) era. Currently, Archeozoic and Azoic have been replaced by Archean ("first", "primary") and Hadean the latter term referring to the hellish conditions of the very early Earth.

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).

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

f03_03_pg31 Famous Geologists Adam Sedgwick( ) –Protégé: Sir Roderick Murchison

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 Principle of superposition Principle of lateral continuity Principle of original horizontality Principle of cross-cutting relationships Principle of faunal succession

f03_02_pg30 Principles of Relative Dating

f_pg26 Principles of Relative Dating

Time Scale Development Much of the geologic time scale had been constructed prior to the 20th century. The modern Geologic Time Scale is a compendium of both relative and absolute age dating and is used to study the rates of environmental and biologic change occurring on Earth.

Time Scale Development Geologists began in the late 1700's to examine fossils from the Phanerozoic Eon. They recognized that the fossils appeared in an orderly fashion in stratigraphic units. The fossilized biota was found to demonstrate rather large changes in overall composition and showed both similarities with, and differences from living taxonomic groups.

Time Scale Development The majority of fossil organisms did not match with modern groups, which led to the classification of three major eras within the period of time when the Earth's surface was populated with advanced life forms of the Phanerozoic Eon: –Paleozoic (meaning ancient life), –Mesozoic (meaning middle life), and –Cenozoic (meaning recent life).

Origin of the Periods of the Phanerozoic Eon

Units of the Paleozoic Era 1830’s: Cambrian – Cambria, Roman name for Wales (Adam Sedgwick) 1835: Silurian – Silures, Celtic Tribes (Sir Roderick Murchison) 1830’s: Ordovician, Taken from upper Cambrian and Low Silurian landforms. 1839: Devonian, Fossil Evidence, Sedgewick & Murchison. 1822: Carboniferous, Coal Beds in Europe and U.S. 1840’s: Permian, Permia – an ancient Russian Kingdom

Units of the Mesozoic Era 1834: Cretacious, Rock Outcrops in France, Belgium, and Holland (Chalk), White Cliffs of Dover 1859: Jurassic, Strata of the Jura Mountains of France and Switzerland 1834: Triassic, For trifold division of rocks of this age

Units of the Cenozoic Era 1760s/1770s: Tertiary - Giovanni Arduino coined the term based on rocks found in Tuscany, Italy. Early 19 th Century: Quaternary – From deposits in the Paris Basin by Jules Desnoyers. 2008: The International Commission on Stratigraphy (ICS) replaced the Tertiary Period with the following 2 periods: –Neogene –Paleogene

Absolute Dating “Radioactive Dating”

Early Attempts to Determine Earths Age Biblical Calculation - 6,000 Evolution of Fossils – 80 Million Sediment Deposition Rate:1 Million to1Billion 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

t03_03_pg39

f03_10_pg38

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

f03_14_pg41 How Carbon-14 Enters the Environment

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 uranium- bearing minerals.

Fission Track Dating Method

Analyzes the number of tracks that are produced from the break-down of uranium- bearing minerals.

f03_15_pg42 Fission Track Dating Method

Application of Absolute Dating

f03_09_pg37 INDEX FOSSILS

f03_08_pg37 CROSS-CUTTING RELATIONSHIPS

THE END