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

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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

• 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

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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   Currently, Archeozoic and Azoic have been replaced by Archean ("first", "primary") and Hadean the latter term referring to the hellish conditions of the very early Earth.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

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Famous Geologists

• Adam Sedgwick(1785-1873)– 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

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Principles of Relative Dating

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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 19th 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

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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

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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

Fission Track Dating Method• Analyzes the number of tracks that are

produced from the break-down of uranium-bearing minerals.

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Fission Track Dating Method

Application of Absolute Dating

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INDEX FOSSILS

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CROSS-CUTTING RELATIONSHIPS

THE END