Time and Geology

What is the age of the Earth?

Uniformitarianism - key concept (James Hutton) means the present is the key to the past.

It was Charles Lyell that brought the principle of uniforitarianism to the attention of the public

How old is the planet earth?

Two ways to look at time - Relative and Absolute terms

1) Relative Time

Seven Principles used to determine relative age (or Fundamental Geologic Principles)

1. Original horizontality

2. Superposition - for undisturbed rock sequence, younger

3. Lateral continuity - rocks story on top

4. Cross-cutting relationships

5. Inclusions - included rock is older than the surrounding rock

6. Fossils (William Smith)

7. Unconformity - missing period in rock formation

Unconformities

a) Disconformity

b) Angular

c) Non conformity

Correlation - Correlating an event or a geologic unit with another using:

1. Physical continuity

2. Similarity of rock types -

3. Correlation by fossils --- funnel succession in index fossil - short lived species known to exist during a specific period of geologic time. Fossil assemblage

Fossil Types

petrified

replacement

molds - imprint

casts - filled with minerals

carbonization/impression

amber

Indirect types of fossils

Tracks

Burrows

Coprolites

Gastroliths - a highly polished stomach stone

4 major time units

Eons, Eras, Periods, Epochs

The Standard Geologic Time Scale
This is based on fossil assemblages
	(Time - Rock Units)
Eon	Era (Erathem)	Period (System)	Epoch (Series)	Age (Stage)
			Recent (Holocene)	0.01
		Quaternary	Pleistocene	1.6
			Pliocene	5
	Cenozoic		Miocene	24
	(new life)	Tertiary	Oligocene	37
			Eocene	58
			Paleocene	65 my
			Cretaceous	144
Phanerozoic	Mesozoic	Jurassic		208
	(middle life)	Triassic		245 my
		Permian		286
		Pennsylvanian		320
		Mississippian		360
		P & M = Carboniferous
	Paleozoic	Devonian		408
	(old life)	Silurian		438
		Ordovician		505
		Cambrian	545	570 my
Precambrian	Precambrian
Proterozoic				2500
Archean
	Hadean	Formation of Earth		4.5 by

Obviously, we cannot date Igneous and most Metamorphic rocks because they lack fossils, hence the use of radiometric method.

2) Absolute Time

a) Use of tree rings (Dendrochronology) to tell the age of a tree or varves (sed. layers in lakes)
b) Fission track,
c) Lichenometry,
d) surface-exposure (cosmogenic isotopes)

Radiometric Dating

Radioactive decay is the spontaneous nuclei disintegration of certain isotopes.
Examining the Radioactive decay or change

Isotopes and radioactive decay

Atoms - electrons and nucleus (neutrons/protons)

The number of neutrons need not be the same for atoms of the same element

Isotopes of a given element have different number of neutrons but the same number of protons

e.g. U-238 92 protons 148 neutrons & U-235 92 protons 143 neutrons

As neutron and proton leave, energy is release which can be detected with a Geiger counter or something similar

With the lost of protons, the atom becomes a different element.

Rate of decay is constant for most radioactive elements. Half-life is used for the decay rate c is the time it takes for the element to be reduced to half

Uses of Radiometric Dating				Daughter Product
	K-40	½ % Fe	1.3 by	Ar-40
	U-238		4.5 by	Pb-206
	U-235		704 713 my	Pb-207
Thorium	Th-232		14.1 by	Pb-208
Rubidium	Rb-87		49 by	Sr-87
	C-14		5730 yr	N-14

This is difficult in the case that cosmic rays from space produce neutrons in the atmosphere this collides with Neutron, knocks out P ¹⁴₇ N +¹n 14₆C + iH

Carbon Dating....used for materials that are few thousand years old.
C14 decays by beta emission to nitrogen ₆¹⁴C ¹⁴₇N + B^-

Problem - need highly skilled technician

Usually only unweathered Igneous are used for radiometric dating.

Above is based on the assumptions that no changehas taken place in the rock

Absolute age and Relative age could be combined