1

Geologic Time Terms• Hadean• Archean• Proterozoic• Phanerozoic• Paleozoic• Mesozoic• Cenozoic(Tertiary)• Cambrian• Unconformity• Angular unconformity

• Half-life• Alpha particle• Beta particle• Gamma ray• Neutron

How do we determine if layers separated by large distances formed at the same time?

UT

CO

WY

How would we recognize a “gap” in the rock record – if part of the rock record is missing and how much time the “gap” represents?

Timing of Geologic Events1) relative-age dating (fossils,

stratigraphy, structure)

2) absolute-age dating (isotopes, tree rings, etc.)

Laws / Principlesof Stratigraphy

Nicolaus Steno (1669)• Law of Superposition

• Principle of Original Horizontality

• Principle of Lateral Continuity

(1638-1686)

William Smith (1793)• Principle of Fossil Succession (1769-1839)

2

Oldest rocksOldest rocks

Youngest rocksYoungest rocks

Law of Superposition

In a sequence of undisturbed layered rocks, the oldest rocks are on the bottom.

Principle of Original Horizontality

Layered strata are deposited horizontally or nearly horizontally

Use of Fossils to Correlate Rock Formations

Principle of Fossil SuccessionWilliam “Strata” Smith (1793)• Recognized that different strata

contained different fossils

• Recognized an order or succession of fossils and strata

• Used fossils to correlate formations from different outcrops

Types of UnconformitiesAngular Unconformity

NonconformityDisconformity

A surface that represents a break in the rock record due to erosion or

nondeposition.

Unconformity

Angular Unconformity

3

DevonianOld Red Sandstone

Older tilted strata(shales and slates)

Siccar Point, Scotland

Nonconformity

Disconformity

Several unconformities are present in the Grand Canyon

1

2

3

4

Several unconformities are present in the Grand Canyon

1

2

3

4

Cambrian Tapeats Sandstone

Precambrian Wapatai Shale

4

South rim of the Grand Canyon250 million years old250 million years old

550 million years old550 million years old1.7 billion years old1.7 billion years old

Paleozoic StrataPaleozoic Strata

PrecambrianPrecambrian

South rim of the Grand Canyon250 million years old250 million years old

550 million years old550 million years oldNonconformity 1.7 billion years old1.7 billion years old

Vishnu SchistVishnu Schist

Principle of Cross-cutting Relationships

Host rocks (red) are older than the intruding rocks (black).

What is this surface?Lava Flow

(bed H)

Your turn Use the geologic principles to

place the events in order

5

How old is the Earth?

• By the mid 19th century a relative time scale had been worked out for the sedimentary rocks of Europe (Phanerozoic).

• They lacked an absolute time scale.• Kelvin and classical physicists advocated

40 million max. • Darwin and evolutionary biologists

advocated billions of years.• Discovery of radioactivity at about 1900

confirmed billions.

Phan

eroz

oic

Geologic Time Scale

Time divisions (units) of Earth’s history as recorded by rock formations – based originally on relative-dating methods:

Fossil groups or assemblagesFossil succession (order of fossils)Stratigraphic relationshipsCross-cutting relationshipsand later…Absolute (isotopic) ages

Geologic Time ScaleThe Age of the Earth

• Bishop Ussher - 17th Cent. (biblical): 4004BC

• Buffon - 18th Cent. (Cooling of spheres): ~50000 Y

• Hutton - late 18th Cent. (Geological cycles): Infinite

• Darwin - late 19th Cent. (Biological changes): Billions

• Kelvin - late 19th C (Sun’s energy): 40 Million Max

• Modern - (Radiometric): 4.55 Billion

6

Absolute-Age Dating

Absolute ages of geologic events and rock formations are based on radioactive elements and the rates at which they decay.

Many isotopes of each element occur naturally

Isotope: a variety of an element with the “normal” number of protons, but different number of neutrons

http://ie.lbl.gov/education/isotopes.htm

Rocks are composed of minerals, Minerals are composed of atoms of different

elements

1. Proton: positive charge2. Neutron: no charge3. Electron: negative charge

The number of protonsdetermines the element

“the atomic number”

The neutrons of a given element may vary

ISOTOPE: variations of the same element, with different # of neutrons, and so different atomic mass number

Isotope

7

Isotopes of Carbon• Types of radioactive decay

1. Alpha emission: Emission of 2 protons and 2 neutrons

Which of the following accurately describes alpha emissionA) Atomic number lower by 2; atomic mass unchanged B) Atomic number lower by 2; atomic mass lower by 2 C) Atomic number lower by 2; atomic mass lower by 4 D) Atomic number lower by 4; atomic mass lower by 4

Radioactive decay: Spontaneous changes in structure of atomic nuclei

What is change in 1) Atomic number? 2) Atomic mass?

-2 -4

Example of alpha emissionU238 Th234

2. Beta emission–A neutron loses an electron and turns into a

proton; the electron is ejected from the nucleus

Th234 Pa234

Which of the following accurately describes beta emissionA) Atomic number unchanged; atomic mass unchanged B) Atomic number increases by 1; atomic mass unchanged C) Atomic number decreases by 1; atomic mass unchanged D) Atomic number increases by 1; atomic mass dec by 1

3. Electron capture: An electron is captured, combines with a proton to form a neutron

What is change in 1) Atomic number? 2) Atomic mass?

-1 0 K40 Ar40

• Parent: an unstable radioactive isotope• Daughter product: isotopes resulting from

decay of parent

Parents and Daughters

Keep track of the ratio # of daughter (D) to

# of parents (N): D/N

8

Radiometric Dating:Establishing an absolute time scale

• Minerals contain naturally radioactive elements– K, U, Th, Rb, Sm

• These radioactive parent isotopes decay to stable daughter isotopes

• When minerals crystallize from melt, they contain parent only.

• If we measure the concentration of daughter isotope in a mineral and we know the decay rate, we can calculate when the mineral crystallized.

Types of Radioactive Decay

• Particle composed of: Mass# Atomic # Example• alpha 2 neutrons+ 4 2 U, Th,

2 protons• beta- electron 0 -1 40K• beta+ positron 0 +1 40K• gamma photon 0 0 all

nuclear reactions

• neutron neutron 1 0 235U

Common types of radioactive decay An Example: U238 to Pb206

The half-life of a radioactive isotope is the time required for half of the original number of radioactive parent atoms to decay to stable daughter products.

Frac

tion

of e

lem

ents

pre

sent

Radioactive decay curve

9

Half-lives: If the amount of radioactive isotope (the parent) is ¼ the amount originally present, how many half-lives have gone by?

A. 1B. 2C. 3D. 4

Naturally Radioactive Isotopes

• Parent DaughterHalf life Decay

• 40K 40Ar 1.3 x 109 y β+

• 87Rb 87Sr 4.9 x 1010 y β-

• 238U 206Pb 4.5 x 109 y 8α, γ• 235U 207Pb 7.1 x 108 y 7α, γ• 232Th 208Pb 1.4 x 1010 y 6α, γ• 14C 14N 5.7 x 103 y β-

Radiometric DatingExample: 40K - 40Ar

• A K-feldspar (KAlSi3O8) crystallizes in a granite and initially contains no Ar.

• Natural K is 0.012% 40K• 40K decays to 40Ar with a half-life of

1.31 x 109 years (1.3 billion years).• If we measure the 40Ar content of the

feldspar, we can get a crystallizationdate of the mineral.

• Isotope measurements are made with a mass spectrometer.

Some Major Events• Latest warming 7000y • Ice age ~1.8 MY • Dinosaur extinction 66 MY• Dinosaurs ~245 MY• Vertebrates ~400 MY• Multi-cell life forms ~550 ‘Cambrian Explosion’• ‘Snowball earth’ 600 MY• Free O2 ~ 2.5 GY (CH4 and NH3 decline)• Single cell life forms ~3.7 GY• Oceans: at least by 4.3 GY• Accretion: 4.55 GY

10

The era of dinosaurs is subdivided into Triassic,

Jurasssic, and Cretaceous. Together these are known as the:

A. ArcheanB. ProterozoicC. PaleozoicD. MesozoicE. Cenozoic

Dike @ 66 million years old

Ash bed @ 160 million years old

This surface represents:A. A faultB. A foldC. An unconformityD. The Phanerozoic

Geologic Time Terms• Hadean• Archean• Proterozoic• Phanerozoic• Paleozoic• Mesozoic• Cenozoic(Tertiary)• Cambrian• Unconformity• Angular unconformity

• Half-life• Alpha particle• Beta particle• Gamma ray• Neutron

Why can’t Why can’t 1414C be used to C be used to date date limestoneslimestones??

• A. No carbon in limestone• B. 14C half-life too short• C. 14C half-life too long• D. Daughter 14N not retained by limestone