Grand Canyon in Arizona (stratification, bedding)

stratification, bedding, stratum (strata)

Rock Record and Geologic Time

Timescale (Relative)

Timescale (Absolute)

How do we tell geologic time?

Relative time• Principles of stratigraphy - 17th – 18th century• Fossils - 18th century• Paleomagnetism – 20th century

Absolute time • Radiometric dating – late 19th-20th century

Relative dating

Placing rocks and events in sequence • Law of superposition – oldest rocks

are on the bottom • Principle of original horizontality –

sediments are deposited in flat, horizontal layers

Grand Canyon in Arizona

Glen Canyon Dam, Lake Powell

Rio Colorado – ‘Red River’

Red sediment now trapped in Lake Powell so the river is no longer red.

Superposition is well illustrated by the strata in the Grand Canyon

Original Horizontality

Relative dating

Other geologic principles • Cross-cutting relationships - fractures,

faults and intrusions must be YOUNGER than rocks they cut

• Inclusions – one rock contained within another (rock containing the inclusions is younger)

Cross-cutting Relationshipsfractures, faults and intrusions must be

YOUNGER than rocks they cut

Inclusions

(12) Youngest Event: Deposition of unit C

(11) Erosion of unit G and intrusion A

(10) Intrusion of A

(9) Deposition of unit G

(8) Erosion of folded unit and of intrusion B

(7) Intrusion of B (or before 6)

(6) Folding of all previously deposited layers

(5) Deposition of layer E

(4) Deposition of layer I

(3) Deposition of layer F

(2) Deposition of layer H

(1) Oldest Event: Deposition of layer D

Telling Time with Fossils

use first occurrence and last occurrencerapidly evolving (short-lived) organisms

divide time into the finest divisionsbest index fossils have a wide geographic

range (planktonic ocean organisms)

Radioactivity and radiometric dating

Radioactivity • Spontaneous breaking apart (decay) of

atomic nuclei• Radioactive decay

Parent – an unstable isotope Daughter products – isotopes formed

from the decay of a parent

Radioactivity and radiometric dating

Radiometric dating • Half-life – the time for one-half of the

radioactive nuclei to decay • Requires a closed system• Cross-checks are used for accuracy • Complex procedure • Yields numerical dates • http://www.youtube.com/watch?v=1920gi3s

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The radioactive decay curve

Proportion of Parent Atoms Remaining as

a Function of Time

Half-lives and remaining parent isotope

Radiometric/Isotopic dating

Radioactive elements (parents) decay to nonradioactive (stable) elements (daughters).

The rate at which this decay occurs is constant and knowable (measurable).

Therefore, if we know the rate of decay and the amount present of parent and daughter, we can calculate how long this reaction has been proceeding.

Radioactivity and radiometric dating

Carbon-14 dating • Half-life of only 5730 years • Used to date very recent events • Carbon-14 produced in upper atmosphere

Incorporated into carbon dioxide Absorbed by living matter

• Useful tool for anthropologists, archeologists, historians, and geologists who study very recent Earth history

Parent Isotope Stable Daughter Product Currently Accepted Half-Life Values

Uranium-238 Lead-206 4.5 billion years

Uranium-235 Lead-207 704 million years

Thorium-232 Lead-208 14.0 billion years

Rubidium-87 Strontium-87 48.8 billion years

Potassium-40 Argon-40 1.25 billion years

Samarium-147 Neodymium-143 106 billion years