Geologic Timeand Paleontology

How Old isthe Earth?

Lisa Herzog, Chief PreparatorPaleontology and Geology Research Lab

Concepts to UnderstandingHow Old the Earth is

Relative Aging Absolute AgingEstablishes events in the order that they occurred

A numeric age of a given sediment is calculated based on radiometric isotopes

Does not pinpoint an exact calendar age

Produces a numeric age, regardless of surrounding sediments/rock.

Only concerned with before or after

Measures amount of time lapsed

Must be measured in conjunction with surrounding sediments and fossils.

Can be determined in isolation, regardless of surrounding materials.

Relative Time Relative Geologic time has Four Main

Principles that refer to sedimentary strata, and a few others that further corroborate the conclusions of deep time.

One of the first main principles is….

UniformitarianismThe present is the key to the past

-James Hutton

The Principles ofRelative Geologic Time

Principle of original horizontality: Layers of sediment are deposited flat (horizontally)x

The Principles ofRelative Geologic Time

Principle of superposition: The older rocks are on the bottom

The Principles ofRelative Geologic Time

Principle of crosscutting relationships: Any rock that cuts through another MUST be younger.

The Principles ofRelative Geologic Time

Principle of included fragments: A Rock that includes fragments of other rock must be younger than the included fragments.

Relative Geologic Time & Fossils

Principle of faunal successionspecific fossils always appear in the samestratigraphic order in rocks.

Unconformitiesand Correlation

Disconformity – sedimentary layers parallel to each other

Angular unconformity – tilted layers with newer flat material atop

Nonconformity – sedimentary rocks atop igneous or metamorphic rock

Unconformitiesand Correlation Correlation – matching up rocks of similar age in different locales Index fossils – accurately indicates the age of a rock Key bed – a thin, widespread, synchronous sedimentary layer

Absolute Time

In Geology, Absolute Time should be thought of as ‘chronometric’ or ‘calendar’ dating. This is more accurate than the word ‘absolute’ as it inaccurately implies certainty and precision.

A numeric age of an event is calculated based on radiometric isotope dating. Absolute dating is the process of

determining an approximate computed age.

Chronometric Dating Measuring absolute age relies on:

Process that occurs at a constant rate Some cumulative record of that process

Isotopes – radioactive varieties of an element Vary by the number of neutrons

Half-life – the time it takes for one-half the atoms of a radioisotope to decompose to another isotope or element

Application to Rocks

Age of rock is determined by ratio of parent : daughter atoms The decay constant must be known

Determines whether half-life is short (min), or long (millions of years)

Rock can be accurately dated if fewer than four half-lives have passed (not enough parent atoms left to measure ratio)

Original rock should not have any daughter atoms present

Common Sources Carbon-14

Decays to nitrogen-14

Present in all living things from plants to animals

Half-life is approximately 5,000 years Can date fossils and

tissues up to 20,000 years old

Common Sources

Potassium-40 Decays to argon-40 and

Calcium-40 Present in igneous

minerals (apatite and muscovite)

Half-life is 1.3 billion years.

Most commonly used isotopes for radiometric dating

Thank YouPlease remember that no one single piece of evidence proves the time scale, or can definitively place an item/artifact in absolute historical position.

All of Geologic time and Paleontology is reliant on the interconnectedness of the data, the principles, and the overall evidence.

Lisa HerzogChief PreparatorPaleontology and Geology Research Laboratory