Dating Fossils : Relative Dating and Absolute Dating

DATING FOSSILS

Knowing the age of a fossil can help a scientist establish its position in the geologic time scale and find its relationship with the other fossils.

Fossils, Fossil Types and Fosilization

There are two ways to measure the age of a fossil: relative dating and absolute dating.

1. RELATIVE DATING
I. Based upon the study of layer of rocks
II. Does not tell the exact age: only compare fossils as older or younger, depends on position in rock layer
III. Fossils in the uppermost rock layer/ strata are younger while those in lowermost deposition are oldest


How Relative Age is Determined

I. Law of Superposition: if a layer of rock is undisturbed, the fossils found on upper layers are younger than those found in lower layers of rocks
II. However, because the Earth is active, rocks move and may disturb the layer making this process not highly accurate

Rules of Relative Dating

A. LAW OF SUPERPOSITION:
Sedimentary layers are deposited in a specific time- youngest rocks on top, oldest rocks at the bottom

B. LAW OF ORIGINAL HORIZONTALITY: Deposition of rocks happen horizontally- tilting, folding or breaking happened recently

C. LAW OF CROSS-CUTTING RELATIONSHIPS: If an igneous intrusion or a fault cuts through existing rocks, the intrusion/fault is YOUNGER than the rock it cuts through

INDEX FOSSILS (guide fossils/ indicator fossils/ zone fossils): fossils from short-lived organisms that lived in many places; used to define and identify geologic periods

2. ABSOLUTE DATING
• Determines the actual age of the fossil
• Through radiometric dating, using radioactive isotopes carbon-14 and potassium-40
• Considers the half-life or the time it takes for half of the atoms of the radioactive element to decay
• The decay products of radioactive isotopes are stable atoms.


Carbon-14 Dating:

A living organism has carbon-14.
For the amount of Carbon in the organism’s body to become half, it will take about 5,700 years; which is the half-life of carbon-14.

Take a look at the table below. Then answer the exercise below.

Read the post on Carbon Dating : Measurement and Accuracy for more facts and information on Carbon Dating.

Exercise Questions :


A.) Fill up the remaining data in the table.

4 , Mass Remaining : _____________ ,  Number of Years : __________
5 , Mass Remaining : _____________ ,  Number of Years : __________
6 , Mass Remaining : _____________ ,  Number of Years : __________

B.) What is the limit in using carbon-14 as a measure to determine a fossil’s age?

C.) Do you think Carbon Dating is an accurate method of determining the correct age of dead organisms? Please state your reason.

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Goelogic Time Scale - History of Life on Earth

The Geologic Time Scale is a tabular representation of the major divisions of the Earth’s history. The time intervals are divided and described from the longest to the shortest as EONS, ERAS, PERIODS and EPOCHS. Each period has an approximated time frame and characterized by distinctive features (events and organisms).



The Geologic Time Scale (GTS)
A. Four eras - Precambrian; Paleozoic; Mesozoic; Cenozoic
B. Periods under the Paleozoic era - Cambrian, Ordovician, Silurian, Devonian, Carboniferous, Permian
C. Periods under the Mesozoic era - Triassic, Jurassic, Cretaceous
D. Periods under the Cenozoic era - Tertiary and Quaternary

The Geologic time is divided into four large segments called Eons:
Hadean, Archean, Proterozoic and Phanerozoic.

The Phanerozoic is divided into Eras: Paleozoic, Mesozoic, and Cenozoic.
Extinction events and appearance of new life forms characterized the divisions among Eras.
Smaller divisions, called Periods, characterized by a single type of rock system, make up each Era.
Some Periods are further divided into smaller time frame called Epochs.

CAMBRIAN EXPLOSION is the belief that there was a sudden, apparent explosion of diversity in life forms about 545 million years ago. The explosion created the complexity of multi-celled organisms in a relatively short time frame of 5 to 10 million years. This explosion also created most of the major extant animal groups today.

The start of the Cambrian was characterized by the breaking up of supercontinent Gondwana into smaller land masses opening up new environmental niches where organisms can colonize and specialize.

Memory Aid / Mneumonic Technique to memorize GTS :

Eras: Precambrian, Paleozoic, Mesozoic, Cenozoic:
* Please Pay My Children!

Periods: Cambrian, Ordovician, Silurian, Devonian, Mississippian, Pennsylvanian, Permian, Triassic, Jurassic, Cretaceous, Tertiary, Quaternary:
* Cold Oysters Seldom Develop Many Precious Pearls, Their Juices Congeal Too Quickly.
* Cows Often Sit Down Carefully. Perhaps, Their Joints Creak

Epochs of Cenozoic Era: Paleocene, Eocene, Oligocene, Miocene, Pliocene, Pleistocene, Holocene (or Recent):
* Put Eggs On My Plate Please Honey.

Precambrian Archean, Proterozoic


What are the evidences that suggest that we are entering / have entered a new epoch?

A new proposed Epoch, the Anthropocene.

- derived from the Greek and means the “ recent age of man ”
- interval of geologic time, (2.6 million years ago to the present), characterized as the time in which the collective activities of human beings (Homo sapiens) began to substantially alter Earth's surface, atmosphere, oceans, and systems of nutrient cycling.
- denote the present time interval, in which many geologically significant conditions and processes are profoundly altered by human activities.

These include changes in:
erosion and sediment transport ,
the chemical composition of the atmosphere, oceans and soils,
environmental conditions.

Fossils , Fossil Type and Fosilization
Dating Fossils : Relative Dating and Absolute Dating

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