Eighth Edition
by Harold L. Levin

Chapter 3 - page 1

Time and Geology

Introduction

The science that deals with determining the ages of rocks is called geochronology.

There are two basic methods of dating rocks:

Relative dating

Using fundamental principles of geology (Steno's Laws, Fossil Succession, etc.) to determine which rocks are older and which are younger.
"A is older than B."


Absolute dating

Quantifying the date in years.
This is done primarily by radiometric dating (or analysis of the breakdown of radioactive elements in the rocks over time).

Geologic Time Scale

The geologic time scale has been determined bit-by-bit over the years through relative dating, correlation, examination of fossils, and radiometric dating.

Boundaries on the time scale are drawn where important changes occur in the fossil record, such as extinction events.

The time scale is divided into a number of types of units of differing size. From the largest units to the smaller units, they are:

• Eons.
• Eras.
• Periods.
• Epochs.

These units are geochronologic units. Geochronologic units are time units.

Geologic time scale.

Eons

Eons are the largest division of geologic time.

In order from oldest to youngest, the three eons are as follows:

1. Archean Eon - "ancient or archaic" (oldest rocks on Earth).
2. Proterozoic Eon - "beginning life" (2.5 billion to 542 million years ago).
3. Phanerozoic Eon - "visible life" (542 million years ago to present).

The Archean and Proterozoic are collectively referred to as the Precambrian (meaning "before the Cambrian Period"), which covers 87% of geologic history.

Eras

There are three eras in the Phanerozoic Eon. In order from oldest to youngest, they are as follows:

1. Paleozoic Era - "ancient life" (such as trilobites).
2. Mesozoic Era - "middle life" (such as dinosaurs).
3. Cenozoic Era - "recent life" (such as mammals and flowering plants).

Periods

Eras are divided into periods.

Paleozoic Era

• Cambrian Period (oldest).
• Ordovician Period.
• Silurian Period.
• Devonian Period.
• Carboniferous Period (Mississippian and Pennsylvanian Periods in North America).
• Permian Period.

Mesozoic Era

• Triassic Period (oldest).
• Jurassic Period.
• Cretaceous Period.

Cenozoic Era

• Paleogene Period.


• Neogene Period (youngest).

Epochs

Periods can be subdivided into epochs.

The epochs are listed for the periods in the Cenozoic Era. They are as follows:

• Paleogene Period
• Paleocene Epoch (oldest)
• Eocene Epoch
• Oligocene Epoch
• Neogene Period
• Miocene Epoch
• Pliocene Epoch
• Pleistocene Epoch
• Holocene Epoch (youngest - today)

Chronostratigraphic units

Chronostratigraphic units are the actual rocks formed or deposited during a specific time interval. (They are sometimes called time-rock units.)

Chronostratigraphic units include:

1. Eonothem (all rocks corresponding to a given eon).
2. Erathem (all rocks corresponding to a given era).
3. System (all rocks corresponding to a given period).
4. Series (all rocks corresponding to an epoch).
5. Stage (all rocks corresponding to a particular age).

Geochronologic units have the same names as the chronostratigraphic units that they represent.
For example, the Cambrian System is a rock unit, and the Cambrian Period is a time unit.
The rocks of the Cambrian System were deposited during the Cambrian Period.