Earth Science
Chapter 9
Section 4
A. The Geologic Time Scale:
Because the time span of Earth’s past is so great, geologists use the
geologic time scale to show Earth’s history.
The geologic time scale is a record of the life forms and geologic
events in Earth’s history.
B. Divisions of Time:
Geologic time begins with a long span of time called the
Precambrian Time, which covers about 88% of the Earth’s history.
The Precambrian time ended 544 million years ago.
After the Precambrian time, the basic units of geologic time scale
are eras, periods and epochs.
C. Eras, Periods, and Epochs:
1. Eras – Units of time between the Precambrian and the present.
Geologists have divided this time into three eras.
a. The Paleozoic – A unit of 300 million years. Paleomeaning ancient or early, and zoic meaning life.
Many of the animals that lived during the Paleozoic Era
were animals without backbones, or invertebrates.
b. The Mesozoic – A unit of 180 million years. Meso –
meaning middle, and zoic meaning life. This period is
known as the “Age of the Dinosaurs”. Mammals began to
develop during this era.
c. Cenozoic Era – A unit of time that began about 65 million
years ago and continues to the present. Ceno meaning recent
and zoic meaning life. The Ear is known as the Age of the
Mammals. Mammals became common during this time.
2. Periods - Smaller units of time within an era. Geologic periods
range form tens of millions to less that two million years.
Periods are named for rocks and fossils of that period first
described.
Examples:
The Cambrian Period refers to Cambria, the old Roman name
for Wales.
Jurassic refers to the Jura Mountains of France.
The Carboniferous Period is named for the large coal
deposits that formed during that period.
3. Epochs – Subdivision of the Cenozoic Era, used to sequence
events due to a more complete fossil record.