Lesson 1: Geologic Time
Evidence That Earth Has Changed
• Geologists use the principle of superposition, which
states that in rock layers that have not been folded
or deformed, the oldest rock layers are on the
bottom.
• The principle of
superposition gives the
relative ages of rock
layers, which tells you
whether the layers are
younger or older than
other rock layers
Lesson 1: Geologic Time
Evidence That Earth Has Changed
• The preserved remains or evidence of past living
organisms are called
fossils.
• Many fossils represent
species that no longer
live on Earth.
Lesson 1: Geologic Time
Evidence That Earth Has Changed
• During radioactive decay, one element changes into
another element.
• The radioactive parent element transforms to the
stable daughter element.
• The half-life is the time it takes for half of the parent
element to transform, or decay, to the daughter
element.
• By comparing the amount of parent element to the
amount of daughter element in a sample, scientists
can calculate the age of the sample.
Lesson 1: Geologic Time
Evidence That Earth Has Changed
• The geologic time scale is a visual record of Earth’s
history, with the individual units based on changes in
the rocks and fossils.
Lesson 1: Geologic Time
The Geologic Time Scale
• Geologists divide Earth’s
history into eons, eras,
periods, and epochs
• According to the
geologic time scale,
Earth has changed over
billions of years
Lesson 1: Geologic Time
The Geologic Time Scale
• The beginning of the Cambrian period is marked by
an abrupt appearance of complex life-forms.
• Earth is 4.6 billion years old. Humans have
experienced only a small part of Earth’s history.
Lesson 1: Geologic Time
Review
1) The preserved remains of organisms are called
__________________.
2) Define geologic time scale in your own words.
3) Use the terms radioactive decay and half-life in a
sentence.
Lesson 1: Geologic Time
Review
4) What is the longest unit of time on the geographic
time scale?
a) Eon
b) Epoch
c) Era
d) Period
5) How were fossils used in the development of the
geologic time scale?
6) Compare the ages of rocks determined by the
principle of superposition and ages determined by
radioactive decay.
Lesson 1: Geologic Time
Review
7) Suggest how the geologic time scale might be
different if there were no radioactive elements.
8) In 1978, the start of the Paleozoic era was placed at
570 million years ago. Today, it is placed at 542 million
years. Discuss how this change is possible.
9) If you begin with 140g of an element, how much
remains after 3 half-lives?
Lesson 1: Geologic Time
Exit Ticket Quiz
1.Which items contain evidence that Earth is very old?
A. rock layers
B. living organisms
C. daughter elements
2.The geologic time scale was developed using
A. fossils and superposition.
B. volcano and earthquake data.
C. parent and daughter elements.
3. If Earth’s history lasted only one year, humans would have
appeared
A. in July.
B. in February.
C. in December.
Lesson 1: Geologic Time
Exit Ticket Quiz
4. process in which one element changes into
another
5. states that older rock layers are generally under
younger rock layers
6. the longest units of geologic time
7. time it takes for half of a radioactive element to
break down into another element
8. preserved remains or evidence of ancient
life-forms
9. stable substance formed from a radioactive one
10. a scale of Earth’s natural history
A. Eons
B. Daughter
Elements
C. Fossils
D. Half-Life
E. Geologic
Time Scale
F. Radioactiv
e Decay
G. Superposition