Geologic Time – Answers

Chapter 9

The earth is 4.6 billion years old and during that time life as developed, adapted to the environment and changed through time.  Some life forms that have existed in the geologic past no longer exist today.  These extinct life forms can help us establish the relative age of rock layers while radioactive decay is used to establish absolute age.  The 4.6 billion years of history is broken down into smaller time units of millions of years called the geologic time scale.

Learning objectives

1.        Understand the difference between relative and absolute age.


2.        Understand the geologic time scale and the changes in life forms through time.

Performance Criteria

1.        Quality of your vocabulary
Factor #1: Completion of vocabulary.
Factor #2  Strength of your “own word”  definitions in your log

2.        Your understanding of geologic history.

Factor # 1.ability to answer the questions using well-formed answers.


1.        Textbook

2.        Class notes

Read the chapter
2.        Construct an outline of the reading.  Include any key questions.
3.        Complete the vocabulary list.  Definitions should be the classic “book” version then put into your own words.
4.        Answer the critical thinking questions.

Critical thinking questions - Answers


1.        What is the difference between absolute and relative dating?

Absolute age dating uses radioactivity to help establish the age of a rock while relative age uses comparisons of older and younger than a rock.

2.        What is the law of superposition?  How are crosscutting relationships used in relative dating?

The law of super position states that the youngest rocks are found on the top of a sequence while the older rocks are on the bottom

Crosscutting relationships help to establish relative age since a rock can not be folded, faulted or intruded into unless it existed prior to the event.

3.        When you see an outcrop of steeply inclined rocks, what principle allows you to assume they were tilted after they were deposited?

The principle of original horizontality tells us that sedimentary rocks were deposited in a horizontal or nearly horizontal layer and any change must have taken time.

4.        A mass of granite is in contact with a layer of sandstone.  Using a principle from your vocabulary list, explain how you might determine whether the sandstone was deposited on the top of the granite or the granite was intruded after the sandstone was deposited.

You would use the principle of inclusion to help determine if the sandstone was deposited upon the granite or the granite was intruded into the sandstone.  If the granite were there first and exposed at the surface, we would expect to find inclusions of the granite in the lower most section of the sandstone.

If the granite intruded into the sandstone we might expect to find sandstone xenoliths or contact metamorphism.

  5.        Distinguish the difference between a disconformity, nonconformity and an angular unconformity.

Disconformity is an erosional or non-depositional surface within essentially parallel sedimentary rocks.

Nonconformity is a contact between igneous intrusive or metamorphic rocks and overlying sedimentary rocks.

An angular unconformity happens when horizontal sedimentary rocks are overlying tilted or folded sedimentary rock layers.

  6.        Do all the practice examples from your GEODE disk that came with the textbook.  If one isn’t available, you may use the disks available in the computer lab located in room 5250.

Do this exercise.  It is worth it.

  7.        If an isotope in a rock has a half-life of 10,000 years and the ratio of parent to stable daughter product is 1:3, how old is the rock?

Two half-lives have past for the 1:3 ratio so the age of the rock must be 20,000.

8.        What are the four major eras in geologic time?  What is the predominate life form associated with each?


Precambrian – no life

Paleozoic – Ancient Life- Invertebrates

Mesozoic – Middle life – Dinosaurs

Cenozoic –Recent Life - Mammals

9.        Let’s assume the earth is 5 billion years old (hey, what’s a few million years between friends?) What percent has been represented by recorded history?  Assume 5000 years for recorded history.


(5000/ 5,000,000,000) X 100 = 0.0001% 

Not much.

10.     Using the same assumptions as #9 and assume the Cambrian began 500 million years ago, what percentage of the earth’s history is represented by abundant life?

(500,000,000/5,000,000,000) X 100 = 10%



last updated 06/23/04