Geologic Time – Answers
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.
2. Class notes
Critical thinking questions - Answers
What is the difference between absolute and
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.
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
When you see an outcrop of steeply inclined
rocks, what principle allows you to assume they were tilted after they
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.
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.
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.
Do this exercise. It is worth it.
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%
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%