Cycle 3 Earth History Supplement
Part 2 - In-Class Activity to Introduce Earth History
Purpose
Scientists trying to reconstruct the history of Earth need to know the timing of
various kinds of geologic events. Such events include the deposition of
sedimentary layers; the intrusion of magmas; the faulting and folding of rock
layers; the appearance (or extinction) of different kinds of organisms; and the
erosion of rocks at the surface. In this exercise, you will be given a list of some
important events in Earth history and asked to speculate their chronologic order
(i.e. which event happened first? second? and so on). Next you will place these
sequenced events on a geologic timeline of Earth history. In subsequent
homework you will examine and practice with the tools and principles that
scientists use to study rock layers, fossils, and relationships to figure out the
chronologic order in which geologic events occurred in the evolution of Earth.
You will each need
 a randomized set of 23 cards depicting fossils or geologic events
Your class will need
 a time line (string) with ages of Geologic Time from earth formation to the
present with Eras and Periods marked. This will be strung across your
classroom
Step 1
On your own, place the cards (on the table or in your hand) in order from oldest to
youngest. Guess, if you are not sure.
Step 2
Discuss with your classmates the relative order of the fossils or events. If
possible, come to a consensus about the relative order and, if needed, readjust your
positioning. You can do this by placing the cards on the ledge of the whiteboard
in the position (relative to other events).
How did your initial ideas differ from the consensus ordering of the geologic
events and fossils?
1
Step 3
Your instructor will now give you a list that shows Scientists’ Ideas for relative
positioning of fossils and events.
How did the consensus ordering of the geologic events and fossils differ from the
Scientists’ Ideas?
Step 4
Reposition the class set of cards in the order of events as established in Scientists’
Ideas.
Step 5
On your own, use the Scientists Ideas table to estimate the time in years before
present that you think the events occurred.
Step 6
Discuss your ideas with students in your group and try to develop a group
estimate. How did your initial ideas differ from the group estimate?
Step 7
Discuss as a class the time estimates and position the class set of cards accordingly
on the Geologic Timeline from Earth formation to the present (use clothes pins or
paper clips.
2
Step 8
Your instructor will now tell you Scientists’ Ideas for the ages of the events. Write
the years before present into the appropriate column on the table
How did the consensus estimates of the ages differ from the Scientists’ Ideas?
Summarizing Questions
S1: Reflect on at least one surprise about order or timing of the Earth history
events.
S2: Discuss this question with your group and record a few of their ideas.
3
Scientists’ Ideas
The table below shows the relative order of the sequence of selected geologic
events established by scientists.
Geologic Event
Scientists’ Ideas
of order from oldest to youngest
Origin of Earth
Your Ideas
Millions of
years ago
Scientists’ Ideas
Millions of
years ago
Oldest Dated Crustal Rocks
Oldest Fossils (single-celled)
1st Oxygen in Atmosphere
Oldest multi-celled fossils
Oldest fish fossils
Oldest land plant fossils
Oldest land amphibian fossils
Huge extinction (~90% of species)
Oldest mammal fossils
Oldest bird fossils
Atlantic Ocean first opens
Oldest flowering plant fossils
Dinosaur extinction
Basalt floods in eastern Washington
Age of Lucy (Australopithecus)
fossils
Ice Age begins
Age of Homo erectus fossils
Age of oldest Homo sapiens fossils
Ice melts from Puget Sound
Jesus born
Columbus lands in “New” World
Your instructor born (add yours)
4