Age and Formation of the Earth
Age and Formation of the Earth
Objective:
· Visualize the age of the Earth and how it compares with human time scales.
· Relate movement rates of plate tectonics with human time scales and describe how—in conjunction with knowledge of the age of the earth—slow movement rates create large changes.
· Compare and contrast the philosophies of Uniformitarianism and Catastrophism
Level/Subject: Eight Grade/Earth Science
Time Duration: 45 min
Curriculum Standard: Unit E3.3 Plate Tectonics Theory
Materials Needed: Rolls of Toilet Paper, markers, chalkboard
Vocabulary:
Geologic Time—Geologic Time refers to considering the entire 4.6 billion year history of the Earth; also referred to as “Deep Time”
Uniformitarianism—the conceptual notion that geologic structures form as the result of very slow processes continuously operating for long periods of time
Catastrophism—the notion that discrete events (e.g., large volcanic eruptions) are responsible for large changes in the Earth’s structure.
Introduction/Description:
The crucial components of today’s lesson—though it does not invoke a “strong” learning verb—is that students will be able to visualize the age of the earth and geologic time. This means that the students will become aware of the relatively miniscule time frame in which humans have lived on the planet. This visualization will also help students realize how slow plate movements, which they will be able to compare to understandable time scales, can cause big changes.
State Objectives: (1-2 min)
Visualize the age of the Earth and how it compares with human time scales. Relate movement rates of plate tectonics with human time scales and describe how—in conjunction with knowledge of the age of the earth—slow movement rates create large changes.
Review/Assessment of Prior Knowledge: (4-5 min)
A) What are three types of plate boundaries?
Convergent, Divergent, Transform/Strike-Slip
B) Can you provide an example or an explanation of each?
Moving towards one another (e.g. Himalayas); Spreading apart (Mid-Atlantic Ridge); Sliding past each other (San Andreas Fault, California)
C) What is the age of the Earth?
4.6 billion years old
D) Can you see plate tectonics happen?
This question is a bit more nebulous, and I expect kids to say no. Continue the question and ask them if they can list any evidence that plates have moved in the past and/or are still moving today. Lead them to the realization that the examples they previously gave are in fact evidence of plate movements.
The students might surprise me at this point and immediately make this conclusion on their own and say that plates are moving very slowly. Even better, the students might point out events such as earthquakes being direct results of plate movements.
E) How fast to plates move?
Plates generally move about 2.5 cm per year, which is approximately the growth rate of human fingernails. The point is, plate tectonics operates very slowly, and this explains why we can’t “see” plates move.
Lecture—Formation of the Earth: (15 min)
A) How Do Plate Tectonics Create the Structures of the Earth? UNIFORMITARIANISM
Hopefully, our warm-up questions will get students to realize that plates are continuously moving (usually very slowly) but nevertheless have created great structures such as Mt. Everest, the Rockies, the Pacific Ocean, etc. Explain that this line of thought is known to geologists as “UNIFORMITARIANISM.” This is the idea that the same processes have been going on very slowly, but they have big results because they go on for a long time. An analogy might be baking a cake. The ingredients of a cake must sit in an oven for a long time until it is baked properly. While in the oven, it undergoes the same process, but it is essential that the proper amount of time be completed for the cake to be ready.
B) How Do Plate Tectonics Create the Structures of the Earth? CATASTROPHISM
The last bit of the warm-ups should have also addressed the San Andreas Fault and the notion that plate tectonics do sometimes occur rapidly. This is the concept of “CATASTROPHISM.” It is the idea that the Earth is sometimes formed by very large events that happen quickly. An example of a catastrophic event might be a huge eruption like that of Mt. St. Helens. An analogy, similar to the kitchen example from before, could be making a sandwich. To make a sandwich, you must take out two slices of bread, slap on a piece of ham and cheese, add tomatoes and other vegetables, and put it all together. Each step happened very quickly, but afterwards, the sandwich was completely different than before, and it all happened very quickly.
C) UNIFORMITARIANISM or CATASTROPHISM?
The truth is that both concepts explain the formation of the Earth. The Earth is shaped by long processes, but large, quick events often interrupt and change the surface as well. Uniformitarianism and Catastrophism are opposite ideas, but they are not mutually exclusive.
Activity—Toilet Paper Time Line: (15 min)
Introduction to the Activity:
Explain that you will use a roll of toilet paper as a scaled model of the age of the Earth’s history.
Reiterate that the Earth is 4.6 billion years old, and the entire roll of toilet paper (all 308 sheets) will represent the age of the Earth. Each sheet of toilet paper will equal approximately 15 million years.
The loose end of the toilet paper will represent ‘NOW—Current Day’ and the end of the toilet paper roll (the end attached to the cardboard tube) will represent the beginning of Earth’s formation.
We will unroll enough toilet paper to make a timeline of the Earth’s last 540 million years, which is when life became widespread on Earth. After we unroll enough sheets to reach 540 million years, the students will work their way through geologic history by marking progressively younger and younger events on their timeline.
Start the Activity:
Divide the class into groups of four. Ask students to take turns marking different events on the timeline.
Starts off the exercise by walking the class through how to begin their time scale. The first event will by 540 million years ago at an event called the Cambrian explosion. 540 million years corresponds to 36 sheets of paper, so unroll 36 sheets of paper. Have the students number each sheet with #1 starting at the loose end. Place the numbers in the upper right hand side of the sheet. At sheet #36, mark “Cambrian Explosion.” This is when life first started to become dominant on our planet, but before this time, there had already been 3 super continents that formed and split up since the Earth’s beginning. The oldest of these continents was 7 times older than the Cambrian Explosion. The youngest of these continents was called Rodinia and is still in the process of splitting up during the Cambrian Explosion.
Because the continents had just split up from a super-continent, this means they were already moving around and starting to form new arrangements and collisions with each other. One such collision happened 466 million years ago and formed the Appalachian Mountains. Have the students mark 466 million years ago and write Appalachian Mountains.
With this guidance, the students should be able to complete the rest of their timescale. Have them plot the formation of Pangea, the break-up of Pangea into Gondwana and Laurasia, the break-up of Gondwana, the formation of the Atlantic Ocean, the Rockies, Hawaii, and the Himalayas.
Conclusion: (2-3 min)
A) Conceptual Realization
Once the students have completed this exercise, ask them to consider when Humans should be plotted on the timescale. Tell them that Humans have been around for 50,000 years (about one hundred times less than the Himalayas).
Assessment/Evaluation: (3-8 min—finish up)
Pose Questions to the class
A) How can plate tectonics create mountains and oceans if the rate of movement is so slow?
The purpose of this question is to bring back the concept of Uniformitarianism. Students should be able to explain that the Earth is much older than what humans can truly understand. By considering the long duration of geologic time, it is easier to understand how slow plate movements are able to build large mountains and wide oceans.
B) What is an example of a Uniformitarian process on the earth’s surface?
Direct examples of this should have already been given in the class, so try to lead students to original ideas. If possible, try to get students to think of a Uniformitarian process that has directly affected the geography of the land around them.
C) What is an example of a Catastrophic process on the earth’s surface?
Repeat the process of step B for Catastrophism. Try to get them to come up with examples from the local area. The examples need not be from the geologic area. A recent landslide or erosional event in a stream are perfect examples.
Resources
Jennifer M. Wenner. Toilet Paper Analogy for Geologic Time Scale. Department of Geology, University of Wisconsin—Oshkosh. (http://serc.carleton.edu/quantskills/activities/TPGeoTime.html)
University of California Museum of Paleontology. Continental Drift. University of California Museum of Paleontology, University of California, Berkeley. (http://www.ucmp.berkeley.edu/geology/anim1.html)