Download Plate Motions: Clues from the Sea Floor

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Plate Motions: Clues from the Sea Floor
We have been exploring some of the evidence that Earth’s lithosphere is broken into plates that are in constant
relative motion. We have explored such questions as: What causes the plates to move? How fast do they move,
and where have they been in the past? As you may recall from lectures, much of the evidence leading to Plate
Tectonic Theory came from defense-related mapping of the sea floor in the 1940’s and 1950’s. Before then,
almost nothing was known about the rocks of the ocean floor, and there was little reason to suspect that the
geology of the ocean floor was much different from that of the continents. But the ocean floor turned out to
be a wholly unfamiliar realm. The discovery of the global mid-ocean ridge system was among the first surprising
results of the sea floor mapping projects. Another surprise was the spatial pattern in the ages of the ocean
floor rocks.
In this assignment, you will be using information from class notes, the provided figure showing the ages of
oceanic crust around the world, and an article about sea turtles to answer the following questions. You may print
out this worksheet and submit handwritten answers if your work is neatly and clearly written. If you are
concerned about neatness, please type your work. Basic rule for grading: If I can’t read it, I can’t grade it.
The above diagram shows the ages of sea floor (oceanic crust) for the world’s oceans. The color coded key
provides a reference to approximate age ranges. Use this figure to answer the following questions.
1.
Examine the map showing the age of oceanic crust around the world.
Where, in general, does the oldest oceanic crust occur:
along the margins or in the central portion of the ocean basins? (circle the correct answer)
What is the estimated age of this old crustal material? (Remember to include age units!)
2. It is not unusual to find continental crust that is 2 billion years old. Some process must prevent ocean crust
from reaching such an old age. What is the process? (Be specific!)
The increasing age of oceanic crust with distance from the mid-ocean ridges records the continuous creation of
new lithosphere by volcanic activity at the ridges. As magma wells up and is extruded at a ridge, existing
volcanic rock is displaced progressively outward away from the ridge crest. By this process of sea floor
spreading, oceans grow and continents move apart.
3. Study the ages of ocean floor rocks in the Atlantic Ocean.
Where does the youngest oceanic crust occur:
along the margins or in the central portion of the Atlantic basin? (circle the correct answer)
Where does the oldest oceanic crust occur:
along the margins or in the central portion of the Atlantic basin? (circle the correct answer)
4. Study the ages of ocean floor rocks in the Atlantic Ocean.
Compare the northern and southern sections of the Atlantic Ocean.
Which section of the Atlantic Ocean (north or south) shows the earliest evidence of sea floor spreading?
What evidence from the map can you use to make your decision?
Read the article about Sea Floor Spreading and Green Sea Turtles to answer the following questions.
5. The Green Sea Turtle has a migration pattern that involves traveling great distances between separate
feeding grounds and nesting grounds. Based on the information in the article, what might be the advantage
to having separate feeding and nesting grounds?
6. The Green Sea Turtle (Chelonia) and its ancestors have long inhabited the region presently known as
northern South America. According to the article, the turtles travel about 2000 kilometers between
modern nesting and feeding grounds. What was the migration distance 70 million years ago?
7. The change in migration distance can be attributed to sea floor spreading. Calculate the average spreading
rate (in centimeters per year) based on the change in distance compared to the amount of time elapsed.
Hint: Change in distance is distance traveled today minus distance traveled in the past. Time elapsed is 70
million years. This is a simple distance equals rate multiplied by time…make sure your units are consistent.
8. According to the article, how are the islands produced?
Look at a world map and find an example of another island produced in this same manner.
(Hint: Hawaii is not an example!)
9. The article mentions that the turtles must move to other islands over time.
Explain what happens to the older islands; why can the turtles not continue to migrate to the older islands?
(Hint: Remember, the islands are volcanic in origin.)