Seafloor Spreading and Hot Spots
Sea-floor spreading — In the early 1960s, Princeton geologist Harry Hess proposed the hypothesis of sea-floor spreading, in which basaltic magma from the mantle rises to create new ocean floor at mid-ocean ridges. On each side of the ridge, seafloor moves from the ridge towards the deep-sea trenches, where it is subducted and recycled back into the mantle (Fig. 2). A test of the hypothesis of sea-floor spreading was provided by studies of the Earth's magnetism.
Who was Harry Hess?
Sea floor spreading model. (from "Sea-floor Spreading Made Easy" by C. McLelland)
Materials: 2 crayons (different colors), 2 sheets of white paper
1. Place two sheets of paper in the space between two desks (the spreading ridge).
2. Pull paper up through the space between the desks, a little at a time and at the same (slow) rate.
3. Stop about every 5 cm, and color the space at "the spreading ridge". (new crust forming; the colors represent polarity; choose one color to represent normal (N) polarity and the other color to represent reversed (S) polarity). (See image below).
4. The model is finished when the last part of the paper emerges through the desk space and is colored.
5. Tape the ends of the paper together, and label the following:
a. rift zone
b. which color represents normal (N) polarity and which color represents reversed (S) polarity
c. oldest rocks
d. newest rocks
6. On the back of the model, write group names and a detailed summary of what the model represents.
7. Use an iPod to take a picture of the model. Insert picture to a Google Presentation (Level 4).
Level 4: Analyze
8. Create a Google Presentation. Share with your team and Mrs. Wheeler.
Slide 1: (Title Page) Seafloor Spreading; names of group members
Slide 2: Picture of model
Slide 3: Summary of model
Slide 4: Answer Question 1: The Earth is about 4.6 billion years old. Based on observations of your sea-floor spreading model, why do you think that the oldest ocean floor is only about 200 million years old?
Slide 5: Answer Question 2: On the real ocean floor, alternating stripes of normal and reversed polarity are not all of equal width. What does this tell you about the lengths of time represented by normal and reversed polarity?
NASA Article about Magnetic Reversal
Who was Tuzo Wilson?
Hot Spots! Determining the Rate of Plate Motion
