Alessandro Grippo's Earth Sciences Pages

Alessandro Grippo, Ph.D.

A key to lab 6 (Earth History), Sea-Floor Spreading and Plate Tectonics

answers to Lab 6 questions Last Updated  •  October 8, 2016   


question 1, page 55
Younger (it is closer to the mid-ocean ridge)

question 2, page 55
Because the ocean spreads in opposite direction at essentially the same rate

question 3, page 55
Most sediment in the ocean is terrigenous sediment; it comes from land, and is deposited in shallow water.
In addition, the edge of an ocean (passive margin) is older than the middle of the ocean, so there is more time for sediment to pile up on the shelf or in the rise, at the foot of the continent.
Once you have a wide ocean, its deeper parts are too far from land to receive much terrigenous sediments, and there would be deposition of either calcareous or siliceous ooze,or both, or very thin layers of red pelagic clay.

question 4, page 56

  • No
  • Because you cannot put sediment on top of materials that do not exist yet

question 5, page 56
This answer is similar to that provided for question 3. But you have to take into account that even pelagic sediment thickness increases away from the ridge, because with time we are creating more and more oceanic crust that will only eventually receive sediment (chalk, siliceous ooze, or red clays)

question 6, page 56
- Distance measured on the image is about 7 mm. At a scale of 1 mm per 50 km, the distance from C to the ridge is 350 km.
- If the age of basalt at C is 10,000,000 years, the rate of separation would be expressed by the ratio 350 km / 10,000,000 years.
- You need to express the rate in cm/yr, so you need to convert km in cm: 350 km = 35,000,000 cm.
- 35,000,000 cm / 10,000,000 yrs = 3.5 cm/yr

question 7, page 56
- You can measure the distance of W from C (23 mm) or from the ridge (30 mm).
- In the first case, the distance will be, at a scale of 1 mm per 50 km, 1150 km; in the second case 1500 km.
- Converting these two distance to cm, you obtain respectively, 115 million cm and 150 million cm.
- Time (age) is distance over speed, hence 115 million cm / 3.5 cm * yr = 32.86 million years.
- (This number is the difference in age between C and W, and must be added to the age of C (10 my/), which adds up to 42.86 million years)
- If instead you used distance from the ridge, 150 million cm / 3.5 cm * yr = 42.86 million years.

question 8, page 56
The isochrons should be labeled with the numbers you obtained through questions 6 and 7 above (10 my and 42.86 my)

Back to Top | Back to Labs | Back to Images and Notes | Back to Home Page

© Alessandro Grippo, since 1994
Los Angeles, California