PHYSICAL SCIENCE 103 HOMEWORK 5 ANSWERS
GEOPHYSICS (PLATE TECTONICS, EARTH'S MAGNETIC FIELD)
FALL 1999

CONCEPTUAL QUESTIONS

1. When Alfred Wegener's 1915 book was translated into English in 1924 many geologists read it, and there was much strong criticism of Wegener's theories. What was the major weakness in Wegener's arguments?

Wegener could not explain how the "continental drift" which he proposed could occur. He could not explain how a continent could plow through solid rock.
2. A compass is shown to the left below as it points toward the magnetic pole of the earth which is near the north geographic pole. To the right the compass is shown placed next to one pole of a bar magnet. Is this pole of the bar magnet a north pole or a south pole?

The pole of the bar magnet is a north pole.

3. List the four types of evidence for continental drift presented by Wegener in his book.

  1. The "fit" of the continents, like a giant jigsaw puzzle.
  2. Fossil evidence from identical plant and animal fossils found on two continents now separated by a large ocean.
  3. Rock types and structural similarities, such as mountain ranges, found on two
     continents now separated by a large ocean.
  4. Paleoclimatic evidence from evidence of glaciers and ice sheets in regions that are now warm to fossils of tropical plants and animals in regions which are now cold.

4. Was Wegener correct when he suggested that the continents drift around on the Earth?

Not exactly. The tectonic plates move, and a typical tectonic plate will be partly continent and partly ocean bottom. The continents move with the plate they are a part of. So continental drift is not the correct word to describe what is happening.

5. Which tectonic plate do we live on?

The Pacific Plate

6. There are three types of plate boundaries. Name the type of boundary for each of the locations given.

a) The Mid-Atlantic-Ridge

a divergent boundary

b) The Peru-Chile trench

a convergent boundary

c) The San Andreas Fault

a transform boundary

7. Where would you expect to find the oldest rock under the Pacific Ocean?

Near the Aleutian trench, because the ocean bottom was produced at the East-Pacific-Rise and moves toward the Aleutian trench where it subducts. The oldest
 sea floor would be at the location just before subduction occurs.

8. If geophysicists find a sample of volcanic rock which is magnetized in a horizontal direction, what latitude was this rock at when it cooled below its Curie point and became magnetized by the Earth's magnetic field?

This rock must have been at the magnetic equator of the Earth when it cooled.

9. Where would the Pacific Ocean Bottom have the thickest sediment on it?

Near the subduction zone at the Aleutian trench, because that is the oldest ocean bottom and the sediments will have collected to the greatest thickness on the oldest ocean bottom.

10. Explain how geophysicists know that the direction of motion of the Pacific Plate changed about 40 million years ago.

As the Pacific Plate moved across the hot spot near Hawaii a series of volcanoes created the Emperor Seamounts and the Hawaiian Islands, and this chain of seamounts and islands has an abrupt bend in it which has been dated to about 40 million years ago.

11. Explain the most direct evidence for sea floor spreading. This evidence also showed geophysicists that plate tectonic theory was correct and that the plates were being created at mid-ocean-ridges.

Paleomagnetism of the seafloor showed regions of the same width and magnetic polarity on each side of the mid-ocean-ridges. This pattern showed that the seafloor was being created and magnetized as it cooled and moved away from the ridge.

12. How can geophysicists figure out when the Earth's magnetic field has reversed in the past?

The paleomagnetism of the seafloor rock shows the reversals as do some lava flows on land. When the rocks at the locations where the field is reversing can be dated, those dates tell us when the magnetic reversals occurred.

13. If a dip needle points directly down, where on the Earth is that dip needle located?

Over the magnetic pole near the north geographic pole, since this magnetic pole is presently a south magnetic pole.

14. If you were a girl/boy scout in the state of New York, you would probably know that a compass does not point exactly toward the north geographic pole. Instead the compass points somewhat . . . . . of geographic north (also called true north). This difference between true north and magnetic north is called the . . . . .

west, magnetic declination

15. How can we most easily map the boundaries of the Earth's tectonic plates?

Earthquakes occur much more frequently along the plate boundaries than at other locations, so a map of earthquake epicenters shows the plate boundaries very clearly.
16. Draw the magnetic field of the Earth. The letters ngp stand for north geographic pole.

17. Where would you look for the oldest rocks on earth, on the seafloor or on the continents? Explain why.

On the continents, because the seafloor subducts and is recycled. Continents are recycled slowly too by weathering and erosion, but in some locations rocks about 4 billion years old can still be found.

18. Name a region on Earth of each type.

a) An island arc.

The Japanese Islands

b) A continental-continental collision producing a range of high mountains.

The Himalayan Mountain range produced as a continental section of the Australian-Indian plate rams into the Eurasian plate.

c) An oceanic-continental convergent boundary near us which produces volcanic activity on the continent.

The example closest to home would be the Pacific Northwest where the Juan de Fuca plate is subducting under the North American plate producing the volcanic from northern California into British Columbia.

19. Earthquakes occur at various depths. The subduction zone east of Japan is called the Japan trench and the earthquakes produced by the subduction at this convergent boundary mostly occur west of the trench. Would the deepest earthquakes be those with epicenters closest to this trench, or farthest west of this trench?

Farthest west of the trench.


QUANTITATIVE PROBLEMS

20. The motion along the San Andreas Fault is about 2.0 cm per year. The last earthquake on the section of this fault which passes through San Francisco was in 1906. The fault has been stuck since then but the land on either side of the fault has been moving along faithfully. If the fault comes unstuck tomorrow across San Francisco and suddenly moves to make up for all of the accumulated motion, how far will the fault move?

d = vt = (2.0 cm)(1996 - 1906) = 180 cm = 1.8 m



21. Use the following table to answer these questions about the motion of a tectonic plate. This table gives some data on sea floor basalt rock samples from drill cores taken at various distances from the mid-Atlantic ridge.

sample number   age in millions of years   distance from the ridge in km
      1                     7.1                         120
      2                    32.5                         550
      3                    53.2                         900
a) Determine the average velocity of the three samples in centimeters per year. Use the fact that
1 km = 10^5 cm. Use scientific notation (powers of 10) if you know how (you really should learn scientific notation if you don't know it yet). In case you don't, I will tell you that 105 = 100000. The basic equation you need to use is simply d = vt (distance = velocity x time), but expressed as v = d/t .

velocity of sample 1:

v = d/t = (120 x 105 cm)/(7.1 x 106 y) = 1.69 cm/y

velocity of sample 2:

v = d/t = (550 x 105 cm)/(32.5 x 106 y) = 1.69 cm/y

velocity of sample 3:

v = d/t = (900 x 105 cm)/(53.2 x 106 y) = 1.69 cm/y

b) Is the data in the table on rock age versus distance from the ridge consistent with the plate tectonic model? Explain clearly why or why not.

Yes, the data is consistent with the plate tectonic model. A tectonic plate should move as one piece, and the fact that the velocities of regions of the plate which are at different locations are all the same indicates that the plate is moving as one solid piece.

22. The Mid-Atlantic-Ridge is about the same distance from North America as it is from Europe. This distance is about 2500 km. Using this fact and what you learned from the previous problem you should be able to calculate about how long ago these two continents began to drift apart. I will help you a little by calculating that 2500 km = 2.5 x 108 cm.

t = d/v = (2.5 x 108) /(2)(1.69 cm/y) = 0.74 x 108 y = 74 x 106y = 74 million years I used a speed of two times 1.69 cm/y because each plate is moving away from the the Mid-Atlantic-Ridge at about this rate.

23. The Pacific Ocean bottom stretches approximately 8000 km from the East-Pacific-Rise where it is produced to the Aleutian trench where it is subducted. Calculate how old the oldest part of this ocean bottom is. Assume the Pacific Plate moves about 5 cm per year.

t = d/v = (8000 km x 105 cm/km)/5 cm/y = 1.60 x 108 y = 160 x 106 y = 160 million years