magnetic polarity evidence for continental drift

magnetic polarity on the same continent with rocks of different ages

Geologists noted important things about the magnetic polarity of different aged rocks on the same continent: Magnetite crystals in fresh volcanic rocks point to the current magnetic north pole (Figure 1.2) no matter what continent or where on the continent the rocks are located. Older rocks that are the same age and are located on the same continent point to the same location, but that location is not the current north magnetic pole. Older rocks that are of different ages do not point to the same locations or to the current magnetic north pole. In other words, although the magnetite crystals were pointing to the magnetic north pole, the location of the pole seemed to wander. Scientists were amazed to find that the north magnetic pole changed location over time (Figure Can you figure out the three possible explanations for this? They are: The location of the north magnetic north pole 80 million years before present (mybp), then 60, 40, 20, and now. 1. The continents remained fixed and the north magnetic pole moved. 2. The north magnetic pole stood still and the continents moved. 3. Both the continents and the north pole moved.



magnetic polarity on different continents with rocks of the same age

How do you figure out which of those three possibilities is correct? You decide to look at magnetic rocks on different continents. Geologists noted that for rocks of the same age but on different continents, the little magnets pointed to different magnetic north poles. 400 million-year-old magnetite in Europe pointed to a different north magnetic pole than magnetite of the same age in North America. 250 million years ago, the north poles were also different for the two continents. Now look again at the three possible explanations. Only one can be correct. If the continents had remained fixed while the north magnetic pole moved, there must have been two separate north poles. Since there is only one north pole today, what is the best explanation? The only reasonable explanation is that the magnetic north pole has remained fixed but that the continents have moved.

magnetic polarity evidence

The next breakthrough in the development of the theory of plate tectonics came two decades after Wegeners death. Magnetite crystals are shaped like a tiny bar magnet. As basalt lava cools, the magnetite crystals line up in the magnetic field like tiny magnets. When the lava is completely cooled, the crystals point in the direction of magnetic north pole at the time they form. How do you expect this would help scientists see whether continents had moved or not? As a Wegener supporter, (and someone who is omniscient), you have just learned of a new tool that may help you. A magnetometer is a device capable of measuring the magnetic field intensity. This allows you to look at the magnetic properties of rocks in many locations. First, youre going to look at rocks on land. Which rocks should you seek out for study?


wegener was right

How does this help you to provide evidence for continental drift? To test the idea that the pole remained fixed but the continents moved, geologists fitted the continents together as Wegener had done. It worked! There has only been one magnetic north pole and the continents have drifted (Figure 1.4). They named the phenomenon of the magnetic pole that seemed to move but actually did not apparent polar wander. On the left: The apparent north pole for Europe and North America if the continents were always in their current locations. The two paths merge into one if the continents are allowed to drift. This evidence for continental drift gave geologists renewed interest in understanding how continents could move about on the planets surface.


instructional diagrams

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crystals that point in the direction of the magnetic field.

-->  a) magnetite

b) granite

c) peridotite

d) hematite

a device capable of measuring the magnetic field intensity.

a) magnetoscope

b) magnetite

-->  c) magnetometer

d) magnemometer

wegener died without knowing that continental drift was accepted.

-->  a) true

b) false

magnetite crystals in young volcanic rocks point to the geographic north pole.

a) true

-->  b) false

magnetic crystals in older rocks of the same age on the same continent point to

a) the current magnetic north pole.

-->  b) the same point, but not the current pole.

c) the magnetic south pole since the pole had flipped.

d) the geographic north pole.

older rocks that are different ages on different continents indicate that

-->  a) the continents have moved.

b) everything is stationary.

c) there are multiple north poles.

d) the poles do wander.

if the continents remained fixed while the magnetic pole moved there must have been two separate magnetic north poles.

-->  a) true

b) false

there is and was only one magnetic north pole.

-->  a) true

b) false

magnetite crystals pointing to the magnetic north pole show are evidence for continental drift if

a) the pole is moving across the landscape.

b) the pole is flipping with the magnetic south pole.

-->  c) the pole is stationary but the continent is moving.

d) the pole is stationary and the continent is stationary.

what line of evidence made scientists realize that wegener was right about continental drift?

a) the puzzle-like fit of the continents.

b) the distribution of exact fossils on widely separated continents.

c) the change in polarity of the magnetic field from normal to reverse.

-->  d) apparent polar wander.

diagram questions

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