The Earth’s
magnetic field – the final piece of the puzzle
EPS 106, Dec. 7th, 1999
The final piece of the puzzle relates to the Earth’s’ magnetic field. Because of the liquid core, there is a
magnetic field that is closely oriented parallel to the rotation axis of the
planet. The line of magnetism are shown
in the following figure. A free
floating bar magnet (one held up on a string) would be perfectly parallel with
the magnetic field.
There are minerals in rocks that behave as small magnets, particularly an iron oxide called magnetite. Magnetite is found both in sedimentary and igneous rocks. But magnetite is only magnetic at low temperatures. Above 580°C, magnetite is above its Curie point, a temperature above which the minerals are not magnetic. The thermal agitation of the minerals is too high. Below this temperature, the small domains which constitute microscopic magnets, line up with the Earth’s magnetic field.
If a lava erupts on the Earth’s surface at high temperatures (above 700°C), the magnetite will not be magnetized. As it cools below 580°C it acquires the Earth’s magnetic field. This it will keep until the rock is destroyed, as the magnetite grains are locked into place. The science is called paleomagnetism.
Not only can we reconstruct the direction to north, on the basis of declination (the degree of vertical tilt of the magnetic direction) we can reconstruct where the north pole actually was. Samples at the equator will have no vertical declination. Samples at the North Pole will point straight down, samples at the S. Pole will point straight up.
In the 1950’s, geophysicists began reconstructing the position of the N. Pole through time.
- Determine the age of a rock
- Measure its paleomagnetic magnetization
- Calculate where the North Pole was at the time the rock cooled below its Curie temperature.
Strangely, the found that the pole appeared to wander with time, although this didn’t make sense, as the pole and orbit or rotation were thought to be the same.
Weirder still was that the apparent position of the North Pole from North America and Europe didn’t agree. Only when the two were placed next to each other, did they line up.
This was further evidence that the plates lined up. The coup de grace was based on another weird discovery. There are apparent reversals in the direction of polar north. Over a very short time interval, the north pole points south, and the south pole points north. No one quite knows why this flip occurs, but it does so periodically. We can date rocks in a lava flow and measure their polarity, thus developing a time scale on the basis of whether it is a normal or reversed polarity.
When ocean measurements of the polarity of the Atlantic ocean were made, geologists were astonished to find that there was a regular flip in polarity, and that the flips were symmetrical about the Mid-Ocean ridge (see following figure). How could this be?
With this final piece of evidence, the seafloor spreading model was constructed. In it, the ocean spread apart at the ridge, creating new ocean floor from the mantle, with older material being pushed outward symmetrically from the center.
But where does the crust go? We can’t have a continuously expanding crust unless the Earth is growing (which is not happening). The answer lay in the earthquakes that are found in the “ring of fire”. These are subduction zones, areas where the cool oceanic crust is sinking back down into the mantle. The answer to this question was solved by seismologists, geophysicists who study earthquakes. They recognized that there was a definite spatial distribution of earthquakes along plate boundaries.
Thus we have areas of newly-created crust (spreding centers) and areas
of consumption of older oceanic crust (subduction zones). The subduction zones also explain why there
are volcanoes just ‘landward’ of the subduction zone. The subducting slab dewaters and the water floats upward causing
melting and volcanism.
What drives this cycle? It is the heat in the mantle that forces convection which pushes the overlying slabs outward. Crust is formed in the oceanic plates, and, because it is heavier (especially as it cools) than the continental crust, it sinks under the continents. (Something has to give!).
Why do the continents float? They are lighter, and ride higher, like a cork. They float on the asthenosphere, while the oceanic crust, being denser, sinks.
The Earth can be divided up into a number of plates, each defined by a plate boundary (Fig. 4.15 in book). Up to now, we have two kinds of plate boundaries, spreading centers and subduction zones. But there are others. What happens when two continental plates collide, such as the example where the Australian-Indian plate is colliding with the Eurasian plate?
Neither of the two continents wants to sink. So they bunch up against each other, forming tremendous mountain chains. The Himalayas are the type locality for this type of collision. A final collision is when two plates slide past each other. There isn’t always a direct collision. These cause earthquakes, such as in California along the San Andreas fault.
The four types of plate boundaries are the following:
1) Divergent margins (spreading centers)
2) Convergent margins- subduction
3) Convergent margins – collision
4) Transform fault margins
Finally, we can have divergent margins occurring under the continents. First a rift valley opens up and then eventually new sea floor is formed, creating an ocean. This is happening in New Mexico right now. We will eventually have seaside housing!
What about Hawaii? Hawaii is a hot spot, and tracks the movement of the oceanic crust over the fixed heat source.
