LECTURE 17

PALEOMAGNETICS AND THE DEVELOPMENT OF CONTINENTAL DRIFT; PLATE TECTONICS - WHY IT IS A WINNING HYPOTHESIS!

The Earth has, in general, (since geology became a science) been viewed as a stable thing with respect to the general position of its continents and ocean basins. By now we think of continents drifting across the Earth and new oceans forming and disappearing. This is a radical change of opinion and I want to see how this alteration in thought came about.

First, keep in mind that there have been, since geology started, only four general hypotheses to explain the Earth. When I was in college, the idea popular (at least in the United States) was the continents and ocean basins stayed about where they are through all time. The idea was that continents could not drift through solid rock and the geophysical evidence about the mantle was it transmitted S waves and therefore, had to be solid. A second idea, especially popular in the 19th century, was that the Earth started hot and then shrank and compressed the mountains. Most mountains are compressional and so the idea had some appeal. However, it was eventually shown that the folding of mountains was too great to be explained by shrinkage and the idea died at the dawning of the 20th century. A third idea was the expanding Earth (phase changes in minerals to lighter structures). This was popular in the 1940's and fifties because some of the data supporting plate tectonics helped the idea, but new data (subduction -- see below) killed the idea. The winner now is plate tectonics (explained below) but is essentially continental drift with the explanation of many otherwise peculiar facts.

I personally learned in college that Earth was stable and Wegener, who constructed a comprehensive theory of continental drift, was thought to be silly. In 1957, I was already teaching at the University of Illinois in Chicago (then CUD), and went to Northwestern to hear a "talk" by S. K. Runcorn (a Physics Professor at Newcastle upon Tyne). I had heard he was interesting and was using paleomagnetics to defend continental drift. I assumed this had to be baloney because I had so learned that in college. Nonetheless, I went anyway.

The talk was very impressive (and I don't often learn much from them). The idea was based on the concept that you could find the former magnetic directions on Earth and from this determine past direction toward the magnetic poles.

This is based on the following. Well before Christ it was recognized that compasses (lodestone then) pointed on Earth toward the North Pole (actually the North magnetic pole that is slightly south of the true pole). Originally, since the stone (now needle) pointed north they called it the north end and the other end was south. On a compass, that is the north seeking end of the needle. We understand magnets better now. That is in a bar magnet (or an electromagnet) the north end is attracted to a south end of another magnet and is repelled by the north end. Thus the north seeking end of one's compass must be a south end of a magnet. However, we retain the several thousand year old practice of calling it the north end. One does want to keep this in mine while discussing magnets and not get confused.

Well enough for the present, but what about rocks? In France, in 1907, an engineer noticed that bricks retained the direction of magnetism that they got while they cooled. Recall bricks are sharply heated (a practice that removes all prior magnetism) and then pallets of them are taken out of the oven and left to cool.

The engineer noticed that the bricks picked up, on cooling, magnetism induced by the earth's field that was oriented like the Earth's field. When the bricks were moved (particularly -- rotated when moved) they retained the originally induced direction of magnetism. This observation was mostly ignored over the years, although some people in Japan studied the phenomena in rocks a little. After the war (World War II), Runcorn and some others became interested.

First of all, may rocks contain iron and other magnetic materials. These, in the simplest case are magnetite (a common accessory mineral in all major forms of rock. Beach sand, limestones, shales, granite, basalt, many metamorphics etc. all have some magnetite. However, there are other magnetic materials (e.g., amphibole and pyroxene etc.) and so generally all rocks have magnetic material. In a sediment, the materials settle into the sediment and take on the magnetic orientation of the Earth at the time.

Essentially all rocks form with recognizable paleomagnetic directions when formed. Then these rocks can be dated and collected in carefully oriented samples (typically small cores) that reveal former directions to north and south Also, because of the different tilt at different latitudes, former latitudes are revealed.

Runcorn's work revealed that the direction toward north for each continent changed through time and moreover (see illustration) the direction toward the north magnetic pole was different and diverging more and more through time. here are only two possible explanations. The first is that every continent has a different magnetic pole (an impossible situation), or second that the continents moved with respect to each other.

This for me in 1957 was a very good argument -- this was particularly so because Runcorn also looked well into all the ways that paleomagnetism could go wrong. From that moment on I accepted continental drift.

Plate tectonics is the refined idea that integrates ocean basin rock formation, spreading ridges, subduction, the idea that continents are permanent (slightly overstated) while ocean basin rocks are temporary. In plate tectonics rocks (basalt) are formed along the ridges. As new rocks are formed there the older ones drift away and become cooler. The ocean basin gets deeper farther away from spreading because the heat flow and temperature are less farther away and so the rocks are denser and sink down isostatically further. Meanwhile, because the rocks away from the ridges are older, more sediments has accumulated on them and so the sediment pile gets thicker. Volcanic peaks follow a similar history. They form and are active near the ridge and then drift away into deeper water. Waves plane off the tops and shallow water animals form there and are killed, while the tops are buried (guyots) as the peaks are forced away from the ridge. Meanwhile, calcium carbonate forms in the shallow sediments near the ridges and drifts out into deep water and is buried by younger sediments. In the ocean, below about 4500 meters (15000 feet) CaCO₃ does not persist on the ocean bottoms because deep ocean chemistry dissolves it. However, this buried CaCO3 (mostly dead animals) surprised people, but was explained by the drift of materials from the ridges into deep water.

The navy in the 1940's noticed a complex magnetic pattern in the ocean bottom. Later, part of this pattern was shown to be magnetic reversals (of the earth's magnetic field). The pattern on either side of the ridge (spreading ridge) is nearly a mirror image. This further emphasizes the spread of rocks from the ridges. Of course the ages of the reversals comes from the ages of the rocks.

Interestingly, most of the items above were first described in the Atlantic ocean and were absolutely, by themselves (along with paleomagnetic striping), consistent with the "Expanding Earth" hypothesis. However, the Pacific ocean (and the Caribbean, but not most of the Atlantic) has subduction, and subduction is absolutely inconsistent with all hypotheses except plate tectonics. In plate tectonics, the cooling plates become denser and are drawn down into the mantle, mostly at continental boundaries. These subduction zones have plates covered with ocean sediments (including silica from deep water life) and so the surface is scraped off by the overlying crustal material creating a deformed mix of sediments (including chert and some carbonates) basalt and gabbro altogether called ophiolites. The deeper parts of the top surface of the subduction plate get under enormous pressure during subduction. At the same time the temperature there is somewhat low in that a subducting plate is cold. The result is high pressure and low (somewhat) temperature metamorphism which results in blue schists. Subduction also creates the deep ocean trenches associated with subduction along with the important gravity anomalies.

Most earthquakes in the world are associated with plate tectonics. Spreading ridges and transform faults (associated always with spreading ridges) have only shallow earthquakes because their effects are near surface events. Faults do occur at both places -- normal faults (grabens) at spreading ridges and strike slip as a part of transform faults, but these are all shallow. Subduction zones have earthquakes of all depths. At the surface where subduction starts the faults are shallow; progressively they get deeper and deeper as the subduction plate deepens.

For example a subduction zone starts just off most of the west coast of South America and so the earthquakes there are shallow. The plate gets deeper and deeper under South America and so the earthquakes get deeper and deeper for a quarter of the way through South America. Similar depth changes progress from the South East coast of Japan toward China. In fact all subduction zone earthquake depths are organized in that fashion.

Geophysicists used to say continental drift was impossible because how could solid rocks sail through solid rock (evidence was the S wave penetrated the mantle showing it was solid). However the aesthenosphere was discovered as they came to understand that there was a low velocity zone for both the P and S waves in the upper 5 or 600 miles of the mantle. This discovery was late in arriving because all crustal and upper mantle velocities are hard to obtain because of the confusing records caused by variable depths to earthquakes. Anyway, since both P and S waves slow down in weaker materials, this low velocity zone was finally analyzed as a weak zone.

Heat makes rocks weaker -- eventually, as heat increases, they melt. Pressure, on the other hand, makes rocks stronger and raises the melting point. In the aesthenosphere, pressure has increased but barely enough to keep the rocks of the upper mantle from being melted. The idea is the weakness there is caused by partial melting.