Tag Archive: plate tectonics


Plate Tectonics Index

Here are the links to posts on Plate Tectonics. I have more in mind on this subject.

Plate Tectonics Index

Plate Tectonics Part I 7/1/11
Plate Tectonics Part II 7/8/11
Plate Tectonics Part III 8/5/11

Plate Tectonics Part III

The next “a-hah!” moment for me in understanding geophysics came in 1965 with a paper by J. Tuzo Wilson, “A new class of faults and their bearing on continental drift.” At the time it seemed a light had gone off in my head, even though by then I was deeply enmeshed in ice fog studies.

The red lines are spreading centers; the blue line is a transform fault which has its origin in the offset between portions of the spreading center. The part of the transform fault between the offset segments is active; the plates are sliding past each other. The parts to either side are "fossil"--although the offsets are still visible, there is no movement.

Up to that point, it was assumed that if a feature—such as a stream, a mountain range, or a magnetic stripe—was displaced by a fault, it was evidence that they had once been continuous, and movement along the fault had separated them. This paper suggested the existence of a new kind of fault, now called a transform fault, where the apparent displacement was due to an offset in a spreading center. Spreading centers were themselves new, though grabens (places where the crust on either side was moving apart, resulting in a downdropped block in the middle) had previously been recognized.

Suddenly the apparent faults in the Pacific Ocean floor, faults that stopped at the coast, made sense. The rocks on either side of the transform fault did not move relative to each other. The only motion was between the offsets on the spreading center.

We now know that plates are bounded by three kinds of faults:

Spreading centers, where plates move apart,

Converging boundaries, where plates collide,

Transform boundaries, which often connect the two, and allow plates to slide past each other.

The familiar faults such as the San Andreas are the active portions of transform faults, but the offset magnetic stripes in the Pacific are generally the “fossil” parts of transform faults. Looked at this way, the “end” of an active fault makes sense: it is where the transform fault joins another type of fault to make part of a plate boundary.

The full plate tectonics hypothesis had not yet been fully formulated, but the essential pieces had been found.

Plate Tectonics: Part II

My next step in understanding geophysics came when my father took me along to a lecture. At that point I was somewhat immunized against continental drift by the professor at Harvard, in spite of my unanswered questions. Since the topic of the talk was something to do with continental drift, I was prepared to be quite critical. Remember this was in the early ‘60’s, before the idea of plate tectonics. The notion that sea floor was the youngest, not the oldest, crust on Earth had not even entered anyone’s mind, and the lack of traces of the continents plowing through the sea floor seemed definitive.

That lecture totally changed my attitude.

The lecture was about paleomagnetism, the fact that when lava cools, it retains the signature of the terrestrial magnetic field present at the time. The horizontal part of the field gives the direction to the north pole; the vertical part gives how far the pole is from the site — the latitude. There were complications – sometimes two lava flows close enough in time and space that they should have pointed to the same pole had exactly opposite directions. But if the north and south poles were considered interchangeable it was found that rocks of the same age on the same continent pointed to a consistent pole location at any given time in the past.

(Why the magnetic signature seemed to reverse at times was a mystery at the time and is still not totally understood, though it now known to be a reversal of the magnetic field rather than a reversal of the magnetism of the rocks.)

The next step was to produce what are called apparent polar wander curves: plots of how the pole moved through time as seen from the site of the lava flow. Again, it was found that these curves were quite consistent for a given continent. (There are exceptions, but I’ll get to them in a later post.)

But the curves for different continents were quite different.

In particular, if the curves for north America, Africa and Europe were compared, and the continents were assumed to move in such a way that they “saw” the same pole, those continents must have been snuggled together back in the late Triassic.

I walked into that lecture convinced that the apparent fit of the continents (and the geology) of the continents across the Atlantic was a coincidence, and that Wegener’s continental drift hypothesis was wrong. Certainly his mechanisms were; there was no evidence that continents had ever plowed through seafloor. But I walked out convinced that while Wegener’s hypothesis was wrong in detail, the continents had indeed moved.

But how?

Formally, I shifted my studies for the next few years to ice fog. Informally, I kept trying to make sense of  solid-earth geophysics. Could there be some sort of underground erosion going on? What about those faults with hundreds of miles of displacement that disappeared when they reached continents?

Luckily, the major journal of my field was the Journal of Geophysical Research, so I was able to follow the steps people were making in the gradual emergence of plate tectonics. More of that later – but in the order I remember finding out, rather than in the order the discoveries were made.

This collection, containing 13 programs on 4 discs, looks at the geological history of specific places on the Earth. The series is grounded in plate tectonics and geological activity, with plate tectonics and glaciology being foremost.

In some ways it is a good introduction to how geological forces act, but as a geophysicist I do have some caveats.

First, while the geological dangers are very real, the program has a tendency to emphasize the “this could happen tomorrow” aspect. There is very little emphasis on what we can do to minimize the effects of possible geological disasters. (All right, there’s not much we can do if Yellowstone blows again, but things like evacuation routes and plans for tsunamis and engineering for earthquakes are hardly touched on.)

Second, the programs do not distinguish among kinds of faults and plate boundaries. While mid-ocean ridges are recognized as divergent boundaries, the difference between transform boundaries such as the San Andreas Fault and convergent boundaries is never clearly described. Nor is the difference between ocean-continent convergence (which produces ocean trenches, volcanoes and massive earthquakes) and continent-continent collisions (responsible for the mountain belt extending from the Himalayas to the Alps.) The latter are capable of producing far greater earthquakes, as the potential area of breakage is far larger, and are also responsible for the “ring of fire” around the Pacific.

Third, the story of each region is told as if the scientists just had to find the missing pieces, and as if they knew what they were looking for. In many cases, the findings were a total surprise and the interpretation we accept today is quite different from what the researchers who found the information at first tried to make of it. I know — I was watching as plate tectonics gradually became the accepted framework of geology.

Overall the series is worth watching if you have any interest at all in how the world came to be as it is today. But take it with a grain of salt – the writers of the narration didn’t always know what they were talking about.

Individual programs are:
The San Adreas Fault
The Deepest Place on Earth (Challenger Deep)
Krakatoa
Loch Ness
New York
Driest Place on Earth (Atacama Desert)
Great Lakes
Yellowstone
Tsunami
Asteroids
Iceland
Hawaii
The Alps

I’m sure you’ve heard, ad nauseum, about the plate tectonics underlying the earthquake and tsunami in Japan. Indeed, it seems that plate tectonics, which produces earthquakes, volcanoes and tsunamis with devastating consequences is a force of destruction, pure and simple. But does it have a positive side as well?

The theory of plate tectonics, which at this point does the best job of explaining the earth’s geology, is based on the idea that the earth’s surface is made up of a number of semi-rigid plates which slide around over the earth’s surface. They interact primarily at their edges, where they may be pulling apart (as in the mid-Atlantic and the African rift valleys) sliding past each other (as in the San Andreas fault of California) or colliding.

Plates are made up of ocean crust, sometimes with relatively light continental crust on top. Ocean crust is dense enough to slide under other plates; the lighter continental rock above it resists being pulled under, and buckles or folds if it is on top of two colliding plates. Thus collisions of two plates with continents on top generally leads to mountain ranges such as the Himalayas.

Collisions between ocean plates and plates with light continental rock atop generally lead to subduction zones, such as the one off the west coast of South America, where the oceanic crust is pulled under the lighter continental crust. The sediments pulled down with the ocean crust are gradually heated and melted, reappearing as volcanic magma. Thus the volcanic spine of the Andes.

If two oceanic plates collide one is normally pulled under the other, but it is less obvious which will be subducted, and in fact this may change over time. The same melting of sediments occurs, and a line of volcanoes, such as the Aleutian Islands, normally develops next to the subduction zone.

Plates don’t slide past each other smoothly. They stick and then break loose, producing earthquakes. If they are just sliding past each other they may produce earthquakes but there is generally not much vertical movement. If one plate is being pulled under another, however, the sticking normally results in a bowing up of one plate, and when that sticking is released, there may be considerable vertical movement. If that movement is under water, a tsunami is created. This is what happened with the great Alaska earthquake, and has now happened off the coast of Japan.

But what would happen if the plates all just stopped? If there were no more plate tectonics? More, if there had never been any plate tectonics?

First, the earth would be flat and completely covered with water, if there were any water on the face of the earth. Mountains are constantly being eroded by the forces of weather. Given far less geologic time than has actually passed, any initial irregularities in the surface of the earth would have been smoothed out. Plate tectonics is and has been the main mountain builder on our planet.

Second, there is some question as to whether we would have an atmosphere. Certainly we’d have a hard time breathing the mixture of carbon dioxide, water vapor and other compounds put out by volcanoes, but then we’d have a hard time breathing the atmosphere prevailing when life evolved. Plants convert the gasses produced by volcanoes into an atmosphere we can breathe.

Third, plate tectonics is part of the way radioactive heating in the earth’s core is transferred to the surface. It’s one of the reasons we don’t have the radical resurfacing we think we see on Venus.

Plate tectonics can certainly produce devastation, but like weather, it’s something we have to live with. Japan has actually done a superb job of preparation, but there are prices we must pay for living on a dynamic planet, one which can support life. One of those prices has just come due.

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