GeologyRocks

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Jon
Imperial College London

Introduction

The evidence for plate tectonics comes many fields, such as palaeontology, geophysics and climate study. The first evidence was the matching of continents and their rock types across oceans. This had been realised since the 17th Century. Theories of how continents moved were ad hoc at best. Alfred Wagner, an early 1900's German Meteorologist proposed that the continents of South America, Africa, India and Australia had been one continent in the past due to their similarity of fossils and palaeoclimate. Wegner plotted the position of the poles for these climate belts. The climate belts had moved or the continents had moved in order to fit these belts together. However, no mechanism was present for the continents to move, so Wegner proposed that the continents moved through the ocean crust, forming a mountain belt at their front edge. Not many geologists believed him though, especially in the U.S where most geologists were at that time.

Magnetic Stripes

However, it wasn't until the 1960's when Fred Vine and Drummond Matthews, two British geologists, recorded symmetric magnetic anomalies across ocean ridges that plate tectonics took off. What they recorded was a series of magnetic stripes in the oceanic crust (Fig. 1). These stripes correspond to times when the Earth's magnetic field reversed polarity and when it was the same polarity as it is now. Figure two below shows the current situation, by swapping the north and south poles around, you get a reversed field. By studying theses stripes rates of spreading could be calculated. Figure one shows a record of magnetic stripes and how they are interpreted. It is clear that in order to get this pattern new oceanic crust must be formed continuously (on a geological timescale). The conclusion was that the mantle melted at these ridges, moving the ocean floor. The idea of moving ocean crust had just been published by an American geologist, Hess, called sea-floor spreading.

Figure 1: Magnetic stripes that are found at mid-ocean ridges. Redrawn from Plummer & McGeary, 1997.

Figure 2: The Earths magnetic field. Redrawn from Plummer & McGeary, 1997.

Palaeomagnetism

Another form of evidence came from continental rocks. Old rocks had different magnetic fields recorded in them than the field today. More precisely, the inclination of the field was different. Figure two shows the magnetic field of the Earth. The dip (inclination) of the field lines change as you move north or south. This is recorded in igneous rocks the instant they cool and is set permanently. From this you can tell the latitude that the rock form in. So, if you have a rock with a horizontal inclination, then it formed at the equator. Rocks from all over the world were found to have inclinations that did not agree with the continents present locations. Two things could have caused this - the magnetics poles move over time or the continents move. The only way to match the data from all the samples was to move the continents around. However, this movement is plotted as if the poles moved in a "Apparent Polar Wander Path". Don't be confused though, it is the continents that move, not the poles. These "Polar Wander Paths" were also used by Wegner by calculating pole position from climate belts.

Other Evidence

How do we know what the plates do when they are subducted or how they are formed? Most of the evidence of this come from geophysical studies. The most modern way of looking at plate tectonics is using GPS. The results from these studies, lasting 5 or more years, the rates of plate movement have been calculated with great accuracy. However, these values agree near perfectly with results from sea-floor spreading and other methods of calculating spreading rates.

Seismic tomography is a method of using seismic waves from earthquakes (plus some other data) to create 3D images of the mantle. These studies pick out areas of fast or slow mantle, which correspond to areas of high and low temperature. From these studies you can actually see subducting slabs and upwelling at mid-ocean ridges. Figure three is a seismic tomographic map at 50km depth. You can see features such as the East-Pacific rise and the Indian ridge systems, the volcanoes around the Pacific rim and even the Hawaiian hotspot (just), all of which show up as magenta (Fig. 3).

Figure 3: A seismic tomography map. Magenta shows fast crust, interpreted as being hot, blue shows slow crust, interpreted as being cool.

More traditional evidence comes from gravity studies. The gravity anomaly over a subduction zone shows many features. Figure four shows a typical gravity anomaly. A high gravity anomaly shows area of high density, whereas a low anomaly shows areas of low density.

Figure 4: Gravity anomaly over a subducting slab. Redrawn from Keary & Vine, 1996.

Mechanisms

The mechanisms of plate tectonics relies on two things: convection in the mantle and forces on the plate. Figure five shows the forces that tectonic plate experience. For an oceanic plate, pulling forces are most important as the asthenosphere beneath the oceanic lithosphere acts as a lubricating layer, uncoupling the lithosphere from the mantle. The continental plates move mainly due to convention in the mantle. This is due to the fact that the asthenosphere is missing in older parts of the continents. The continents in these areas are much thicker than usual, maybe 200km thick. These form keels, which anchor the continent into the mantle. The continents hence move with mantle and its convection. Note in many texts, the ridge force is called a ridge push. This is a bad name as the main force in this area is due to gravity pulling the plate downwards, not the magma pushing the plates apart.

Figure 5: Forces acting on a plate. Redrawn from Keary & Vine, 1996.

Conclusions

The evidence for plate tectonics comes from numerous sources. The most obvious is the fit between some coastlines, for example South America and Africa. The fact that ancient sedimentary sequence record a vast range of climatic conditions in the same physical location also points ot the continent moving through the climatic belts, rather than the climatic belts moving. Evidence for sea-floor spreading comes from magnetic anomalies from the ocean floor. These anomalies record periods of reversals of the Earth's magnetic field. As the sea floor is being continuously generated on both sides of a mid-ocean ridge, the patterns are symmetrical. Dating these stripes allows the calculation of spreading rates. Palaeomagnetism data shows that the latitude os rocks during deposition/formation. This allow the construction of apparent polar wandering curves. Evidence for subduction comes from seismic tomography and gravity data.

References

P. Keary & F.J Vine, 1996. Global Tectonics.

C. Plummer, D. McGeary, 1997. Physical Geology.

W. Lowrie, 1997. Fundamentals of Geophysics.