Plate tectonics (Part 2): A closer look
Helen Gould (UK)
As we saw last time (Plate tectonics (Part 1): What are they?), the Earth is a pretty dynamic place, with tectonic plates moving about on the surface, driven by convection cells in the upper mantle. But producing a workable theory, which combined most of the observations of geological evidence, took years. It was known that the centres of continents were extremely old, and that some areas around the continental “cratons” didn’t seem to belong because they contained completely different types of rocks.
Combining continental drift with seaﬂoor spreading and mantle convection currents produced the idea of plate tectonics, and provided an explanation for the odd rocks on areas fringing some cratons. These “microplates” had come from other areas of the Earth, where different geological processes had produced different rock types.
The role of density in recycling: oceanic and continental crust
The physical features of the ocean basins and continental mountain ranges are known as the “crustal dichotomy” (splitting of the crust into two equal parts), and because these types of feature are essentially dissimilar, they have their own rock types. Basalt is the commonest rock both in the Solar System and on Earth, where it forms the ocean ﬂoor, along with various sedimentary rocks deposited underwater which make up another 5% of the total oceanic crust.
Continents typically consist of coarse-grained rocks related to granites, which solidify below ground. Comparing similar-sized pieces of basalt and granite in the hand will establish obvious physical differences between them. Basalt’s density allows oceanic crust to be subducted beneath granitic continental crust (Table 1). Different types of volcanic rocks are produced by changes in chemical composition related to the amount and type of silicate minerals in the melt.
However, water is an important factor as well, and plate tectonics couldn’t occur without it. It is subducted along with the seafloor at oceanic trenches – its presence as a film on individual grains of rocks in the upper mantle permits partial melting, which fuels volcanism; and the differentiation of magmas, producing silica-rich rocks, is a direct consequence of this process. An important realisation is that rocks can only form from the chemical elements that are locally available (Fig. 1).
Here’s the really surprising part: different types of boundary have their own eruptive styles, types of volcanoes, rock chemistry and volcanic landforms (Table 2).
So, basaltic eruptions found at hot spots and mid-ocean ridges are quiet, fluid and not particularly dangerous. By contrast, intermediate rocks, for example, andesite, occur in subduction zones. They contain more silica, which makes lavas stiffer and more prone to explosive eruption. Some of these volcanoes erupt lavas and solid fragments (ash) at alternate eruptions – composite or strato-volcanoes, such as Mount St Helens in the Cascades. Acidic rocks (for example, rhyolite and dacite) are formed from even more viscous lavas, which often form domes on top of the volcanoes’ central vents. These explode violently, as dissolved gas pressure inside the volcanic vent reaches critical levels. In addition, pyroclastic ﬂows may be produced at acidic volcanoes – making them extremely dangerous.
Because the percentages of elements in rocks vary, they have different names, such as MOR basalt, which is the kind of basalt found at mid-ocean ridges. The science of differentiating between these rock types is a branch of geochemistry, and Earth scientists use specially-developed computer packages to produce graphs that allow them to differentiate between rock types on a locational basis. For example, back-arc basin rocks are chemically distinct from those at mid-ocean ridges (Tables 3 and 4).
The Earth isn’t the only dynamic planet in our Solar System. Io is very geologically active as well, because its orbit is so close to Jupiter that it is squeezed by the giant planet’s gravitational ﬁeld. As a result, it pumps sulphurous lavas out of giant volcanoes with every cycle of activity.
Mars is now thought to have once had plate tectonics. The presence of magnetic striping, detected by Mars Global Surveyor, has confirmed earlier suspicions. This is not surprising considering the sharp contrast of its landscapes, which resemble continents and ocean basins, mirroring the rise and fall of terrestrial land levels.
Even Venus may still be volcanically active. Its runaway global warming suggests Venusian lavas may resemble komatiites – runny, high-temperature lavas no longer found on Earth, but which erupted here before the current plate tectonic regime was fully initiated, when our planet’s interior was hotter than today.
The role of magnetism
The study of magnetism in ancient rocks is known as palaeomagnetism. Magnetic elements include iron, nickel and cobalt. Rocks containing magnetic minerals only become magnetic as they cool below a certain temperature, known as the Curie Point or Temperature, and this is always lower than the melting temperature of the mineral.
Magnetic studies in France in the 1900s revealed that some rocks were magnetised in one direction and others in the opposite direction, and this was conﬁrmed in a number of locations around the world in the 1920s. Scientists realised that the Earth’s magnetic ﬁeld reversed at irregular intervals, and by combining this information with the ages of the rocks studied, they have worked out when recent reversals occurred (Fig. 3).
Against this background came Vine and Matthews’ discovery (see box, The Vine–Matthews–Morley hypothesis below), by which time it was possible to put the new data in an appropriate context.
|The Vine–Matthews–Morley hypothesis|
|This was the first key scientific test of the seafloor spreading theory of continental drift and plate tectonics. Its key impact was that it allowed the rates of plate motions at mid-ocean ridges to be calculated.|
The evidence from polar wandering curves suggested that the Earth’s magnetic poles moved around over the whole of the Earth. However, by assuming that they stayed close to the geographical poles (for which good evidence is available), it became apparent that it was instead the continents which moved about and their movements could be tracked.
This spawned the science of palaeogeography and led to the detection of the supercontinents of Pangaea and Gondwanaland (Fig. 4), and the discovery of magnetic striping on the seaﬂoor was the real breakthrough that proved the existence of plate tectonics.
Graphics produced by Stuart Handley.
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