Plate tectonics (Part 1): What are they?
Helen Gould (UK)
What does “plate tectonics” really mean?
The Earth’s surface bears about 20 plates, which are able, over millions of years, to move about on layers beneath the crust. Some of the surfaces of these plates consist of continental crust, some of oceanic crust, some both (Fig. 1).
Who came up with the idea?
The idea didn’t develop overnight as a result of one person’s efforts. In 1915, Alfred Wegener suggested “continental drift”, in which the continents moved around on the Earth’s surface. Arthur Holmes later suggested continents could be moved by convection currents in the mantle, fuelled by the heat of radioactive decay. Harry Hess was an American geologist who came up with the idea of seaﬂoor spreading. In the 1960s, J Tuzo Wilson developed the convection current idea further, proposed “hot spots” and “plates” and, in 1963, Fred Vine and Drummond Matthews proved the existence of seaﬂoor spreading using “magnetic striping”.
What proof is there that plate tectonics really exists?
The ﬁt of continents against each other, particularly Africa and South America, shows that they were once joined (Fig. 2). This branch of geology – palaeogeography – has led to the detection of several ancient supercontinents and oceans. Their existence is supported by matching similar geological features, such as ancient crystalline rocks and glaciated areas, in adjacent regions of South America and Africa, and North America and Europe. Two massive continents, which existed in the past, have been named Laurasia and Gondwanaland, which, when joined, formed the supercontinent, Pangaea.
Furthermore, the same dinosaur fossils were found in Africa and South America (for example, Mesosaurus); and plant fossils from both India and the southern continents (for example, Glossopteris, a common form of seed fern, and Gangamopteris, a genus of Carboniferous-Permian plants, very similar to Glossopteris) are so distinctive that their presence can only be explained in this way.
The position on the Earth of an igneous rock during cooling can be used to ﬁnd the magnetic pole position when it was formed. Plotting these measurements can show us the position of continents in the past. To understand the idea of magnetic striping, imagine a bar magnet inside the planet. The Earth’s internal magnetic ﬁeld reverses periodically, so, as material erupted at mid-ocean ridges cools, if the magnetic ﬁeld of the Earth ﬂows in one direction, this is preserved on the seafloor, as it will when the ﬁeld is reversed. Finding “mirror-image” magnetised strips of seaﬂoor on either side of the mid- ocean ridges proved the existence of seafloor spreading (Fig. 3).
There are three types of plate boundary, each with multiple names (Table 1).
At mid-ocean ridges, new crust is produced. The ridge is marked by a graben, which is a section of the ocean ﬂoor that has dropped below the rest of the ocean ﬂoor as tension in the crust pulls the two halves of the seaﬂoor apart and new materials well up in the ridge itself (Fig. 4).
This happens on land as well – Iceland is probably the most famous example (Fig. 5).
There are parts of the Earth where plates slide past one another, known as passive plate boundaries. An example of these is the San Andreas Fault in California (Fig. 6). It should be mentioned at this point that earthquakes and volcanoes clearly mark the positions of plate boundaries. So, while crust at passive margins is neither destroyed nor created, the sliding of rocks past one another can be anything but peaceful. Yet, although these processes go on all the time, the movement of the rocks is so slight that we only become aware of it when the strain builds up to such an extent that an earthquake is the result of sudden slippage.
No oceanic crust older than about 200 million years is accessible. Older ocean crust has been entirely removed, because, at subduction zones (also called destructive plate boundaries), oceanic ﬂoor is literally destroyed. The oceans are ﬂoored with basalt, while typical continents consist of granitic rocks. At subduction zones, the denser oceanic rock slides beneath the lighter continental rocks. Density is literally what makes the world go round. The mark of a subduction zone is the deep-sea trench. These can be up to 10,000m deep (Fig. 7).
Where plates meet, they slide past each other, make new crust or collide. Hence, there are various landforms associated with each type of boundary (Table 2).
There are three types of plate collision:
- Ocean-ocean collision, which leads to the formation of volcanic island arcs such as Japan, New Zealand and the Aleutian Islands.
- Ocean-continental collision, as with the Andes.
- Continent-continent collision. An example of this is the collision of India with Asia, producing the Himalayas.
Look at a map of the Earth and you will quickly see a pattern to the distribution of volcanoes and earthquakes on our planet. They follow, and mark, the plate boundaries. Earthquakes may occur near to the Earth’s surface (shallow-focus) or deeper in the crust (deep-focus). Shallow-focus earthquakes occur at mid-ocean ridges and along oceanic trenches at subduction zones, while deep- focus quakes mark the angle of the subduction zone itself (Fig. 8).
What are hotspots?
You may be wondering how “hotspots” fit into this scenario. It’s thought that plumes of hot, light magmatic material rise up through the mantle and crust, and during continental drift, the Earth’s crust passes over them, producing volcanoes. The best- known example is Hawaii (although there are many other, including Iceland), where several volcanoes have formed on one island and a string of previous volcanoes have passed over the plume.
The thing we don’t know – and there is some debate over this at present – is how deep the plumes are located in the Earth. It’s impossible to imagine the impact that plate tectonics had on geology. Before, Earth scientists had only part of the information and were, to a certain extent, groping about in the dark. Crucial insights switched the light on, in the sense that here was a theory that unified many strands of investigation. It truly was a revolution in geological thinking, and in understanding how our planet works.
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