Plate Tectonics 22/12/2017

Transcription

1 Map of the tectonic plates. Plate Tectonics In 1912 the meteorologist Alfred Wegener independently developed what he called continental drift, (expanded in his 1915 book The Origin of Continents and Oceans). He started the scientific debate that became the theory of plate tectonics 50 years later, in the early 1960s. The fit of Africa to South America was noted with early maps. The speculation that continents might have 'drifted' was first put forward by Abraham Ortelius in Text Antonio Snider-Pellegrini's Illustration of the closed and opened Atlantic Ocean (1858). Early in the 20 th Century Alfred Wegener proposed strongly, with good evidence (as follows), published in 1912, that the continents had moved around (Continental Drift). He found evidence that the continents had been assembled as one, a megacontinent, Pangaea (meaning "all lands"). Over time they have drifted apart into their current distribution. He believed that Pangaea was intact until the late Carboniferous period, about 300 million years ago. Wegener was a geophysicist and meteorologist who went on 4 Greenland Expeditions. He died in Greenland in 1930 and was buried there. Mountain ranges match on either side of the Atlantic. Wegener also showed geological similarities on both sides. 1

2 Wegener used fossil evidence to demonstrate ancient links. Fossils of the same land species are found in matching continents, showing the land masses were once linked. By about 1930 continental drift was not at all favoured by most geo scientists, but Arthur Holmes pioneered radiometric dating and later suggestions on the age of the Earth as 3,000 million years. He also supported Wegener s Continental Drift (against most current thinking), and published ideas on mantle convection currents. Part of his mantle convection model was the origin of ideas on seafloor spreading. Holmes published a widely used textbook (Principles of Physical Geology), which some older, (and some not so old), U3A members may remember. Still on my bookshelves. In the very early 1960s plate tectonics became an accepted theory following reassessment of the evidence produced in support of Continental Drift, and some new developments. Evidence of ocean floor spreading was becoming quite convincing. Palaeomagnetism was showing strong evidence of plate movements. The pattern of World earthquakes suggests plates with boundaries (some very deep). Map of World Seismicity Key shows depth of earthquake in kilometres. At any point the magnetic field can be measured. The measured magnetic field vector is divided into declination (D), inclination (I) and intensity (F) The vertical inclination of the preserved magnetic direction can give an estimate of the latitude at the time the rock was formed. 2

3 Palaeo magnetism studies made in Europe suggested the magnetic pole moved as shown. But then N.American studies gave different positions. The magnetic field signal locks in as the lava cools. In sediments magnetic particles align with the magnetic field as they settle at quiet river or ocean deposition sites. Plate tectonics explains this if the continents moved then the apparent previous position of the pole will be moved. An important step in developing understanding of plate tectonics was the recognition of a big difference between Continental Crust and Oceanic Crust. Continental crust has a higher percentage of silica and aluminium (and other chemicals) favouring formation of feldspars & quartz. Palaeomagnetic studies showed the polarity of the Earth s magnetic field reverses from time to time! Sequences of magnetic rocks (e.g. A series of basalt lavas) can record these magnetic reversals. Continental crust has a lower density, and typical igneous rocks are granite (and rhyolite). Oceanic crust has a higher percentage of magnesium and iron (ferrous) more mafic minerals. Hence oceanic crust is denser, with typical igneous rocks being basalt and gabbro. Typical oceanic/mafic igneous rocks are also darker in colour. Oceanic Crust is much thinner than Continental Crust and also much younger ( my maximum, cf billion yrs!). In the 1950s observations and ideas began to show that there are mid-ocean ridges where new basaltic crust was being formed. Even more surprising, there were symmetrical bands of normal and reversed polarity rocks parallel to the mid ocean ridges e.g. the Mid Atlantic ridge. Reversals of magnetic polarity through time produce symmetrical bands of magnetic basaltic rock with alternating polarity parallel to a spreading mid ocean ridge. 3

4 This diagram shows how bands of rock showing magnetic reversals have developed. Dating of the rocks on the floor of the Atlantic Ocean further supports the idea of formation of new ocean crust over time. S The tectonic plates of the world were mapped in the second half of the 20th century. Very like the seismicity map! World Seismicity Key shows depth in kilometres. The rigid lithosphere moves over the asthenosphere which is hotter and capable of plastic flow. 4

5 Now we recognise three types of plate boundary (at top). Transform Plate Boundary, Divergent Plate Boundary and Convergent Plate Boundary Diagram of an oceanic-continental convergent plate margin. The denser oceanic crust subducts beneath the lighter continental crust. Friction with the subducting oceanic crust causes heating and volcanic activity. (W coast S.America) Continental-continental convergent margin. One plate may subduct under the other, or there may simply be a lot of crumpling. In either case a mountain range results. e.g. The Indian Plate being thrust under the Eurasian Plate, creating the Himalayas and the Tibetan Plateau beyond (also e.g. the Alps). Oceanic crust subducting under oceanic crust will produce a sub ocean trench, and an island arc of active volcanic islands. There are several examples in the Pacific. The water in the subducting crust (ocean sediments) both lowers the melting point of the rock, and gives especially explosive volcanic activity. A transform fault or transform boundary, (also known as conservative plate boundary since these faults neither create nor destroy lithosphere) is a type of fault whose relative motion is mainly horizontal, left or right handed parallel to the fault. Transform faults are commonly found linking segments of midoceanic ridges or spreading centres. The movement does not increase the distance between the two ridges because new crust is being created. Transform faults are also found at continental margins e.g. the well known San Andreas Fault on the USA Pacific coast. The North American Plate moving SSE slides past the Pacific Plate moving NNW. It runs for around 800 miles (1300km). Another example of a transform fault is the Alpine Fault that runs almost the entire length of New Zealand's South Island. It forms a transform boundary between the Pacific Plate and the Indo-Australian Plate. 5

6 View of the San Andreas Fault,one of the few transform faults exposed on land. Age of the Crust under the main oceans. Reds are youngest (0 to 33 million years), then yellow (48-56my), green (68-120my), pale blue ( my) & dark blue(150 to 180my). Plate motion based on Global Positioning System (GPS) satellite data from NASA JPL. The vectors show direction and magnitude of motion. About 250 million years ago the land masses of the world were all assembled together a supercontinent (named Pangea). The breakup of Pangea 6

7 Formation of supercontinents seems to have been cyclical. There is evidence of at least 3 previous supercontinents. There is general agreement that there were earlier supercontinents, but the detail gets less certain further back in time. Continents and continental crust has a long and complex history (about 3.5 billion years), and complex structures in the older parts. There have been many episodes of crumpling and of parts being thrust over other parts. Continental crust is less dense (2.7 g/cm 3 ) but on average much thicker (25 to 70 km). Oceanic crust is much younger (270my maximum and most is less than 180 my). Oceanic crust develops at ocean floor spreading sites. Oceanic crust is denser (2.9 g/cm 3 ) and on average much thinner (7-10km). Plate tectonics explains a lot about the evolution of the present world and explains many large scale geological features. We can understand how the highest parts of Everest, the Alps etc can be sediments that were once beneath the oceans. We can understand why the World s main mountain belts are where they are. It explains the distribution of many of the World s volcanoes. And also explains many other features. World Seismicity Key shows depth in kilometres. Thank you for your patient attention. 7