Volcanic and Seismic events are major pieces of evidence towards proving that plate tectonics theory is valid. A seismic event is the transient motion and release of kinetic energy caused by sudden failure of the earth’s crust, usually felt as shaking or tremors in the rock mass. Seismic events range in size from barely perceptible tremors to major earthquakes. Volcanic events occur when there is a release of magma, gas and ash from the Earth’s crust. The entire outer surface of the planet is divided into these plate formations with approximately 30 in total. These different plates vary in size from over 100,000,000 square miles, for example The Pacific Plate, to the Galapagos micro plate at 12,000 square miles. Fault lines separate each of the individual plates. These fault lines extend from the surface all the way to the asthenosphere, at which point the fault disappears due to the plasticity of the rock that exists there. The plate tectonics theory was first conceived by a man named Alfred Wegener in 1912.
The theory of plate tectonics states that the Earth’s lithosphere is fragmented into a dozen or more large and small rigid sections called plates that are moving relative to one another as they float on top of the underlying semi-molten mantle. If the divisions between these plates were mapped out, it would correlate significantly with the distribution of volcanoes around the world. These plates are either continental, The North American Plate, or oceanic, The Nazca Plate. Furthermore, tectonic plates are powered by convection currents, which is the circular movement of magma within the mantle. These currents are powered by the core, which heats the magma, causing it to rise, cool and fall back down. This circular motion causes the plates to move. Volcanoes represent spectacular releases of energy from inside the earth’s crust and upper mantle. There are about 500 active volcanoes which are closely associated with plate boundaries. Magma rises at the plate margin to create the volcanic activity. During the course of the year only a small number (20-30) of the active volcanoes erupt. Their eruptive events are relatively short and are separated by periods of low activity. Earthquakes however are a series of vibrations and shock waves which are initiated by volcanic eruptions or movements along the plate boundaries.
They result from the build-up and the release of pressure created by plate tectonic activity. Wherever there is movement in the earth’s crust earthquakes will form. Earthquakes occur when a build-up of pressure within the Earth’s crust is suddenly released and the ground shakes violently. The point within the crust where the pressure release occurs is called the focus. This can be: 1. Shallow 0 – 70km. Earthquakes at constructive margins are shallow in focus caused by rising magma and tensional forces in the crust. Most are submarine (except along the East African Rift valley) and pose little hazard usually lass that 4 on the Richter scale. This is due to the fact the plates are moving away from each other and little tension builds up. 2. Intermediate 70-300km. Earthquakes at destructive plate boundaries (oceanic-oceanic or oceanic-continental) tend to be intermediate focus or deep focus and result in major events, for example the Japan Tsunami March 2011. 3. Deep 300-700km.
This is normally on a destructive plate boundary. The seismic shock waves have their highest level of energy at the focus; energy decreases as the waves spread outwards. The place on Earth’s surface immediately above the focus is called the epicentre. It receives the highest amount of energy and so the most potentially dangerous location. Seismic waves travel out from the focus, there are three types of waves, and these are: 1. Primary waves (P waves) travel fastest and are compressional they vibrate in the direction in which they are travelling. 2. Secondary waves (S waves) which travel at half the speed of P waves and shear rock by vibrating at right angles to the direction of travel. 3. Love/Surface waves (L waves) which travel slowest and near to the ground surface. Some surface waves (Raleigh waves) shake the ground at right angles to the direction of wave movement and some have rolling motion that produces vertical ground movement. Modern understanding of the structure of Earth is derived in large part from the interpretation of seismic studies that measure the reflection of seismic waves off features in Earth’s interior. There are many types of seismic waves released by the rupture of rocks at the focus. Different materials transmit and reflect seismic shock waves in different ways, and of particular importance to the theory of plate tectonics is the fact that liquid does not transmit S-wave.
This is because the mantle transmits S-waves, it was long thought to be a cooling solid mass. Geologists later discovered that radioactive decay provided a heat source within Earth’s interior that made the asthenosphere plasticine (semi-solid). Although solid-like with regard to transmission of seismic S-waves, the asthenosphere contains very low velocity (inches per year) currents of magma-like molten materials. They are more destructive due to subduction zones or compression forces along the Benioff Zone. The Benioff Zone is a sloping plane where two plates ride over one another. The friction created will produce earthquakes, and heat which will destroy the two plates. At (continental-continental) destructive plate margins, shallow focus earthquakes occur over a broad zone, for example Sichuan, China 2008. At the constructive boundaries the convection currents rise forcing the crust to crack. New Basaltic magma rises to the surface, cools and pushes the plates apart. The convection currents also help drag the plates apart and then pull them down at convergent boundaries. This can be seen at the mid-ocean ridges. Eruptions are non-violent with the exceptions of the East African Rift Valley where the recent eruption in 2002 in the Republic of Congo had a devastating effect on the local population, and where the ocean ridges rise up out of the sea such as in Iceland. The theory of plate tectonics also explains the formation of destructive plate boundaries which account for creating 80% of the world’s active volcanoes.
The melting crust at Subduction Zones (oceanic-oceanic or continental-oceanic) are due to differences in density between oceanic and continental lithospheres, where the less dense plate is pushed beneath the more dense plate, and becomes part of the asthenosphere forming magma that is andesitic in its nature. This creates the most violent volcanic activity. Island Arc volcanoes such as Pinatubo and Fold Mountain Volcanoes such as Nevado Del Ruiz (Andes) are very destructive. The evidence for deep, hot, convective currents combined with plate movement (and concurrent continental drift) also explained the mid-plate “hot spot” formation of volcanic island chains for example the Hawaiian Islands and the formation of rift valleys such as The Rift Valley of Africa. Mid-plate earthquakes, such as the powerful New Madrid earthquake in the United States in 1811, are explained by interplate press res that bend plates much like a piece of sheet metal pressed from opposite sides. At continental-continental destructive plate margins both continental crusts resist subduction, the momentum of collision causes an uplift of crust, forming mountain chains. An example of this type of collision is found in the ongoing collision of India with Asia that has resulted in the Himalayan Mountains that continue to increase in height each year.
Conservative plate margins or Transform faults are cracks in the crust which are produced at right angles to the plate margin. They occur because the earth is round and the rate of spreading along the constructive boundary varies horizontally and vertically through the crust and so it cracks or faults at right angles to the margin, the plates grind past each other and can result in earthquakes as the grinding rocks suddenly snap into new positions, supporting the theory of plate tectonics. The best example of which is the San Andreas Fault in California. The San Andreas Fault zone which is about 1,300 km long (and in places tens of kilometres wide) slices through two thirds of the length of California. Along it, the Pacific plate has been grinding horizontally past the North American plate for 10 million years, at an average rate of about 5cm/yr. As the plates try to pass over each other the stresses build up until the pressure is released in a violent jolt. This jolt can produce very violent, destructive earthquakes which have shallow foci and so can be a major hazard in the area. In 1961, scientists began to theorize that mid-ocean ridges mark structurally weak zones where the ocean floor was being ripped in two lengthways along the ridge crest. New magma from deep within the Earth rises easily through there weak zones and eventually erupts along the crest of the ridges to create new oceanic crust. This process is called sea floor spreading and has been operating over millions of years.
This hypothesis is supported by several lines of evidence. 1. At or near the crest of the ridge, the rocks are very young, and they become progressively older away from the ridge. 2. The youngest rocks at the ridge crest always have present day (normal) polarity. 3. Stripes of rock parallel to the ridge crest alternated in magnetic polarity (normal, reversed, normal. Etc.), suggesting that the magnetic striping and the construction of the id-oceanic ridge system, the sea floor spreading hypothesis gained converts and represented another major piece of evidence towards the theory of plate tectonics. As methods of dating improved, one of the most conclusive lines of evidence in support of plate tectonics derived from the dating of rock samples. Highly supportive of the theory of sea floor spreading (the creation of oceanic crust at a divergent plate boundary for example The Mid-Atlantic Ridge) was evidence that rock ages are similar in equidistant bands symmetrically centred on the divergent boundary.
More importantly, dating studies show that the age of the rocks increases as their distance from the divergent boundary increases. Accordingly, rocks of similar ages are found at similar distances from divergent boundaries, and the rocks near the divergent boundary where crust is being created are younger than the rocks more distant from the boundary. Eventually, radioisotope studies offering improved accuracy and precision in rock dating also showed that rock specimens taken from geographically corresponding areas of South America and Africa showed a very high degree of correspondence, providing strong evidence that at one time these rock formations had once coexisted in an area subsequently separated by movement of lithospheric plates. Similar to the age of rocks, studies of fossils found in once adjacent geological formations showed a high degree of correspondence. Identical fossils are found in bands and zones equidistant from divergent boundaries. Accordingly, the fossil record provides evidence that a particular band of crust shared a similar history as its corresponding band of crust located on the other side of the divergent boundary.
The technological advances necessitated by the Second World War made possible the accumulation of significant evidence now underlying modern plate tectonic theory. The line of evidence, however, that firmly convinced modern geologists to accept the arguments in support of plate tectonics derived from studies of the magnetic signatures or magnetic orientations of rocks found on either side of divergent boundaries. Just as similar age and fossil bands exist on either side of a divergent boundary, studies of the magnetic orientations of rocks reveal bands of similar magnetic orientation that were equidistant and on both sides of divergent boundaries. Tremendously persuasive evidence of plate tectonics is also derived from correlation of studies of the magnetic orientation they recognised that rock generally belonged to two groups according to their magnetic properties. One has ‘normal polarity’, characterised by the magnetic minerals in the rock having the same polarity as that of the Earths present magnetic field. The other group has ‘reversed polarity’, indicated by a polarity alignment opposite to the earth’s present magnetic field. These results from grains of magnetite which behave like small magnets and align themselves with the orientation of the Earth’s magnetic field. When magma cools to form solid volcanic rock, the alignment of the magnetite grains is ‘locked in’, recording the Earths polarity at the time of cooling.
This revealed recognizable patterns, a zebra like pattern occurred with alternating strips of magnetically different rock, shown on either side of the Mid Atlantic Ridge. Paleomagnetic studies and discovery of polar wandering, a magnetic orientation of rocks to the historical location and polarity of the magnetic poles as opposed to the present location and polarity, provided a coherent map of continental movement that fit well with the present distribution of the continents. Another major piece of evidence towards the theory of when Hess reasoned that as the earth’s crust was expanding along the oceanic ridges, it must be shrinking elsewhere. He suggested that new oceanic crust continuously spread away from the ridges in a conveyor belt like motion. Many millions of years later, the oceanic crust eventually descends into the oceanic trenches (for example the rim of the Pacific Ocean basin). According to Hess, the Atlantic Ocean was expanding while the Pacific Ocean was shrinking. As old oceanic crust was consumed in the trenches, new magma rose and erupted along the spreading ridges to form new crust. In effect, the ocean basins were perpetually being ‘recycled’, with the creation of new crust and the destruction of old oceanic lithosphere occurring simultaneously. He explained why the earth doesn’t get bigger with sea floor spreading and why there is so little sediment accumulation on the ocean floor, and why oceanic rocks are so much younger than continental ones.
Maps of the global distribution of earthquakes made during the Cold War readily identified stressed plate boundaries. Improved mapping also made it possible to view the retrofit of continents in terms of the fit between the true extent of the continental crust instead of the current coastlines that are much variable to influences of weather and ocean levels. Another line of evidence in support of plate tectonics came from the long-known existence of ophiolte suites (slivers of oceanic floor with fossils) found in upper levels of mountain chains. The existence of ophiolte suites are consistent with the uplift of crust in collision zones predicted by plate tectonic theory. Lastly, when all the continents are placed together, they all appear to fit easily with similar outlines. This implies that once they could’ve all been connected in a way that the continents would’ve fitted. It can then be taken once step further if the continental shelves are used because they all fit extremely well, like a jigsaw. It means that it would be completely possible for them to have all been one super continent at one point because they all fit the same.
The consideration of jigsaw fit being evidence for continental was presented by Alfried Wegener. The feature of the jigsaw fit that he noticed was the fit of the coastlines on the opposite sides of the Atlantic Ocean. This observation meant that Wegener believed the two continents were once joined. Although this theory seemed promising in providing evidence of continental drift occurring, Wegener could not prove his theory. However in the 1960’s Sir Edward Bullard and his associates showed that at a depth of 900 metres the continents of South America and Africa almost fit together perfectly (Tarbuck and Lutgens, 2002). In conclusion, volcanic and seismic events are important when trying to provide evidence for plate tectonics theory. However there are also many other separate areas of evidence which are just as important. For example the idea of paleomagnetism and fossils are also significant in trying to prove this particular theory. In order for a sufficient conclusion to be created as to whether plate tectonics theory is valid, all of the evidence must be considered.