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The theory of plate tectonics

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To begin with we can see that many of the landmasses appear as separated jigsaw pieces with their complementary shaped coastlines. One example of this is how the North Western coastline of Africa could quite easily ‘slot into’ the North Eastern Coastline of America. This suggests that in the history of the Earth, these two land masses were once connected and through the movement of tectonic plates on the lithosphere via convection currents, the one land mass has been separated into different pieces, much like breaking up a jigsaw. Running from the top the bottom of the map, central in the ocean is what appears to be line. This is actually a mid-oceanic ridge, an indicator of tectonic movement, specifically along a constructive plate margin. As the two plates move apart, a gap forms in the ocean floor, and magma from the mantle wells up and solidifies as mountain ranges, this is also known as sea floor spreading.

One argument against this map showing tectonic activity, specifically looking at the jig-saw fit theory is that many land masses have no complementary coastline shape to the others around them, including much of England’s coastline, however this may be because of erosion over time removing the jig-saw shape we are looking for. All across the Atlantic we can see many small Islands, located well away from the tectonic plate boundaries, these could be areas where there are hotspots in the tectonic plates much like Hawaii. At a hot spot, magma in the mantle can be abnormally hot, or the crust could be abnormally thin and weak. In either case magma is able to burn through the tectonic plate and well up through the crust regardless as to whether it is at a plate boundary.

Describe the characteristics of, and explain the formation of, minor forms of extrusive volcanic activity. There are three main types of minor extrusive volcanic activity I am going to focus on. The first are geysers. A geyser, by definition is an intermittent turbulent discharge of superheated water ejected and accompanied by a vapour phase, a short term geological feature. Geysers occur in areas where typical active volcanoes may not even exist, water heated at depth in the crust by magma builds up pressure, and the water periodically escape as steam and hot water. Where hot water on its way up and out, it can mid with mud near the surface of the crust and a boiling mud volcano can form. For Geysers to form, you need three things, heat, water and a plumbing system. This includes a reservoir to hold the water while it is heated they form in cracks and fissures in the earth, often are aligning along faults formed by earthquakes. Geysers are dangerous as they eject scalding water and sulphurous gases hundreds of feet into the air without warning, but they do bring their benefits, the main one being tourism, such as Old Faithful at Yellowstone.

The second are hot springs. Hot springs are very similar to Geysers, apart from the fact they lack the plumbing system that contains the water while it is heated, so there is no build up of pressure, and hence no fountain of water associated with geysers. Hot springs form because of the density of the water. Being less dense then normal water, it means this super heated water can rise through very small cracks and fissures in the crust much more easily than say normal water. This super heated water is also a much better solvent than is cooler water dissolving lots of silica enlarging many cracks and fissures, while others clog up trhough deposited material. Hot springs differ from geysers in that their underground systems allow rapid circulation of water. The rising hot water dissipates heat energy by evaporation or runoff, while convection currents return the cooler water to the underground system.

Hot springs rich in minerals, which are often the case as it can dissolve them, become very colourful and appear as boiling pools of colourful mud, common in New Zealand. Thirdly we have features known as fumaroles. A fumarole is an opening in a planet’s crust, often in the neighborhood of volcanoes, which emits steam and gases such as carbon dioxide, sulfur dioxide, hydrogen chloride, and hydrogen sulfide. The steam is created when superheated water turns to steam as its pressure drops when it emerges from the ground. The name solfatara, is given to fumaroles that emit sulfurous gases. They can often persit hundreds of years after a volcano has become extinct, and people around them should be cautious because of the hazards posed from the gases expelled from the openings.

In what ways does volcanic activity vary in relation to the type of plate margin along which it occurs? A volcano is an opening in the earth’s crust that allows molten rock from the mantle to flow out onto the surface as lava. Volcanic activity can occur at convergent or divergent plate margins, but it can also occur at hotspots in which no plate margin is involved. At convergent margins two plates which are moving together can be either both oceanic plates or one continental and the other oceanic. In the case of one continental plate and one oceanic plate, volcanic eruptions are very violent and emit andesitic or rhyolitic lava. These types of lava are very viscous due to its high silica content. This is because the lava rises from the subduction zone through continental lithosphere which has a low density and is filled with air spaces containing gases which become incorporated into the lava.

This very viscous lava often blocks off vents of volcanoes and when the pressure building up in the vent is eventually released, the top of the volcano can be blown off leaving a huge crater, such as in the 2002 eruption of Mount Etna in Sicily. When the two plates involved are oceanic, explosions tend to be less violent than this as the melted lithosphere which forms the lava is denser and so contains fewer gases. At divergent boundaries where plates are moving apart from one another, basaltic lava is erupted between the gap. This type of lava is not very viscous due to its low silica content. This is because no subduction of crust is involved so the lava is not made of melted lithosphere but has risen from the mantle itself. The low viscosity of this lava causes it to flow very far before cooling and solidifying. This forms shield volcanoes with very gentle slopes and a much wider base than more conical shaped volcanoes involved with convergent plate margins. The low viscosity of the basaltic lava also causes vey gentle eruptions, so shield volcanoes have a low explosivity.

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