Volcanoes occur along convergent plate margins only when subduction occurs. When two plates move towards each other, the denser, heavier plate is forced to dip under the lighter one (oceanic plates are denser than continental plates). As the subducted plate moves down the mantle, great heat melts the rocks.
When an oceanic plate subduct beneath a continental plate, an oceanic trench may be formed. One example is the Peru-Chilean trench. The convergence of the Nazca (oceanic) and South American (continental) plates created the Andes Fold Mountains
Eventually, through convection currents, the magma rises to the earth surface, forming volcanoes on the folded continental plate.
When two oceanic plates collide, with one sinking beneath the other, the rising magma from the subduction zone eventually forms volcanic islands (e.g. the Philippines). A series of these islands form an island arc.
The global distribution of earthquakes is similar to that of volcanoes. By and large, the distribution of earthquakes divides the earth's surface into a number of areas that are aligned very well with plate boundaries. These follow two main sets of lines, the first being shallow earthquakes, that follow the line of sub-oceanic ridges. The second set, mostly deep earthquakes, follow the lines of subduction along deep-sea trenches. Earthquakes are known to induce an eruption (as in the case of mount St Helens in 1980). Volcanic eruptions, being 1000 times stronger than a nuclear explosion, also cause earthquakes to occur. Thus, it is not surprising that the Circum-Pacific region (the area where the Pacific Ring of Fire is found) has the highest (70% of the worlds earthquakes) incidences of earthquakes.
The belt stretching from the Mediterranean Sea to the Himalayan Mountains, have about 20% of the earth's earthquakes. The rest of the earthquakes occur in diverging plate boundaries such as the Mid-Atlantic ridge as well as transform plate boundaries (San Andreas Fault).
Earthquakes occur when the energy that is built up in an area in the earth's crust or upper mantle is suddenly released.
When plates converge, the downward movement off the subducting plate is not smooth. Where the plates diverge, and where the plates slip past each other, the movement is also not smooth. The plates get stuck easily by tremendous friction and large amounts of energy build up. Due to this, the rocks are under enormous pressure and stress as the plates stretch or compress them. The stress on the rocks becomes unbearable and it finally causes the rocks to bolt and jerk free into new positions along the fault line. The movement results in a release of stored energy in the form of seismic waves that make the ground vibrate.
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b) Using examples explain why volcanic and earthquake activity occurs in areas away from tectonic plate boundaries. [9]
Most volcanic eruptions occur near plate boundaries but there are some exceptions. For example, the islands of the US state of Hawaii, which are entirely of volcanic origin, have formed in the middle of the Pacific Ocean more than 3,200 km from the nearest plate boundary.
This phenomenon occurs due to 'hotspots' (relatively small, long-lasting, and exceptionally hot regions) under the Earth's surface.
The hotspots provide localised sources of high heat energy to sustain volcanism. In the case of Hawaii, the string of volcanoes resulted from the Pacific Plate moving over a deep, stationary hotspot in the mantle, located beneath the present-day position of the Island of Hawaii. Heat from this hotspot produced a never-ending source of magma by partly melting the overriding Pacific Plate. The magma, which is lighter than the surrounding solid rock, then rises through the mantle and crust to spew onto the seafloor. The magma released is basic in nature and it forms shield volcanoes. Over time, countless eruptions cause the tiny volcano to grow until it finally emerges above sea level to form an island volcano. Further plate movement eventually pushes the island beyond the hotspot, cutting it off from the magma source, and volcanism ceases. As one island volcano becomes extinct, another develops over the hotspot, and the cycle is continues. This process of volcano growth and death, over millions of years, has left a long trail of volcanic islands and seamounts across the Pacific Ocean floor.
Earthquakes can also occur away from plate boundaries. However, less than 10% of the Earth's earthquakes occur in these areas. These earthquakes are called intra-plate earthquakes. They occur because the strain induced by tectonic plates as they move against each other is not only released along the plate boundaries, but is also spread through the plates into their interiors. These strains can build up over very long periods of time, ranging from centuries to millennias only to be released in massive but very rare earthquakes in the interiors of plates.
Such earthquakes are often associated with extensional faulting deep in the crust, where continental rifting may be at its early stages. An example is the New Madrid Fault, which gave rise in the 1810s to a series of huge earthquakes in the region of New Madrid, Missouri, USA.
In 1886, a major intra-plate earthquake struck Charleston, South Carolina. Smaller earthquakes can also occur on old faults in plate interiors.
The British Isles, which are located well away from any plate boundary, experience minor earthquakes of this sort from time to time. One occurred in 1990 in Shropshire County, and was felt as far away as London, which was about 100 kilometres away.
Earthquakes are also associated with volcanic activity. As magma rises through the plumbing system of a volcano, it forces its way up by exploiting lines of weakness in the crust, opening up cracks and fissures, and this frequently gives rise to swarms of earthquakes as the pressure is released. It was, for example, one such earthquake that triggered the explosive eruption of Mount St. Helens in 1980, because the earthquake caused the mountainside to give way, releasing the pressure on the magma chamber inside the volcano.
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