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5. Volcano Structure
The volcano that emerges at the surface takes on different shapes depending on the nature of the magma that emerges. If the magma is rich in silica it tends to be thick and sticky. The magma that emerges (known as lava) does not flow easily and produces a steep sided conical, composite volcano. In other locations the magma is less sticky and produces a broad based volcano called a shield volcano. Of the two types it is the conical volcano that is the most violent. This is because the sticky magma does not allow gases to escape easily.The classic cone shaped volcano has steep sides and a crater or opening at its top. The crater is fed magma through a channel that is connected to a magma chamber deep within the earth's crust. In addition to this main channel there may be any number of channels or vents that extend from the main channel to the sides of the volcano. An eruption is often preceded by an escape of gas from within the volcano, visible as a permanent cloud over the volcano. An eruption is caused when the magma finally reaches the surface and lava pours down the hill. A cloud of ash that is thrown into the sky may accompany the eruption.
When a volcano erupted in 1995 on the small island of Montserrat the ash fallout and molten material covered the island. Plymouth, the island's capital, was destroyed and most of the population was evacuated. Two years later most of the evacuees had not returned.
In some volcanoes the path of the lava flows is well known and causes little loss of human life. At Mount Etna (where Africa meets Europe geologically) eruptions are common and the people of Sicily cope with ash fallout and lava flows. Volcanic soils are rich in nutrients and ash provides a natural fertilizer. However, the society must be ready to adjust at all times and perhaps be ready to evacuate.
In other volcanoes eruptions are unpredictable and explosive. In 1903 on the island of Martinique, Mount Pelee finally exploded after spewing ash and steam over the previous days. As the magma built up within the volcano the solid 'cap' was not releasing the pressure as quickly as it was generated. Finally, a substantial portion of the mountain collapsed causing the volcano to explode outwards. A dense cloud of gases, magma and solid rock descended down the mountainside and engulfed the town of St Pierre. Only one person of 30,000 survived, a prisoner located in his cell below the ground.
We can get some idea of what this eruption must have been like by looking at Mount St Helen's in the US. For months beforehand scientists had monitored activity on the volcano. On one side an enormous bulge had been growing as magma beneath the surface built up. The area is not densely populated and most had been evacuated before May 18, 1980 when the eruption finally happened. A massive landslide moved the bulge down hill finally releasing the pressure inside. A massive explosion removed the upper 396 metres off the mountain and destroyed an area of 596 square km. A flood of mud and ash (lahar) moved downhill wiping out everything in its path. The photographs of this eruption remain the most dramatic ever captured.
