dioxide is one of the main volcanic gases, and increasing amounts of it usually herald the arrival of more and more magma near the surface. For example, on 13 May 1991, 500 tonnes of sulfur dioxide per day was released from Mount Pinatubo in the Philippines. On 28 May, just two weeks later, sulfur dioxide emissions had increased to 5,000 tonnes per day, ten times the earlier amount. Mount Pinatubo erupted on 12 June 1991. Gas emissions can increase 5 to 10 times during eruptions compared to normal levels.
4. Seismic patterns
Volcanoes trigger earthquakes before they erupt. These quakes have a pattern that can help predict when the volcano will erupt. Volcanic earthquakes have three major forms: short-period earthquakes, long-period earthquakes and harmonic tremor. • Short wavelength earthquakes: These are related to the fracturing of brittle rock as the magma forces its way upward from the magma chamber. These earthquakes signify the growth of a magma body near the surface.
• Long wavelength quakes: These earthquakes are believed to indicate increased gas pressure in a volcano's ‘plumbing system’. They are similar to the clanging sometimes heard in your home/school plumbing system.
• Harmonic tremor quakes occur when there is sustained movement of magma below the surface. The volcano is continuously shaking as if it is humming.
Patterns of seismic activity are complex and often difficult to interpret. However, increasing activity is very serious especially if episodes of harmonic tremor appear. These methods were combined to help avert major loss of life before the Mount St
Helens (USA) eruption in 1980 and the Mount Pinatubo (Philippines) eruption in 1995. They were also used to predict the 2010 eruption of Mount Merapi in Indonesia.
Fig. 17 Geologists measure the shape of the volcanic dome to help predict when it will erupt. 47