Causes of Volcanic Eruptions

 More than 2,000 years ago, Pompeii was a busy Roman city near the sleeping volcano Mount Vesuvius. People did not see Vesuvius as much of a threat. Everything changed when Vesuvius suddenly erupted and buried the city in a deadly blanket of ash that was almost 20 ft. thick!

 Today, even more people are living on and near active volcanoes. Scientists closely monitor volcanoes to avoid this type of disaster. They study gases coming from active volcanoes and look for slight changes in the volcano’s shape that could indicate that an eruption is near. Scientists know much more about the causes of eruptions than the ancient Pompeiians did, but there is much more to be discovered.

 Magma forms in the deeper regions of the Earth’s crust and in the uppermost layers of the mantle where the temperature and pressure are very high.  Changes in pressure and temperature cause magma to form.

 Even though the upper mantle is very hot, the rock remains a puttylike solid because of pressure caused by the weight of the rock above the mantle.  The rock is pressed so close together that it will not melt until the temperature increases or the pressure on the rock decreases.

 Magma often forms at the boundary between separating tectonic plates, where pressure is decreased.  Once formed, the magma is less dense than the surrounding rock, so the magma slowly rises toward the surface like an air bubble in a jar of honey.

 A large number of volcanoes lie directly on tectonic plate boundaries.  About 80% of active volcanoes on land form where plates collide, and about 15% form where plates separate.

 When oceanic crust scrapes past the continental crust, the temperature and pressure increase.  The combination of increased heat and pressure causes the water contained in the oceanic crust to be released.  The water then mixes with the mantle rock, which lowers the rock’s melting point, causing it to melt.  This body of magma can rise to form a volcano.

 Volcanoes are classified in three categories:  Extinct volcanoes  Dormant volcanoes  Active volcanoes

 Extinct volcano – a volcano that has not erupted in recorded history and probably never will erupt again.

 Dormant volcano – currently not erupting, but the record of past eruptions suggests that it may erupt again.

 Active volcano – currently erupting or showing signs of erupting in the near future.

 Most active volcanoes produce small earthquakes as the magma within them moves upward and causes surrounding rock to shift.  Just before an eruption, the number and intensity of the earthquakes increase.  Monitoring these quakes is one of the best ways to predict an eruption.

 Other indications of a possible eruption include:  The ratio of certain gases like sulfur dioxide to carbon dioxide  The swelling of Earth’s surface  The angle of a volcano’s slope  Changes in surface temperature and gas emissions of a volcano over time