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CHAPTER 6: VOLCANOES: Magma: Viscosity, Sources & Composition
Currently, more than 1,500 volcanoes around the
world are classified as active.
Just 20 active volcanoes worldwide have a complete
network of monitoring instruments.
The resistance to flow of a liquid is called viscosity;
increases in temperature will reduce viscosity.
Viscosity of magma is control by it composition and
temperature. Silica-rich/iron-poor magma has high
viscosity, while .silica-poor/iron-rich magma has
low viscosity.
More-violent eruptions occur where gases cannot
escape easily, this happens due to high viscosity
conditions.
Most of the world’s active volcanoes are located
along convergent plate boundaries. NOTE:
divergent boundaries have a great deal of volcanic
activity, but generally volcanoes do not form along
them.
Volcanoes may be located in the interior of tectonic
plates above hot spots.
Silica-rich minerals have a lower melting
temperature than silica-poor (iron-rich) minerals.
Limited melting, involving only some minerals, is
termed partial melting. This leads to a more silicarich magma than was the original source rock.
Different plate boundaries produce different magma
compositions because each setting generates magma
from melting a different source rock.
Basaltic magmas are characteristic of divergent
plate boundaries and hot spots.
On May 18, 1980, Mount St. Helens in Washington
State generated the most destructive volcanic
eruption in U.S. history.
Three reliable indicators prior to an eruption are:
increasing frequency of earthquakes, rapid changes
in the shape of the volcano, and large-scale
emissions of common volcanic gases.
In the days before the eruption, 50 to 100
earthquakes per day, several greater than magnitude
4, were common.
The rising bulge caused the north flank of Mount St.
Helens to grow by approximately 150 meters (490
feet) from its original elevation.
Enormous blocks collapsed and slid down slope,
merging to form a massive debris avalanche when
Mount St. Helens erupted.