|MadSci Network: Earth Sciences|
In one way your question is simple, in another it is very complex.
Simply: increase the temperature and the viscosity decreases and the magma flows more easily. There is, however, no simple way to predict viscosity from temperature alone.
Magma is not a particularly simple substance. It is a mixture of many types of rock and gases. The rocks crystallize at different temperatures. The fraction of the magma that is crystalline has a tremendous effect on viscosity. Magmas that have more silica, like rhyolite, are much more viscous than low silica magma, like basalt, at the same temperature. High silica magmas also tend to erupt at lower temperatures than low silica magmas.
Dissolved gases like water (steam) have very strong effects on viscosity, especially in high silica magmas. As the magma approaches the surface, the pressure drops and the dissolved gases can bubble out and increase the viscosity of the remaining magma.
The volcanos in your part of the world generally have high silica magmas and tend to have explosive eruptions like Mount St. Helens. I visit the Mount St. Helens VolcanoCam fairly often. This site has a number of interesting links.
I found several other web sites that you may find useful.
http://www.geo.ua.edu/volcanology/lecture_notes_files/controls_on_magma_viscos.html> The Physical Nature of Magmas.
http://www.geology.sdsu.edu/how_volcanoes_work/Controls.html> Physicochemical Controls on Eruption Style which has links to rare types of magmas.
http://www.earth.northwestern.edu/people/seth/107/Rocks/magviscosity.htm> Magma Viscosity.
Try the links in the MadSci Library for more information on Earth Sciences.