The Basics, What's the Red Stuff?
Over 500 hundered active volcanoes are present on earth with many more lying dormant or even undiscovered.
I'll be using Lava and Magma interchangably, they essentially mean the same thing. Magma is intrusive, or molten rock under the surface of the earth. Lava is extrusive, or on the surface of the earth.
When you think of volcanoes, do you think of glowing red stuff flowing down the side of a steep mountain with a thin whisp of smoke at the top? This is a common misconception. Lava gets it's ability to flow from something called viscosity.(resistance to flow) The higher viscosity, the thicker the lava becomes. If there is a low viscosity, the magma flows like water over the land.
There are four generalized magma types.
Ultramafic
Mafic
Intermediate
Felsic
Though I said before that flowing lavas are a misconception, I was probably unclear.
Mafic lava, or Basalt, has a low viscosity(45%) and is very fluid. Perhaps the most well known place it can be found is Hawai'i. The fluid lava was released through something known as a hotspot which I will cover a little later. As it erupted, it flowed and spread out, making more of a gentle hill than a steep sided mountain.
Intermediate lava, or Andesite, is higher on the viscosity scale(50-60%) and there for is an entirely different material. Andesitic magmas, and the intrusive form known as Dacite, built the North American High Cascades. Mount Saint Helens is a well known volcano built by overlapping eruptions of andesite, basalt and even felsic lava called Rhyolite.
Felsic magma is the most viscous of the four and the most explosive as a result. Yellowstone Volcano erupted 600,000 years ago in a cataclysmic event, driven by a magma chamber of molten rhyolite. Because it is so thick, the magma retains much of the gas disolved in the mixture, preventing it's escape. Mafic magma, because of it's low viscosity, allows gas to escape freely, and the explosivity is usually low.
I said there were four magma types. I haven't touched Ultramafic yet and will only do so breifly. Ultramafic magma does not really exist today anywhere on the surface of the earth. It is believed that when the planet formed and the surface was molten, the crust was ultramafic in composition. There are few deposits of the original crust left. This is due to something called partial melting.
Viscosity, which you know affects the fluid aspects of magma, is caused by something called the silicic content of the magma. The more silica, the harder it is to flow and vice versa. When the crust was ultramafic, a material called peridotite, it went through a compositional change as it was remelted. When you melt something low in silica, certain minerals crystalize or evaporate out first. Quartz, which is entirely made of silica is the last mineral to leave the mix. When you remove the less silicic material first and leave the more silicic in, you get a magma with a higher viscosity. Partially melting Ultramafic magma, produces Mafic. Melting Mafic produces Andesite and melting that produces Rhyolite.