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Volcanoes and Volcanism

A volcano is simply an outlet where magma from the Earth's mantle and crust escape to the surface.  When magma reaches the Earth’s surface, geologists now refer to it as lava.  As the lava cools at the Earth’s surface, this newly formed igneous rock builds up to form the characteristic hill or mountain that we imagine when we think of a volcano.  (Incidentally, the word “volcano” comes from the Latin, Vulcan, the god of fire and metalworking and not Mr. Spock from Star Trek.)

How molten rock forms

So, we need to ask:  What is the nature of this molten rock?  To answer this question, we have to think back to your discussions on minerals and rocks.  If we can remember back to the Rocks and Minerals Module, we will remember that the most abundant group of rock-forming minerals in the Earth’s crust is Silicates group, minerals which are composed primarily of silicon and oxygen.  It turns out that amount of silica in the magma is what determines the type of volcano we have, and more importantly for us humans, the type of eruption it will have.  So, just like earthquakes, all we need to know to understand volcanoes is what type of rock we have (how strong its bonds are) and the plate movement involved.

Take a look at 8.2: Types of Volcanoes (p 245), specifically the column that says, “Silica Content of Magma.” Each type of volcano has a characteristic style of eruptive activity that is a result of the mineral content of the rock and the temperature at which it is expelled; both of these determine the viscosity (thickness) of the magma. In general, the more viscous (thick) the magma, the more explosive our eruption will be. So, our question now becomes: What affects the mineral content of our volcano’s magma?

Well, that’s pretty simple:  the quality of our magma is determined by the quality of the rocks that melted to create it!  To understand this, all we need to think about is the tectonic setting, which includes the movement and the type of plate involved (continental or oceanic).  If you haven’t read it already, this next section compliments the “A Closer Look:  How Magma Forms” on pages 254 – 255 of your textbook.

• Continental crust is the less dense of our two crusts.  It is made mostly of silicate minerals like feldspars and other silicates.  By combining the two words, geologists coined the word felsic (feldspar + silica).  This means that when continental crust melts, the magma that forms is going to be rich in silicate minerals and, consequently, the lava that forms our volcano will be rich in silicate minerals.

• Oceanic crust is the denser of our two crusts.  And while it is still made of silicate minerals it is enriched with other elements, specifically, magnesium and iron.  This means that when oceanic crust melts, the magma that forms is going to be rich in magnesium and iron and, consequently, the lava that forms our volcano will be rich in magnesium and iron.