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Volcanoes

3. Volcanic Activity

A. How magma reaches the surface

1. Begins as magma in mantle (asthenosphere = zone of partial melting; but this isn't a big pool of lava; just a few percent melt).
   a. What causes melting?
   1. Increase in temperature
   2. Decrease in pressure
   3. Addition of water and other fluids
      

b. Magma formed by partial melting of mantle is basalt.

2. Rises due to lower density than solid rock.
3. Hot magma may melt rocks of lithosphere on way up (this adds silica if the melt is moving through continental crust). [Rocks of continent (granite and related rocks) have more silica than does rock found on the ocean floor (basalt) - more later]
4. As magma rises, pressure is released, allowing carbon dioxide and other gases to escape. Rock formed when lava cools may have holes left by bubbles of gas (vesicles).

B. Once magma reaches surface, it is called lava.

C. Characteristics of Magma

1. Viscosity = resistance to flow. Depends on:
   a. Temperature: ~ 700 degrees C (rhyolite) to ~ 1200 degrees C (basalt)
   b. Chemistry, especially silica content
   
   Demonstration: viscosity of different liquids
   
   
2. Chemical composition
   
   The chemical composition of magmas is diverse, but silica (Si02) is always an abundant component. Plus Al₂O₃, MgO, FeO, Na₂O, CaO, K₂O
   
   
3. Gases (~ 1-5 % of total weight). Mostly H₂O vapor and CO₂, plus SO₂, H₂S, HCl

Property	Basaltic	Andesitic	Rhyolitic
Silica Content	~ 50%	~ 60%	~ 70%
	Least	Intermediate	Most
Color (reflects mineral composition)	Dark	Medium (gray)	Light (may be pink)
Rock name(note: (E = Name for extrusive rock; I = name for intrusive equivalent)	Basalt(E) Gabbro(I)	Andesite(E) Diorite(I)	Rhyolite(E) Granite(I)

Table 1: Properties of magmas

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updated January 28, 2002

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