Geology 101 - Lecture 7

(Brief Outline Only)

I. Relationship of basalt to andesite to rhyolite: Magma differentiation

• As a magma cools and crystallizes, the minerals it crystallizes do not have the same composition as the magma itself
• Thus if the crystals are removed, the magma becomes a different composition than it was before
• Minerals crystallize in a particular order, with the highest temperature minerals crystallizing first, and the lower temperature minerals crystallizing last (Bowen's Reaction Series)
• The highest temperature minerals are the richest in Mg and Fe, and the poorest in Si.
• Thus, progressive crystallization will cause a magma to become depleted in Mg and Fe and enriched in Si. (i.e., basalt goes to rhyolite)

II. Extrusive (Volcanic) Rocks

A. Several types of material erupted (lava, pyroclastic material, volcanic gases)

Pyroclastic = "fire broken". Exploded magma

B. What determines whether an eruption will be a lava flow or an explosive eruption?

• viscosity of magma (resistance to flow) - depends on
  • composition of magma (more felsic the magma, more viscous, more potential for explosion)
    
  • temperature of magma (the lower the temperature of the magma, the more viscous, the more potential for explosion)
• amount of dissolved gases (more dissolved gases, more potential for explosive eruption)

1. Lava flows
a. Basalt lava - very fluid, erupts at approx. 1000-1200 degrees C
• Pahoehoe
• Aa
• Pillow lava
  
  Scoria - highly vesicular basalt

b. Rhyolite lava - very viscous, erupts at approx 800-1000 degrees C
• Plugs and domes
• Dangerous - plug vents and trap gases
  
  Obsidian - rhyolitic glass

c. Andesite lava - intermediate

2. Pyroclastic ("fire-broken") materials: Fragmented volcanic material ejected into the air during an eruption

a. Basalt: Not common, but basaltic pyroclasts do exist
• Ash - exploded lava with an average diameter less than 2mm
• Bombs/blocks - particles with an average diameter greater than 64 mm

b. Andesite and rhyolite: typically form the truly explosive eruptions
• Ash - exploded lava with diameter less than 2mm
• Pumice - exploded lava of larger size, lots of gas bubbles
• Bombs/blocks - particles have an average diameter greater than 64 mm

3. Gas(see next lecture)

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