Volcanoes 1: Igneous Rocks and Making Magma

What is a Volcano?

• volcano: a hole in the Earth’s crust where molten rock, rock fragments, and hot gas emerge from Earth’s crust and mantle

How Do Volcanoes Form?

• magma reaches the surface through fractures and is extruded as lava or explodes as pyroclastic material
  • magma: created by melting pre-existing rock below Earth’s surface

Volcanic Products

• gases: dissolved in magma, low pressure near surface allows escape
  • mostly H2O and CO2 (also SO2, H2S, HCl, and others)
• pyroclastic material: fragments or rock blasted up into the air
  • pyro=fire, clast=fragments = fire fragments
• lava: melted rock at the Earth’s surface
  • melt + crystals + bubbles
• magma: melted rock below Earth’s surface
  • melt + crystals + bubbles

Igneous Rocks

• ignis=fire, so fire rocks
• one of the 3 types of rocks
  • other two are metamorphic and sedimentary
• crystallize from melted material (magma or lava)
• form at volcanoes
• extrusive igneous rocks: form on surface from cooled and crystallized lava
• intrusive igneous rocks: form beneath the surface from cooled and crystallized magma

Intrusive Igneous Rocks

• magma cooled very slowly deep below the surface
• formation takes years to centuries (possibly millenia)

Extrusive Igneous Rocks

• lava cooled quickly on surface
• formation takes days to months

Composition of Igneous Rocks

• felsic igneous rock features:
  • light colored
  • form the continental crust
  • common rock types = granite (intrusive) or rhyolite (extrusive)
• intermediate igneous rock features:
  • intermediate colored
  • form the continental crust
  • common rock types = diorite (intrusive) or andesite (extrusive)
• mafic igneous rock features:
  • dark colored
  • form oceanic crust
  • common rock types = gabbro (intrusive) or basalt (extrusive)

How Does Magma Form?

• at depth, rock is under high pressure and high temperature

How to Make Magma (Liquid)

• 3 ways:

  1. increase temperature

     1. hotter things melt

  2. decrease pressure

     1. move it to lower pressure

     2. called “decompression melting” (temperature stays the same)

  3. add water or other volatiles

     1. lowers melting temperature

Why Should We Study Volcanoes?

• pose a threat (loss of life)
• destroy ecosystems
• avoid them
  • we can predict them
• aftermath can be worse than initial eruption
  • air quality