Themes of the Day:

• Igneous Rocks Basics
• Making Magmas: Melting the Mantle
• Types of Volcanoes
• Mafic Magmas
• Basaltic Lavas
• Hawaiian Volcanoes

Igneous Rocks Basics

• Form by cooling and crystallization of magmas - often showing crosscutting relations
• Usually tight, interlocking crystals, generally silicate minerals
• Crystal size (grain size) varies based on cooling rate - fast cooling = fine-grained, slow cooling = coarse-grained
• Intrusive (Plutonic) vs. Extrusive (Volcanic) Igneous Rock Textures
• Composition (chemical, mineralogical) varies from mafic to felsic
• Types of Igneous Rocks - an excellent companion to "The Big Chart" from the US Geological Survey

Making Magmas: Melting the Mantle

• Three ways to cause melting:
  • Increase Temperature (T) - rare on Earth
  • Decrease Pressure (P) - most effective - upwelling mantle at spreading centers (mid-ocean ridges) and hotspots due to mantle convection - partial melts (10-30%) of basaltic composition (dry melting)
  • Add fluids (water) - decreases melting temperature - common at subduction zones where fluids are driven from the subducting oceanic crust into the overlying mantle causing melting (wet melting)
• Important Associations between Plate Tectonics and Magmatism:
  • Depressurization melting - divergent plate boundaries and hotspots - basalts and gabbros are most common rock types - thin crust so little magma differentiation - ophiolite suite (oceanic crust) - Block Diagram of a Mid-Ocean Ridge Magmatic System
  • Melting by Addition of Water - subduction zones (convergent plate boundaries) - granites and andesites are most common rock types - evolved compositions due to magma differentiation by fractional crystallization as magmas rise through the crust - Block Diagram of a Subduction Zone Magmatic System

Types of Volcanoes

• volcanoes, lava flows
• Profile of a volcano, and types of eruption dictated by composition of the magma (lava) - Types of Igneous Rocks
• Main types of volcanoes and volcanic products:
  • Shield Volcanoes - low viscosity basaltic magmas - lava lakes and flows common - not explosive - flood basalts and submarine pillow lavas are basaltic, too
  • Cinder Cones - variable composition, often basaltic - usually a single batch of magma - steep cones formed by cinders piled around vent at angle of repose
  • Composite Volcanoes (Stratovolcanoes) - interayered lava flows and pyroclastic deposits - ranges from intermediate to felsic in composition (andesite-dacite-rhyolite) - in volcanic arcs above subduction zones
  • Dome Volcanoes - similar in composition and often genetically related to composite volcanoes - usually more viscous (more silicic) magma type (dacite/rhyolite) - pyroclastic flows abundant - explosive eruptions common
  • Continantal Calderas - in continental regions the result of the largest types of volcanic eruptions - large, shallow, silicic magma chambers empty catastrophically with unparalleled violence; roof of magma chamber collapses into emptied portion of magma chamber
• Crater vs. Caldera - Aniakchak Volcano
• USGS Photo Glossary of Volcanic Terms - lots of good illustrations of volcanoes and volcanic features

Mafic Magmas

• formed by melting of the mantle
  • by depressurization melting (upwelling of the mantle) at divergent plate boundaries and hotspots
  • by addition of fluids at subduction zones
• Mafic magmas undergo reltively little magmatic evolution in the thin oceanic crust, thus they crystallize to form basalt or gabbro
• At subduction zones, mafic magmas generally rise through thicker, more felsic crust, therefore tend to evolve into intermediate to felsic composition magmas before crystallizing

Basaltic Lavas

• low viscosity
• minerals - olivine, pyroxene, Ca-rich plagioclase
• high crystallization temperatures - ~1200^oC
• shield volcanoes
• effusive eruptions
• fissure eruptions
• lava fountains
• lava lakes
• lava channels & tubes
• pahoehoe
• 'a'a
• pillow lava

Hawaiian Volcanoes

• Photo Gallery: LSSU Geology Club Trip to Hawaii - January 2004
• There's lots more interesting information at the Hawaiian Volcano Observatory