Handouts:

Themes of the Day:

• Igneous Rocks Basics
• Introduction to the Big Chart
• Making Magmas: Melting the Mantle

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

Introduction to the Big Chart

• The Big Chart
• Types of Igneous Rocks - an excellent companion to "The Big Chart" (see Wednesday's Lecture) 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