IGNEOUS ROCKS

• most rocks in the earths crust are igneous in origin
• Formation of Igneous Rocks
• from the solidification of magmas
• silicates, gases, solid minerals, country rock
• temperature, pressure, and water are catalysts of rock melting
• partial melting
• since melted rock (liquid) has a lower density than solid rock, it rises in the earths crust where it accumulates in magma chambers
• Classification based on texture and composition
• Texture: size of individual grains / minerals
• phaneritic vs. aphanitic
• glassy
• porphyritic
• fast cooling = "fine" texture or "glassy"
• slow cooling = "course" texture

• Composition
• Felsic = high in silica
• crystallize at low temperatures
• light color
• poor in Fe and Mg
• Granite, rhyolite
• Intermediate
• midway between felsic and mafic in silica content
• diorite (intrusive), andesite (extrusive)
• Mafic
• low in silica
• high in olivine and pyroxene
• crystallize at higher temperatures
• gabbro, basalt
• Ultramafic
• mafic minerals with < 10 feldspar
• peridotite (intrusive)

• Intrusive Igneous Rock Bodies
• magmas from the melting of mantle crustal rocks
• magmas rise, intrude into the crust and crystallize forming plutons
• Plutons - concordant vs. disconcordant
• Batholith
• Lacoliths
• Stocks
• Dikes
• Sills
• Veins



• Extrusive Igneous Rocks
• Lavas - pahoehoe, aa (viscosity)
• Pyroclastics (fire fragments)
• volcanic ash and tuff
• Volcanic Glass (pumice, scoria, obsidian)
• Porphyritic Igneous Rocks

• viscosity, temperature, and color
• magmatic differentiation
• continuous vs. discontinuous series
• fractional crystallization

• different kinds of rocks melt at different temperatures (ie. sedimentary rocks melt at lower temps than do igneous rocks)
• different kinds of rocks melt at different depths
• plate tectonics can explain the formation of two kinds of magmas
• MOR
• basaltic magma of the oceanic crust
• subduction zones
• sedimentary and basaltic rocks melt
• intermediate to high silica content and gaseous content causes volcanoes to erupt violently
• OC - OC produce andesitic magma
• OC - CC produce granitic magma
• mantle plumes
• hot spots beneath OC basaltic magma
• hot spots beneath CC explosive ash flow and later basaltic magma