Chapter 3 Section 2 Igneous Rocks From the core of the continents to nearly all of the oceanic crust—igneous is everywhere!
IGNEOUS ROCKS magma lava “fire” rocks cooling andesite pumice Formed from cooled/hardened _________ or ______ Are called _______________ [ignis Latinfire] The rate of __________ determines the size of the mineral’s crystals. basalt
INtrusive: below groundcools slower many many &large crystals Ex: granite, gabbro
Intrusive Igneous • Visible due to erosion & weathering • Magma= mainly silicon & oxygen • Gases: water vapor (gases trapped) • Magma less dense than surrounding rock moves towards surface & cools • Elements combine minerals form & ‘grow’ crystals! • Example: • Granite • Cumberlandite
Intrusive Igneous: Cumberlandite • Only found in Cumberland, RI (official state rock) • 4-acre lot in Blackstone Valley (Iron Mine Hill) & trace amts. Narr. Bay • Dense & prone to rust (iron magnetic); titanium, transitions to peridotite (intr. coarse ign.) • Formed 1.5 bya volcanic activity • Valuable cannon and farm tools 18th/19th cent.
Obsidian EXtrusive: above groundcools quicker f e w &small crystals Air bubbles are sometimes trapped (pumice, scoria) Basalt
Extrusive Igneous Basalt • Magma = below ground// Lava= above ground • Lava most gases escaped • Lava hardens extrusive igneous rock • Extruded onto surface • Example: Rhyolite, Basalt
Classifying Igneous Rocks • Classification based on two major characteristics: • Texture: appearance based on size, shape & arrangement of crystals • Composition: proportions of light and dark minerals
Classification: Texture • Coarse-grained: slow cooling large crystals • Allows charged atoms (ions) to move large distances within magma & link together (crystallize) • Fine-grained: rapidcooling small crystals • Ions lose motion and quickly combine; all compete for available ions • Glassy: lava on surface little time for ions to crystallize randomly distributed glassy appearance (tiny crystals) • Porphyritic: different-sized minerals experience different rates of cooling • Large crystals (phenocrysts) may be surrounded by fine grained minerals • Minerals don’t crystallize @ same rate (location and melting point.)
Classification: Composition • Granitic: light-colored silicates (quartz & feldspar) • 70 % silica • 10 % dark silicate minerals (i.e. biotite mica, amphibole) • Major component of continental crust • Ex: rhyolite = extrusive granitic rock • Basaltic: contain many dark silicate materials (plagioclase feldspar) • Rich in magnesium and iron • Iron darker and denser than granitic rocks • Ex: gabbro = intrusive basaltic rock • Andesitic: composition between granitic and basaltic • Volcanic rock = andesite: • At least 25 % dark silicates (amphibole, pyroxene, biotitemica) • Dominant mineral plagioclase feldspar :type of feldspar dark w/ridges (striations) • Ultramafic: composed almost entirely of dark silicate minerals • Rare @ Earth’s surface • Ex: peridotite—upper mantle composition