Rocks – how do we classify themTexture and compositionIgneous rocks– what are they? How do they form? What are their characteristics?• Crystallization of Magma/Lava…form from magma, Silicate minerals dominate; some oxides2 categories of igneous rocks – how does each form and how does this influence texture?Plutonic (intrusive) versus Volcanic (extrusive)Slow cooling – Intrusive (plutonic) • Large, visible interlocking crystals • Phaneros = “to see”• PHANERITIC textureTexture: • Vesicular– holes (Vesicles) in Volcanic rock left as contained gases exsolve from lava as a result of Pressure decrease• Porphyritic Texture• Well formed crystals (phenocrysts)• Fine grained matrix (groundmass)• Aphanitic rocks• Complex cooling history• Initial stage of slow cooling• Large, well formed crystals form• Later stage of rapid cooling• Remaining magma crystallizes more rapidlyComposition: Mafic, intermediate, felsic• Composition = the mineralogy of the magma and hence, rocks• While texture allows us to separate Plutonic from Volcanic rocks, composition allows us to identify and name the rocks in each category• Composition based on Silica content and the relative abundance of Ferromag and Sialic mineralsRapid cooling – Extrusive (volcanic)- Small, interlocking crystals- Crystals generally too small to see without a handlens- APHANITIC textureTypes of minerals found in mafic or intermediate or felsic rocksComposition (color)Volcanic PlutonicUltramafic (Green) Komatitie PeridotiteMafic (Darks) Basalt GabbroIntermediate (B&W, Gray)Andesite DioriteFelsic (pink,whites,lights)Rhyolite Granite Bowens’s reaction series: Know the minerals, relative temperature constraints, silica and mafic metal content. Be able to associate Bowen’s with igneous rock formation and silica tetrahedral structuresCompositional equivalents between Volcanic and Plutonic rocksBasalt – andesite – rhyolite – granite – diorite- gabbro Obsidian, pumice scoria, tuffHow magma forms• Lowering Pressure– Mantle convection moves deep mantle rocks upwards. Less confining pressure near the surface• Raising Temperature– Hot mafic magma intrudes into the crust• Changing composition– Adding small amounts of water (lowers melting point)– Fractional Crystallization of magma– Partial melt of surrounding rocksMagma Differentiation• Fractional crystallization- – high T mafic minerals crystallize first and settle outof magma. Magma becomes more Felsic over timePartial melt- Low T minerals in surrounding country rock melt into magma, making it more Felsic over timeMagma differentiation- fragments of country rock (sometimes ultra mafic mantle) Xenoliths-Magma MixingPlutonic (intrusive processes)• Intrusions are classified by their size and shape• Large intrusions (plutons)• Batholiths – 100 km2 or greater• Stocks – Smaller than 100 km2 • Small intrusions• Dikes –discordant – Tabular shape– cuts across rock layers• Sills – concordant – Tabular shape– intrudes between rock layers• Laccoliths – Concordant – mushroom shapedRock types associated by compositionDikes, sill, laccoliths, batholiths, stocksPlate tectonic settings for plutonic rock formationVeins- Fractures filled with minerals, Pegmatites- Very coarse-grained textures, large crystals develop from late-stage crystallization of vaporVolcanic (extrusive processes)• Aphanitic texture – small, interlocking crystals not visible to unaided eye• Volcanism produces Rocks such as Basalt, Andesite, and Rhyolite, but also other “products”Tuff- Ash and glass fragments “welded” together from intense heatScoria- volcanic rockPumice- frothy” volcanic glassAsh- Effect depends upon the volume of material ejected but can be potentially produce a large impact on a global scalePyroclastics,1) Blocks – pre-existing country rocks or already solidified volcanic rock ejected into the air during eruption2) Tephra – Molten material that solidifies in the air before falling back to earthVolcanoes:**Mafic, Intermediate and felsic lava – how does this control volcanic landforms and hazards? How does this affect viscosity?- Mafic lavas – less viscous; very “runny”- Felsic lavas - viscous and resistant to flowVolcanoes divided into two broad categories- Fissure eruptionso Lava and/or pyroclastic material flow outward from a large cracks or “fissures”o Flood basalts and basalt plateaus are the result Pyroclastic floods can occur, too- Volcanic “cones”o Lava and pyroclastic material emanates from a central vento Generally build conical peakso Several varieties of volcanic cones, the type dependent upon the material producedother- Lava tubes-Quickly congealed outer rinds form pipes in which lava can rapidly travel- columnar basalt- Usually a product of vast outpourings of Basaltic lava (But can form from other products)- pillow basalt- In undersea lava flows, rapidly quenched lava forms “pillow” like formso can be formed at mid ocean ridges & convergent zones on land - Fissure eruptions-o Lava and/or pyroclastic material flow outward from a large cracks or “fissures”o Flood basalts and basalt plateaus are the resulto Pyroclastic floods can occur, too- flood basalts- Vast outpourings of sequences of lava can build very large (100’s – 100,000’s of miles2) broad, flat plateauso they flooded the surface with low viscosity molten rock that rapidly spread.- Volcanic “cones”o Lava and pyroclastic material emanates from a central vento Generally build conical peakso Several varieties of volcanic cones, the type dependent upon the material producedCentral Vent volcanoes:• Many different landforms as a result of the magma composition, explosivity, etc.• The morphology (shape) of a volcano can tell you much about the hazards and materialsTypes: Shield, Cinder, Composite (aka stratovolcanoes), Dome and Caldera. 1. Shield volcanoes Example. Hawaii- Gentle, broad, flat volcanoes- Can be the largest volcanoes- Built from lava flows- Result of lava viscosity- Basalt (low viscosity)- Little magma mixing or differentiation3. Composite cones- Large steep-sided cones- Alternating layers of lava andpyroclastic material- Common volcanic form along subduction zones- Produces “stratified” layers- Often a result of viscous lavaa. Intermediate compositionb. Can be explosivec. Produce