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Rocks and Rock Cycle Intro. The Rock Cycle Rocks are not what they seem. They are constantly changing. Changes can take millions of years or happen very quickly. sediment is compacted (squashed) and particles get cemented Sediment shells mud sand weathering and erosion Sedimentary Rock limestone eg chalk mudstone sandstone Any rock can be weathered or eroded to mud or heat and sand pressure fast cooling Igneous Rock small crystals large crystals eg basalt eg granite Metamorphic Rock heat and pressure marble slate slow cooling cooling and crystallisation (solidification) Magma (molten rock) very hot! very high temperatures causing melting The Rock Cycle Causes of Change Causes of rocks to change from one type to another. Weathering Erosion Compaction Cementation Heat & Pressure Melting and cooling Causes of Change Igneous Rocks Earth’s original!! Igneous Types Intrusive (magma) Slow cooling Large crystals (course-grained) Extrusive (Lava) Fast cooling Small Crystals (finegrained) Magma Composition Composition Contains the same elements/compounds of earth’s crust Therefore silica & oxygen most abundant Categorized by the amount of silica & oxygen content Igneous Classification Magma Origins Minerals begin melting between 8001200oC Factors that affect melting 1. Temperature 2. Pressure 3. Increases w/depth (geothermal gradient) Increases w/depth Water content More water lower melting point How Rocks Melt Partial melt Some minerals melt faster than others Creates a magma slush Fractional crystallization Some minerals cool at lower temps Cools in reverse order of melt Crystal Settling Bowen’s Reaction Series Bowen’s Reaction Series Illustrates the relationship between cooling magma and mineral formation 2 main branches Continuous Feldspar group Discontinuous iron-magnesium group Bowen’s Reaction Series Continuous Feldspar Group As magma cools composition gradually changes Calcium-rich cools first @ high temps. Sodium-rich cools last @ lowest temps. This means the previously cooled minerals react with the magma to form new (different) minerals Bowen’s Reaction Series Iron-rich minerals Minerals change suddenly at dif. Temps Olivine cools 1st @ 1800oC using up iron & magnesium When too little Iron & magnesium is left for olivine formation, pyroxene will form As more iron & magnesium get used the minerals that follow are more silica & oxygen rich Therefore rocks abundant in silicate minerals form last (Quartz) Chemical Makeup Felsic (comes from Feldspar & Silica) Light color & low density Make up most of the continental crust. Ex/ mica, quartz, feldspar Chemical Make-Up Mafic (comes from Magnesium & Ferros/Iron) Dark colored and high in density Main type of rocks in the oceanic crust. Ex/ Gabbro & Basalt Ultramafic –very low silica and high iron & magnesium Extremely dark in color Glassy Texture No crystal shapes and are either massive (obsidian) or frothy (pumice or scoria) Coarse Texture Course textures have mineral crystals large enough to be clearly seen Slow cooling Fine Grained Texture Fine grains and crystal are not able to be distinguished w/the naked eye Fast cooling Porphyritic Textures Course-grained crystals surrounded by finegrained Two cooling histories: slowly then quickly