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Igneous Rocks: Solids from Melts Igneous Rocks are formed from liquid or molten rock Means to identify igneous rocks: Texture and Minerals Texture Are there visible crystals? Are there large visible crystals? Coarse vs. Fine Grained Where were they formed? Intrusive Rocks – Plutonic Rocks 1) Molten rock (magma) cooling below the Earth’s surface forms crystalline rocks that cool slowly, we look for large mineral crystals 2) Country rock – is the existing rock that magma intrudes Extrusive Rocks – Volcanic Rocks 1) Molten rock (lava) cools at the Earth’s surface forming smooth rocks with few crystals visible with the unaided eye 2) Types of material from Volcanic Rocks a) Lava – is the erupted molten rock b) Pyroclasts – broken shards expelled by violent eruptions c) Volcanic Ash – the finest Pyroclastic material d) Tuff – Pyroclastic layers lithified by heat from cooling particles e) Pumice – Frothy volcanic glass with a high content of vesicles (holes) f) Obsidian – smooth volcanic glass, often black, dense g) Porphyry – large visible crystals, visible with the unaided eye, and set into a fine crystalline matrix h) Phenocrysts – large visible crystals that formed just prior to the molten rock material being extruded Chemical composition and Minerals Plutonic and Volcanic rock composition Felsic vs. Mafic Felsic – feldspar plus silica – light in color Alkali feldspar – K-spar Plagioclase feldspar – Ca,Na Feldspar Muscovite Mafic – Magnesium and F for Fe Iron – dark in color Olivine Pyroxene Amphibole Biotite 1 Discuss Magma, Composition, Temperature and Rock Type Chart Magma Chamber - is an open area in the lithosphere where magma sits. We know magma exists as liquid in these chambers from seismic work done below volcanoes. Partial Melt – this has been conducted in labs to show that all minerals do not melt at the same temperature (therefore they didn’t crystallize at the same temperature.) Only a portion of the magma forms crystals while the rest remains in a liquid state. Places of Magma Activity 1- Ocean Island Arcs (Mafic and Intermediate – Volcanic) 2- Magma Chamber (Mafic and Intermediate – Plutonic) 3- Mid-Oceanic Ridges (Basaltic – Volcanic) 4- Continental Volcanic Arc (Mafic to Silicic – Plutonic) & 2 Mantle Plumes – are thin pencil-like streams of lava that rise from great depths allowing hot basaltic magma to rise and erupt far from plate boundaries. Basaltic Magmas come from : 1) upper mantle beneath mid-ocean ridges 2) mantle plumes Magmatic differentiation – a uniform parent magma develops a variety of composition. This occurs because different minerals crystallize at different temperatures. Fractional Crystallization – Is where the first crystals in a magma are separated from the remaining magma. 1) crystals settle to the magma chamber floor 2) crystals are pressed into an area by deformation The Palisades Sill – This shows fractional crystallization and lead geologists to examine how magmas can differentiate. Triassic seds above and below, a chilled zone above and below, plagioclase (no olivine), plagioclase and pyroxene (no olivine), olivine. Magmatic Intrusions Magma chamber – is a large subterranean “room” where magma intrudes and crystallizes. Pluton – large igneous body formed at depth 1) wedges open overlying rock 2) breaks off large existing blocks of rock 3) melts surrounding rock 2 Granitization – is the melting and recrystallization of rocks to form granites Batholiths – huge (100 square Km) plutons (magma chambers) of coarse-grained igneous rocks that cut across rock layers Stock – are small plutons that cut across rock layers Dikes – are tabular igneous intrusions that cut across rock layers Sills – are tabular igneous intrusions that follow along rock layers Discordant intrusions – they cut across the layers of country rock that they intrude Batholith, Stock, Dike Concordant intrusions – follow along the layers of country rock that they intrude Sill Veins – deposits of minerals found within a rock fracture that are foreign to the host rock (different composition). Pegamtite – veins of extremely coarse-grained granite that cut across a much finer grained country rock – they are from late stage water-rich magma. They often contain the exotic mineral suites. Plate Tectonics Batholiths form cores for many mountain ranges (like the Sierra Nevada Mountains) that are formed by converging plates (Pacific Plate and the North American Plate) Mention Juan de Fuca plate and the Cascade Range 3