Igneous Rocks 1 51 What are Igneous Rocks

Igneous Rocks 1 / 51

What are Igneous Rocks? from the Latin word for “fire” - ignis Thus, rocks that are “fire-formed” Molten rock (magma) cools to form a solid rock 2 / 51

What is magma? Magma is molten rock, and contains: Melt Solids Volatiles Liquid, composed of mobile ions Silicate minerals that have already crystallized from the melt gases dissolved in the melt, including water vapor (H 2 O), carbon dioxide (CO 2), and sulfur dioxide (SO 2) 3 / 51

Identifying Igneous Rocks 4 / 51

How to classify igneous rocks 1. Composition 2. Texture 5 / 51

Igneous Rocks formed on the surface Volcanic (extrusive) rocks Rocks formed inside Earth Plutonic (intrusive) rocks 6 / 51

Igneous Rocks (two types) Extrusive Lava and Pyroclastic Debris Extruded at surface or at very shallow levels. Basalt is Extrusive Intrusive Magma crystallized slowly within the crust. No exposure to the cool atmosphere. Granite is Intrusive Plutonic– intrusive igneous rock at great depth within crust or mantle.

Magma Compositions Magma is composed mainly of silicate minerals Dark Light iron and/or magnesium potassium, sodium, & calcium Olivine Pyroxene Amphibole Biotite mica Quartz Muscovite mica Feldspars 8 / 51

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Bowen’s Reaction Series Magma crystallizes over a temperature range of several hundred degrees Minerals crystallize in a predictable order Last minerals to crystallize are very different in composition from the earlier formed minerals 10 / 51

Bowen’s Reaction Series Animation 11 / 51

Bowen’s Reaction Series Al Mg Si Si Ca Al Na Ca Si Fe Si K Fe Ca Al Mg Na K Si Si Al Si Pyroxene K Mg Amphibole Si Ca Na Mg Mg Si Fe Olivine Na Ca K Fe Al Mg Al Na K Plagioclase Fe Biotite Mg Si Fe Al Si Al Fe Mg Na Ca Fe Al K Muscovite Orthoclase Quartz 12 / 51

Partial Melting Silica-rich compounds melt before other compounds. Magma Differentiation Four types: • Magma Mixing • Crystal Settling • Magma Assimilation • Magma Migration

Changing a Magma’s Composition Crystal settling Earlier-formed minerals are denser than the liquid portion and sink to the bottom of the magma chamber 14 / 51

Changing a Magma’s Composition Assimilation Changing a magma’s composition by the incorporation of foreign matter (surrounding rock bodies) into a magma 15 / 51

Changing a Magma’s Composition Magma mixing Involves two bodies of magma intruding one another Two chemically distinct magmas may produce a composition quite different from either original magma 16 / 51

Texture is estimated using visual grain size (depends on crystallization history)

Composition is estimated using visual color Can be misleading

Mafic minerals crystallize early and felsic minerals crystallize late in magma Minerals at the top of Bowen’s Reaction Series: Minerals at the bottom of Bowen’s Reaction Series: • Dark in color • Mafic to ultramafic • Iron and magnesium • Light in color • Felsic • Sodium plagioclase, potassium plagioclase, and quartz are light.

Igneous rocks are named on the basis of their texture and composition

Igneous Textures The size, shape, and arrangement of the minerals. ü Glassy – Made of solid glass or glass shards. ü Interlocking crystals – Minerals that fit like jigsaw pieces. ü Fragmental – Pieces of pre-existing rocks. 21 / 51

Igneous Textures ü Aphanitic Phaneritic Porphyritic Glassy Pyroclastic Pegmatitic • Rapid rate of cooling of lava or magma • Microscopic crystals • Typically occurs in extrusive / volcanic rocks 22 / 51

Basalt: Aphanitic Texture 23 / 51

Igneous Textures Aphanitic ü Phaneritic Porphyritic Glassy Pyroclastic Pegmatitic • Slow cooling • Crystals can be identified without a microscope • Typically occurs in intrusive / plutonic rocks 24 / 51

Granite: Phaneritic Texture 25 / 51

Thin section showing APHANITIC TEXTURE 26 / 51

Thin section showing PHANERITIC TEXTURE 27 / 51

Igneous Textures Aphanitic Phaneritic ü Porphyritic Glassy Pyroclastic Pegmatitic • Minerals form at different temperatures & rates • Large crystals (phenocrysts) are embedded in a matrix of smaller crystals 28 / 51

Granodiorite: Porphyritic Texture Potassium Feldspar Phenocryst 29 / 51

Igneous Textures Aphanitic Phaneritic Porphyritic ü Glassy Pyroclastic Pegmatitic • Very rapid cooling of molten rock • Resulting rock is called obsidian 30 / 51

Obsidian: Glassy Texture 31 / 51

Igneous Textures Aphanitic Phaneritic Porphyritic Glassy ü Pyroclastic Pegmatitic • Various fragments ejected during a violent volcanic eruption • Textures often appear similar to sedimentary rocks 32 / 51

Igneous Textures Aphanitic Phaneritic Porphyritic Glassy Pyroclastic ü Pegmatitic • Exceptionally coarse grained • Form in late stages of crystallization of granitic magmas 33 / 51

Quartz Vein: Pegmatitic Texture 34 / 51

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Crystal Shapes • Euhedral - grains completely bounded by crystal faces • Subhedral - grains only partly bounded by crystal faces • Anhedral - grains completely devoid of crystal boundaries Source: 1, 2 36 / 51

Igneous Textures Arrangement of the grains Equigranular Slightly inequigranular 37 / 51

Igneous Textures 38 / 51

Magma Compositions Igneous rocks are often classified by mineral (chemical) composition Ultramafic Mafic (or basaltic) Intermediate (or andesitic) Felsic (or granitic) 39 / 51

Ultramafic Compositions Magma Composition: Rare high in magnesium and iron composed entirely of ferromagnesian silicates Fe Fe Olivine Mg Mg Fe Mg Pyroxene 40 / 51

Mafic (or basaltic) Compositions Magma Composition: Mafic: magnesium and ferrum (iron) Contains substantial dark silicate minerals and calcium-rich plagioclase feldspar More dense than granitic rocks Fe Al Fe Na Mg Mg Ca Si Fe Si Ca Ca Mg Na Fe Al Si Olivine Pyroxene Plagioclase 41 / 51

Mafic (or basaltic) Compositions Extrusive / Volcanic Rock: Basalt Intrusive / Plutonic Rock: Gabbro ü Aphanitic texture ü Composed mainly of pyroxene and calcium-rich plagioclase ü Most common extrusive igneous rock ü Phaneritic texture ü Composed of pyroxene and calcium-rich plagioclase ü Makes up a significant portion of the oceanic crust 42 / 51

Intermediate (or andesitic) Compositions Magma Composition Contains at least 25% dark silicate minerals Associated with explosive volcanic activity Fe Al Fe Na Mg Mg Ca Si Si Ca Ca Mg Na Fe Na Al Si Amphibole Plagioclase 43 / 51

Intermediate (or andesitic) Compositions Extrusive / Volcanic Rock: Andesite Intrusive / Plutonic Rock: Diorite ü Aphanitic texture ü Dark grey color ü May have plagioclase phenocrysts ü Phaneritic texture ü Composed of intermediate feldspar and hornblende ü “Black and white makes DIORITE” 44 / 51

Felsic (or granitic) Compositions Magma Composition Felsic: feldspar and silica Composed almost entirely of light-colored silicates Amphibole Al Si Fe Al Fe K Na Mg Mg Ca Si K Si Ca Al Ca Mg Na K Fe K Na Al Si Plagioclase Biotite Muscovite Orthoclase Quartz 45 / 51

Felsic (or granitic) Compositions Extrusive / Volcanic Rock: Rhyolite Intrusive / Plutonic Rock: Granite ü Aphanitic texture ü Buff or pink color ü May have glass phenocrysts ü Phaneritic texture ü Over 25 % quartz, about 65 % or more feldspar ü The term “granite” covers a wide range of mineral compositions 46 / 51

Varied Compositions Obsidian Pumice ü Volcanic ü Dark colored ü Glassy texture ü Volcanic ü Glassy texture ü Frothy appearance with numerous voids 47 / 51

Varied Compositions Pyroclastic rocks Composed of fragments ejected during a volcanic eruption lvo. wr. usgs. gov/gallery/Bishop. Tuff_1. html Tuff – ash-sized fragments Volcanic breccia – particles larger than ash www. gc. maricopa. edu 48 / 51

Igneous Rock Classification 49 / 51

All rocks on Earth have evolved from the first igneous rocks All Igneous Rocks Result from Magma Differentiation.

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