CHAPTER 4 IGNEOUS ROCKS. Magma: The parent material of igneous rock Lava Extrusive – volcanic...
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Transcript of CHAPTER 4 IGNEOUS ROCKS. Magma: The parent material of igneous rock Lava Extrusive – volcanic...
• Magma: The parent material of igneous rock
• Lava• Extrusive – volcanic• Intrusive – plutonic• Nature of magma• Melt• Volatiles• From magma to crystalline rock• Crystallization
IGNEOUS TEXTURES• Factors affecting crystal size• Rate at which magma cools• Amount of silica present• Amount of dissolved gases (volatiles) in the magma• Types of igneous textures• Aphanitic (fine-grained) texture• Vesicular• Phaneritic (coarse-grained) texture• Porphyritic• Phenocrysts• Groundmass• Glassy• Pyroclastic (fragmental) • Pegmatitic
ORIGIN OF MAGMA
• Generating magma from solid rock
• Role of heat
• Role of pressure
• Role of volatiles
ORIGIN OF MAGMA
• Generating magma from solid rock
• Role of heat
• Role of pressure
• Role of volatiles
ORIGIN OF MAGMA
• Generating magma from solid rock
• Role of heat
• Role of pressure
• Role of volatiles
HOW MAGMAS EVOLVE
• BOWEN’S REACTION SERIES
and the composition of igneous rocks
• Discontinuous reaction series
• Continuous reaction series
• Magmatic differentiation (crystal settling)
• Assimilation and magma mixing
IGNEOUS ROCK COMPOSITIONS
• Silica content is an indicator of composition
• Composition categories
• Granitic – felsic• Andesitic – intermediate • Basaltic – mafic• Peridotitic – ultramafic
NAMING IGNEOUS ROCKS• Felsic (granitic) igneous rocks – intrusive & extrusive• Granite• Ryolite• Obsidian• Pumice (vesicular)• Intermediate (andesitic) igneous rocks – intrusive & extrusive• Diorite• Andesite• Mafic (basaltic) igneous rocks – intrusive & extrusive• Gabbro• Basalt• Pyroclastic rocks – airborne• Tuff• Welded tuff• Volcanic breccia
OBSIDIAN - Glassy (cooled too rapidly to form crystals). Photomicrographs are magnified
about 27 times.
Photomicrograph of a thin section of andesite porphyry to illustrate texture. Notice that the few large crystals (phenocrysts) are surrounded by
much smaller crystals (groundmass).
Because replacement was not complete, this feldspar crystal has a calcium-rich interior surrounded by zones
that are progressively richer in sodium.
Outcrop of welded tuff (tan) interbedded with obsidian (black) near Shoshone, California. Caption:Tuff is composed mainly of ash-sized particles and may
contain larger fragments of pumice or other volcanic rocks
Close up of diorite. The white crystals are plagioclase feldspar, and the black crystals are
amphibole and biotite.