CHAPTER 4 IGNEOUS ROCKS

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CHAPTER 4IGNEOUS ROCKS
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• Magma The parent material of igneous rock
• Lava
• Extrusive volcanic
• Intrusive plutonic
• Nature of magma
• Melt
• Volatiles
• From magma to crystalline rock
• Crystallization

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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

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ORIGIN OF MAGMA

• Generating magma from solid rock
• Role of heat
• Role of pressure
• Role of volatiles

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ORIGIN OF MAGMA

• Generating magma from solid rock
• Role of heat
• Role of pressure
• Role of volatiles

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ORIGIN OF MAGMA

• Generating magma from solid rock
• Role of heat
• Role of pressure
• Role of volatiles

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MORE VOLATILES LOWER MELTING TEMPERATURES
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ADDING A SECOND MINERAL LOWERS MELTING POINT OF
FIRST MINERAL
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HOW MAGMAS EVOLVE

• BOWENS REACTION SERIES
• and the composition of igneous rocks
• Discontinuous reaction series
• Continuous reaction series
• Magmatic differentiation (crystal settling)
• Assimilation and magma mixing

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BOWENS REACTION SERIES
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IGNEOUS ROCK COMPOSITIONS

• Silica content is an indicator of composition
• Composition categories
• Granitic felsic
• Andesitic intermediate
• Basaltic mafic
• Peridotitic ultramafic

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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

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OBSIDIAN - Glassy (cooled too rapidly to form
crystals). Photomicrographs are magnified about
27 times.
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Pumice, a glassy rock containing numerous
vesicles
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Vesicles are small holes left by escaping gas
bubbles.
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Rhyolite, the aphanitic equivalent of granite, is
less abundant.
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Aphanitic (fine-grained).
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PEGMATITE
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Volcanic rock - angular rock fragments embedded
in a light-colored matrix of ash.
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Andesite porphyry - a common volcanic rock.
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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).
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Because replacement was not complete, this
feldspar crystal has a calcium-rich interior
surrounded by zones that are progressively richer
in sodium.
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Basalt is aphanitic and a very common extrusive
rock.
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Outcrop of welded tuff (tan) interbedded with
obsidian (black) near Shoshone, California.
CaptionTuff is composed mainly of ash-sized
particles and may contain larger fragments of
pumice or other volcanic rocks
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Granite - one of the most common phaneritic
igneous rocks.
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Diorite - phaneritic igneous rock of intermediate
composition.
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Close up of diorite. The white crystals are
plagioclase feldspar, and the black crystals are
amphibole and biotite.
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Gabbro, the phaneritic equivalent of basalt, is
less abundant.
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