CHAPTER 4

IGNEOUS ROCKS

• 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

MORE VOLATILES = LOWER MELTING TEMPERATURES

ADDING A SECOND MINERAL LOWERS MELTING POINT OF FIRST MINERAL

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

BOWEN’S REACTION SERIES

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.

Pumice, a glassy rock containing numerous vesicles

Vesicles are small holes left by escaping

gas bubbles.

Rhyolite, the aphanitic equivalent of granite, is less abundant.

Aphanitic (fine-grained).

PEGMATITE

Volcanic rock - angular rock fragments

embedded in a light-colored matrix of ash.

Andesite porphyry - a common volcanic rock.

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.

Basalt is aphanitic and a very common extrusive rock.

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

Granite - one of the most common phaneritic

igneous rocks.

Diorite - phaneritic igneous rock of intermediate composition.

Close up of diorite. The white crystals are plagioclase feldspar, and the black crystals are

amphibole and biotite.

Gabbro, the phaneritic equivalent of basalt, is less abundant.