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) 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.