Chapter 4! Igneous Rocks

“Chapter 4: Igneous “

Submitted by Group 3-B

I. Magma: The Parent Materialof Igneous Rock

Igneous rocks form as molten rock cools and solidifies.

A.The Nature of Magma 3 distinct parts of the magma:(1.)liquid component,(2.)solid component & (3.) gaseous phaseB. From magma to crystalline rock Crystallization- is the formation and growth of a

crystalline solid from a liquid or gas.

II. Igneous Textures The size, shape, and mutual relationships of the

minerals in an igneous rock is called texture.A. Factors Affecting Crystal Size(1.) the rate at which magma cools(2.) the amount of silica present(3.) the amount of dissolved gases in the magmaB. Types of Igneous rock Textures(1.) Basic Igneous Rock Textures Phaneritic- this term applies to igneous rock in

which the constituent minerals are megascopic in size.

Aphanitic – this term is used to describe te texture of an igneous rock composed entirely of microscopic crystals.

Glassy - few , no mineral grains are present and the main material present is volcanic glass.

Porphyritic- this term applies to an igneous rock in w/c larger grains(phenocrysts) are embedded in a matrix of distinctly smaller grains(groundmass) which are either phaneritic or aphanitic.

• Porphyro-phaneritic - both the large phenocrysts as well as the smaller crystals in the groundmass are large enough to see without magnification.

• Porphryo-aphanitic- if the phenocrysts are large enough to see but the groundmass is microcyrstalline.

Pyroclastic- this terms applies to igneous rocks composed of fragments of volcanic material ejected during an eruption. This material may include dust, ash, lapilli, bombs, and blocks, as well as individual mineral grains.

(2.) Special Igneous Rock Textures Vesicles

vesicles are spherical or subspherical cavities in igneous rock produced by gas which was present in the original magma. Igneous rocks containing vesicles are termed vesicular igneous rocks.

Pegmatite texture- applied to rocks consisting of exceptionally large mineral grains.

A granite pegmatite composed mainly of quartz and feldspar (salmon color). The elongated, dark quartz crystal on the right is about the size of a person’s index finger.

Aplitic Texture- applies to granitic dike rocks consisting of fine-grained, sugar-sized crystals.

III. Igneous Compositions

IV. Naming Igneous rocks

A. Felsic (granitic) Igneous rock(1.) Granite –perhaps the best known of all igneous rocks.

(2.)Rhyolite- extrusive equivalent of granite and light-colored silicates. Aphanitic and contains glass fragments and voids.

(3.) obsidian- dark-colored glassy rock that usually forms when silica-rich lava is quenched quickly.

(4.) Pumice- volcanic rock that has a glassy texture.

• B. Intermediate (Andesitic) Igneous rocks(1.) Andesite- medium gray fine-grained rock. Its name came from South America’s Andes Mountains.

(2.) Diorite- plutonic equivalent of Andesite. Course-grained intrusive rock that looks somewhat similar to gray granite.

• C. Mafic (Basaltic) Igneous rock• (1.) Basalt- Very dark green to black fine grained

volcanic rock composed primarily of pyroxene and calcium-rich plagioclase feldspar, with lesser amounts of olivine and amphibole present.

(2.) Gabbro- intrusive equivalent of basalt.

D. Pyroclastic Rock- composed of fragments ejected during a volcanic eruption.

Bowen’s Reaction Series- A concept proposed by N. L. Bowen that illustrates the relationships between magma and the minerals crystallizing from it during

the formation of igneous rocks.