Geology 111G

Lecture 5. Igneous Rocks

Rocks

Rock Cycle

Igneous Rocks

Igneous Rock Textures

Igneous Rock Classification

Origins of Magma

I. Rocks: Rocks are combinations of minerals. They are classified and named by the minerals present. Different rock types form in different geologic environments and thus are key to understanding geologic processes. There are three major rock groups.

A. Igneous rocks: Crystallized from magma, a general term for molten rock.

1. Lava: magma erupted to earth's surface.

B. Sedimentary rocks: Deposits formed on land or in the sea and subsequently lithified upon burial.

C. Metamorphic rocks: formed by transformation /recrystallization of existing rocks under the influence of high pressure and/or temperature.

D. Rock cycle

II. Igneous rocks are divided into two classes

A. Intrusive rocks: formed by slow cooling and crystallization of magma at depth.

B. Extrusive rocks: formed by rapid cooling and crystallization of magma erupted to surface (lava).

III. Igneous Rock Textures. Texture is the size, shape and arrangement of interlocking mineral grains in the rock. It is key to the cooling history of the rock-rapid cooling creates small grains (or absence of grains); slow cooling creates large grains.

A. Granular or phaneritic texture: Rock is composed entirely of large mineral grains visible to unaided eye in hand specimen. Indicates slow cooling; intrusive rocks generally have phaneritic texture.

B. Aphanitic texture: rock is composed of very small, microscopic grains. This results from rapid cooling of magma and is characteristic of melts erupted to the surface.

C. Glassy texture: No minerals present; the rock is a solid homogeneous mass. Results from quenching, with no time for crystals to form.

D. Porphyritic texture: Rock composed of coarse mineral grains set in a groundmass of microscopic grains or glass. Indicates a two-stage cooling history. This is the most common texture found in extrusive rocks.

IV. Igneous Rock Classification. Rocks are classified by a combination of texture and composition, essentially their silica content (SiO2). Compositions range from mafic (low silica, high Fe, Mg) to silicic (high silica, low Fe, Mg).

A. Dark colored, mafic rocks.

1. Basalt: fine grained, mafic rock, black

2. Gabbro: phaneritic equivalent of basalt.

3. Ultramafic rocks: dunite, peridotite

B. Intermediate rocks

1. Diorite: granular

2. Andesite: aphanitic or porphritic

C. Light colored, silicic rocks

1. Granite, poor in Fe, Mg silicates, therefore light in color.

2. Rhyolite, aphanitic or porphryitic equivalent of granite.

3. Obsidian

V. Origins of magma. Magma rises because it is more buoyant than the solid rocks around it. It is a very efficient agent of heat transfer, because it carries the heat by convection, or bodily as it rises through fracture systems. The other way that heat is transferred through rock is by conduction, which is a very slow and inefficient process.

A. Magmas form in different, specific plate-tectonic settings

1. Partial melting of mantle

a. Partial melting by decompression at midocean ridge. Forms mafic lavas and basalt. Resulting ocean crust is basaltic.

b. By fluxing of mantle above a subducted slab. Presence of H2O and other volatiles facilitates melting and creates intermediate magmas, results in formation of andesites.

2. Partial melting of deep continental crust. In continental collision zones, crust, which is intermediate in composition, may be heated sufficiently to form low silica melts that crystallize to form granite.