Igneous Rocks (Chapters 4 and 5)

I. Introduction

A. Rock = collection of ³ one mineral in varying proportions (Fig. 3.3).

B. One of the three major rock types making up the rock cycle

C. > 95% of outer 10 km of Earth is made up of igneous rocks.

 

II. Formation of igneous rocks

A. Form from solidification of molten rock (magma).

B. Magma-liquid solution of randomly distributed elements

at high temperature (at least 600oC).

1. Carefully read later section of Chap. 4 on Origin of Magma

2. Lava = magma extruded at the surface of the Earth

C. Solidification of the magma is called crystallization.

 

III. Most common igneous minerals = olivine, pyroxene, amphibole, mica, feldspar, quartz

 

IV. Source of the heat to melt rocks

A. Comes from the interior of the Earth, largely as a result of the decay of radioactive elements.

B. Geothermal gradient-rate at which temperature of the Earth increases with increasing depth (Figure 4.18).

C. Pressure competes with temperature to keep rocks from melting at very shallow depths in the Earth.

 

V. Bowen's Reaction Principle

A. Wide range in chemistry and mineralogy is observed in igneous rocks. Without mechanisms to change the composition of magmas as crystallization proceeded, there would have to be thousands of different original magma compositions to produce the thousands of different igneous rocks observed on Earth.

B. Fractional crystallization or crystal settling-process by which magma crystallizes a little bit at a time so that the early-formed crystals can separate from the remaining molten rock - often by settling down through the magma (Figure 4.24 & 4.25).

C. If the crystals aren't separated from the melt a different composition of the solid products results as the first-formed minerals undergo continuous modification due to reaction with the liquid remaining after they crystallize. N. L. Bowen first outlined this process in 1922.  On the basis of laboratory experiments and observations of naturally occurring igneous rocks, Bowen was able to arrange the rock-forming silicates into a Reaction Series (Figure 4.23 and Box 4.2):

  Discontinuous Series                Continuous Series             

Olivine                           Ca-plagioclase (Anorthite)                         

     Pyroxene

            Amphibole

                  Biotite    Na-plagioclase (Albite)      

                          ↓                 

                 Muscovite, K-feldspar, Quartz 

D. Effect of rates of crystallization

1. Like fractional crystallization the rate at which magma solidifies affects the nature of the resultant rock (Figure 4.4).

 

VI. Classification of igneous rocks is based on texture and composition (Figures 4.10, 4.11, 4.12).  

A. Texture-physical appearance of the rocks=size, shape, & arrangement of interlocking mineral grains (Figs.4.3-4.6, 4.12)

1. Coarse-grained, slow cooling, large crystals, intrusive (Phaneritic) (Figs. 4.3, 4.4B, 4.13)

2. Fine-grained, fast cooling, small crystals, extrusive (Aphanitic) (Figs. 4.4A, 4.5)

3. Glassy (very rapid cooling, no crystals; Figs. 4.6, 4.14-4.15)

4. Porphyritic (diff. cooling rates, diff. size crystals) (Figs. 4.4d)

5. Pegmatitic (abnormally large grains; Fig. 4.9)

B. Chemical differentiation (Fig. 4.10)

1. Dark-colored igneous rocks = mafic/basaltic

Basalt and Gabbro

2. Intermediate igneous rocks = Andesitic

3. Light-colored igneous rocks = felsic/granitic

Rhyolite and Granite

C. Learn Bowen's Reaction Series and understand the crystallization of igneous rocks by carefully studying Fig. 4.23.

 

VII. Masses of intrusive rocks (plutons) (Figs. 5.32-5.34, 5.36-5.37)

A. Crystallized from magmas that never reach the surface.

              Gabbro        Diorite      Granite

B. Read about the general shapes in text, but you are only responsible for dike, sill, batholith, and stock

C. Discordant plutons = intruded across layers of preexisting rocks = dikes, batholiths and stocks

D. Concordant plutons = intruded parallel to layers = sills.

 

VIII. Features of extrusive igneous rocks

A. Volcano=surface expression of subsurface igneous activity

B. Materials extruded during an eruption

1.  Lava = magma extruded onto the surface of the Earth.

a. Pahoehoe  &  aa  (Figure 5.7)

2. Gases such as  H2O, CO2, N2, SO2, Cl2, H2, Ar

a. Gases propel the magma from the volcano & probably create conduit connecting magma chamber to surface.

3. Pyroclastics = volcanic rock ejected from volcano

a. Ash, bombs, pumice, and blocks (Fig. 4.8, 4.17, 4.22, 5.9, 5.16)

C. Characteristics of eruptions and volcanoes are a function of the composition of the magma, its temperature, and its content of dissolved gas (Table 5.1, Figure 5.14). All these factors affect the viscosity (resistance to flow) of the magma which is a measure of its fluidity. Viscosity affects mobility. Increased silica content, decreased temperature, and decreased gas content make magma more viscous.

1. "Quiet" = shield volcanoes (Figs. 5.5, 5.7, 5.8, 5.13); gentle slopes; low viscosity, mafic lavas (e.g., Hawaiian Islands);  also fissure flows and plateau basalts (Fig.5.26-5.27)

2.Explosive = composite volcanoes (Figs. 5.1, 5.2, 5.6, 5.11-5.12, Boxes 5.1 & 5.2), steep slopes, high viscosity, felsic lavas, much pyroclastic debris (Mt.St. Helens)

D. Be aware of other features associated with volcanism: lava tubes (Fig. 5.8), cinder cones (Fig. 5.16), lava domes (Fig. 5.28), craters (Figs. 5.12 & 5.29), calderas (Fig. 5.24, 5.25)