Igneous Rocks

Introduction: One of the greatest challenges in geology is deciphering processes which occur deep within the Earth's crust and mantle - regions of the Earth which we cannot directly observe. The study of igneous rocks is important because as products of deep Earth processes such as melting of solid rock and crystallization of molten rock (magma), they provide us with important information regarding the mineralogical and chemical composition of the Earth's interior. The study of igneous rocks allows geologists to answer questions such as: How and why rocks within the earth melt?; What changes occur to magma as it slowly makes its way toward the Earth's surface? Importantly, many magmas are eventually erupted at volcanoes and then cool and solidify to form volcanic rocks. So, through the study of igneous rocks we learn much about processes which occur within volcanoes, and the forces which power volcanic eruptions.

I. Definitions

Rock - a consolidated mixture of one or more minerals

Rock Cycle - The sequence of events in which rocks are formed, destroyed, altered and reformed by geologic processes. (Fig. 3.2)

Magma - molten rock beneath the earth's surface

Lava- molten rock above the earth's surface; also, a rock which solidifies from molten rock above the earth's surface

Igneous Rock - a rock formed by the solidification of magma or lava

II Types/Textures of Igneous Rocks (Make sure to see all of the following textures in you lab specimens!)

Extrusive Igneous Rock (Volcanic Rock) - forms when lava erupts and is solidified on the Earth's surface. (Fig 3.5 A,B)

Intrusive Igneous Rock (Plutonic Rock) - forms when magma solidifies beneath the Earth's surface. (Fig 3.5 C,D)

Pyroclastic Igneous Rock - an extrusive rock made up of material ejected explosively from a volcano's vent. ( Fig 3.5 A)

Rock Texture - the size, shape, and arrangement of mineral grains.

Phaneritic Texture - all grains are large enough to be identified without the aid of a microscope or hand lens; typical of most intrusive rocks (Fig. 3.5D)  

Aphanitic Texture - some or all grains are too small to be seen without the aid of a microscope or hand lens; typical of most extrusive rocks (Fig. 3.5B)  

Porphyritic Texture - two distinctly different grain sizes are observed in a rock; typical of igneous rocks which have two distinctly different cooling rates. The larger crystals are called phenocrysts; smaller crystals collectively make up the groundmass. (Fig 3.5C)

Glassy Texture - no crystals form because of very rapid cooling, resulting in a glassy appearance. (Fig 3.5 A, 3.7)

Vesicular Texture - rock contains abundant gas vesicles, bubbles formed by expanding gasses in volcanic rock. (Fig 3.6)  

Fragmental Texture - pyroclastic rock consisting of fragments of pre-existing rock embedded within volcanic ash


III. Classification of Igneous Rocks (Figure 3.9, 3.11) (Make sure to identify all of the following rock types (except peridotite and komatiite) in lab!)

Pictures of igneous rock types defined below also are available on the GEODe II CD-ROM that accompanies your text.

Classification of Igneous Rocks is based upon:

a) whether the rock is intrusive or extrusive; if extrusive, did it form by solidification of lava or does it represent pyroclastic material?
b) the chemical and mineralogical composition of the rock

Terms describing the chemical composition of igneous rocks (Names of common examples of each type are in green):

ultramafic - very high Fe, Mg, Ca; very low Si, Na , K content; typically very dark in color (intrusive = peridotite, extrusive = komatiite 

mafic - high Fe, Mg, Ca content; low Si, Na, K content; typically dark in color (intrusive = gabbro, extrusive = basalt)

intermediate  - low Fe, Mg, Ca content; high Si , Na, K content; typically gray in color (intrusive = diorite, extrusive = andesite)                    

felsic - very low Fe, Mg, Ca content; very high Si, Na, K content; typically light in color (intrusive = granite, extrusive = rhyolite)                     

Glassy igneous rocks:

obsidian - black or dark colored volcanic glass; usually similar in composition to rhyolite (Fig 3.7)

- frothy magma solidified into a glassy rock with abundant gas vesicles; usually similar in
composition to rhyolite (Fig 3.8)

IV The Chemical Evolution of Igneous Rocks According to Bowen's Reaction Series (Fig 3.13)

Norman Bowen - in early part of 20th century proposed that mafic magmas may evolve by cooling and crystallization to produce more silica-rich magmas. Using laboratory experiments, Bowen monitored the cooling history of basaltic liquid and established the following:

a) There is a regular sequence of silicate mineral crystallization, such that minerals common to mafic rocks crystallize at the highest temperatures, and minerals common to felsic rocks crystallize at the lowest temperatures.
b) Once a mineral forms, it will undergo a chemical reaction with the surrounding melt to produce the next  lower temperature mineral in the sequence
(example: Olivine undergoes a reaction with the surrounding melt to form pyroxene. Pyroxene reacts with the surrounding melt to form amphibole, etc.).

There are two important parts of the reaction series:

a) The discontinuous series - includes minerals with differing arrangement of Si-O tetrahedra; (olivine, pyroxene, amphibole, biotite)

b) The continuous series - includes plagioclase feldspar minerals, all of which are framework silicates; (Ca-rich plagioclase (anorthite), Na-rich plagioclase (albite))


Importance of Bowen's reaction series:

a) Explains how a variety of igneous rock types can be derived from a single (mafic) magma composition (Fig 3.14):

Fractionation (Fractional Crystallization)- The removal of crystals from contact with the surrounding melt, stopping further chemical reactions as described in Bowen's reaction series. Fractional crystallization is commonly caused by crystal settling and/or volcanic eruption.

Crystal Settling - the process by which early-formed crystals in a cooling magma sink to the bottom of the magma chamber if their density is greater  than that of the magma.

Volcanic Eruption - Erupted lavas must cool very quickly, not allowing time for the reaction series to proceed further.

b) Allows interpretation of crystallization temperature based upon mineralogical composition: Ultramafic rocks represent highest temperatures of crystallization. Felsic rocks represent lowest temperatures of crystallization.

Study Questions - Igneous Rocks