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This document last updated on 12-Feb-
EENS 212 Petrology Prof. Stephen A. Nelson Tulane University
Textures of Igneous Rocks
Introduction to Igneous Rocks
An igneous rock is any crystalline or glassy rock that forms from cooling of a magma.
A magma consists mostly of liquid rock matter, but may contain crystals of various minerals, and may contain a gas phase that may be dissolved in the liquid or may be present as a separate gas phase.
Magma can cool to form an igneous rock either on the surface of the Earth - in which case it produces a volcanic or extrusive igneous rock , or beneath the surface of the Earth, - in which case it produces a plutonic or intrusive igneous rock.
Characteristics of Magma
Types of Magma
Types of magma are determined by chemical composition of the magma. Three general types are recognized, but we will look at other types later in the course:
1. Basaltic magma -- SiO 2 45-55 wt%, high in Fe, Mg, Ca, low in K, Na 2. Andesitic magma -- SiO 2 55-65 wt%, intermediate. in Fe, Mg, Ca, Na, K 3. Rhyolitic magma -- SiO 2 65-75%, low in Fe, Mg, Ca, high in K, Na
Gases in Magmas
At depth in the Earth nearly all magmas contain gas dissolved in the liquid, but the gas forms a separate vapor phase when pressure is decreased as magma rises toward the surface. This is similar to carbonated beverages which are bottled at high pressure. The high pressure keeps the gas in solution in the liquid, but when pressure is decreased, like when you open the can or bottle, the gas comes out of solution and forms a separate gas phase that you see as bubbles. Gas gives magmas their explosive character, because volume of gas expands as pressure is reduced. The composition of the gases in magma are:
z (^) Mostly H 2 O (water vapor) with some CO 2 (carbon dioxide) z Minor amounts of Sulfur, Chlorine, and Fluorine gases
The amount of gas in a magma is also related to the chemical composition of the magma. Rhyolitic magmas usually have higher dissolved gas contents than basaltic magmas.
Temperature of Magmas
Temperature of magmas is difficult to measure (due to the danger involved), but laboratory measurement and limited field observation indicate that the eruption temperature of various magmas is as follows:
z (^) Basaltic magma - 1000 to 1200o^ C z (^) Andesitic magma - 800 to 1000oC z Rhyolitic magma - 650 to 800o^ C.
Viscosity of Magmas
Viscosity is the resistance to flow (opposite of fluidity). Viscosity depends on primarily on the composition of the magma, and temperature.
z Higher SiO 2 (silica) content magmas have higher viscosity than lower SiO 2 content magmas (viscosity increases with increasing SiO 2 concentration in the magma).
z Lower temperature magmas have higher viscosity than higher temperature magmas (viscosity decreases with increasing temperature of the magma).
Thus, basaltic magmas tend to be fairly fluid (low viscosity), but their viscosity is still 10, to 100,0000 times more viscous than water. Rhyolitic magmas tend to have even higher viscosity, ranging between 1 million and 100 million times more viscous than water. (Note that solids, even though they appear solid have a viscosity, but it is very high, measured as trillions time the viscosity of water). Viscosity is an important property in determining the eruptive behavior of magmas.
Summary Table Magma Type
Solidified Rock
Chemical Composition Temperature Viscosity Gas Content
Basaltic Basalt
45-55 SiO 2 %, high in Fe, Mg, Ca, low in K, Na
1000 - 1200 oC 10 - 10 3 PaS Low
Andesitic Andesite
55-65 SiO 2 %, intermediate in Fe, Mg, Ca, Na, K
800 - 1000 oC 103 - 10 5 PaS Intermediate
Rhyolitic Rhyolite
65-75 SiO 2 %, low in Fe, Mg, Ca, high in K, Na.
650 - 800 oC 105 - 10 9 PaS High
Textures of Igneous Rocks
The main factor that determines the texture of an igneous rock is the cooling rate (dT/dt)
Other factors involved are:
z The diffusion rate - the rate at which atoms or molecules can move (diffuse) through the liquid.
many crystals, they will run into each other before they have time to grow and the resulting texture will be a fine grained texture. If the size of the grains are so small that crystals cannot be distinguished with a handlens, the texture is said to be aphanitic.
- At high degrees of undercooling, both the growth rate and nucleation rate will be low. Thus few crystals will form and they will not grow to any large size. The resulting texture will be glassy, with a few tiny crystals called microlites. A completely glassy texture is called holohyaline texture.
Two stages of cooling, i.e. slow cooling to grow a few large crystals, followed by rapid cooling to grow many smaller crystals could result in a porphyritic texture , a texture with two or more distinct sizes of grains. Single stage cooling can also produce a porphyritic texture. In a porphyritic texture, the larger grains are called phenocrysts and the material surrounding the the phenocrysts is called groundmass or matrix
In a rock with a phaneritic texture, where all grains are about the same size, we use the grain size ranges shown to the right to describe the texture:
<1 mm fine grained 1 - 5 mm medium grained 5 - 3 cm coarse grained
3 cm very coarse grained
In a rock with a porphyritic texture, we use the above table to define the grain size of the groundmass or matrix, and this table to describe the phenocrysts:
0.03 - 0.3 mm microphenocrysts 0.3 - 5 mm phenocrysts
5 mm megaphenocrysts
Another aspect of texture, particularly in medium to coarse grained rocks is referred to as fabric. Fabric refers to the mutual relationship between the grains. Three types of fabric are commonly referred to:
- If most of the grains are euhedral - that is they are bounded by well-formed crystal faces. The fabric is said to be idomorphic granular.
- If most of the grains are subhedral - that is they bounded by only a few well-formed crystal faces, the fabric is said to be hypidiomorphic granular.
- If most of the grains are anhedral - that is they are generally not bounded by crystal faces, the fabric is said to be allotriomorphic granular.
If the grains have particularly descriptive shapes, then it is essential to describe the individual grains. Some common grain shapes are:
z (^) Tabular - a term used to describe grains with rectangular tablet shapes.
z Equant - a term used to describe grains that have all of their boundaries of approximately
equal length.
z (^) Fibrous - a term used to describe grains that occur as long fibers.
z Acicular - a term used to describe grains that occur as long, slender crystals.
z Prismatic - a term used to describe grains that show an abundance of prism faces.
Other terms may apply to certain situations and should be noted if found in a rock.
z (^) Vesicular - if the rock contains numerous holes that were once occupied by a gas phase, then this term is added to the textural description of the rock.
z Glomeroporphyritic - if phenocrysts are found to occur as clusters of crystals, then the rock should be described as glomeroporphyritic instead of porphyritic.
z Amygdular - if vesicles have been filled with material (usually calcite, chalcedonay, or quartz, then the term amygdular should be added to the textural description of the rock. An amygdule is defined as a refilled vesicle.
z Pumiceous (^) - if vesicles are so abundant that they make up over 50% of the rock and the rock has a density less than 1 (i.e. it would float in water), then the rock is pumiceous.
z Scoraceous - if vesicles are so abundant that they make up over 50% of the rock and the rock has a density greater than 1, then the rock is said to be scoraceous.
z Graphic - a texture consisting of intergrowths of quartz and alkali feldspar wherein the orientation of the quartz grains resembles cuneiform writing. This texture is most commonly observed in pegmatites.
z Spherulitic - a texture commonly found in glassy rhyolites wherein spherical intergrowths of radiating quartz and feldspar replace glass as a result of devitrification.
z Obicular - a texture usually restricted to coarser grained rocks that consists of concentrically banded spheres wherein the bands consist of alternating light colored and dark colored minerals.
Other textures that may be evident on microscopic examination of igneous rocks are as follows:
z Myrmekitic texture - an intergrowth of quartz and plagioclase that shows small wormlike bodies of quartz enclosed in plagioclase. This texture is found in granites.
z Ophitic texture - laths of plagioclase in a coarse grained matrix of pyroxene crystals, wherein the plagioclase is totally surrounded by pyroxene grains. This texture is common in diabases and gabbros.
z Subophitic texture - similar to ophitic texture wherein the plagioclase grains are not completely enclosed in a matrix of pyroxene grains.
z Poikilitic texture - smaller grains of one mineral are completely enclosed in large, optically continuous grains of another mineral.
