Petrology

Prof. Stephen A. Nelson

Introduction & Textures & Structures of Igneous Rocks

Petrology & Petrography

Petrology - The branch of geology dealing with the origin, occurrence, structure, and history of rocks.

Petrography - The branch of geology dealing with the description and systematic classification of rocks, especially by microscopic examination of thin sections. Petrography is a subfield of Petrology.

In this course, most of the lecture material falls under the field of Petrology, while most of the laboratory material falls in the field of Petrography.

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 --  SiO₂ 45-55 wt%, high in Fe, Mg, Ca, low in K, Na
   
2. Andesitic magma --  SiO₂ 55-65 wt%, intermediate. in Fe, Mg, Ca, Na, K
   
3. Rhyolitic magma --  SiO₂ 65-75%, low in Fe, Mg, Ca, high in K, Na
   

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:  

• Mostly H₂O (water vapor) with some CO₂ (carbon dioxide)
• 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.

• Basaltic magma - 1000 to 1200^oC
  
• Andesitic magma -  800 to 1000^oC
  
• Rhyolitic magma -  650 to 800^oC.

Viscosity of Magmas

Viscosity is the resistance to flow (opposite of fluidity).  Viscosity depends on primarily on the composition of the magma, and temperature.
 

• Higher SiO₂ (silica) content magmas have higher viscosity than lower SiO₂ content magmas (viscosity increases with increasing SiO₂ concentration in the magma).

• 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 10,000 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 very high, measured as trillions time the viscosity of water).  Viscosity is an important property in determining the eruptive behavior of magmas.

Summary Table

Plutonic (Intrusive) Igneous Rocks

Hypabyssal Intrusions

Intrusions that intrude rocks at shallow levels of the crust are termed hypabyssal intrusions.  Shallow generally refers to depths less than about 1 km.   Hypabyssal intrusions always show sharp contact relations with the rocks that they intrude.  Several types are found:

• Dikes are small (<20 m wide) shallow intrusions that show a discordant relationship to the rocks in which they intrude.  Discordant means that they cut across preexisting structures.  They may occur as isolated bodies or may occur as swarms of dikes emanating from a large intrusive body at depth.

• Sills are also small (<50 m thick) shallow intrusions that show a concordant relationship with the rocks that they intrude.  Sills usually are fed by dikes, but these may not be exposed in the field. 

• Laccoliths are somewhat large intrusions that result in uplift and folding of the preexisting rocks above the intrusion.  They are also concordant types of intrusions.

Plutons are generally much larger intrusive bodies that have intruded much deeper in the crust.  Although they may show sharp contacts with the surrounding rocks into which they intruded, at deeper levels in the crust the contacts are often gradational.  

• Lopoliths are relatively small plutons that usually show a concave downward upper surface.  This shape may have resulted from the reduction in volume that occurs when magmas crystallize, with the weight of the overlying rocks causing collapse of into the space once occupied by the magma when it had a larger volume as a liquid.
  

• Batholiths are very large intrusive bodies, usually so large that there bottoms are rarely exposed.  Sometimes they are composed of several smaller intrusions.
  
  
• Stocks are smaller bodies that are likely fed from deeper level batholiths.  Stocks may have been feeders for volcanic eruptions, but because large amounts of erosion are required to expose a stock or batholith, the associated volcanic rocks are rarely exposed.

Volcanic (Intrusive) Igneous Rocks

Volcanic Eruptions

• In general, magmas that are generated deep within the Earth begin to rise because they are less dense than the surrounding solid rocks.

• As they rise they may encounter a depth or pressure where the dissolved gas no longer can be held in solution in the magma, and the gas begins to form a separate phase (i.e. it makes bubbles just like in a bottle of carbonated beverage when the pressure is reduced).
  

• When a gas bubble forms, it will also continue to grow in size as pressure is reduced and more of the gas comes out of solution.  In other words, the gas bubbles begin to expand.

• If the liquid part of the magma has a low viscosity, then the gas can expand relatively easily.  When the magma reaches the surface, the gas bubble will simply burst, the gas will easily expand to atmospheric pressure, and a non-explosive eruption will occur, usually as a lava flow (Lava is the name we give to a magma on the surface of the Earth).

• If the liquid part of the magma has a high viscosity, then the gas will not be able to expand easily.  Thus, pressure will build inside the gas bubble(s).  When the magma reaches the surface, the gas bubbles will have a high pressure inside, which will cause them to burst explosively on reaching atmospheric pressure.  This will cause an explosive volcanic eruption.
  

Explosive Eruptions
Explosive eruptions are favored by high gas content and high viscosity (andesitic to rhyolitic magmas).

• Explosive bursting of bubbles will fragment the magma into clots of liquid that will cool as they fall through the air.  These solid particles become pyroclasts (meaning - hot fragments) and tephra or volcanic ash, which refer to sand-  sized or smaller fragments.

• Blocks are angular fragments that were solid when ejected.
  
  
• Bombs have an aerodynamic shape indicating they were liquid when ejected.
  
  
• Bombs and lapilli that consist mostly of gas bubbles (vesicles) result in a low density highly vesicular rock fragment called pumice.

• Clouds of gas and tephra that rise above a volcano produce an eruption column that can rise up to 45 km into the atmosphere. Eventually the tephra in the eruption column will be picked up by the wind, carried for some distance, and then fall back to the surface as a tephra fall or ash fall.

• If the eruption column collapses a pyroclastic flow  will occur, wherein gas and tephra rush down the flanks of the volcano at high speed.  This is the most dangerous type of volcanic eruption.  The deposits that are produced are called ignimbrites if they contain pumice or pyroclastic flow deposits if they contain non-vesicular blocks.

Nonexplosive Eruptions
Non explosive eruptions are favored by low gas content and low viscosity magmas (basaltic to andesitic magmas).

• If the viscosity is low, nonexplosive eruptions usually begin with fire fountains due to release of dissolved gases.
  
• Lava flows are produced on the surface, and these run like liquids down slope, along the lowest areas they can find.
  
• Lava flows produced by eruptions under water are called pillow lavas.
  
• If the viscosity is high, but the gas content is low, then the lava will pile up over the vent to produce a lava dome or volcanic dome.
  
  

Volcanic Landforms

Shield Volcanoes

• A shield volcano is characterized by gentle upper slopes (about 5^o) and somewhat steeper lower slopes (about 10^o). 

• Shield volcanoes are composed almost entirely of thin lava flows built up over a central vent.

• Most shields are formed by low viscosity basaltic magma that flows easily down slope away form a summit vent. 

• The low viscosity of the magma allows the lava to travel down slope on a gentle slope, but as it cools and its viscosity decreases, its thickness builds up on the lower slopes giving a somewhat steeper lower slope.

• Most shield volcanoes have a roughly circular or oval shape in map view.

• Very little pyroclastic material is found within a shield volcano, except near the eruptive vents, where small amounts of pyroclastic material accumulate as a result of fire fountaining events. 
  
  

• Have steeper slopes than shields, with slopes of 6 - 10^o  low on the flanks to 30^o near the summit.

• Steep slope near the summit result from thick, short  viscous lava flows that don't travel far  from the vent.

• The gentler slopes near the base are due to accumulations of material eroded from the volcano and to the accumulation of pyroclastic material.

• Stratovolcanoes show inter-layering of lava flows and pyroclastic material, which is why they are sometimes called composite volcanoes.  Pyroclastic material can make up over 50% of the volume of a stratovolcano.

• Lavas and pyroclastics are usually andesitic to rhyolitic in composition.

• Due to the higher viscosity of magmas erupted from these volcanoes, they are usually more explosive than shield volcanoes.

• Stratovolcanoes sometimes have a crater at the summit, that is formed by explosive ejection of material from a central vent.  Sometimes the craters have been filled in by lava flows or lava domes, sometimes they are filled with glacial ice, and less commonly  they are filled with water.

• Long periods of repose (times of inactivity) lasting for hundreds to thousands of years, make this type of volcano particularly dangerous, since many times they have shown no historic activity, and people are reluctant to heed warnings about possible eruptions.
  

Tephra Cones (also called Cinder Cones)

• Tephra cones are small volume cones consisting predominantly of tephra that result from strombolian eruptions.  They usually consist of basaltic to andesitic material.

• They are actually fall deposits that are built surrounding the eruptive vent.

• Slopes of the cones are controlled by the angle of repose (angle of stable slope for loose unconsolidated material) and are usually between about 25 and 35^o.

• They show an internal layered structure due to varying intensities of the explosions that deposit different sizes of pyroclastics.
  

• On young cones, a depression at the top of the cone, called a crater, is evident, and represents the area above the vent from which material was explosively ejected.  Craters are usually eroded away on older cones.

• If lava flows are emitted from tephra cones, they are usually emitted from vents on the flank or near the base of the cone during the later stages of eruption.

• Cinder and tephra cones usually occur around summit vents and flank vents of stratovolcanoes.

• An excellent example of cinder cone is Parícutin Volcano in Mexico.   This volcano was born in a farmers corn field in 1943 and erupted for the next 9 years.  Lava flows erupted from the base of the cone eventually covered two towns.

• Cinder cones often occur in groups, where tens to hundreds of cones are found in one area
  

• Maars result from phreatic or phreatomagmatic activity, wherein magma heats up water in the groundwater system, pressure builds as the water to turns to steam, and then the water and preexisting rock (and some new magma if the eruption is phreatomagmatic) are blasted out of the ground to form a tephra cone with gentle slopes.

Parts of the crater walls eventually collapse back into the crater, the vent is filled with loose material, and, if the crater still is deeper than the water table, the crater fills with water to form a lake, the lake level coinciding with the water table.

Lava Domes (also called Volcanic Domes)

• Volcanic Domes result from the extrusion of highly viscous, gas poor andesitic and rhyolitic lava.  Since the viscosity is so high, the lava does not flow away from the vent, but instead piles up over the vent. 

• Blocks of nearly solid lava break off the outer surface of the dome and roll done its flanks to form a breccia around the margins of domes.

• The surface of volcanic domes are generally very rough, with numerous spines that have been pushed up by the magma from below.

• Most dome eruptions are preceded by explosive eruptions of more gas rich magma, producing a tephra cone into which the dome is extruded.
  
  
• Volcanic domes can be extremely dangerous. because they form unstable slopes that may collapse to expose gas-rich viscous magma to atmospheric pressure.   This can result in lateral blasts or Pelean type pyroclastic flow (nuee ardent) eruptions.

Craters and Calderas

• Craters are circular depressions, usually less than 1 km in diameter, that form as a result of explosions that emit gases and tephra.

• Calderas are much larger depressions, circular to elliptical in shape, with diameters ranging from 1 km to 50 km.  Calderas form as a result of collapse of a volcanic structure.  The collapse results from evacuation of the underlying magma chamber. 

• In shield volcanoes, like in Hawaii, the evacuation of the magma chamber is a slow drawn out processes, wherein  magma is withdrawn to erupt on from the rift zones on the flanks. 

• In stratovolcanoes the collapse and formation of a caldera results from rapid evacuation of the underlying magma chamber by voluminous explosive eruptions that form extensive fall deposits and pyroclastic flows.

• Calderas are often enclosed depressions that collect rain water and snow melt, and thus lakes often form within a caldera.

• Plateau or Flood basalts are extremely large volume outpourings of low viscosity basaltic magma from fissure vents.  The basalts spread huge areas of relatively low slope and build up plateaus. 

• The only historic example occurred in Iceland in 1783, where the Laki basalt erupted from a 32 km long fissure and covered an area of 588 km² with 12 km³ of lava.  As a result of this eruption, homes were destroyed, livestock were killed, and crops were destroyed, resulting in a famine that killed 9336 people.
  

• In Oregon and Washington of the northwestern U.S., the Columbia River Basalts represent a series of lava flows all erupted within about 1 million years 12 million years ago.  One of the basalt flows, the Roza flow, was erupted over a period of a few weeks traveled about 300 km and has a volume of about 1500 km³.

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:

• The diffusion rate - the rate at which atoms or molecules can move (diffuse) through the liquid.
  

• The rate of nucleation of new crystals - the rate at which enough of the chemical constituents of a crystal can come together in one place without dissolving.
  

• The rate of growth of crystals - the rate at which new constituents can arrive at the surface of the growing crystal.  This depends largely on the diffusion rate of the molecules of concern.
  

In order for a crystal to form in a magma enough of the chemical constituents that will make up the crystal must be at the same place at the same time to form a nucleus of the crystal.  Once a nucleus forms, the chemical constituents must diffuse through the liquid to arrive at the surface of the growing crystal.  The crystal can then grow until it runs into other crystals or the supply of chemical constituents is cut off.

All of these rates are strongly dependent on the temperature of the system.  First, nucleation and growth cannot occur until temperatures are below the temperature at which equilibrium crystallization begins.  Shown below are hypothetical nucleation and growth rate curves based on experiments in simple systems.  Note that the rate of crystal growth and nucleation depends on how long the magma resides at a specified degree of undercooling (DT = T_m - T), and thus the rate at which temperature is lowered below the the crystallization temperature.  Three cases are shown.

1. For small degrees of undercooling (region A in the figure to the right) the nucleation rate will be low and the growth rate moderate.  A few crystals will form and grow at a moderate rate until they run into each other.  Because there are few nuclei, the crystals will be able to grow to relatively large size, and a coarse grained texture will result.  This would be called a phaneritic texture.
   

2. At larger degrees of undercooling, the nucleation rate will be high and the growth rate also high. This will result in many crystals all growing rapidly, but because there are so 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.
   
   

3. 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.
   

1. 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.
   
   
2. 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.
   
   
3. 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.
   

• Tabular - a term used to describe grains with rectangular tablet shapes.
  
  
• Equant - a term used to describe grains that have all of their boundaries of approximately equal length.
  
  
• Fibrous - a term used to describe grains that occur as long fibers.
  
  
• Acicular - a term used to describe grains that occur as long, slender crystals.
  
  
• Prismatic - a term used to describe grains that show an abundance of prism faces.
  

• 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.
  
  
• Glomeroporphyritic - if phenocrysts are found to occur as clusters of crystals, then the rock should be described as glomeroporphyritic instead of porphyritic.
  
  
• 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.
  
  
• 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.
  
  
• 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.
  
  
• 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.
  
  
• Spherulitic - a texture commonly found in glassy rhyolites wherein spherical intergrowths of radiating quartz and feldspar replace glass as a result of devitrification.
  
  
• 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.
  

• Graphic textures - an intergrowth of quartz and feldspar which gives the appearance of cuneiform writing. This texture is found in granites and pegmatites.
  
  
• Myrmekitic texture - an intergrowth of quartz and plagioclase that shows small wormlike bodies of quartz enclosed in plagioclase. This texture is found in granites.
  
  
• 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.
  
  
• Subophitic texture - similar to ophitic texture wherein the plagioclase grains are not completely enclosed in a matrix of pyroxene grains.
  
  
• Poikilitic texture - smaller grains of one mineral are completely enclosed in large, optically continuous grains of another mineral.
  
  
• Intergranular texture - a texture in which the angular interstices between plagioclase grains are occupied by grains of ferromagnesium minerals such as olivine, pyroxene, or iron titanium oxides.
  
  
• Intersertal texture - a texture similar to intergranular texture except that the interstices between plagioclase grains are occupied by glass or cryptocrystalline material.
  
  
• Hyaloophitic texture - a texture similar to ophitic texture except that glass completely surrounds the plagioclase laths.
  
  
• Hyalopilitic texture - a texture wherein microlites of plagioclase are more abundant than groundmass, and the groundmass consists of glass which occupies the tiny interstices between plagioclase grains.
  
  
• Trachytic texture - a texture wherein plagioclase grains show a preferred orientation due to flowage, and the interstices between plagioclase grains are occupied by glass or cryptocrystalline material.
  
  
• Coronas or reaction rims - often times reaction rims or coronas surround individual crystals as a result of the crystal becoming unstable and reacting with its surrounding crystals or melt. If such rims are present on crystals they should be noted in the textural description.
  
  
• Patchy zoning - This sometimes occurs in plagioclase crystals where irregularly shaped patches of the crystal show different compositions as evidenced by going extinct at angles different from other zones in the crystal.
  
  
• Oscillatory zoning - This sometimes occurs in plagioclase grains wherein concentric zones around the grain show thin zones of different composition as evidenced by extinction phenomena.
  
  
• Moth eaten texture (also called sieve texture)- This sometimes occurs in plagioclase wherein individual plagioclase grains show an abundance of glassy inclusions.
  
  
• Perthitic texture - Exsolution lamellae of albite occurring in orthoclase or microcline.

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