Class Notes - Igneous Rocks

Introduction

The study of igneous activity requires understanding processes initiated at depth and at temperatures high enough to produce liquids (magmas). In general, both temperature and pressure increase with increasing depth and it is the rate of increase that is important. The geothermal gradient measures the rate at which temperature increases within the Earth. Near surface studies indicate that the geothermal gradient is about 30 ^o C per kilometer.
Near surface studies indicate that the geothermal gradient is about 30 ^o C per kilometer. If the radius of the Earth is about 6,000 kilometers what is the expected temperature at the center (in ^o C)?

1,800 degrees C
18,000 degrees C
180,000 degrees C
I don't have my calculator


Therefore, the 30 ^oC per kilometer gradient must be confined to the near-surface and must "flatten out" with increasing depth. Otherwise the temperatures predicted are too high.

Pressure increases at a rate of about 333 bars per kilometer in the crust. A bar is about one atmosphere (pressure at the surface of the Earth at sea level = 1 atmosphere = 14.7 pounds per square inch). Therefore the pressure gradient is about one-third of a kilobar (1000 bars) per kilometer.

Diamonds require about 100 kilobars to form. At what depth within the Earth should this pressure be reached?

30 kilometers
300 kilometers
3000 kilometers
I don't have my calculator


Partial Melting

Most rocks are mixtures of minerals and each mineral has its own set of physical characteristics. For example, Quartz melts at about 1725 ^oC at one atmosphere total pressure; in this case, melting is defined as the temperature at which solid and liquid of the same composition are in equilibrium. In general we must specify the pressure in order to state a unique melting point.

If Quartz is mixed with Alkali Feldspar in some proportion (80% feldspar and 20% quartz, for example) melting occurs but not in the same way that the melting of a pure compound occurs. In general, there is no single temperature at which the entire mixture goes from solid to liquid. Rather, there is a range of temperatures at which liquid and solid are present. This is the interval of partial melting or partial crystallization.

    1000o C  liquid
      900o C   solid + liquid
      800o C   solid + liquid
      700o C   solid + liquid
      600o C   solid

The amount of liquid decreases as the temperature drops until all of the liquid is used up in producing solids. The amount of total solids decreases as the temperature is raised.

In this hypothetical example partial melting is initiated at about 700 degrees centigrade and completed at 1000 ^o C. As the temperature increases the amount of solid decreases. Cooling is the reverse. This mixture would be 100% liquid until a temperature of about 1000 ^o C. Crystallization begins and the amount of solids increase and the amount of liquid decreases as the temperature drops. At about 700 ^o all of the liquid is gone.

The concept of partial melting plays a crucial role in igneous processes. For example, many (most? all?) magmas are formed by partial melting which did not reach the temperature at which all of the material was molten. In general, liquids tend to be less dense than the solids that crystallize from them. In a mixture of crystals and liquids the liquid (less dense) will attempt to migrate upwards whereas the crystals may sink. In other words, this hypothetical mixture need not be heated to 1000 ^o C to produce a magma. Magma generation is initiated at about 700 ^o .



When magma reaches the surface it is called lava. Magmas which cool at the surface of the Earth are extrusive whereas those that cool within the Earth are intrusive.

Composition of the Earth

Review the relationships between plate boundaries and igneous activity.

Classification of Igneous Rocks

Why do we need to classify things? In part because it requires us to focus on attributes (characteristics) of the things that we are interested in and to focus on those that we feel are the most important. In part we classify things because we want to be able to efficiently communicate information. Giving something a name can aid but only if everyone understands the basis for assigning the name in the first place. For example, if your Universe consisted of Fords and Chevrolets and you wanted to devise a classification scheme that focused on differences between the two brands you would probably not use color as an important attribute.

Two properties of igneous rocks that we will focus on are texture and mineralogy.

Texture refers to the size, shape and arrangement of the grains in the rock.

• phaneritic - coarse grained - you can see the individual crystals

• aphanitic - fine grained - you can't see the individual crystals or grains

• porphyritic - big grains and small grains
  • phenocryst - big
    
  • groundmass - small

In general, igneous rocks have an interlocking texture. As minerals crystallize from a liquid they compete for space. There is a tendency for the minerals to inter grow making for "jagged grain boundaries". A number of variables control the texture of an igneous rock but, the rate of cooling is certainly important. The more rapid the cooling rate the finer the grain size. When lava at 1000 degrees C pours out on the surface of the Earth it is in contact with the air at about 25 degrees C. Heat flows from high values to low values in an attempt to bring the two bodies into thermal equilibrium. Therefore, the lava would cool rapidly and probably develop an aphanitic texture.

Consider a body of magma at 800 degrees C which is intruded into country rock which is 400 degrees C. Heat will flow from the magma to the country rock and its temperature will increase. At the margin of the magma chamber the rate of cooling is quite high as compared with the rate of cooling at the center of the chamber. Therefore, we might expect some variation in grain size from the margin to the center of the chamber; finer grained at the margin and coarser toward the center. Since thermal equilibrium is reached at 400 degrees, the rock will likely have a phaneritic texture.

A porphyritic rock probably had a two stage cooling history.

True - rapid cooling for the phenocrysts
True - slow cooling for the phenocrysts
False


Cooling Rate and Crystal Size

Mineralogy - Recall that the most abundant mineral groups in the crust are plagioclase (Oceanic Crust) and alkali feldspar (Continental Crust) s. Norman Bowen (about 1915) proposed the following sequences of crystallization of silicates from a magma.



1. With the singular exception of quartz, the other phases present represent solid solution series.

2. The viscosity (resistance to flow) of a melt (magma/lava) increases with decreasing temperature.

3. The complexity (amount of sharing of the oxygens of the silicon-oxygen tetrahedrons) increases with decreasing temperature.

4. The dashed lines are drawn to reflect three mineral assemblages :
   • high temperature - olivine, pyroxene and Ca-rich plagioclase;
     
   • intermediate temperature - amphibole, biotite and Na-rich plagiolase; and
     
   • low temperature - muscovite, alkali feldspar and quartz.
     

5. Bowen's Reaction Series points out that there are commonly occurring mineral assemblages (based on similar temperatures of formation/crystallization). For example, quartz and olivine (at least the magnesium-rich variety) are not expected to occur together as an equilibrium assemblage.

At high temperatures two sequences exist. One includes those minerals which are rich in Fe and Mg (ferromagnesian):
• high temperature
  • olivine
  • pyroxene
  • amphibole
  • biotite
• low temperature

Bowen referred to this sequence as the discontinuous side of the reaction series. He determined (by laboratory experiments) that as a magma containing olivine cooled, the olivine would react with liquid (dissolve in the liquid) and the liquid would crystallize a pyroxene. He thought that pyroxene would yield to amphibole which would yield to biotite. He knew that pyroxene melted incongruently to form an olivine and assumed that amphibole and biotite would do something similar.

The fact that the series does not work exactly as Bowen predicted does not lessen the value of the series as a framework for working with igneous rocks.

Bowen was not aware that biotite, muscovite and amphibole must contain (OH)^- or fluorine or chlorine. If one of these volatile components is not present in the melt then these phases will not form.

At the same time the discontinuous reactions are taking place a continuous reaction is occurring"

• high temperature
  • calcium-rich plagioclase
  • calcium-sodium plagioclase
  • sodium-rich plagioclase
• low temperature

Recall that the plagioclase series is one continuous solid solution series. At high temperatures the plagioclase is rich in calcium, and at low temperatures it is rich in sodium.

At temperatures below the continuous and discontinuous sides of the reaction series the following minerals crystallize:

• high temperature
  • muscovite
  • alkali feldspar
  • quartz
• low temperature

If we focus on the feldspars, we can take advantage of the following relationship in relating the mineralogy of an igneous rock to its temperature of formation:

• high temperature
  • calcium-rich plagioclase
  • sodium-rich plagioclase
  • alkali feldspar (a solid solution between K and Na feldspars)
• low temperature

Quartz has the highest melting point of the individual minerals in Bowen's Reaction Series but it crystallizes at the lowest temperature from a magma. Thus, the importance of understanding the properties of a mixture.

Thought Questions

Note that the composition of the melt is important. If melt did not contain any Fe or Mg, which minerals would not crystallize? If the melt did not contain any water, which mineral would not crystallize?

olivine
alkali feldspar
amphibole
quartz


Certain mineral assemblages give us information about temperature. Which of the following pairs of minerals represents the highest temperature?

olivine and Ca-rich plagioclase
alkali feldspar and quartz
amphibole and Na-rich placioclase
olivine and quartz


Certain mineral assemblages are unlikely to occur. Minerals that crystallize at aboutthe same temperature tend to occur together. Which of the following pairs of minerals is highly unlikely to be found in nature?

olivine and Ca-rich plagioclase
alkali feldspar and quartz
amphibole and Na-rich placioclase
olivine and quartz


Finally, we are able to construct a classification scheme for igneous rocks using texture and mineralogy. The temperature will be estimated by the feldspar(s) present and the cooling rate by the texture. In constructing the following chart a number of short-cuts have been taken.

						
                         Alkali Feldspar   Sodium Plagioclase   Calcium Plagioclase

Phaneritic		Granite	                     Diorite			Gabbro	

Aphanitic		Rhyolite                      Andesite                   Basalt



Be able to identify each of the six common igneous rocks with the mineral assemblages that they contain. Be able to describe when you would expect to use the aphanitic name and when you would use the phaneritic name.

The is a rather specific relationship between temperature, plate tectonic setting and type of igneous rock. In general, basalts are found associated with a spreading center, andesites with a subduction zone, and rhyolites with continent-continent collision. However, look at the Hawaiian Islands. They are a long way from the nearest plate boundary or spreading center. Therefore, there must be exceptions to these generalizations.

Take a Tour of a Rock Garden and look at some other igneous rocks.

Granite is a coarse grained igneous rock which contains abundant alkali feldspar. Granites also contain quartz. This is a low-temperature assemblage. Rhyolite is the mineralogical equivalent of granite but it formed as a result of rapid cooling giving the rock the fine grained texture. Think about the relationships between Diorite and Andesite and Gabbro and Basalt. Look at the steps leading up to this building from the parking lot. Does it look similar to this picture? The reddish mineral is alkali feldspar and quartz is the gray, glassy mineral. The black material is a mixture of biotite and amphibole; thus, this rock formed in the presence of (OH).

If the rock is a granite but with a porphyritic texture it would be a granite porphyry. It if is a rhyolite but with a porphyritic texture it would be a rhyolite porphyry.

Viscosity is a measure of "resistance to flow". A liquid with high viscosity flows with difficulty. In general, as the temperature of the liquid increases the viscosity of the liquid decreases and the liquid flows more easily. Water modifies the viscosity of a melt. In general, the greater amount of water dissolved in the melt, the lower its viscosity and the more readily it flows.

Shapes of Intrusive Igneous Bodies

• Tabular Bodies - relatively low viscosity to allow magma to follow relatively narrow openings.
  • Dikes - tabular bodies that cut across the "structure" of the enclosing rock.

  • Sills - tabular bodies that are oriented parallel to the "structure of the enclosing rock. Look at the picture at the start of the chapter on Geologic Time. The sill is essentially parallel (conformable) with the enclosing sedimentary rocks.

  • Laccoliths - bodies that "dome up" the overlying rocks

  • Lopoliths - bodies with a bottom that has "subsided" into the underlying rokcs

• Irregular bodies include stocks and batholiths which typically were formed from highly viscous melts.

Dikes and sills are tabular intrusive bodies that are favored by melts with low viscosities. Which of the following magmas is most unlikely to form dikes or sills?

basalt
gabbro
rhyolite
andesite


Water (or other volatiles) tends to reduce the viscosity of a melt. If you found a granite dike which of the following minerals would indicate the presence of water in the magma?

biotite mica
quartz
alkali feldspar
Na-rich plagioclase


Shapes of Extrusive Igneous Bodies

The cross-section or profile of a volcano can give us a clue as to the composition of the lava that it is composed of.

Lavas with low viscosity can flow a great distance down relatively gentle slopes. These large shield volcanoes are very wide relative to their height. The top of some of the Hawaiian Islands is nearly two miles above sea level. Imagine how wide they must be.

Lavas with intermediate viscosities then to produce composite or strato volcanoes. These volcanoes (with the classic profile of Mt. Fuji in Japan) are tall relative to their width. They often consist of lava flows interlayered with fragments that were erupted. Given their intermediate viscosity, many are highly explosive - such as Mt. St. Helens.

Lavas with high viscosities often form cinder cones which are made up of fragmented particles. These lavas are often highly explosive and may contain large quantities of gas. A nuee ardente is a firely gass clound.

Which of the following lavas is highly likely to form a shield volcano?

rhyolitic
andesitic
granitic
basaltic


Strato volcanos are associated with which of the following plate margin types?

continent/continent collision
divergent
convergent - subduction and island arc
all of the above


Source of Heat To Partially Melt Solid Rock

At one time geologists thought that there was a world-wide layer of molten material relatively close to the surface of the Earth. Under certain conditions these magmas wouild enter the crust or flow out on the surface and produce igneous rocks. A study of earthquake waves, however, will show that such a permanent layer of liquid material does not exist in either the crust or the mantle.

Perhaps there is some process which produces heat within the Earth which is responsible for episodes of magma generation. When radioactive elements (U235, U238, Th 232 or K40) decay, heat is given off. Each decay gives off a very small amount of heat but given sufficiently long time periods, this heat can result in temperature increases sufficient to initiate partial melting.

Fractional Crystallization

Imagine a magma that contains 25g Olivine, 50g Ca-plagioclase and 25g Pyroxene. If nothing is added or subtracted from the magma, it will crystallize a rock with 25% Olivine, 50% Ca-plagioclase and 25% Pyroxene. The first mineral to crystallize is olivine. Olivine is denser than the liquid it is crystallizing from and, unless convection stirs the melt, the early formed olivine may settle to the bottom of the magma chamber and effectively be separated from the liquid.

Imagine that 25g Olivine and 25g Plagioclase are removed from the melt. They form a rock of 50% Olivine and 50% Plagioclase. The liquid contains 25g Plagioclase and 25g Pyroxene. They eventually crystallize and form a rock containing 50% Plagioclase and 50% Pyroxene. Thus, two different rocks formed as a result of fractional crystallization:

1. 50% Olivine and 50% Plagioclase
   
2. 50% Pyroxene and 50% Plagioclase

Thus, fractional crystallization has increased the "complexity" of the situation -- two rocks rather than one.

Anything that separates crystals from liquids may cause crystal fractionation.

Magma Mixing and Assimilation

Imagine a basaltic magma and a rhyolitic magma. Take 1 part basalt and mix with one part rhyolite and ..... you have something like an andesite. This works well on paper but think about the conditions under which you expect each of these magmas to form. Basalts are typically associated with divergent margins or hot spots. Rhyolites are associated with continent/continent convergent zones. Thus, although the chemistry is OK, the conditions needed to form the two magmas are drastically different. Magma mixing is probably rather rate.

Magmas may partially melt the surrounding country rocks thus changing the composition of the magma. However, it takes energy to melt the colder country rocks and the magma would have to cool. The more it cools the more it crystallizes thus reducing the amount of country rock that could be dissolved. Assimilation does occur but its extent is probably minor in most cases. Blocks of country rock (called xenoliths - "strange" rocks) are often found near the margins of a magma chamber. Sometimes these xenoliths have margins indicating that they were dissolving in the liquid.

Thought Question

The amount of heat generated by radioactive decay has decreased over time (Why?). Therefore, might there be a time on Earth when igneous process stop?

Partial Melting

The following problem is taken in modified form from the National Association of Geoscience Teachers.

Massive outpourings of molten material from the Earth's interior have taken place several times in different geographic areas. Typically, only a relatively small portion of the source material melts - somewhere between 1% and 10%.

An area of 1,000,000 km² is covered by a thick sequence (10 km) of shallow intrusive and extrusive igneous rocks.

Assuming that all of the magma and lava was derived from the deep crust and upper mantle directly underneath the area covered by these rocks, what thickness of the crust and upper mantle must have been involved if partial melting was 1%. That is, 1% of what thickness melted to yield a sequence 10 km thick.

100 km
1,000
1,000 km
10,000


What volume of the deep crust and upper mantle was involved in the partial melting process?

1.0 * 10⁹ km²
1.0 * 10⁹ km³
1,000,000,000 km
1.0 * 10¹⁰ km²


If partial melting was 5%, the volume of deep crust and upper mantle taking part would be greater than that involved in 1% partial melting

True
False


Heat necessary to melt basaltic material is 300 cal/cm³. Recall that a cal (calorie) is the amount of heat required to raise the temperature of 1 cm³ of water 1^oC. You could show that 1 km³ = 10¹⁵cm³. How much heat, in calories, is required to melt the known volume of igneous rock?

3.0 * 10²⁶ cal
3.0 * 10²⁴ cal
3.0 * 10¹⁰⁵cal
1.0 * 10²² cal/km



Heat generated by the radioactive decay of material in the deep crust and upper mantle is 5 * 10^-14cal/cm³/sec. Assume that heat is generated throughout the volume that will be partially melted and that all of the heat will accumulate - none is loss to the surroundings. How much heat is generated in the volume that was partially melted?

5 * 10 ¹⁰cal/sec
5 * 10 ¹⁰cal/cm³/sec
5 * 10 ⁸cal/sec
3 * 10 ⁸cal/sec



You now know how much heat is required to melt the volume of igneous rocks produced (cal) and the rate of heat production in the volume partially melted (cal/sec). The ratio of these two numbers will give you the length of time (in seconds) required to accumulate enough heat.

5 * 10 ¹⁰cal/sec
5 * 10 ¹⁰cal/cm³/sec
5 * 10 ⁸cal/sec
3 * 10 ⁸cal/sec


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Copyright by John C. Butler, July 29, 1995

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