Study Notes Magmas, Igneous Rocks, Volcanoes, and Plutons

Kinds of Igneous Rock

Igneous Rocks are  formed by crystallization from a liquid, or magma. They include two types  

• Volcanic or extrusive  igneous rocks form when the magma cools and crystallizes on the surface of the Earth
  
  
• Intrusive or plutonic igneous rocks wherein the magma crystallizes at depth in the Earth.

Magma is a mixture of liquid rock, crystals, and gas. Characterized by a wide range of chemical compositions, with high temperature, and  properties of a liquid.

Magmas are less dense than surrounding rocks, and will therefore move upward. If magma makes it to the surface it will erupt and later crystallize to form an extrusive or volcanic rock. If it crystallizes before it reaches the surface it will form an igneous rock at depth called a plutonic or intrusive igneous rock. Because cooling of the magma takes place at a different rate, the crystals that form and their interrelationship (texture) exhibit different properties.

• Fast cooling on the surface results in many small crystals or quenching to a glass. Gives rise to aphanitic texture (crystals cannot be distinguished with the naked eye), or obsidian (volcanic glass).

• Slow cooling at depth in the earth results in fewer much larger crystals, gives rise to phaneritic texture.

• Porphyritic texture develops when slow cooling is followed by rapid cooling. Phenocrysts = larger crystals, matrix or groundmass = smaller crystals.

Types of Magma

Chemical composition of magma is  controlled by the abundance of elements in the Earth. Si, Al, Fe, Ca, Mg, K, Na, H, and O make up 99.9%. Since oxygen is so abundant, chemical analyses are usually given in terms of oxides. SiO₂ is the most abundant oxide.

1. Basaltic or gabbroic --  SiO₂ 45-55 wt%, high in Fe, Mg, Ca, low in K, Na
   
2. Andesitic or Dioritic --  SiO₂ 55-65 wt%, intermediate. in Fe, Mg, Ca, Na, K
   
3. Rhyolitic or Granitic --  SiO₂ 65-75%, low in Fe, Mg, Ca, high in K, Na.
   

Gases - At depth in the Earth nearly all magmas contain gas.  Gas gives magmas their explosive character, because the gas expands as pressure is reduced. 

• Mostly H₂O with some CO₂
  
• Minor amounts of Sulfur, Cl , and F
  
• Rhyolitic or granitic magmas usually have higher gas contents than basaltic or gabbroic magmas.
  

Temperature of Magmas

• Basaltic or Gabbroic - 1000-1200^oC
  
• Andesitic or Dioritic -  800-1000^oC
  
• Rhyolitic or Granitic -  650-800^oC.

Viscosity of Magmas -

• Higher SiO₂ content magmas have higher viscosity than lower SiO₂ content magmas
  
• Lower Temperature magmas have higher viscosity than higher temperature magmas.
  

Summary Table

Eruption of Magma

When magmas reach the surface of the Earth they erupt from a vent.  They may erupt explosively or non-explosively.  

• Non-explosive eruptions are favored by low gas content and low viscosity magmas (basaltic to andesitic magmas).
  
  
• Usually begin with fire fountains due to release of dissolved gases
  
  
• Produce lava flows on surface
  
  
• Produce Pillow lavas if erupted beneath water
  

• Explosive eruptions are favored by high gas content and high viscosity (andesitic to rhyolitic magmas).
  
  
• Expansion of gas bubbles is resisted by high viscosity of magma - results in building of pressure
  
  
• High pressure in gas bubbles causes the bubbles to burst when reaching the low pressure at the Earth's surface.
  
  
• Bursting of bubbles fragments the magma into pyroclasts and tephra (ash).
  
  
• Cloud of gas and tephra rises above volcano to produce an eruption column that can rise up to 45 km into the atmosphere.

Tephra that falls from the eruption column produces a tephra fall deposit.

Image-Left: Section of tephra seen just S of Anatahan's active crater on 18 July showing deposits laid down in the eruptions that began in May 2003. The section contains a lower (brown) pumice-fall deposit (~ 25 cm thick) covered by multiple layers (~ 20 cm thick) of gray ash from phreatic eruptions. Courtesy of S. Nakada, University of Tokyo.

If eruption column collapses a pyroclastic flow  may 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.

Lateral blasts and debris avalanches occur when gas is released suddenly by a large landslide or debris avalanche taking out part of the volcano.

Images: USGS

Plutons

Igneous rocks cooled at depth. Name comes from Greek god of the underworld - Pluto.

• 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.

Image: USGS

Until erosion began its relentless work, Devils Tower (left) was not visible above the overlying sedimentary rocks. The forces of erosion, began to wear away the sandstones and shales. The much harder igneous rock (dike) survived the erosional forces, and the gray columns of Devils Tower began to appear above the surrounding landscape.

• 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.

• Batholiths are very large intrusive bodies, usually so large that there bottoms are rarely exposed.  Sometimes they are composed of several smaller intrusions.
• (from the Greek bathos, depth + lithos, rock)
  
  
• 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.

Methods of intrusion

• Melting - crystallization
  
  
• Stoping - xenoliths
  
  
• Injection

Why do we see intrusive igneous rocks at the surface of the Earth?  Answer - They are exposed by erosion which has removed all of the material above the intrusion.

Origin of Magma

In order for magmas to form, some part of the Earth must get hot enough to melt the rocks present. Under normal conditions, the geothermal gradient is not high enough to melt rocks, and thus with the exception of the outer core, most of the Earth is solid.  Thus, magmas form only under special circumstances.  To understand this we must first look at how rocks and mineral melt.

As pressure increases in the Earth, the melting temperature changes as well.  For pure minerals, there are two general cases.

• For a pure dry (no H₂O or CO₂ present) mineral, the melting temperate increases with increasing pressure.

• For a mineral with H₂O or CO₂ present, the  melting temperature first decreases with increasing pressure

• Melting of dry rocks is similar to melting of dry minerals, melting temperatures increase with increasing pressure, except there is a range of temperature over which there exists a partial melt.  The degree of partial melting can range from 0 to 100%

• Melting of rocks containing water or carbon dioxide is similar to melting of wet minerals, melting temperatures initially decrease with increasing pressure, except there is a range of temperature over which there exists a partial melt.

Diverging Plate Boundaries

Diverging plate boundaries are mostly beneath the oceans and occur at oceanic ridges.  Here, basaltic magma is erupted at the oceanic ridge and is intruded beneath the ridge where it forms new oceanic crust.

Converging Plate Boundaries

Where lithospheric plates converge, oceanic lithosphere subducts beneath either another plate composed of oceanic lithosphere or another plate composed of continental lithosphere.

• If an oceanic lithospheric plate subducts beneath another oceanic lithospheric plate, we find island arcs on the surface above the subduction zone. These are volcanoes built of mostly andesitic lavas pyroclastic material, although some basalts and rhyolites also occur.
  

• If an oceanic plate subducts beneath a plate composed of continental lithosphere, we find continental margin arcs.  Again, the volcanoes found here are composed mostly of andesitic lavas and pyroclastics.  It is likely that some magmas cool beneath the volcanic arc to form dioritic and granitic plutons.  

Areas where rising plumes of hot mantle reach the surface, usually at locations far removed from plate boundaries are called hot spots. 

Much evidence suggests that Basaltic magmas result from dry partial melting of mantle.

• Basalts make up most of oceanic crust and only mantle underlies crust.
  
  
• Basalts contain minerals like olivine, pyroxene and plagioclase, none of which contain water.
  
  
• Basalts erupt non-explosively, indicating a low gas content and therefore low water content.
  

The Mantle is made of garnet peridotite (a rock made up of olivine, pyroxene, and garnet) -- evidence comes from pieces brought up by erupting volcanoes. In the laboratory we can determine the melting behavior of garnet peridotite.

Origin of Granitic Magma

Most Granitic or Rhyolitic magma appears to result from wet melting of continental crust. The evidence for this is: 

• Most granites and rhyolites are found in areas of continental crust.
  
• When granitic magma erupts from volcanoes it does so very explosively, indicating high gas content.
  
• Solidified granite or rhyolite contains quartz, feldspar, hornblende, biotite, and muscovite. The latter minerals contain water, indicating high water content.

Still, the temperature in continental crust is usually not high enough to cause melting, and thus another heat source is necessary.  In most cases it appears that this heat source is basaltic magma.  The basaltic magma is generated in the mantle, then rises into the continental crust.  But, because basaltic magma has a high density it may stop in the crust and crystallize, releasing heat into the surrounding crust. This raises the geothermal gradient and may cause wet partial melting of the crust to produce rhyolitic magmas.

Origin of Andesitic Magma

Average composition of continental crust is andesitic, but if andesite magma is produced by melting of continental crust then it requires complete melting of crust. Temperatures in crust unlikely to get high enough. Andesitic magmas erupt in areas above subduction zones - suggests relation between production of andesite and subduction. One theory involves wet partial melting of subducted oceanic crust. But, newer theories suggest wet partial melting of mantle.

Magmatic Differentiation

When magma solidifies to form a rock it does so over a range of temperature. Each mineral begins to crystallize at a different temperature, and if these minerals are somehow removed from the liquid, the liquid composition will change. Depending on how many minerals are lost in this fashion, a wide range of compositions can be made. The processes is called magmatic differentiation by crystal fractionation.

Crystals can be removed by a variety of processes. If the crystals are more dense than the liquid, they may sink. If they are less dense than the liquid they will float. If liquid is squeezed out by pressure, then crystals will be left behind. Removal of crystals can thus change the composition of the liquid portion of the magma. Let me illustrate this using a very simple case.

Imagine a liquid containing 5 molecules of MgO and 5 molecules of SiO₂. Initially the composition of this magma is expressed as 50% SiO₂ and 50% MgO. i.e.

Now let's imagine I remove 1 MgO molecule by putting it into a crystal and removing the crystal from the magma. Now what are the percentages of each molecule in the liquid?

If we continue the process one more time by removing one more MgO molecule

Thus, composition of liquid can be changed.

Volcanoes and Volcanic Rocks

Basalts, Andesites, Dacites, and Rhyolites are all types of volcanic rock distinguished on the basis of their mineral assemblage. Depending on conditions present during eruption and cooling, any of these rock types may form one of the following types of volcanic rocks.

• Obsidian - dark colored volcanic glass showing concoidal fracture. Usually rhyolitic or dacitic.
  
  
• Pumice - light colored and light weight rock consisting of mostly holes (vesicles) that were once occupied by gas, Usually rhyolitic, dacitic or andesitic.
  

• Vesicular rock - rock filled with holes (like Swiss cheese) or vesicles that were once occupied by gas. Usually basaltic and andesitic.
  
  
• Amygdaloidal basalt. If vesicles in a vesicular basalt are later filled by precipitation of calcite or quartz, the fillings are termed amygdules and the basalt is termed an amygdaloidal basalt.

Pyroclasts and Tephra

• Pyroclasts = hot, broken fragments. Result from explosively ripping apart of magma. Loose assemblages of pyroclasts called tephra. Depending on size, tephra can be classified as bombs. lapilli, or ash.
  
  
• Rock formed by accumulation and cementation of tephra called a pyroclastic rock or tuff. Welding, compaction and deposition of other grains cause tephra (loose material) to be converted in pyroclastic rock.

Volcanoes

• Shield volcano - volcanoes that erupt low viscosity magma (usually basaltic) that flows long distances from the vent.

• Pyroclastic cone or cinder cone - a volcano built mainly of tephra fall deposits located immediately around the vent.

• Stratovolcano (composite volcano) - a volcano built of interbedded lava flows and pyroclastic material.

• Crater  -  a depression caused by explosive ejection of magma or gas.
  
  

• Caldera - a depression caused by collapse of a volcano into the cavity once occupied by magma.

• Lava Dome - a steep sided volcanic structure resulting from the eruption of high viscosity, low gas content magma

• Thermal Springs and Geysers - hot water resulting from heating by magma at depth in the Earth.  Springs flow, geysers erupt.
  
  
• Fissure Eruptions - An eruption that occurs along a narrow crack or fissure in the Earth's surface.
  
  
• Pillow Lava - Lavas formed by eruption beneath the surface of the ocean or a lake.