Magmas, Igneous Rocks, Volcanoes, and Plutons
of Entire Course
Kinds of Igneous Rock
Types of Magma
Eruption of Magma
Origin of Magma
Origin of Basaltic Magma
Origin of Granitic Magma
Origin of Andesitic Magma
Bowen's Reaction Series
Volcanoes and Volcanic Rocks
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.
cooling on the surface results in many small crystals or quenching to a
glass. Gives rise to aphanitic texture (crystals cannot be
with the naked eye), or obsidian (volcanic glass).
Slow cooling at depth in the
results in fewer much larger crystals
, gives rise to phaneritic texture.
texture develops when slow cooling
is followed by rapid cooling.
Phenocrysts = larger crystals, matrix or groundmass = smaller crystals.
rock textural terms
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.
Basaltic or gabbroic
SiO2 45-55 wt%, high in Fe, Mg, Ca, low in K, Na
Andesitic or Dioritic -- SiO2 55-65 wt%, intermediate. in Fe, Mg, Ca, Na, K
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.
|Magma Type||Solidified Volcanic Rock||Solidified Plutonic Rock||Chemical Composition||Temperature||Viscosity||Gas Content|
|Basaltic||Basalt||Gabbro||45-55 SiO2 %, high in Fe, Mg, Ca, low in K, Na||1000 - 1200 oC||Low||Low|
|Andesitic||Andesite||Diorite||55-65 SiO2 %, intermediate in Fe, Mg, Ca, Na, K||800 - 1000 oC||Intermediate||Intermediate|
|Rhyolitic||Rhyolite||Granite||65-75 SiO2 %, low in Fe, Mg, Ca, high in K, Na||650 - 800 oC||High||High|
- Tephra that falls from the eruption column produces a tephra fall deposit.
Lateral blasts and debris
occur when gas is released suddenly by a large landslide or debris
taking out part of the volcano
As pressure increases in the Earth, the melting temperature changes as well. For pure minerals, there are two general cases.
a pure dry (no H2O or CO2 present) mineral, the melting
temperate increases with increasing pressure.
a mineral with H2O or CO2 present, the melting
temperature first decreases with increasing pressure
Since rocks mixtures of minerals, they behave somewhat differently. Unlike minerals, rocks do not melt at a single temperature, but instead melt over a range of temperatures. Thus it is possible to have partial melts, from which the liquid portion might be extracted to form magma. The two general cases are:
of dry rocks is similar to melting of dry minerals, melting
increase with increasing pressure, except there is a range of
over which there exists a partial melt. The degree of partial
can range from 0 to 100%
of rocks containing water or carbon dioxide is similar to melting of
minerals, melting temperatures initially decrease with increasing
except there is a range of temperature over which there exists a
Under normal conditions the temperature in the Earth, shown by the geothermal gradient, is lower than the beginning of melting of the mantle. Thus in order for the mantle to melt there has to be a mechanism to raise the geothermal gradient. Once such mechanism is convection, wherein hot mantle material rises to lower pressure or depth, carrying its heat with it. If the raised geothermal gradient becomes higher than the initial melting temperature at any pressure, then a partial melt will form. Liquid from this partial melt can be separated from the remaining crystals because, in general, liquids have a lower density than solids. Basaltic or gabbroic magmas appear to originate in this way.
Origin of Granitic Magma
Most Granitic or Rhyolitic magma appears to result from wet melting of continental crust. The evidence for this is:
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.
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?
Pyroclasts and Tephra
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.
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
around the vent.
(composite volcano) - a volcano built of interbedded lava flows and
Crater - a depression caused
by explosive ejection of magma or gas.
- a depression caused by collapse of a volcano into the cavity once
Dome - a steep sided volcanic structure resulting from the eruption
of high viscosity, low gas content magma
Question - 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