How Does Mineralogy Control Volcano Structure, And Volcanic Eruption Type

How Volcanoes Work

PHYSICOCHEMICAL CONTROLS ON ERUPTION STYLE

In that location is a great range in the explosivity of volcanic eruptions. Many eruptions are relatively quiescent and are characterized by the calm, nonviolent extrusion of lava flows on the globe's surface. Other eruptions, however, are highly explosive and are characterized by the violent ejection of fragmented volcanic debris, chosen tephra , which tin can extend tens of kilometers into the atmosphere higher up the volcano.


Nonexplosive eruption with effusive lava flows	Explosive eruption with voluminous plume of tephra

Whether or not an eruption falls into 1 of these end-member types depends on a variety of factors, which are ultimately linked to the composition of the magma (molten rock) underlying the volcano. Magma limerick is discussed below, followed past a clarification of the controlling factors on explosivity -- viscosity , temperature , and the corporeality of dissolved gases in the magma.

MAGMA Composition AND ROCK TYPES

Merely ten elements make up the majority of most magmas: oxygen (O), silicon (Si), aluminum (Al), iron (Iron), magnesium (Mg), titanium (Ti) calcium (Ca), sodium (Na), potassium (K), and phosphorous (P). Because oxygen and silicon are by far the two most abundant elements in magma, it is convenient to describe the unlike magma types in terms of their silica content (SiO 2 ). The magma types vary from mafic magmas , which accept relatively depression silica and high Atomic number 26 and Mg contents, to felsic magmas , which have relatively high silica and low Atomic number 26 and Mg contents. Mafic magma will absurd and crystallize to produce the volcanic stone basalt , whereas felsic magma volition crystallize to produce dacite and rhyolite . Intermediate-limerick magmas volition crystallize to produce the rock andesite . Because the mafic rocks are enriched in Iron and Mg, they tend to exist darker colored than the felsic rock types.

SiO two CONTENT	MAGMA Type	VOLCANIC ROCK
~50%	Mafic	Basalt
~60%	Intermediate	Andesite
~65%	Felsic (low Si)	Dacite
~70%	Felsic (loftier Si)	Rhyolite

There also exists more than unusual magmas that erupt less unremarkably on the Earth's surface as ultramafic, carbonatite, and strongly alkaline lavas.

For an historical note on rock terminology run across: Basic/Acidic vs. Mafic/Felsic

MAGMA VISCOSITY, TEMPERATURE, AND GAS CONTENT

The viscosity of a substance is a measure out of its consistency. Viscosity is defined as the ability of a substance to resist flow. In a sense, viscosity is the inverse of fluidity. Cold molasses, for example, has a college viscosity than h2o because it is less fluid. A magma's viscosity is largely controlled past its temperature, limerick, and gas content (meet downloadable programs at the lesser of this folio). The effect of temperature on viscosity is intuitive. Similar most liquids, the college the temperature, the more fluid a substance becomes, thus lowering its viscosity.

Limerick plays an even greater role in determining a magma's viscosity. A magma's resistance to flow is a function of its "internal friction" derived from the generation of chemical bonds within the liquid. Chemical bonds are created between negatively charged and positively charged ions (anions and cations, respectively). Of the 10 most abundant elements institute in magmas (see above), oxygen is the only anion. Silicon, on the other mitt, is the well-nigh arable cation. Thus, the Si-O bond is the single virtually important cistron in determining the caste of a magma'south viscosity. These ii elements bond together to class "floating radicals" in the magma, while it is still in its liquid state (i.e., Si-O bonds begin to grade well above the crystallization temperature of magma). These floating radicals contain a small silicon atom surrounded by four larger oxygen atoms (SiO 4 ). This atomic configuration is in the shape of a tetrahedron. The radicals are therefore called silicon-oxygen tetrahedra , as shown here.

These floating tetrahedra are electrically charged compounds. As such, they they are electrically attracted to other Si-O tetrahedra. The outer oxygen atoms in each tetrahedron tin can share electrons with the outer oxygen atoms of other tetrahedra. The sharing of electrons in this style results in the development of covalent bonds between tetrahedra. In this way Si-O tetrahedra can link together to form a multifariousness shapes: double tetrahedra (shown here, C), chains of tetrahedra, double chains of tetrahedra, and complicated networks of tetrahedra. As the magma cools, more and more bonds are created, which eventually leads to the development of crystals within the liquid medium. Thus, the Si-O tetrahedra course the edifice blocks to the common silicate minerals plant in all igneous rocks. Yet, while however in the liquid state, the bonding of tetrahedra results in the polymerization of the liquid, which increases the "internal friction" of the magma, so that it more readily resists flow. Magmas that take a high silica content volition therefore exhibit greater degrees of polymerization, and have higher viscosities, than those with depression-silica contents.

The amount of dissolved gases in the magma can also affect it's viscosity, but in a more than ambiguous way than temperature and silica content. When gases begin to escape (exsolve) from the magma, the effect of gas bubbles on the bulk viscosity is variable. Although the growing gas bubbling will exhibit depression viscosity, the viscosity of the balance liquid volition increase as gas escapes. The overall majority viscosity of the chimera-liquid mixture depends on both the size and distribution of the bubbles. Although gas bubbling exercise accept an effect on the viscosity, the more than important role of these exsolving volatiles is that they provide the driving force for the eruption. This is discussed in more particular below.

VESICULATION

Equally dissolved gases are released from the magma, bubbles volition begin to form. Bubbles frozen in a porous or frothy volcanic rock are called vesicles , and the procedure of bubble formation is called vesiculation or gas exsolution . The dissolved gases can escape merely when the vapor force per unit area of the magma is greater than the confining pressure of the surrounding rocks. The vapor pressure is largely dependent on the amount of dissolved gases and the temperature of the magma.

Explosive eruptions are initiated by vesiculation, which in turn, tin can exist promoted in two ways: (1) by decompression, which lowers the confining pressure, and (2) by crystallization, which increases the vapor pressure. In the first case, magma rise can lead to decompression and the formation of bubbles, much similar the decompression of soda and the formation of CO two bubbles when the cap is removed. This is sometimes referred to as the first boiling . Alternatively, equally magma cools and anhydrous minerals begin to crystallize out of the magma, the residual liquid will go increasingly enriched in gas. In this instance, the increased vapor pressure in the residual liquid can also lead to gas exsolution. This is sometimes referred to as second (or retrograde) boiling . Both mechanisms tin trigger an explosive volcanic eruption.

CONTROLS ON EXPLOSIVITY

The corporeality of dissolved gas in the magma provides the driving forcefulness for explosive eruptions. The viscosity of the magma, however, is also an of import factor in determining whether an eruption volition be explosive or nonexplosive. A low-viscosity magma, like basalt, will allow the escaping gases to drift chop-chop through the magma and escape to the surface. However, if the magma is viscous, like rhyolite, its high polymerization will impede the up mobility of the gas bubbles. Equally gas continues to exsolve from the sticky melt, the bubbles will be prevented from rapid escape, thus increasing the overall pressure on the magma column until the gas ejects explosively from the volcano. As a general rule, therefore, nonexplosive eruptions are typical of basaltic-to-andesitic magmas which accept low viscosities and low gas contents, whereas explosive eruptions are typical of andesitic-to-rhyolitic magmas which have loftier viscosities and high gas contents.

SiO two	MAGMA Type	TEMPERATURE (centigrade)	VISCOSITY	GAS CONTENT	ERUPTION Mode
~fifty%	mafic	~1100	low	low	nonexplosive
~threescore%	intermediate	~1000	intermediate	intermediate	intermediate
~70%	felsic	~800	high	high	explosive

At that place are, however, two exceptions to this general dominion. Andesitic-to-rhyolitic lavas that have been degassed often erupt at the surface nonexplosively as glutinous lava domes or obsidian flows . Similarly, many of the so-called hydrovolcanic eruptions involve basaltic-to-andesitic magmas that erupt explosively in the presence of groundwater or surface water. For more than information on the variability of explosivity, see the Volcano Explosivity Alphabetize .

Intermediate to avant-garde users may be interested in the following programs for computing viscosity. Click image to download.

Viscosity for Windows . Developed past Dr. Jon Dehn, this program calculates the viscosity of silicate magmas from the magma composition and temperature.

Magma for Windows . Developed by Dr. Ken Wohletz, this program calculates the density and viscosity of silicate magmas from the magma limerick and temperature.