What Causes the Iron in the Magma to Switch From + to - and Back Again?

EENS 3050	Natural Disasters
Tulane University	Prof. Stephen A. Nelson
Volcanoes, Magma, and Volcanic Eruptions

Since volcanic eruptions are acquired by magma (a mixture of liquid rock, crystals, and dissolved gas) expelled onto the Earth'southward surface, we must first talk over the characteristics of magma and how magmas grade in the World. Characteristics of Magma Types of Magma Types of magma are determined by chemic composition of the magma. Three full general types are recognized: Basaltic magma --  SiO2 45-55 wt%, high in Fe, Mg, Ca, depression in Thousand, Na Andesitic magma --  SiO2 55-65 wt%, intermediate. in Fe, Mg, Ca, Na, K Rhyolitic magma --  SiOtwo 65-75%, low in Fe, Mg, Ca, loftier in K, Na Gases in Magmas At depth in the Earth nearly all magmas contain gas dissolved in the liquid, just the gas forms a split up vapor phase when pressure is decreased equally magma rises toward the surface of the Earth.   This is like to carbonated beverages which are bottled at high pressure. The high pressure keeps the gas in solution in the liquid, just when pressure is decreased, like when y'all open the can or bottle, the gas comes out of solution and forms a separate gas phase that you see equally bubbles.   Gas gives magmas their explosive grapheme, considering volume of gas expands as force per unit area is reduced.  The composition of the gases in magma are: Mostly H2O (water vapor) & some CO2 (carbon dioxide) Minor amounts of Sulfur, Chlorine, and Fluorine gases The corporeality of gas in a magma is as well related to the chemic composition of the magma. Rhyolitic  magmas usually take college gas contents than basaltic  magmas.

Temperature of Magmas

Temperature of magmas is difficult to measure (due to the danger involved), but laboratory measurement and limited field observation signal that the eruption temperature of diverse magmas is as follows:

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

Viscosity of Magmas

Viscosity is the resistance to menses (contrary of fluidity).  Viscosity depends on primarily on the limerick of the magma, and temperature.

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

• Lower temperature magmas have higher viscosity than college temperature magmas (viscosity decreases with increasing temperature of the magma).

Thus, basaltic magmas tend to be fairly fluid (low viscosity), only their viscosity is still ten,000 to 100,0000 times more glutinous than water.  Rhyolitic magmas tend to have fifty-fifty higher viscosity, ranging between 1 million and 100 million times more than sticky than h2o.  (Annotation that solids, even though they appear solid have a viscosity, but it very high, measured every bit trillions times the viscosity of water).  Viscosity is an important holding in determining the eruptive behavior of magmas.

Summary Table
Magma Type	Solidified Rock	Chemic Composition	Temperature	Viscosity	Gas Content
Basaltic	Basalt	45-55 SiO2 %, high in Iron, Mg, Ca, low in Grand, Na	1000 - 1200 oC	Low	Low
Andesitic	Andesite	55-65 SiO2 %, intermediate in Atomic number 26, Mg, Ca, Na, K	800 - 1000 oC	Intermediate	Intermediate
Rhyolitic	Rhyolite	65-75 SiO2 %, low in Fe, Mg, Ca, loftier in G, Na.	650 - 800 oC	High	Loftier

How Magmas Form in the Globe

Equally nosotros have seen the simply office of the earth that is liquid is the outer cadre.  But the core is not likely to exist the source of magmas because it does not have the correct chemical composition.  The outer cadre is mostly Iron, just magmas are silicate liquids.  Thus, magmas DO NOT Come FROM THE MOLTEN OUTER CORE OF THE World.  Since the rest of the earth is solid, in order for magmas to form, some role of the earth must get hot enough to melt the rocks present.

Nosotros know that temperature increases with depth in the earth along the geothermal slope.  The earth is hot inside due to heat left over from the original accession process, due to oestrus released past sinking of materials to form the core, and due to rut released by the decay of radioactive elements in the earth.  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 class merely under special circumstances, and thus, volcanoes are just found on the Earth'south surface in areas above where these special circumstances occur. (Volcanoes don't simply occur anywhere, every bit we shall soon see). To sympathise this we must first await at how rocks and mineral melt. To sympathize this we must kickoff look at how minerals and rocks cook.

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

If the mineral contains no water (HiiO) or carbon dioxide (CO2) and there is no water or carbon dioxide present in the surroundings, then melting occurs at a unmarried temperature at any given pressure and increases with increasing pressure or depth in the Earth.  This is called dry melting .

If water or carbon dioxide are present within or surrounding the mineral, so melting takes place at a unmarried temperature at any given force per unit area, but first decreases with increasing pressure level

Since rocks are mixtures of minerals, they comport somewhat differently.  Unlike minerals, rocks do not melt at a unmarried temperature, merely instead cook 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:

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

Melting of wet rocks is like to melting of moisture minerals, except in that location is range of temperature range over which partial melting occurs.  Again, the temperature of offset of melting first decreases with increasing pressure or depth, and then at high pressure level or depth the melting temperatures once more brainstorm to rise.

3 ways to Generate Magmas

From the above nosotros tin conclude that in order to generate a magma in the solid part of the earth either the geothermal gradient must exist raised in some mode or the melting temperature of the rocks must be lowered in some way.

The geothermal gradient can be raised past upwelling of hot material from below either by uprise solid material (decompression melting) or past intrusion of magma (heat transfer). Lowering the melting temperature can be achieved past calculation water or Carbon Dioxide (flux melting).

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

Decompression Melting - Under normal conditions the temperature in the Earth, shown past the geothermal gradient, is lower than the beginning of melting of the mantle.  Thus in order for the mantle to melt there has to exist a machinery to raise the geothermal gradient.  Once such mechanism is convection, wherein hot mantle material rises to lower force per unit area or depth, carrying its rut with information technology.

If the raised geothermal slope becomes higher than the initial melting temperature at any force per unit area, then a partial melt volition form.  Liquid from this partial melt tin be separated from the remaining crystals because, in full general, liquids have a lower density than solids.  Basaltic magmas announced to originate in this way. Upwelling mantle appears to occur beneath oceanic ridges, at hot spots, and below continental rift valleys.  Thus, generation of magma in these three environments is likely caused past decompression melting.

Transfer of Oestrus-  When magmas that were generated by some other mechanism intrude into cold chaff, they bring with them rut.  Upon solidification they lose this rut and transfer it to the surrounding crust.   Repeated intrusions can transfer enough heat to increase the local geothermal gradient and cause melting of the surrounding rock to generate new magmas.

Rhyolitic magma tin likewise be produced by changing the chemical composition of basaltic magma equally discussed after.

Transfer of rut by this mechanism may be responsible for generating some magmas in continental rift valleys, hot spots, and subduction related environments.

Flux Melting - As we saw higher up, if water or carbon dioxide are added to rock, the melting temperature is lowered.   If the addition of water or carbon dioxide takes place deep in the world where the temperature is already high, the lowering of melting temperature could cause the rock to partially melt to generate magma.  One place where water could be introduced is at subduction zones. Here, water nowadays in the pore spaces of the subducting body of water floor or h2o present in minerals like hornblende, biotite, or dirt minerals would be released by the rising temperature and then move in to the overlying mantle.   Introduction of this water in the drapery would then lower the melting temperature of the drape to generate partial melts, which could then separate from the solid curtain and rise toward the surface.

Chemical Composition of Magmas

The chemical composition of magma can vary depending on the rock that initially melts (the source rock), and process that occur during partial melting and transport.

Initial Limerick of Magma

The initial composition of the magma is dictated past the composition of the source stone and the degree of partial melting.   In general, melting of a mantle source (garnet peridotite) results in mafic/basaltic magmas.  Melting of crustal sources yields more siliceous magmas.

In general more siliceous magmas course by low degrees of partial melting. As the degree of partial melting increases, less siliceous compositions tin can exist generated. And so, melting a mafic source thus yields a felsic or intermediate magma. Melting of ultramafic (peridotite source) yields a basaltic magma.

Magmatic Differentiation

But, processes that operate during transportation toward the surface or during storage in the crust can alter the chemical composition of the magma.   These processes are referred to as magmatic differentiation and include absorption, mixing, and partial crystallization.

• Absorption - Equally magma passes through cooler rock on its way to the surface information technology may partially melt the surrounding stone and comprise this melt into the magma. Because small amounts of partial melting result in siliceous liquid compositions, addition of this melt to the magma will brand information technology more siliceous.
• Mixing - If two magmas with different compositions happen to come in contact with one another, they could mix together. The mixed magma volition accept a composition somewhere between that of the original two magma compositions. Evidence for mixing is often preserved in the resulting rocks.
• Crystal Fractionation - 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 tin can be made. The processes is chosen magmatic differentiation past crystal fractionation.

  Crystals tin can be removed past a diversity of processes. If the crystals are more dumbo than the liquid, they may sink. If they are less dumbo than the liquid they will float. If liquid is squeezed out past pressure, so crystals volition be left backside. Removal of crystals can thus change the composition of the liquid portion of the magma. Let me illustrate this using a very unproblematic case.

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

.

Now let'south 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 go on the process one more time by removing one more MgO molecule.

Thus, limerick of liquid tin be changed.  This process is called crystal fractionation.  A mechanism by which a basaltic magma beneath a volcano could modify to andesitic magma and eventually to rhyolitic magma through crystal fractionation, is provided by Bowen'south reaction series, discussed next.

Bowen's Reaction Series
Bowen institute past experiment that the guild in which minerals crystallize from a basaltic magma depends on temperature.  As a basaltic magma is cooled Olivine and Ca-rich plagioclase crystallize outset.  Upon further cooling, Olivine reacts with the liquid to produce pyroxene and Ca-rich plagioclase react with the liquid to produce less Ca-rich plagioclase.  Simply, if the olivine and Ca-rich plagioclase are removed from the liquid by crystal fractionation, so the remaining liquid volition exist more than SiO₂ rich.  If the procedure continues, an original basaltic magma tin alter to start an andesite magma then a rhyolite magma with falling temperature.

Volcanic Eruptions

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

• Every bit they ascension they may encounter a depth or pressure where the dissolved gas no longer tin can be held in solution in the magma, and the gas begins to form a dissever phase (i.due east. it makes bubbles just like in a bottle of carbonated beverage when the pressure is reduced).

• When a gas bubble forms, it will likewise continue to abound in size equally pressure is reduced and more of the gas comes out of solution.  In other words, the gas bubbles begin to aggrandize.

• If the liquid part of the magma has a low viscosity, then the gas tin aggrandize relatively easily.  When the magma reaches the Earth'due south surface, the gas bubble will simply burst, the gas will easily aggrandize to atmospheric pressure, and a non-explosive eruption volition occur, normally equally a lava menses ( Lava is the proper noun nosotros give to a magma when it on the surface of the Globe).

• If the liquid part of the magma has a loftier viscosity, then the gas will not be able to expand very easily, and thus, force per unit area will build up inside of the gas chimera(due south).  When this magma reaches the surface, the gas bubbling will take a high force per unit area inside, which will crusade them to outburst explosively on reaching atmospheric pressure.  This volition cause an explosive volcanic eruption.

Effusive (Not-explosive) Eruptions

Non explosive eruptions are favored past low gas content and low viscosity magmas (basaltic to andesitic magmas). If the viscosity is low, non-explosive eruptions usually begin with burn down fountains due to release of dissolved gases.

When magma reaches the surface of the globe, it is called lava.  Since information technology its a liquid, information technology flows downhill in response to gravity as a lava flows.  Dissimilar magma types behave differently equally lava flows, depending on their temperature, viscosity, and gas content.

Lava Flows

Pahoehoe Flows - Basaltic lava flows with depression viscosity outset to cool when exposed to the low temperature of the temper.  This causes a surface skin to form, although it is yet very hot and behaves in a plastic fashion, capable of deformation.  Such lava flows that initially accept a smooth surface are called pahoehoe flows.  Initially the surface peel is smooth, just often inflates with molten lava and expands to course pahoehoe toes or rolls to course ropey pahoehoe.  (See figure half-dozen.17 in your text).   Pahoehoe flows tend to be thin and, because of their low viscosity travel long distances from the vent.

A'A' Flows - Higher viscosity basaltic and andesitic lavas also initially develop a smooth surface skin, but this is chop-chop broken upward by flow of the molten lava within and by gases that continue to escape from the lava.   This creates a rough, clinkery surface that is characteristic of an A'A' period (meet figure 6.18 in your text).

Pillow Lavas - When lava erupts on the body of water flooring or other body of water, the surface skin forms rapidly, and, similar with pahoehoe toes inflates with molten lava.  Somewhen these inflated balloons of magma drop off and stack up like a pile of pillows and are called pillow lavas.  Ancient pillow lavas are readily recognizable considering of their shape, their glassy margins and radial fractures that formed during cooling.

Siliceous Lava Flow s - High viscosity andesitic and rhyolitic lava flows, because they can't flow very easily, class thick chubby flows that don't move far from the vent.

Lava  Domes or 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 most solid lava break off the outer surface of the dome and ringlet down its flanks to course a breccia effectually the margins of domes. The surface of volcanic domes are generally very crude, with numerous spines that have been pushed upward by the magma from below.

Explosive Eruptions

Explosive eruptions are favored past high gas content and high viscosity (andesitic to rhyolitic magmas). Explosive bursting of bubbling volition fragment the magma into clots of liquid that will cool as they autumn through the air.  These solid particles get pyroclasts (meaning - hot fragments) and tephra or volcanic ash, which refer to sand- sized or smaller fragments.

Tephra and Pyroclastic Rocks
Average Particle Size (mm)	Unconsolidated Cloth (Tephra)	Pyroclastic Stone
>64	Bombs or Blocks	Agglomerate
two - 64	Lapilli	Lapilli Tuff
<2	Ash	Ash Tuff

• 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 by and large of gas bubbling ( vesicles ) result in a low density highly vesicular rock fragment chosen pumice .

Clouds of gas and tephra that ascension to a higher place 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 altitude, and then autumn 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 downwardly the flanks of the volcano at high speed.  This is the nearly dangerous blazon 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.

If the gas pressure inside the magma is directed outward instead of upward, a lateral blast tin can occur.  When this occurs on the flanks of a lava dome, a pyroclastic flows called a glowing avalanche or nu�due east ardentes (in French) can also consequence. Directed blasts often result from sudden exposure of the magma past a landslide or collapse of a lava dome.

Pyroclastic Deposits

Pyroclastic textile ejected explosively from volcanoes becomes deposited on the land surface. The process of deposition leaves clues that permit geologists to interpret the style of ejection from the volcano.

Fall Deposits

Material ejected into an eruption column eventually falls back to the earth's surface and blankets the surface like to the way snow blankets the earth.

The thickest deposits occur shut to vent and get thinner with distance from the vent.

By measuring the thickness at numerous locations 1 can construct an isopach map.   Such isopach maps aid to locate the source volcanic vent (if it is not otherwise known) and provides information nigh wind direction in the upper levels of the atmosphere during the eruption.

Fall deposits are usually fairly well-sorted, meaning that the clast size does not vary likewise much within the private deposit. The clast size can be ash every bit in a cinder cone

or tin can be clasts of pumice that range in size from blocks close to the vent to lapilli at greater distances from the vent to fine ash at slap-up distances from the vent.  They may likewise contain clasts of rock fragments (called lithic fragments) that are pieces of the volcanic structure ripped from the sides of the conduit during the explosive eruption.

Pyroclastic Flows Pyroclastic flows are also sometimes called pyroclastic density currents (PDCs).   They can range from surges which can have a range of clast densities from low to high with generally low concentration of  of solid clasts (high amonts of gases) to high clast concentration clouds of ash and gas (pyroclastic flows).

If the pyroclastic flows consist of solid clasts with high density along with ash fragments, they are called block and ash flows.   If the pyroclastic flows have low density clasts (pumice) along with ash, they are called ignimbrites.   There are no definitive boundary between pyroclastic flows and surges as they grade into one another continuously.  Similarly, ignimbrites grade into block and ash flows as the clast density increases.

Pyroclastic Flow Deposits Pyroclastic flows tend to follow valleys or depression lying areas of topography. The cloth deposited, thus tends to fill valleys, rather than uniformly blanket the topography similar fall deposits.

Cake and Ash Flow Deposits
As defined above, cake and ash flows consist of an unsorted mixture of blocks and ash with the blocks being mostly rock fragments.

Ignimbrites Ignimbrites contain blocks of pumice in an unsorted mixture of ash, lapilli, pumice blocks, and lithic fragments.  Sometimes one finds concentrated zones of pumice or lithic fragments in the deposits.
Surge Deposits
Surges tend to hug the ground equally they period over the surface and thus tend to produce thicker deposits in valleys with thinner deposits over ridges.  This helps to distinguish surge deposits from menstruation deposits and fall deposits.
Because they motion close to footing, friction with ground tends to produce cantankerous stratification in the deposits.   Individual layers tin be well-sorted, but overall the deposits tend to be poorly sorted.

Types of Volcanic Eruptions

Volcanic eruptions, especially explosive ones, are very dynamic phenomena. That is the behavior of the eruption is continually changing throughout the course of the eruption.  This makes it very difficult to allocate volcanic eruptions.  Nevertheless they can be classified according to the principal types of beliefs that they showroom.  An important point to remember, however, is that during a given eruption the type of eruption may modify between several unlike types.

• Hawaiian - These are eruptions of low viscosity basaltic magma.  Gas discharge produces a burn down fountain that shoots incandescent lava up to 1 km above the vent.  The lava, withal molten when it returns to the surface flows away downwards slope every bit a lava flow.  Hawaiian Eruptions are considered non-explosive eruptions.  Very piffling pyroclastic fabric is produced.

Strombolian - These eruptions are characterized by singled-out blasts of basaltic to andesitic magma from the vent.  These blasts produce incandescent bombs that fall nearly the vent, eventually edifice a small-scale cone of tephra (cinder cone).  Sometimes lava flows erupt from vents low on the flanks of the small cones.  Strombolian eruptions are considered mildly explosive, and produce depression acme eruption columns and pyroclastic autumn deposits. Vulcanian - These eruptions are characterized by sustained explosions of solidified or highly viscid andesite or rhyolite magma from a the vent.  Eruption columns tin reach several km above the vent, and frequently plummet to produce pyroclastic flows.  Widespread pyroclastic falls are mutual that contain mostly angular blocks.  Vulcanian eruptions are considered very explosive. Pelean - These eruptions issue from the collapse of an andesitic or rhyolitic  lava dome, with or without a directed blast, to produce glowing avalanches or nu�e ardentes, equally a type of pyroclastic menstruum known as a block-and-ash menstruation. They may also produce surges with resulting surge deposits.  Pelean eruptions are considered violently explosive. Plinian - These eruptions result from a sustained ejection of andesitic to rhyolitic magma into eruption columns that may extend upwardly to 45 km higher up the vent.  Eruption columns produce wide-spread fall deposits with thickness decreasing away from the vent, and may exhibit eruption column collapse to produce pyroclastic flows and surges.  Plinian ash clouds can circle the Earth in a matter of days. Plinian eruptions are considered violently explosive. Phreatomagmatic - These eruptions are produced when magma comes in contact with shallow groundwater causing the groundwater to flash to steam and be ejected along with pre-existing fragments of the rock and tephra from the magma.  Because the h2o expands and then quickly, these eruptions are violently explosive although the distribution of pyroclasts around the vent is much less than in a Plinian eruption.  Surge deposits are usually produced. Phreatic (too called steam boom eruptions) - result when magma encounters shallow groundwater, flashing the groundwater to steam, which is explosively ejected along with pre-exiting fragments of stone.  No new magma reaches the surface. Surge deposits may event from these eruptions.

Questions on this material that could be asked on an exam What are the three major types of magma and how are they distinguished from one some other in terms of their chemic compositions and physical properties?  (Note that you should be able to answer this question in relative terms - you would non be expected to cite exact numbers). How do each of the three types of magma originate in terms of melting mechanism and part of the earth where they grade? What are the major gases in magma? What are the small gases in magma? Why is the amount of gas in magma important in relation to volcanic eruptions? What chemic and concrete characteristics of magma are most of import in whether the magma erupts explosively or not-exzplosively? Define the post-obit terms (a) viscosity, (b) block, (c) bomb, (d) ash, (e) eruption cavalcade, (f) pyroclastic period, (g) lateral smash. How would i distinguish ash fall deposits from pyroclastic menses and surge deposits? How would one distinguish pyroclastic flow deposits from suge deposits? Compare and contrast the dissimilar types of volcanic eruptions. Return to EENS 3050 Homepage

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Source: https://www.tulane.edu/~sanelson/Natural_Disasters/volcan&magma.htm

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