Whereas basalt forms a'a and pahoehoe surface forms, andesite generally produces blocky lava. Here, the surface contains smooth-sided, angular fragments blocks that are not as splintery or vesicular as a'a lava fragments.
The blocky nature of these flows is attributed to the higher viscosity of andesite. Andesite commonly erupts from stratovolcanoes , where they form small-volume flows that typically advance only short distances down the flanks of a volcano.
The two examples shown here are short andesite flows advancing down the slope of the Lascar volcano in Chile, and the Colima volcano in Mexico. Andsite flows on Lascar Andesite flow on Colima. Although detached blocks occur on the tops of andesite flows, the flow interior is composed of massive lava which grades downward into an autobrecciated self-fragmented basal layer.
The flow moves by the injection of lobes of lava into the collar of blocky rubble that comprises the flow front. The flow-front rubble is then continually overridden by the massive lava core to form the fragmented basal layer of the flow. If felsic magma rises into a magma chamber, it may be too viscous to move and so it gets stuck.
Dissolved gases become trapped by thick magma and the magma chamber begins to build pressure. The type of magma in the chamber determines the type of volcanic eruption. A large explosive eruption creates even more devastation than the force of the atom bomb dropped on Nagasaki at the end of World War II in which more than 40, people died.
A large explosive volcanic eruption is 10, times as powerful. Felsic magmas erupt explosively because of hot, gas-rich magma churning within its chamber. The pressure becomes so great that the magma eventually breaks the seal and explodes, just like when a cork is released from a bottle of champagne. Magma, rock, and ash burst upward in an enormous explosion creating volcanic ash called tephra.
That is why it is so dangerous to inhale the air following an eruption. Pyroclastic flows knock down everything in their path. The temperature inside a pyroclastic flow may be as high as 1,oC 1, degrees F. Prior to the Mount St. Helens eruption in , the Lassen Peak eruption on May 22, , was the most recent Cascades eruption. A column of ash and gas shot 30, feet into the air.
This triggered a high-speed pyroclastic flow, which melted snow and created a volcanic mudflow known as a lahar.
It is probably older than the Nelson Mountain Tuff The Indian Peak field Best et al. The eruptive cycles terminated with a trachydacitic ash flow. Intercalated between these felsic tuffs are small volumes of andesitic lava flows. Outcrop locations of andesite lavas interlayered with dacite and rhyolite ash-flow tuffs in the central Nevada volcanic field black area represents approximate outcrop locations of Cr-spinel-bearing andesite of Pritchards Station; other shading patterns have same significance as in Fig.
As for the San Juan field, the andesite units are named on the basis of their stratigraphic relationships with intercalated ash-flow tuffs Best et al. From oldest to youngest, the units are Fig. Generally, these andesites contain phenocrysts of clino- and orthopyroxene, plagioclase and magnetite. Additionally, hornblende is common in andesites of the Escalante Desert and Isom Formations and rare in the andesite of the Ryan Springs Formation. Cr-spinel occurs as inclusions in olivine of both units.
We restricted our study to two specific areas. These lavas are chiefly two-pyroxene andesites; however, hornblende and biotite are additionally present in some samples from the area along the northern edge of the complex. The andesite of the Wah Wah Springs Formation is defined as intermediate lavas stratigraphically bracketed by the felsic tuffs of the Cottonwood Wash Contacts between this andesite and the felsic tuffs are conformable. The exposure is a m thick sequence of dense to vesicular olivine andesite lava flows.
Locally, this unit is conformably bound by the felsic tuff members; however, some outcrops are underlain by felsic tuffs of the Wah Wah Springs Formation or overlain by Quaternary alluvium. Our samples are from outcrops stratigraphically overlain or underlain by felsic tuff members of the Ryan Springs Formation.
Although outcrops are small and discontinuous, we believe they represent a closely related sequence of lavas: outcrops are distributed over a small area, no other lithologies are interlayered between these lavas and felsic members of the Ryan Spring Formation, and lithologic characteristics of the andesitic lavas at each outcrop are similar.
The andesite of the Lund Formation overlies the dacitic tuff of the Lund Formation We investigated one m thick sequence of vesicular lava flows that outcrops over 1 km 2 along the sides and top of White Rock Peak. This location is in the center of the felsic caldera complex, and lithologies include pyroxene and hornblende andesites.
Volcanic rocks that make up the central Nevada field overlie chiefly Paleozoic sedimentary rocks. Volumetrically minor andesitic lavas are interlayered between these tuffs Fig.
Miocene to Pliocene Basin and Range faulting deformed the caldera complex, and Quaternary basalts covered portions of the complex Ekren et al. Andesitic rocks in this field have been mapped in three separate areas Fig. From oldest to youngest, these will be referred to informally here as the andesite of Pritchards Station Dixon et al.
The andesites of Pritchards Station and Reveille contain phenocrysts of clino- and orthopyroxene, plagioclase, minor magnetite and minor ilmenite. The latter andesite contains hornblende phenocrysts. The volcanics of Citadel Mountain contain phenocrysts of clino- and orthopyroxene, plagioclase, hornblende, Fe—Ti oxides and minor amounts of biotite. These lavas outcrop around the base of Park Range, a km 2 , m high horst bound on its east and west sides by grabens.
This horst is capped by — m of dacitic Windous Butte Tuff Locally, the Stone Cabin Formation Because this andesite outcrops around the base of this horst on the north, east and west sides, we infer that andesite is continuous beneath the Windous Butte Formation from one end of the horst to the other, and that the andesite erupted from a single vent or closely related vents.
The andesite of Reveille underlies the Monotony Tuff The volcanics of Citadel Mountain are a sequence of intermediate lavas and tuffs underlain by the tuffs of Buckskin Point undated and Lunar Cuesta The unit was previously described as an unnamed andesite by Ekren et al. Compositions of phenocrysts were determined by wavelength dispersive X-ray spectrometry using a Cameca SX electron microprobe at the University of South Carolina.
Generally, 15 kV accelerating voltage, a 15 nA beam current and a minimum spot size were utilized, although a relatively large spot size was used in the case of feldspar to minimize Na migration during analyses. Data reduction incorporated procedures that correct for the influence of matrix atomic number, X-ray absorption and secondary fluorescence.
Comprehensive data sets and sampling locations have been given by Askren, or can be obtained by writing to the first author; representative modes and compositions can be found in Table 1. Samples with the lowest modal abundance typically have relatively calcic plagioclase; the most calcic of these are An 89 in the andesite of the Isom Formation and An 88 in the andesite of Pritchards Station Fig.
Mean values within each andesite unit range from An 44 to An 78 Fig. Such compositional variations together with complex zoning are typical of many orogenic andesites Gill, Representative compositions and modal abundances of phenocrysts from andesitic volcanic rocks. Compositions and modal abundances of plagioclase phenocrysts. Crosses indicate mean composition for each unit. Solid boxes indicate one standard deviation from the mean. Stippled boxes mark total observed range of compositions.
Values to the right of each box are ranges of modal abundances of plagioclase phenocrysts observed for each unit. Clinopyroxene is present as phenocrysts in all lavas, whereas orthopyroxene is generally restricted to olivine-free lavas.
Both orthopyroxene, ranging in composition from En 60 to En 79 [classified as enstatite, following Morimoto et al. Some isolated grains in samples IA3, WA4 and EA1 contain significant contents of components outside the pyroxene quadrilateral up to 6. In most cases, amphiboles are associated with two pyroxenes and appear texturally to be in equilibrium with the groundmass phases.
Most amphiboles belong to the calcic amphibole group of Leake, , although those in the andesites of the Isom Formation and Bristol Head are transitional from calcic amphibole to sodic—calcic amphibole.
Biotite is much rarer than amphibole and is always associated with amphibole. Biotites are generally Fe rich and have ubiquitous coronas of iron oxide granules. Luhr, Olivine compositions range from Fo 57 to Fo Texturally, the olivines appear to be in equilibrium except in sample EA1, which also contains hornblende and biotite.
In this sample olivine is rimmed by crystals of orthopyroxene, suggesting a reaction relationship between olivine and melt. In the andesite of Pritchards Station, this mineral occurs as inclusions in clinopyroxene and as discrete grains.
Cr-spinel inclusions are sub- to euhedral, and grain boundaries between the inclusions and host phenocrysts show no indication of disequilibrium. The olivine and clinopyroxene host phenocrysts in the andesites of the Wah Wah Springs Formation and Pritchards Station appear to be in equilibrium with matrix phases, whereas the olivine grains hosting Cr-spinel in the andesite of the Escalante Desert Formation are rimmed by orthopyroxene.
The occurrence of spinel without olivine in the andesite of Pritchards Station may indicate early-formed olivine reacted with melt. Magnetite, with or without ilmenite, is present as phenocrysts and in the groundmass in all units; pervasive secondary oxidation affected these phases in most cases. Approximately one hundred samples were analyzed for major element abundances by X-ray fluorescence spectrometry Askren, Many of these were also analyzed for selected trace elements by X-ray fluorescence spectrometry, and samples of each unit with minimum and maximum SiO 2 contents were analyzed for trace elements by neutron activation Table 2.
Three of the four units from the San Juan field include trachyandesites. There are some intriguing inter-field and intra-field distinctions between andesite units. Here, P 2 O 5 decreases with increasing SiO 2 , whereas it is only poorly correlated with SiO 2 in the other two fields.
Titania shows similar behavior to P 2 O 5. Titania is inversely correlated with SiO 2 in the lavas of the two eastern fields, perhaps because of fractionation of Fe—Ti oxides. In central Nevada, however, TiO 2 content is nearly constant, with the exception of the andesite of Pritchards Station—which has the highest TiO 2 content up to 1. In this andesite, phenocrysts of Fe—Ti oxides are small and rare, and Cr-spinel inclusions are present in clinopyroxene. Therefore these lavas may have relatively high TiO 2 contents because Cr-spinel, rather than Fe—Ti oxides, was an early-fractionating phase.
Lower TiO 2 contents in more SiO 2 -rich lavas from this unit may have resulted from subsequent fractionation of Fe—Ti oxides. Generally, the andesitic lavas from the three volcanic fields have similar abundances of incompatible trace elements Ba, Sr, Zr, Rb, La, Th and Ta; Fig.
However, the relatively alkaline lavas of the San Juan field Fig. Differences also exist between units from the same field. Moreover, contrasting trends of trace element abundances with increasing SiO 2 occur: Ba increases with increasing SiO 2 content in the andesite of Pritchards Station and decreases with SiO 2 content in the andesite of Reveille.
As SiO 2 increases within a single unit, Rb generally increases, whereas Zr most commonly remains constant, Sr decreases or remains constant and Ba may increase or decrease Fig.
Miyashiro, ; Gill, All of the lavas show important trace element similarities with subduction-related lavas: on a chondrite-normalized abundance diagram Fig. The Nb and Ta depletions may result from retention of these elements by a residual mineral in refractory mantle material McDonough, or by reaction between primary melt and mantle peridotite Kelemen et al.
Ti depletions may be enhanced by subsequent crystal fractionation of Fe—Ti oxides. Likewise, P and Eu are depleted relative to elements of similar compatibility, and these depletions probably result from crystal fractionation of apatite and plagioclase.
The secondary oxidation of Fe—Ti oxides and scarcity of ilmenite precluded use of oxide minerals to estimate magmatic temperatures.
These temperatures are similar to estimated magmatic temperatures from orogenic andesites Gill, and are higher than the calculated magmatic temperatures of the associated felsic ash-flow tuffs, except for the tuffs of the Isom Formation Fig. Silica content of the andesites and pyroxene equilibration temperatures are inversely correlated for many units Fig.
All samples from each volcanic field fall within labeled patterned areas except three samples from the Indian Peak field square symbols, from the Escalante Desert Formation and one sample from the San Juan field cross symbol, from the volcanics of Table Mountain. Rock types are from classification of LeBas et al. Although there is overall consistency between calculated pyroxene equilibration temperatures, bulk SiO 2 content and phenocryst populations, in some cases our calculations suggest that the two pyroxenes were not completely in equilibrium.
Grove et al. Similar large temperature ranges are estimated for andesites of the Isom and Escalante Desert Formations. These large temperature ranges may suggest disequilibrium, or such ranges may record a temperature interval of crystallization. The cracks, called fissure s or vents, are tell-tale signs of a volcano. Many volcanoes sit over magma chambers. An eruption reduce s the pressure inside the magma chamber.
Large eruptions can nearly empty the magma chamber. The layers of magma may be document ed by the type of eruption material the volcano emits. Gases, ash, and light-colored rock are emitted first, from the least-dense, top layer of the magma chamber. Dark, dense volcanic rock from the lower part of the magma chamber may be released later.
In violent eruptions, the volume of magma shrinks so much that the entire magma chamber collapses and forms a caldera. All magma contains gases and a mixture of simple element s. Being that oxygen and silicon are the most abundant elements in magma, geologists define magma types in terms of their silica content, expressed as SiO 2.
These differences in chemical composition are directly related to differences in gas content, temperature, and viscosity. This type of magma has a low gas content and low viscosity, or resistance to flow.
Mafic magma also has high mean temperatures, between o and o Celsius o and o Fahrenheit , which contributes to its lower viscosity. Low viscosity means that mafic magma is the most fluid of magma types. This lava cools into basalt , a rock that is heavy and dark in color due to its higher iron and magnesium levels. The Hawaiian Islands are a direct result of mafic magma eruptions. This results in a higher gas content and viscosity. Its mean temperature ranges from o to o Celsius o to o Fahrenheit.
This more gaseous and sticky lava tends to explode violently and cools as andesite rock. Intermediate magma most commonly transforms into andesite due to the transfer of heat at convergent plate boundaries. Andesitic rocks are often found at continent al volcanic arcs, such as the Andes Mountains in South America, after which they are named.
As a result, felsic magma also has the highest gas content and viscosity, and lowest mean temperatures, between o and o Celsius o and o Fahrenheit. These trapped bubbles can cause explosive and destructive eruptions.
These eruptions eject lava violently into the air, which cools into dacite and rhyolite rock. Much like intermediate magma, felsic magma may be most commonly found at convergent plate boundaries where transfer of heat and flux melting create large stratovolcano es. Magma exists as pockets and plumes beneath the surface of the Earth. Photograph by Carsten Peter, National Geographic. Mysterious Magma.
Magma is usually studied as lava or igneous rock. In , the Icelandic Deep Drilling Project created a well that uses magma to generate geothermal energy. Normally, geothermal energy is created by pumping water into hot volcanic bedrock, creating steam that is then harnessed to generate electricity. This huge increase in temperature allowed just one magma well to generate roughly 36 megawatts of electricity, powering 36, homes.
In comparison, one single wind turbine generates between 1 to 3 megawatts. Also called a collision zone.
0コメント