mineralogical and chemical variations of ochreous precipitates from acid sulphate waters asw at the rosia montană gold mine romania the mineralogical and chemical variations of ochreous precipitates forming from acid sulphate waters discharged from the lowest mine adit sf cruci din orlea of the rosia montan gold mine romania were investigated by a multianalytical approach xrpd ir tem icp applied to surface precipitates and associated waters the mineralogy of the precipitates changed significantly as a consequence of the variations in the chemical parameters of the circulating solutions mainly ph eh and sulphate concentrations which were mainly controlled by mixing with unpolluted waters of rosia river ochreous precipitates are characterized by high concentrations of potentially toxic elements ptes in particular cr co ni cu zn as cd and pb and consist of a mixture in variable proportion of jarosite and schwertmannite which represent the stable secondary minerals along the investigated transect of rosia river particular regard is given to the ability of authigenic phases to selectively scavenge selected ptes from contaminated solutions during their genesis and minerogenetic evolution furthermore laboratory kinetic batch experiments on natural heterogeneous samples of ochreous precipitates were carried out to investigate the release processes involving ptes and to verify the type and the amount of elements that can be temporarily permanently trapped by the solid phase from the contaminated solutions the comparative analysis of the precipitates and waters of the rosia montan mining area indicated that the role of secondary minerals as mitigating agents can be limited because even minor ph eh oscillations would cause mineralogical transformations that could lead to trace elements mobilization in the environment

petrology and geochemistry of lava and ash erupted from volcan colima mexico during 1998 2005 abstract lava n 8 and bulk ash samples n 6 erupted between july 1999 and june 2005 were investigated to extend time series compositional and textural studies of the products erupted from volcan colima since 1869 in particular we seek to evaluate the possibility that the current activity will culminate in major explosive plinian style event similar to that in 1913 lava samples continue to show relatively heterogeneous whole rock compositions with some significant mafic spikes 1999 2001 as have prevailed since 1976 groundmass sio2 contents continue trends to lower levels that have prevailed since 1961 in the direction of the still lower groundmass sio2 contents found in 1913 scoriae importantly ash samples from investigated vulcanian style explosive eruptions in 2005 are devoid of particles with micro vesiculated groundmass textures such textures characterized the 1913 scoriae signifying expansion of in situ magmatic gas as the propellant of the 1913 eruption all magmas erupted since 1913 appear to have arrived in the upper volcanic conduit system in a degassed state the small to moderate vulcanian style explosive eruptions which have been common since 1999 16 000 events have blasted ash clouds as high as 11 km a s l and sent pyroclastic flows out to distances of 5 km these eruptions do not appear to be powered by expansion of in situ magmatic gas new small lava domes have been observed in the crater prior to many explosive eruptions these plugs of degassed lava may temporarily seal the conduit and allow the build up of magmatic gases streaming upward from below ahead of rising and degassing magma in this interpretation when gas pressure exceeds the strength of the plug seal in the upper conduit an explosive vulcanian style eruption occurs alternatively these explosive eruptions may represent interactions of hot rock and groundwater phreato magmatic

the colima volcanic complex mexico part ii late quaternary cinder cones since the late pleistocene eleven cinder and lava cones have erupted on the floor of the southern colima graben ne and nw of the large active andesitic volcano colima scoria and lava samples from nine of the cones form a completely transitional basic alkalic series including basanites 9 leucite basanites 3 and minettes 15 the commonest variety of mica lamprophyre these samples represent primitive high temperature magmas with 47 6 50 3 sio2 7 4 15 3 mgo 2 5 4 4 k2o and 2 2 9 9 normative nepheline all members of this basic alkalic suite contain mg olivine fo75 94 chromite augite and late plagioclase and titanomagnetite the petrographic transition from basanite to minette is marked by the appearance of sanidine and the volatile bearing phases phlogopite apatite and analcime during late stages of crystallization as these phases increase in abundance presumably reflecting a rise in magmatic volatile content there are corresponding increases in the whole rock concentrations of 16 incompatible elements although these incompatible elements are relatively abundant even in the basanites many are highly concentrated in the minettes ba 4 200 ppm sr 3 100 ppm zr 550 ppm ce 190 ppm hf 18 ppm among the incompatible elements the degrees of enrichment in the minettes relative to the basanites decrease in the order h th ce la nd zr hf u ba sm eu pb p nb sr ti these enrichments may reflect the increasing importance of minor incompatible element rich mantle phases during partial melting the concentrations of alkali metals k na rb and cs do not correlate with these other elemental enrichments the leucite basanties have similar incompatible element contents to many minettes differing from them only in the presence of leucite rather than analcime and ti f rich groundmass phlogopite rather than hydrous phlogopite phenocrysts thus the leucite basanites represent relatively dry equivalents of minettes two of the eleven cinder cones are calc alkaline in nature and do not belong to the basanite minette group the easternmost cone is constructed of high al basalt and the northernmost of basaltic andesite the high al basalt 49 5 sio2 9 3 mgo 221 ppm ni closely approximates a parental magma to the post caldera andesitic suite of v colima 56 5 61 6 sio2 the basaltic andesite is relatively enriched in incompatible elements compared to the high al basalt v colima trend the ne normative basanite minette samples are highly enriched in incompatible elements while the contemporaneous hy qz normative calc alkaline suite encompassing the high al basalt and v colima s andesites is characterized by relatively low abundances of these elements no likely mineral assemblage can relate the alkaline and calc alkaline suites through crystal fractionation they probably represent fundamentally different melting events during the quaternary the main focus of andesitic volcanism in the southern colima graben has migrated southward with time volcan colima marks its present position 5 km south of the pleistocene volcano nevado de colima and another 15 km from the still older volcan cantaro the eruptions of basic alkalic magma probably occurred during the late stages of nevado s life and through the life of v colima they generally migrated from west to east with time towards v cantaro the most recent cone v apaxtepec is the only one east of the andesitic colima cantaro axis the oldest and the two youngest cones produced basanites while minettes dominated at cones of intermediate ages the cinder cone eruptions may have coincided with a phase of lamprophyre dike injection into plutons solidifying beneath the extinct volcanoes north of v colima the southern end of the colima graben can be viewed then as the volcanic analog of many classical post plutonic hypabyssal lamprophyre localities

evolution of lascar volcano northern chile lascar volcano in the central andes is an ese wnw elongate composite stratocone stage i 3 at 26 45 ka producing the soncor ignimbrite no caldera formed laharic and alluvial fan deposits and a debris avalanche formed from 26 45 to 19 2 ka indicating a humid climatic period the dacitic and andesitic western stratocone stage iii developed over the soncor vent stage iii culminated in the 9 2 ka tumbres eruption which formed andesitic scoria flow fans and a summit collapse crater stage iv activity switched to the east forming an andesite lava three nested summit craters which repeatedly filled with lava and collapsed absence of significant erosion since 9 2 ka is due to the much drier holocene climate activity since 1984 displays cycles of lava dome formation in the summit crater lava dome subsidence with crater collapse vulcanian to plinian explosive eruptions culminating in the major explosive eruption of 19 20 april 1993 the total volume of magma erupted from lascar over a 43 ka period is estimated at 30 to 40 km 3

recent lahars at volcan de colima mexico drainage variation and spectral classification abstract volcan de colima is the most active volcano in mexico and represents a high risk for more than 500 000 people in 1998 the volcano renewed its activity with the extrusion of a lava dome and subsequent lava and block and ash flows during the recent period of activity pyroclastic products did not directly affect villages around the volcano however several lahars did we used lidar topographic coverage aster and landsat images for the recognition of morphological changes in the drainage system and lahar detection for lahar delineation we applied principal components analysis and canonical classification tasseled cap in order to perform a supervised image classification using the maximum likelihood rule algorithm laharz objective delineation of distal debris flow hazard zones has been used and tested using two topographic datasets with different resolutions which provided evidence of the importance of high resolution topographic coverage in hazard assessment finally a hazard map for lahars is presented showing that several villages and ranchos can be affected in contrast due to morphological changes produced by products of the intense explosive activity other populated areas such as la yerbabuena are outside of the high risk zone they could be affected by lahars only in case of cataclysmic eruptions such as the 1913 plinian event

geology of the northern colima and zacoalco grabens southwest mexico late cenozoic rifting in the mexican volcanic belt the south trending northern colima graben intersects two other grabens the northwest trending zacoalco graben and the east trending chapala graben to form a rift rift rift triple junction 50 km south southwest of guadalajara in southwest mexico the northern colima graben represents the northernmost extension of the 190 km long 20 to 65 km wide colima rift which overlies the projection of the subducting boundary of the rivera and cocos plates and represents a rift of probable pliocene pleistocene age of the east trending mexican volcanic belt mvb associated with the northernmost 90 km of the rift there are 4 6 m y old to holocene alkaline volcanic rocks rarely found in continental volcanic arc environments and which are more typically associated with either continental intraplate rifting or oceanic islands these alkaline lavas have erupted side by side with 10 m y old to holocene calc alkaline lavas that represent typical arc volcanism it is proposed that the grabens are associated with the initiation of a rifting of a portion of southwest mexico away from the mexican mainland similar to the rifting of baja california away from the mexican mainland the calc alkaline k ar dates are consistent with post mid miocene development of east trending mvb volcanism

the pleistocene cinder cones surrounding volcan colima mexico re visited eruption ages and volumes oxidation states and sulfur content located at the volcanic front in the western mexican arc in the colima rift is the active volcan colima which lies on the southern end of the massive 450 km3 colima nevado volcanic complex along the margins of this andesitic volcanic complex is a group of 11 scoria cones and associated lavas which have been dated by the 40ar 39ar method nine scoria cones erupted 1 3 km3 of alkaline magma basanite leucite basanite minette between 450 and 60 ka with 99 between 240 and 60 ka two additional cones both the oldest and calc alkaline erupted 0 003 km3 of basalt 0 5 ma and 0 003 km3 of basaltic andesite 1 2 ma respectively cone and lava volumes were estimated with the aid of digital elevation models dems the eruption rate for these scoria cones and their associated lavas over the last 1 2 myr is 1 2 km3 myr which is more than 400 times smaller than that from the andesitic colima nevado edifice in addition to these alkaline colima cones two other potassic basalts erupted at the volcanic front but 200 km to the ese near the historically active volcan jorullo and were dated at 1 06 and 0 10 ma these potassic suites reflect the tendency in the west central mexican arc for magmas close to the volcanic front to be enriched in k2o relative to those farther from the trench

caldera formation at volcan colima mexico by a large large holocene volcanic debris avalanche abstract about 4 300 years ago 10 km 3 of the upper cone of ancestral volcan colima collapsed to the southwest leaving a horseshoe shaped caldera 4 km in diameter the collapse produced a massive volcanic debris avalanche deposit covering over 1550 km 2 on the southern flanks of the volcano and extending at least 70 km from the former summit the avalanche followed a steep topographic gradient unobstructed by barriers resulting in an unusually high area volume ratio for the colima deposit the apparent coefficient of friction fall height distance traveled for the colima avalanche is 0 06 a low value similar to those of other large volume deposits the debris avalanche deposit contains 40 75 angular volcanic clasts from the ancestral cone a small proportion of vesicular blocks that may be juvenile and in distal exposures rare carbonate clasts plucked from the underlying surface by the moving avalanche clasts range in size to over 20 m in diameter and are brecciated to different degrees pulverized and surrounded by a rock flour matrix the upper surface of the deposit shows prominent hummocky topography with closed depressions and surface boulders a thick coarse grained compositionally zoned scoria fall layer on the upper northeastern slope of the volcano may have erupted at the time of collapse a fine grained surge layer is present beneath the avalanche deposit at one locality apparently representing an initial blast event most of the missing volume of the ancestral volcano has since been restored at an average rate of 0 002 km 3 yr through repeated eruptions from the post caldera cone as a result the southern slope of volcan colima may again be susceptible to collapse over 200 000 people are now living on primary or secondary deposits of the debris avalanche and a repetition of this event would constitute a volcanic disaster of great magnitude ancestral volcan colima grew on the southern trenchward flank of the earlier and larger volcano nevado de colima trenchward collapse was favored by the buttressing effect of nevado the rapid elevation drop to the south and the intrusion of magma into the southern flank of the ancestral volcano other such trenchward younging paired volcanoes are known from mexico guatemala el salvador chile and japan the trenchward slopes of the younger cones are common sites for cone collapse to form avalanche deposits as occurred at colima and popocatepetl in mexico and at san pedro volcano in chile

emplacement of pyroclastic flows during the 1998 1999 eruption of volcan de colima mexico after three years of quiescence volcan de colima reawakened with increasing seismic and rock fall activity that reached its peak on november 20 1998 when a new lava dome forced its way to the volcano s summit the new lava rapidly reached the s sw edge of the summit area beginning the generation of merapi type pyroclastic flows that traveled down la lumbre and the el cordoban western and eastern ravines reaching distances of 3 4 5 and 3 km respectively on december 1 1998 the lava flow split into three fronts that in early 1999 had reached 2 8 3 1 and 2 5 km in length advancing down the el cordoban ravines the lava flow fronts disaggregated into blocks forming pyroclastic flows one of the best examples occurred on december 10 1998 as the lava flow ceased moving in early 1999 activity became more explosive strong blasts were recorded on february 10 may 10 and july 17 1999 the last event developed a 10 km high eruptive column from which a pyroclastic flow developed from the base traveling 3 3 km sw from the summit into the san antonio montegrande ravines regardless of the mechanism of pyroclastic flow generation each flow immediately segregated into a basal avalanche that moved as a granular flow and an upper ash cloud in which particles were sustained in turbulent suspension when the basal avalanche lost velocity and eventually stopped the upper ash cloud continued to move independently as a dilute pyroclastic flow that produced a massive pyroclastic flow deposit and an upper dune bedded surge deposit the dilute pyroclastic flow scorched and toppled maguey plants and trees and sandblasted vegetation in the direction of the flow at the end of the dilute pyroclastic flow path the suspended particles lifted off in a cloud from which a terminal ash fall was deposited the basal avalanche emplaced block and ash flow deposits up to 8 m thick that filled the main ravines and consisted of several flow units each flow unit was massive monolithologic matrix supported and had a clast supported steep front ca 1 5 to 2 m thick composed of boulders up to 1 7 m in diameter the juvenile lithic clasts had an average density of 1800 kg m3 the dilute pyroclastic flow emplaced overbank deposits found on valley margins or beyond the tip of block and ash flow deposits they consist from bottom to top of a massive medium to coarse sand size flow layer 2 4 cm thick a dune bedded surge layer 2 10 cm thick and a massive silt size layer 0 5 cm thick the total estimated volume of the pyroclastic flow deposits produced during the 1998 1999 eruption is 24 105 m3

correlation of deposits and vent locations of the proximal campanian ignimbrite deposits campi flegrei italy based on natural remanent magnetization and anisotropy of magnetic susceptibility characteristics abstract correlation of the distal deposits of the campanian ignimbrite with their proximal equivalents in the campi flegrei caldera is complicated by a lack of medial exposures complex and limited proximal stratigraphic sections and large lateral facies changes paleomagnetic data from 10 sites in and near the campi flegrei yield natural remanent magnetizations nrm that are statistically indistinguishable from the distal campanian ignimbrite in addition their virtual geomagnetic pole vgp yields a possible correlation with lac du bouchet france secular variation data that indicate an age of approximately 32 850 14 c years the secular variation curve at this age was only briefly at this vgp and did not return to it for 10 000 years so the paleomagnetic correlation of proximal and distal deposits is unique and robust the date is consistent with 14 c dates from the campanian ignimbrite but younger than 39 ar 40 ar dates for the same rocks this suggests that a better correction factor for cosmic flux for this time period is needed to calibrate older 14 c dates anisotropy of magnetic susceptibility ams data show that the proximal deposits have an oblate disk shaped poorly lineated magnetic fabric the distal deposits are much better lineated the difference may be due to chaotic depositional currents in the proximal areas in which particles were not well aligned with greater distance of travel and loss of energy particles within the flow became aligned and developed stronger ams lineations early eruptions of the piperno tuff were from a central vent north of pozzuoli whereas later tuffs that underlie the breccia museo may have been emplaced by flows associated with ring vents located on the northern and southern caldera margins

giant debris avalanches from the colima volcanic complex mexico implications for long runout landslides 100 km and hazard assessment at least two giant volcanic debris avalanche deposits are associated with the colima volcanic complex located in the western part of the trans mexican volcanic belt one avalanche originated from volcan de colima 3820 m probably 4300 yr ago a much larger avalanche originated from nevado de colima 4240 m 18500 yr ago it traveled more than 120 km from its source and covered an area of at least 2200 km 2 almost twice the area of any previously described avalanche deposit this older debris avalanche is the second largest by volume known has the longest travel distance yet reported and has one of the lowest height length ratios 0 04 the avalanche material was probably hot during emplacement and after initial slope failure may have behaved similarly to a pyroclastic flow such large long runout landslides from volcanic or nonvolcanic constructs present a serious geologic hazard that must be considered in risk assessment

10 000 years of explosive eruptions of merapi volcano central java archaeological and modern implications abstract stratigraphy and radiocarbon dating of pyroclastic deposits at merapi volcano central java reveals 10 000 years of explosive eruptions highlights include 1 construction of an old merapi stratovolcano to the height of the present cone or slightly higher our oldest age for an explosive eruption is 9630 60 14 c y b p construction of old merapi certainly began earlier 2 collapse s of old merapi that left a somma rim high on its eastern slope and sent one or more debris avalanche s down its southern and western flanks impoundment of kali progo to form an early lake borobudur at 3400 14 c y b p hints at a possible early collapse of merapi the latest somma forming collapse occurred 1900 14 c y b p the current cone new merapi began to grow soon thereafter 3 several large and many small buddhist and hindu temples were constructed in central java between 732 and 900 a d roughly 1400 1000 14 c y b p explosive merapi eruptions occurred before during and after temple construction some temples were destroyed and or buried soon after their construction and we suspect that this destruction contributed to an abrupt shift of power and organized society to east java in 928 a d other temples sites though were occupied by caretakers for several centuries longer 4 a partial collapse of new merapi occurred 14 c y b p eruptions 700 800 14 c y b p 12 14th century a d deposited ash on the floors of still occupied candi sambisari and candi kedulan we speculate but cannot prove that these eruptions were triggered by the same partial collapse of new merapi and that the eruptions in turn ended caretaker occupation at candi sambisari and candi kedulan a new or raised lake borobudur also existed during part or all of the 12 14th centuries probably impounded by deposits from merapi 5 relatively benign lava dome extrusion and dome collapse pyroclastic flows have dominated activity of the 20th century but explosive eruptions much larger than any of this century have occurred many times during merapi s history most recently during the 19th century are the relatively small eruptions of the 20th century a new style of open vent less hazardous activity that will persist for the foreseeable future or alternatively are they merely low level background activity that could be interrupted upon relatively short notice by much larger explosive eruptions the geologic record suggests the latter which would place several hundred thousand people at risk we know of no reliable method to forecast when an explosive eruption will interrupt the present interval of low level activity this conclusion has important implications for hazard evaluation

modeling of pyroclastic flows of colima volcano mexico implications for hazard assessment the 18 24 january 1913 eruption of colima volcano consisted of three eruptive phases that produced a complex sequence of tephra fall pyroclastic surges and pyroclastic flows with a total volume of 1 1 km 3 0 31 km 3 dre among these events the pyroclastic flows are most interesting because their generation mechanisms changed with time they started with gravitanional dome collapse block and ash flow deposits merapi type changed to dome collapse triggered by a vulcanian explosion block and ash flow deposits soufriere type then ended with the partial collapse of a plinian column ash flow deposits rich in pumice or scoria the best exposures of these deposits occur in the southern gullies of the volcano where heim coefficients h l were obtained for the various types of flows average h l values of these deposits varied from 0 40 for the merapi type similar to the block and ash flow deposits produced during the 1991 and 1994 eruptions 0 26 for the soufriere type events and 0 17 0 26 for the column collapse ash flows additionally the information of 1991 1994 and 1998 1999 pyroclastic flow events was used to delimit hazard zones in order to reconstruct the paths velocities and extents of the 20th century pyroclastic flows a series of computer simulations were conducted using the program flow3d with appropriate heim coefficients and apparent viscosities the model results provide a basis for estimating the areas and levels of hazard that could be associated with the next probable worst case scenario eruption of the volcano three areas were traced according to the degree of hazard and pyroclastic flow type recurrence through time zone 1 has the largest probability to be reached by short runout b5 km merapi and soufriere pyroclastic flows that have occurred every 3 years during the last decade zone 2 might be affected by soufriere type pyroclastic flows 9 km long similar to those produced during phase ii of the 1913 eruption zone 3 will only be affected by pyroclastic flows 15 km long formed by the collapse of a plinian eruptive column like that of the 1913 climactic eruption today an eruption of the same magnitude as that of 1913 would affect about 15 000 inhabitants of small villages ranches and towns located within 15 km south of the volcano such towns

the colima volcanic complex mexico iii a 34 meter section of ash and scoria fall units has been studied on the upper ne flank of the active volcan colima charcoal and soil horizons are restricted to the topmost 12 m nine 14c dates show a smooth progression from 235 years near the surface to 8 300 years at 10 meters depth indicating a post pleistocene accumulation rate of 1 3 m 1 000 years at 6 km from the vent this figure allows an estimate of the magmatic eruption rate for fallmaterial of 0 31 km3 1 000 years less than 15 of colima s lava eruption rate an unusually thick and coarse grained scoria fall horizon at about 4 m depth in the section appears to have been produced by the caldera forming eruption of colima some 4 300 years ago

modelling morphological processes in the vicinity of shore parallel breakwaters abstract a coastal area morphological modelling system is used to investigate the planform development tombolo or salient behind a single surface piercing detached breakwater a quasi three dimensional q3d description of the flow and the sediment transport together with a revised bed level update scheme were incorporated recently in the morphological modelling suite applied in the present investigation the q3d sediment transport formulation used allows simulating beach profile evolution and planform development simultaneously several breakwater layouts were investigated in a series of tests in which the incident wave condition was taken as constant in time the investigation was done for surface piercing shore parallel breakwaters in a non tidal environment the morphological simulation results for the planform shape agree qualitatively with field observations and predictions from empirical rules using the simulation results simple design curves for surface piercing breakwaters are derived the importance of properly accounting for the influence of wave diffraction when modelling sediment transport and morphology in the vicinity of shore parallel structures is also investigated the present paper summarises much of the work performed at dhi water environment over the past 10 years within the field of coastal area morphological modelling

eruptive and magmatic cycles at fuego de colima volcano mexico the fuego de colima volcano displays a pattern of eruptive cyclicity characterized by lava flows and or slow effusions in an open crater alternating with short explosive events historical accounts field investigations and petrological data allow us to refine our understanding of the relationships between recurrent acidic acidic andesite and mafic andesitic products and this eruptive behaviour the second to last 1818 1913 and present 1913 present day cycles are instructive in this respect the short explosive events in 1818 and 1913 correspond to the initial mixing stage of a differentiated magmatic body with a new mafic input mathematical modeling shows that the pyroclastic products of 1913 result from the mixing between an acidic andesite 61 5 sio2 and an olivine andesite 56 sio2 in various ratios the final andesite mixture is made of 38 basaltic andesite and 62 acidic andesite the long effusive extrusive phases correspond to the ensuing differentiation stage of the new magmatic body for example the 1961 lava flows were derived from the final mixed and homogenized magma after the eruption of 1913 by fractionation of 7 plagioclase 9 pyroxene clinopyroxene orthopyroxene and 0 7 titanomagnetite the fractionation of 11 plagioclase 9 pyroxene and 1 4 titanomagnetite is required to produce the composition of the 1986 summit lava dome from the same parent in conclusion at fuego de colima an eruptive cycle begins with a short violent explosive event related to a mixing process and continues with a long effusive phase characterized by magmatic differentiation this scheme differs from others that have been proposed which consider that the eruptive cycles end with an explosive event

strong interseismic coupling fault afterslip and viscoelastic flow before and after the oct 9 1995 colima jalisco earthquake continuous gps measurements from colima mexico 1 continuous gps measurements from colima mexico during 4 93 6 01 bracketing the oct 9 1995 m 8 0 colima jalisco earthquake provide new constraints on rivera plate subduction mechanics modeling of margin normal strain accumulation before the earthquake suggests the rivera north america subduction interface was fully locked transient postseismic motion from 10 95 6 97 is well fit by a model that includes logarithmically decaying fault afterslip elastic strain from shallow fault relocking and possibly a minor viscoelastic response but is fit poorly by models that assume a dominant maxwell viscoelastic response of the lower crust and upper mantle independent of the assumed viscosities landward margin normal motion since mid 1997 is parallel to but 75 slower than the pre seismic velocity afterslip alone fails to account for this slowdown the viscoelastic response predicted by a fem correctly resolves the remaining velocity difference within the uncertainties both processes thus offset elastic strain accumulating from the relocked subduction interface

eruptive history of the colima volcanic complex mexico abstract the evolution of the colima volcanic complex can be divided into successive periods characterized by different dynamic and magmatic processes emission of andesitic to dacitic lava flows acid ash and pumice flow deposits fallback nuees ardentes leading to pyroclastic flows with heterogeneous magma plinian air fall deposits scoriae cones of alkaline and calc alkaline nature four caldera forming events resulting either from major ignimbrite outbursts or mount st helens type eruptions separate the main stages of development of the complex from the building of an ancient shield volcano 25 30 km wide up to two summit cones nevado and fuego the oldest caldera c1 7 8 km wide related to the pouring out of dacitic ash flows marks the transition between two periods of activity in the primitive edifice called nevado i the first one which is at least 0 6 m y old was mainly andesitic and effusive whereas the second one was characterized by extrusion of domes and related pyroclastic products a small summit caldera c2 3 3 5 km wide ended the evolution of nevado i two modern volcanoes then began to grow the building of the nevado ii started about 200 000 y ago it settled into the c2 caldera and partially overflowed it the other volcano here called paleofuego was progressively built on the southern side of the former nevado i some of its flows are 50 000 y old but the age of its first outbursts is not known however it is younger than nevado ii these two modern volcanoes had similar evolutions each of them was affected by a huge mount st helens type or bezymianny type event 10 000 y ago for the paleofuego and hardly older for the nevado ii the landslides were responsible for two horseshoe shaped avalanche calderas c3 nevado and c4 paleofuego each 4 5 km wide opening towards the east and the south in both cases the activity following these events was highly explosive and produced thick air fall deposits around the summit craters the nevado iii formed by thick andesitic flows is located close to the southwestern rim of the c3 caldera it was a small and short lived cone volcan de fuego located at the center of the c4 caldera is nearly 1500 m high its activity is characterized by an alternation of long stages of growth by flows and short destructive episodes related to violent outbursts producing pyroclastic flows with heterogeneous magma and plinian air falls the evolution of the primitive volcano followed a similar pattern leading to formation of c1 and then c2 the analogy between the evolutions of the two modern volcanoes nevado ii iii paleofuego fuego is described their vicinity and their contemporaneous growth pose the problem of the existence of a single reservoir or two independent magmatic chambers after the evolution of a common structure represented by the primitive volcano

transport and deposition of a pyroclastic surge across an area of high relief the 18 may 1980 eruption of mount st helens washington the 08 32 pdt 18 may 1980 mount st helens eruption began as an explosion blast that produced a laterally flowing pyro clastic current blast surge it moved outward in a 180 arc at supersonic speeds and then slowed to subsonic speeds supersonic flow severely eroded the ground and produced a ground layer formerly layer a0 above the ground layer lies the deposit derived from the blast surge the blast surge deposits formed in three continuous and concurrent stages the first stage was movement of the transport system that is the blast surge carrying fragments most of the distance from the vent the next two stages involved the depositional system this system developed from rapidly accumulating sediment by gravity segregation at the base of the blast surge two stages of sediment gravity flows with independent flow regimes formed sequentially in rapid succession from the moving blast surge each moved downhill due to gravity independently of the generating blast surge that was driven by explosive expansion units repeated in some sections are deposits derived from divergent and crossing surge lobes caused by irregular topography locally ponded massive deposits in valleys and depressions formed from gas rich pyroclastic flows and originated in two ways 1 topographic blocking by steep volcano facing slopes blocking includes diversion or damming of the lower elevation high density part of the blast surge that could not surmount high ridges 2 also occurring to form valley ponds was drainage of just deposited or almost deposited material from steep slopes 30 35 moving downhill in all slope directions into separate valleys or depressions distances traveled by the blast surge and rates at which coarsest fragments decrease laterally are related to the topographic grain surge runout was farther where flow directions paralleled topographic trends and least where trends were at right angles to the flow obstacle interference depositional patterns thinning thickening rapid fine to coarse facies changes caused a spread in median diameter values that are as great in local areas near the volcano as they are over the width of the devastated region

tephrochronology petrology and geochemistry of late holocene pyroclastic deposits from volcan de colima mexico abstract volcan de colima in western mexican volcanic belt is the most active volcano in north america we present a detailed tephrochronological study of its holocene proximal tephra fall pyroclastic surge and pyroclastic flow deposits based on 98 new stratigraphic sections 183 new dendrocalibrated radiocarbon 14 c age determinations 209 new geochemical analyses of pumice and ash 50 texture and mineral point counting mode determinations and 230 grain size analyses these results provide a much clearer understanding of the explosive eruptive behavior of volcan de colima during the past 10 000 years our data reveals that only one of the many eruptions described in historic documents has been large enough to be preserved we discuss the major and trace element geochemistry and petrology of the holocene tephra and compare it with the currently erupting calc alkaline andesites and alkaline minettes and basanites from the adjacent cinder cones it appears that several of the oldest 12 000 6000 yrs bp samples of the holocene tephra may record the last episodes of calc alkaline and alkaline magma mixing as first described in the pioneering work of luhr and carmichael 1981 finally the tephrochronological record from the northern downwind side of the colima volcanic complex is compared with the extensive radiocarbon constrained record of holocene debris avalanche deposits on the south side of the complex to build a more integrated picture of its past volcanic activity