the geomorphology of the southwest guinea margin tectonic volcanic mass movement and bottom current influences abstract slope morphology controls were examined in two topographically different areas on the southern transform margin of the guinea plateau sectors g1 and g2 tectonic control is manifested by steep faulted slopes in both study areas and by basement warping in sector g1 and magmatic emplacements in sector g2 sediment deposition and the formation of bedforms such as sediment waves on the margin are influenced by topographically intensified bottom currents within the intermediate deep water mass the north atlantic deep water nadw as well as in the shallower equatorial surface water esw antarctic intermediate water aaiw at the uppermost slope plateau edge level thus unconsolidated sediments are eroded from steep slopes the upper and lower slopes canyon walls and the slopes of the volcanic cones and concentrated into slope parallel depocentres in sector g1 and ponded in moats between the volcanic cones in sector g2 surficial sediment waves which are apparently oriented at an angle to the slope range from 0 5 to 2 km in wavelength and up to 100 m in height mass movement of the sediment cover is significant on the lower slopes of both sectors and is the result of slope steepness and instability caused by the continuing tectonic activity

the madden julian oscillation barotropic dynamics and north pacific tropical cyclone formation part i observations low level barotropic dynamics may help to explain the modulation of eastern and western north pacific tropical cyclones by the madden julian oscillation mjo during northern hemisphere summer the mjo is characterized by alternating periods of westerly and easterly 850 mb zonal wind anomalies across the tropical pacific ocean when mjo 850 mb wind anomalies are westerly small scale slow moving eddies grow through barotropic eddy kinetic energy eke conversion from the mean flow these growing eddies together with strong surface convergence 850 mb cyclonic shear and high mean sea surface temperatures create a favorable environment for tropical cyclone formation periods of strong mjo easterlies over the pacific are characterized by lesser eke and negligible eddy growth by barotropic conversion the term 2 x is a leading contributor to low level barotropic eke conversion during mjo westerly 2 u9 u periods across the pacific indicating the importance of zonal variations in the westerly jet for producing concentrations of eddy energy this mechanism can be described as wave accumulation associated with variations of the low level zonal flow the conversion term 2 y contributes a smaller portion of the total conversion u9y9 u over the eastern pacific but is of comparable importance to 2 x during westerly mjo events in the 2 u9 u western pacific

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

tholeiitic and calc alkaline magma series at adatara volcano northeast japan 1 geochemical constraints on their origin abstract low alkali island arc by jakes and gill 1970 or low k by gill 1978 tholeiitic rocks are closely associated with calc alkaline rocks in the outer volcanic zone called the nasu volcanic zone of northeast japan major element trace element ni cr co sc v zr nb zn rb cs ba sr ce and y and sr isotope compositions are presented for 9 low alkali tholeiitic and 16 calc alkaline samples of adatara volcano belonging to the nasu volcanic zone in the trace elements vs sio 2 diagram the tholeiitic series and calc alkaline series show different trends to each other for co sc v zr rb sc ba and sr as expected from k 2 o contents the calc alkaline samples are enriched in incompatible elements such as rb cs ba ce and zr compared with the tholeiitic samples on the other hand contents of nb in the calc alkaline samples are somewhat lower than those in the tholeiitic samples plots of ni and cr contents vs sio 2 for the tholeiitic samples come near smooth trend lines whereas those for the calc alkaline samples are scattered sr isotopic ratios in the tholeiitic series lie within the range between 0 7056 and 0 7058 and between 0 7048 and 0 7052 in the calc alkaline series smooth major and trace element compositional trends in the tholeiitic series are successfully modelled by simple fractional crystallization of plagioclase hypersthene augite titanomagnetite and minor olivine similarly crystal fractionation models can closely reproduce major and most of the trace element variations in the calc alkaline series but failed to account for irregular variations of ni and cr the scattering of ni and cr probably reflects simultaneous crystal fractionation and subordinate magma mixing in the subvolcanic chamber the inferred fractionated phases for the calc alkaline series are richer in augite and titanomagnetite and slightly poorer in plagioclase than those for the tholeiitic series the tholeiitic magma and calc alkaline magma are not derived from a common parental tholeiitic magma and the calc alkaline parental magma is richer in sio 2 and incompatible elements in particular rb and zr poorer in radiogenic sr and possibly more oxidized than the tholeiitic parental magma

a possible new mechanism for northward propagation of boreal summer intraseasonal oscillations based on trmm and merra reanalysis boreal summer intraseasonal oscillations bsisos manifest in the active and break spells and act as the primary building block of the indian summer monsoon although recent research has evolved a basic framework for understanding the scale selection and northward propagation of the bsiso the role of different hydrometeors in modulating these processes remains poorly explored in this study trmm 2a12 retrievals and modern era retrospective analysis for research and applications reanalysis data are examined to establish relationship between cloud hydrometeors and other atmospheric dynamical parameters with the northward propagation of the bsisos the study reveals that the cloud liquid water leads the deep convection during the northward propagation of bsisos in the lower troposphere while the cloud ice slightly lags the convection this distribution indicates the occurrence of a possible mechanism of the lower level moistening through the large scale moisture advection in lower atmosphere and boundary layer pbl convergence followed by triggering of the deep convection the analyses of moisture advection and the dynamical fields with respect to the convection center show that low level moistening is a manifestation of the barotropic vorticity and pbl convergence of moisture anomaly north of the convection center a new internal dynamical thermodynamical mechanism is unraveled to understand the reason behind the middle tropospheric heating maximum and its role on the northward propagation it is shown that the enhanced moisture perturbation in lower levels together with the heat transport by the sub grid scale eddies within the pbl induces lower level instability required to precondition the lower atmosphere for triggering the deep convection vigorous upward motion inside the deep convection uplifts the liquid hydrometeors to upper levels and the formation of precipitable ice leads to the heating maxima in the middle troposphere to check the robustness of the proposed hypothesis similar analysis is performed for the weak northward propagating bsiso cases

magma evolution and ascent at volcanic arcs constraining petrogenetic processes through rates and chronologies large 226 ra excesses in a number of mafic arc magmas and geophysical observations of earthquake hypocenters locations indicate that mafic melts can get transferred from source to surface within days to 1 kyr decreasing 226 ra 230 th disequilibria with increasing sio2 in co magmatic suites of individual arc volcanoes suggest that magma differentiation often occurs on timescales of a few thousand years in closed systems or less in open systems however the rapid decrease of global 238 u 230 th disequilibria with increasing sio2 within basaltic andesite compositions suggests that rapid closed system differentiation does not typically produce more evolved compositions from mafic parental magmas with large 238 u excesses in the case of rapid magmatic evolution open system processes are frequently involved in the production of andesites and dacites and the data imply that the open system component is close to 238 u 230 th secular equilibrium melts modeling of magma evolution at santorini in the aegean arc provides corroborating evidence from an individual well studied arc volcano that closed system fractional crystallization fails to produce the more evolved compositions and increasing 87 sr 86 sr ratios with increasing sio2 from andesites to rhyolites suggests assimilation of old crust is involved these observations can be conceptualized through thermal modeling of basaltic sill injections into the lower and upper crust melt production in deep crustal hot zones provides an explanation for low 238 u 230 th disequilibria of the evolved magmatic component as significant accumulation of evolved melts through partial re melting of previously intruded basalts requires n100 kyr incubation time at typical magmatic flux rates thermal modeling also provides a framework for understanding assimilation of old upper crustal rocks in more mature magmatic systems further insights into magmatic evolution can be gained from dating of minerals from mafic intermediate and felsic arc magmas a many u th mineral isochrons in mafic arc magmas yield ages significantly older than those

clustering of synoptic activity by indian summer monsoon intraseasonal oscillations 1 active and break phases of the indian summer monsoon are characterized by enhancement and decrease of precipitation over the monsoon trough region using genesis data of monsoon low pressure systems lps and circulation data for the period 1954 to 1993 it is shown that the frequency of occurrence of lps is nearly 3 5 times higher in the active phase of monsoon as compared to the break phase in addition the tracks of these synoptic systems are also strongly spatially clustered along the monsoon trough during the active phase of the monsoon the enhanced decreased frequency of occurrence of lps during active break phases is due to modulation of meridional shear of zonal winds and cyclonic vorticity along the monsoon trough by the intraseasonal oscillations iso

role of the boundary layer moisture asymmetry in causing the eastward propagation of the madden julian oscillation the moisture budget associated with the eastward propagating madden julian oscillation mjo was diagnosed using 1979 2001 40 yr ecmwf re analysis era 40 data a marked zonal asymmetry of the moisture relative to the mjo convection appears in the planetary boundary layer pbl below 700 hpa creatingapotentiallymoreunstablestratificationtotheeastofthemjoconvectionandfavoringtheeastward propagation of mjo the pbl integrated moisture budget diagnosis indicates that the vertical advection of moisture dominates the low level moistening ahead of the convection a further diagnosis indicates that the leading term in the vertical moisture advection is the advection of the background moisture by the mjo ascending flow associated with pbl convergence the cause of the zonally asymmetric pbl convergence is further examined it is found that heating induced free atmospheric wave dynamics account for 75 90 of the total pbl convergence while the warm sst anomaly induced by air sea interaction contributes 10 25 of the total pbl convergence the horizontal moisture advection also plays a role in contributing to the pbl moistening ahead of the mjo convection the leadingterm in the moistureadvectionis the advectionacross the backgroundmoisture gradient by the mjo flow in the western indian ocean maritime continent and western pacific the meridional moisture advection by the mjo northerly flow dominates while in the eastern indian ocean the zonal moisture advection is greater the contribution of the moisture advection by synoptic eddies is in generalsmall ithas a negativeeffect overthetropicalindianoceanand westernpacificandbecomespositive in the maritime continent region

from sink to volcanic source unravelling missing terrestrial eruption records by characterization and high resolution chronology of lacustrine volcanic density flow deposits lake inawashiro ko fukushima japan

coupling between northward propagating intraseasonal oscillations and sea surface temperature in the indian ocean using a hybrid atmosphere ocean coupled model it is shown that during the boreal summer northwardpropagating intraseasonal oscillations npisos are strongly coupled to the underlying sea surface temperature sst in the indian ocean sector on the one hand the intraseasonal atmospheric convection changes the sst through solar radiation latent heat flux and mixed layer entrainment on the other the induced sst fluctuations feed back to affect the intraseasonal convection the preferential northward rather than southward propagation of boreal summer isos in the indian ocean is partially explained by an interaction among the summer mean climate state the atmospheric disturbances and the ocean surface temperature a solution to an atmosphere only model forced with daily sst produces much stronger npisos than a similar solution forced with monthly mean sst amip type run the atmosphere only model however even when it is forced by daily sst from the coupled model with a small amount of noise in the initial and or boundary conditions is unable to reproduce the npisos in the coupled case in the coupled system intraseasonal sst anomalies are forced by intraseasonal atmospheric convection and hence are in quadrature with the convection in the stand alone atmospheric model however sst acts only as a boundary forcing and the resultant atmospheric convection has almost the same phase with the underlying sst one consequence is that the intensity of the sst forced intraseasonal convection in the stand alone atmospheric model is considerably weaker than in the coupled model finally solutions indicate that the northward movement of the off equatorial convection in the northern indian ocean is more closely related to local intraseasonal sst anomalies than to the equatorial eastward moving madden julian oscillation positive negative sst anomalies in the northern indian ocean lead the active break phases of the intraseasonal convection by about 2 pentads 10 days therefore intraseasonal sst anomalies in the northern indian ocean are potentially a useful index to forecast active break spells of the south asian summer monsoon

the arc lavas of the shirahama group japan sr and nd isotopic data indicate mantle derived bimodal magmatism new sr and nd isotopic data are presented and integrated with previous data for the shirahama group mio pliocene mediumk volcanic arc suite of south central honshu japan main results are 1 the shirahama lavas range in sr sr from 0 70315 to 0 70337 and in mnd 144nd from 0 51298 to 0 51306 the sr and nd isotopic data cluster tightly within the mantle array and all lie within an overlapping field of midocean ridge basalt and ocean island basalt 2 small differences exist among the shirahama tholeiitic series calc alkaline series and mixed lavas the present isotopic data are consistent with a previously published model which proposes that chemical variations in magmas of coexisting tholeiitic and calc alkaline series are produced through crystal fractionation from mantlederived magmas of basalt and magnesian andesite respectively moreover the tholeiitic series and the calc alkaline series are isotopically identical thus both magma series can be derived from a source mantle with the same isotopic composition supporting the hypothesis of simultaneous generation of basalt and magnesian andesite magmas from a single diapir rising through the mantle wedge above the subduction zone the differences of water content and temperature within the diapir are again thought to have been produced through dehydration and heating of an isotopically homogeneous hydrous diapir the isotopic data show that the high sio lavas have the same isotopic compositions as more mafic lavas these data and liquid lines of descent of the shirahama magmas suggest that even rhyolites can be produced by differentiation from mantle derived magmas without crustal contamination analyses from 38 other arc volcanoes have been compiled to investigate the intravolcano variability of sr sr twelve of these display no intravolcano strontium isotopic variability as is the case with the shirahama group but others show greater variation of sr 86sr from individual volcanic centers presumably reflecting crustal contamination most of the latter volcanoes are underlain by thick continental crust it is noteworthy however that the greater variations of sr sr correlate with sio2 content andesites or dacites not basalts from the same volcano have the lowest sr sr and these rocks are calc alkaline in terms of feo mgo and sio theoretically assimilation of continental crust by the isotopically uniform shirahama magmas could produce these relationships given that mantle derived basalt and magnesian andesite both encounter continental crust on their ascent to the surface the hotter basalt magma would assimilate more crustal wallrocks than the cooler andesite resulting in the basalt being more radiogenic fractional crystallization magma mixing and or assimilation fractional crystallization of these magmas in crustal magma chambers could produce large compositional variations but the derivatives of the hotter basaltic magmas tholeiitic series in the broad sense would display greater contamination than those derived from the cooler andesitic magmas calc alkaline series

organic rich sedimentation at the cenomanian turonian boundary in oceanic and coastal basins in the north atlantic and tethys one of the most striking results of the deep sea drilling project is the proof that organic rich sediments have a widespread geographical distribution during the period from upper cenomanian to middle turonian such sediments were drilled at north atlantic dsdp sites 105 135 137 138 367 398 551 603 they are also present from outcrops or oil wells on the shelf of the african continental margin senegal tarfaya and agadir basins in the tethys former alboran block southern spain algeria tunisia and umbrian apennines italy and in the north sea although these sediments have different lithologies and depositional environments from shelf areas to the deep sea their study mainly based on organic geochemistry with additional data on sedimentology biostratigraphy and palaeo bathymetry suggests that a unique pulse of organogenic accumulation characterizes the cenomanian turonian boundary event ctbe the content and type of organic matter are related to the depositional environment and organic preservation the organogenic accumulation is distributed according to various trends off the african continental margin the organic content increases from onshore areas to the shelf casamance area and moreover increases also in deep sea areas with a gradual transition from terrestrial type iii to marine type ii the best preservation of the organic matter being in the deepest areas i e site 367 off the american continental margin site 603 shows the same toc and type of organic matter as at site 105 the ctbe is also well recorded in the northern part of the atlantic celtic margin north sea by a drastic lithological change black shales within chalks but the type of organic matter is mainly terrestrial in the tethyan area the organic matter is of marine origin and well preserved results are compared with those of pratt 1984 from the western interior basin of the usa different hypotheses to explain this synchronous widespread accumulation of organic matter are discussed 1986 the geological society

aleutian tholeiitic and calc alkaline magma series i the mafic phenocrysts diagnostic mafic silicate assemblages in a continuous spectrum of aleutian volcanic rocks provide evidence for contrasts in magmatic processes in the aleutian arc crust tectonic segmentation of the arc exerts a primary control on the variable mixing fractional crystallization and possible assimilation undergone by the magmas end members of the continuum are termed calc alkaline ca and tholeiitic th ca volcanic rocks e g buldir and moffett volcanoes have low feo mgo ratios and contain compositionally diverse phenocryst populations indicating magma mixing their ni and cr rich magnesian olivine and clinopyroxene come from mantle derived mafic olivine basalts that have mixed with more fractionated magmas at mid to lower crustal levels immediately preceding eruption high al amphibole is associated with the mafic end member in contrast th lavas e g okmok and westdahl volcanoes have high feo mgo ratios and contain little evidence for mixing evolved lavas represent advanced stages of low pressure crystallization from a basaltic magma these lavas contain groundmass olivine fo 40 50 and lack ca poor pyroxene aleutian volcanic rocks with intermediate feo mgo ratios are termed transitional tholeiitic tth and calc alkaline tca tca magmas are common e g moffett adagdak great sitkin and kasatochi volcanoes and have resulted from mixing of high al basalt with more evolved magmas they contain amphibole high and low al or orthopyroxene or both and are similar to the japanese hypersthene series tth magmas e g okmok and westdahl contain orthopyroxene or pigeonite or both and show some indication of upper crustal mixing they are mineralogically similar to the japanese pigeonite series high al basalt lacks mg rich mafic phases and is a derivative magma produced by high pressure fractionation of an olivine tholeiite the low pressure mineral assemblage of high al basalt results from crystallization at higher crustal levels

occurrence of marine mid cretaceous sediments along the guinean slope equamarge ii cruise their significance for the evolution of the central atlantic african margin abstract during the equamarge ii cruise 1988 several coring attempts made along the slope edge of the southern guinean plateau led to the recovery of middle cretaceous sediments in two cores one contained a carbonate block dated as middle albian h rischi zone the micropaleontological and sedimentological data suggest an open marine environment and paleodepths of approximately 150 200 m another core included a dark shale level corresponding to the global anoxic event ctbe cenomanian turonian boundary event w archaeocretacea zone as shown by seismic reflection profiles these mid cretaceous beds lie very close to a major unconformity which distinctly divides the sedimentary cover into two units the lower one folded and faulted the upper one undisturbed geodynamic and paleoenvironmental implications of these new data better constrain models of evolution for this crucial area of the atlantic realm the compressive and extensional events which affected the equatorial atlantic during the progressive separation of africa from south america are thus more accurately dated these results also strengthen the hypothesis of an open seaway between the central and south proto atlantic as early as the middle albian

a gigantic bezymianny type event at the beginning of modern volcan popocatepetl the history of volcan popocatepetl can be divided into two main periods the formation of a large primitive volcano approximatively 30 km wide on which is superimposed a modern cone 6 8 km in diameter and 1700m high a major event of bezymianny type marks the transition between these two dissimilar periods the activity of the primitive volcano was essentially effusive and lasted several hundred thousands of years the total volume of products ejected by the volcano is of the order of 500 600 km3 its last differentiated magmas are dacitic a gigantic debris flow d f spread on the southern side is related to the bezymianny type event which destroyed the summit area of the ancient edifice an elliptical caldera 6 5 11 km wide was formed by the landslide its deposits with a typical hummocky surface cover 300 km2 for a volume of 28 30 km3 numerous outcrops belonging to this debris flow show slabs of more or less fractured and dislocated rocks that come from the primitive volcano these deposits are compared to two studied debris flows of similar extent and volume the mount shasta and colima s d f this eruption takes a major place in the volcanologic and magmatic history of popocatepetl pyroclastic products of surge type with laminites and crude layers ashflows and pumiceous airfall layers are directly related to this event and begin the history of the modern volcano probably less than 50 000 years ago in addition a second andesitic and dacitic phase rose both from the central vent forming the basis of modern popo and from lateral vents the terminal cone is characterized by long periods of construction by lava flows alternating with phases of destruction the duration of these episodes being 1000 to 2000 years the cone is composed of two edifices the first volcan el fraile began with effusive activity and was partly destroyed by three periods of intense explosive activity the first period occurred prior to 10 000 years b p the second from 10 000 to 8000 years b p and the third from 5000 to 3800 years b p each period of destruction shows cycles producing collapsing pyroclastic flows or nuees of the st vincent type related to the opening of large craters plinian air fall deposits and minor lava flows the second edifice the summit popo produced lava flows until 1200 years b p and since that time entered into an explosive period two cataclysmic episodes each including major pyroclastic eruptions occurred 1200 and 900 1000 years ago during the pre hispanic and historic times effusive activity was restricted entirely to the summit area alternating with plinian eruptions nevertheless despite the quiet appearance of the volcano the last period of pyroclastic activity which started 1200 years ago may not have ended and can be very dangerous for the nearby populations

the petrology of poas volcano lavas basalt andesite relationship and their petrogenesis within the magmatic arc of costa rica abstract basalt andesite and dacite from poas volcano exhibit a distinct petrogenetic evolution the basalts al 2 o 3 16 17 4 k 2 o 1 43 1 61 are hypersthene normative tholeiites with phenocrysts of labradorite olivine and two pyroxenes orthopyroxene en 67 56 is generally rimmed by clinopyroxene fs 10 18 wo 39 48 in all basalts and in a few andesites pyroxene phenocrysts have al and ti rich salite rims that surround cores of orthopyroxene or augite having lower values the al z values of tetrahedral site occupied by al for these augites are above 6 thus defining on the al z vs tio 2 diagram an intermediate trend between arc related cumulates and rift related tholeiites and ophiolite layered cumulates moreover the presence of olivines with coronae of wormy intergrowths of orthopyroxene and ti magnetite similar to those occurring in metagabbros of the granulite facies and characteristic of arenal gabbroic xenoliths together with anorthite megacrysts an 94 91 suggest a possible contamination origin for these basalts sr isotope ratios for the basalts are between 0 703816 and 0 703849 whereas andesites and dacites have lower ratios 0 703676 0 703715 these relationships together with the mineral compositions suggest that the basalts may have formed from basaltic andesites by contamination with olivine gabbros and anorthosites of the lower crust pearce element ratio analysis confirms that the basalts had a different origin from the rest of the suite and that the dacite may be derived by crystal fractionation of the less silicic basaltic andesite with removal mainly of clinopyroxene and plagioclase

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

predicting extreme phases of the indian summer monsoon abstract extreme active and break phases of the indian summer monsoon ism often bring about devastating floods and severe draughts here it is shown that these extreme phases exhibit distinctive precursory circulation conditions in both the tropics and extratropics over a range of antecedent periods the extremely active monsoon over northern india is preceded by a strengthening of the upper tropospheric central asian high and enhancement of the tropical convection over the equatorial indian ocean and the south china sea the concurrent buildup of the anomalous high over central asia and the arrival of tropical convection over northern india increase the likelihood of occurrence of a heavy rainy period there similarly the concurrent anomalous low over central asia and the arrival of suppressed convection originating from the equatorial indian ocean and the south china sea precede extremely strong monsoon breaks over northern india two predictors can be used to predict the extreme active break phases

teleconnections associated with northern hemisphere summer monsoon intraseasonal oscillation the boreal summer intraseasonal oscillation bsiso has strong convective activity centers in indian i western north pacific wnp and north american na summer monsoon sm regions the present study attempts to reveal bsiso teleconnection patterns associated with these dominant intraseasonal variability centers during the active phase of ism a zonally elongated band of enhanced convection extends from india via the bay of bengal and philippine sea to tropical central pacific with suppressed convection over the eastern pacific near mexico the corresponding extratropical circulation anomalies occur along the waveguides generated by the north african asian jet and north atlantic north european jet when the tropical convection strengthens over the wnpsm sector a distinct great circle like rossby wave train emanates from the wnp to the western coast of united states us with an eastward shift of enhanced meridional circulation in the active phase of nasm large anticyclonic anomalies anchor over the western coast of us and eastern canada and the global teleconnection pattern is similar to that during a break phase of the ism examination of the evolution of the bsiso teleconnection reveals quasi stationary patterns with preferred centers of teleconnection located at europe russia central asia east asia western us and eastern us and canada respectively most centers are embedded in the waveguide along the westerly jet stream but the centers at europe and russia occur to the north of the jet induced waveguide eastward propagation of the iso teleconnection is evident over the pacific north america sector the rainfall anomalies over the elongated band near the monsoon domain over the indo western pacific sector have an opposite tendency with that over the central and southern china mexico and southern us providing a source of intraseasonal predictability to extratropical regions the bsiso teleconnection along and to the north of the subtropical jet provides a good indication of the surface sir temperature anomalies in the nh extratropics

late cretaceous to early quaternary organic sedimentation in the eastern equatorial atlantic abstract reconstructing the long term evolution of organic sedimentation in the eastern equatorial atlantic odp leg 159 provides information about the history of the climate ocean system sediment accumulation and deposition of hydrocarbon prone rocks the recovery of a continuous 1200 m long sequence at odp site 959 covering sediments from albian to the present day about 120 ma makes this position a key location to study these aspects in a tropical oceanic setting new high resolution carbon and pyrolysis records identify three main periods of enhanced organic carbon accumulation in the eastern tropical atlantic i e the late cretaceous the eocene oligocene and the pliocene pleistocene formation of upper cretaceous black shales off west africa was closely related to the tectono sedimentary evolution of the semi isolated deep ivorian basin north of the cote d ivoire ghana transform margin their deposition was confined to certain intervals of the last two cretaceous anoxic events the early turonian oae2 and the coniacian santonian oae3 organic geochemical characteristics of laminated coniacian santonian shales reveal peak organic carbon concentrations of up to 17 and kerogen type i ii organic matter which qualify them as excellent hydrocarbon source rocks similar to those reported from other marginal and deep sea basins a middle to late eocene high productivity period occurred off equatorial west africa porcellanites deposited during that interval show enhanced total organic carbon toc accumulation and a good hydrocarbon potential associated with oil prone kerogen deposition of these toc rich beds was likely related to a reversal in the deep water circulation in the adjacent sierra leone basin accordingly outflow of old deep waters of southern ocean origin from the sierra leone basin into the northern gulf of guinea favored upwelling of nutrient enriched waters and simultaneously enhanced the preservation potential of sedimentary organic matter along the west african continental margin a pronounced cyclicity in the carbon record of oligocene lower miocene diatomite chalk interbeds indicates orbital forcing of paleoceanographic conditions in the eastern equatorial atlantic since the oligocene miocene transition a similar control may date back to the early oligocene but has to be confirmed by further studies latest miocene early pliocene organic carbon deposition was closely linked to the evolution of the african trade winds continental upwelling in the eastern equatorial atlantic ocean chemistry and eustatic sea level fluctuations reduction in carbonate carbon preservation associated with enhanced carbon dissolution is recorded in the uppermost miocene 5 82 5 2 ma section and suggests that the latest miocene carbon record of site 959 documents the influence of corrosive deep waters which formed in response to the messinian salinity crisis furthermore sea level related displacement of higher productive areas towards the west african shelf edge is indicated at 5 65 5 6 5 55 5 2 4 8 ma in view of humid conditions in tropical africa and a strong west african monsoonal system around the miocene pliocene transition the onset of pronounced toc cycles at about 5 6 ma marks the first establishment of upwelling cycles in the northern gulf of guinea an amplification in organic carbon deposition at 3 3 ma and 2 45 ma links organic sedimentation in the tropical eastern equatorial atlantic to the main steps of northern hemisphere glaciation and testifies to the late pliocene transition from humid to arid conditions in central and western african climate aridification of central africa around 2 8 ma is not clearly recorded at site 959 however decreased and highly fluctuating carbonate carbon concentrations are observed from 2 85 ma on that may relate to enhanced terrigenous eolian dilution from africa

lamprophyric lavas in the colima graben sw mexico the colima graben located in sw mexico is one of three grabens which intersect about 50 km ssw of guadalajara forming a triple junction the 90 km long 20 60 km wide colima graben represents a n s rift of the e w trending mexican volcanic belt since the early pliocene the colima graben has served as a locus for the eruption of alkaline lavas the most recent of which are basanites and minettes erupted from late pleistocene cinder cones luhr and carmichael 1981 in this paper we report on older alkaline lavas which crop out in the graben s walls these rocks include phlogopite and hornblende bearing lamprophyres a phlogopite kalsilite ankaratrite and high k andesites these lavas crop out throughout the colima graben area and are intimately associated with calc alkaline lavas in the field compared to these the alkaline rocks are strikingly enriched in the incompatible elements particularly ba sr p and the lree unlike the younger late pleistocene alkaline cinder cone lavas most of the graben wall lamprophyres and the high k andesites represent magmas that appear to have undergone significant evolution since their generation including fractionation crustal contamination and possible magma mixing least squares modeling indicates that the cinder cone minettes represent reasonable parental magmas for the graben lamprophyres the occurrence of these alkaline lavas in an active calc alkaline volcanic arc is unusual and we suggest that they are a manifestation of the rifting processes which produced the colima graben

real time multivariate indices for the boreal summer intraseasonal oscillation over the asian summer monsoon region the boreal summer intraseasonal oscillation bsiso of the asian summer monsoon asm is one of the most prominent sources of short term climate variability in the global monsoon system compared with the related madden julian oscillation mjo it is more complex in nature with prominent northward propagation and variability extending much further from the equator in order to facilitate detection monitoring and prediction of the bsiso we suggest two real time indices bsiso1 and bsiso2 based on multivariate empirical orthogonal function mv eof analysis of daily anomalies of outgoing longwave radiation olr and zonal wind at 850 hpa u850 in the region 10 s 40 n 40 160 e for the extended boreal summer may october season over the 30 year period 1981 2010 bsiso1 is defined by the first two principal components pcs of the mv eof analysis which together represent the canonical northward propagating variability that often occurs in conjunction with the eastward mjo with quasi oscillating periods of 30 60 days bsiso2 is defined by the third and fourth pcs which together mainly capture the northward northwestward propagating variability with periods of 10 30 days during primarily the pre monsoon and monsoon onset season the bsiso1 circulation cells are more rossby wave like with a northwest to southeast slope whereas the circulation associated with bsiso2 is more elongated and front like with a southwest to northeast slope bsiso2 is shown to modulate the timing of the onset of indian and south china sea monsoons together the two bsiso indices are capable of describing a large fraction of the total intraseasonal variability in the asm region and better represent the northward and northwestward propagation than the real time multivariate mjo rmm index of wheeler and hendon

experimental constraints on plagioclase crystallization during h2o and h2o co2 saturated magma decompression experiments simulating magma decompression allow the textures of volcanic rocks to be calibrated against known eruptive conditions interpretation of natural samples may be complicated however by both the decompression path and the composition of exsolving volatiles which affect the time evolution of crystal textures here we present the results of decompression experiments at elevated temperature and pressure designed to assess the effects of degassing path on crystallization of mount st helens rhyodacite three families of experiments were employed to simulate varied ph2o t trajectories single step h2o saturated decompression ssd continuous h2o saturated decompression cd continuous h2o co2 saturated decompression quantitative textural data crystal abundance number density and size are used to calculate plagioclase nucleation and growth rates and assess deviations from equilibrium in run products these are the first experiments to quantify feldspar nucleation and growth rates during h2o co2 saturated decompression we find that reducing the initial melt water content through addition of co2 increases nucleation rates relative to the pure water case an effect most pronounced at low dp dt moreover these early formed textural distinctions persist at the lowest pressures examined suggesting that deep h2o co2 fluids could leave a lasting textural fingerprint on erupted magmas crystals formed prior to decompression during the annealing process also modulate sample textures and growth on pre existing crystals contributes significantly to added crystallinity at a wide range of experimental conditions the phase assemblage itself is a dynamic variable that can be used in conjunction with textural data to infer conditions of magma ascent and eruption finally quantitative textural data from experimental samples are compared with those of natural pyroclasts erupted during the summer 1980 explosive effusive transition at mount st helens this comparison supports a model of magma ejected from multiple storage regions present in the upper crust following the may 18 plinian eruption such that subsequent eruptions tapped magmas that experienced varied decompression and degassing histories

seismic characteristics and identification of negative flower structures positive flower structures and positive structural inversion negative and positive flower structures and positive inverted structures imply specific modes of formation and their distinctive characteristics make them important criteria for the identification of certain structural styles a negative flower structure from the andaman sea consists of a shallow synform bounded by upward spreading strands of a wrench fault that have mostly normal separations paralleling monoclines and oblique en echelon normal faults flank the divergent wrench fault a positive flower structure from the ardmore basin oklahoma consists of a shallow antiform displaced by the upward diverging strands of a wrench fault that have mostly reverse separations en echelon folds are present on either side of this convergent wrench fault flower structures may resemble extensional or contractional fault blocks on individual seismic profiles flower structures are identified by their unique internal fault and fold architecture by their association with a straight and throughgoing zone of deformation and by their lack of a consistent sense of separation or regional upthrown side positive structural inversion at the rambutan oil field south sumatra basin has formed a shallow anticlinorium and has partly uplifted the underlying graben deeper fault segments bounding the graben have retained their normal fault profiles but at shallow levels some of these faults have reverse separations similar changes of separation with depth may be produced by strike slip but sets of faults with listric profiles and nearby blocklike trend patterns identify the inversion as a dip slip event where correlation of reflections is uncertain or deep control is unobtainable positive inverted structures may also resemble contrational fault blocks detached normal fault assemblages and certain negative inverted structures

tectonic controls on tholeiitic and calc alkaline magmatism in the aleutian arc the tectonic position of aleutian arc volcanic centers and their magmatic differentiation trends calc alkaline or tholeiitic appear to correlate from 160 w to 175 e the volcanoes form four major arc segments that coincide with earthquake aftershock zones and major geographic features on both the upper and lower plates the tholeiitic volcanoes are large primarily basaltic centers that occur between or at the end of segments where magmas can more easily reach the surface and undergo shallow closed system differentiation the calc alkaline volcanoes are smaller more andesitic centers that occur in the middle of segments where transit through the upper plate is apparently more difficult differentiation is deeper and the intrusive to extrusive ratios are higher than in the tholeiitic centers the chemistry of the least differentiated basalts at both types of centers is similar and suggests a common parent magma probably derived from mantle peridotite conditions for the derivation of the two trends through crystal fractionation support the proposed model the tholeiitic magmas show characteristics i e no hydrous phenocrysts fe enrichment trend parallel ree patterns vitrophyric and esites and dacites consistent with low pressure high temperature crystallization in large shallow magma chambers the calc alkaline magmas show characteristics i e some hydrous phenocrysts no fe enrichment trend nonparallel rare earth patterns porphyritic lavas consistent with higher pressure and lower temperature of crystallization than the tholeiitic series tertiary plutons also show both calc alkaline and tholeiitic trends and appear to be chemically similar to the quaternary volcanoes the larger plutons are calc alkaline and probably represent the extensive 1002 km magma chambers of the small calc alkaline volcanoes the small shallow tholeiitic plutons complement the large tholeiitic volcanoes and reflect the greater percent of extrusive rocks in the tholeiitic series

geochemistry of basalts from central and southern new hebrides arc implication for their source rock composition abstract the basaltic rocks from the central and southern islands of the new hebrides aneityum tanna erromango efate emae tongoa and epi have geochemical features typical of island arc volcanics they are enriched in lile and depleted in zr hf nb and ta compared to n type morb the rocks were derived from a similar upper mantle source as n type morb but with a higher degree of partial melting in addition their source was enriched in lile k rb sr ba and lree probably by migrating hydrous fluids released during the dehydration of the subducted oceanic slab the basalts from futuna island which is located farther from the trench display characteristics typical of calc alkaline rocks the futuna basalts were generated from a different lile enriched upper mantle source it seems that this upper mantle source was modified by interaction with partial melts from the subducted oceanic lithosphere

genesis of tropical cyclone nargis revealed by multiple satellite observations 1 tropical cyclone tc nargis recently battered myanmar on may 2 2008 is one of the most deadly tropical storms in history nargis was initiated by an abnormally strong intraseasonal westerly event associated with madden julian oscillation mjo in the eastern indian ocean an incipient cyclonic disturbance emerged as an emanation of rossby wave induced vortex when the intraseasonal convective anomaly reached the maritime continent the northeastward movement of mjo convection facilitated further development of the disturbance the incipient disturbance became a tropical disturbance td with a central warm core structure on april 26 the further development from the td to tc formation on april 28 is characterized by two distinctive stages a radial contraction followed by a rapid intensification the processes responsible for contraction and rapid intensification are discussed by diagnosis of multiple satellite data this proposed new scenario is instrumental for understanding how a major tc develops in the northern indian ocean

low frequency equatorial waves in vertically sheared zonal flow part i stable waves abstract the mechanism by which a vertically sheared zonal flow affects large scale low frequency equatorial waves is investigated with two level equatorial plane and spherical coordinates models vertical shears couple barolinic and barotropic components of equatorial wave motion affecting significantly the rossby wave and westward propagating yanai wave but not the kelvin wave this difference results from the fact that the barotropic component is a modified rossby mode and can be resonantly excited only by westward propagating internal waves the barotropic components emanate poleward into the extratropics with a pronounced amplitude while the baroclinic components remain equatorially trapped a westerly vertical shear favors the trapping of rossby and yanai waves in the upper troposphere whereas an easterly shear tends to confine them in the lower troposphere as such their westward propagation is slowed down by both westerly and easterly shears when the strength of the vertical shear varies w

tholeiitic and calc alkaline magma series at adatara volcano northeast japan 2 mineralogy and phase relations abstract low alkali low k by gill 1978 or island arc by jakes and gill 1970 tholeiitic magma suites are closely associated with medium k by gill 1978 calc alkaline suites in the outer volcanic zone called nasu volcanic zone nvz of northeast japan chemistry and phase relations of phenocrysts are studied for representative 5 low alkali tholeiitic and 8 calc alkaline rock samples of adatara volcano in nvz in order to examine the mineralogical evolutionary mechanisms of the two series of magmas also clarified are differences in the magmatic temperature and oxygen fugacity between the two magma series the correlations between whole rock chemical compositions and mineralogy in the tholeiitic suite at this volcano are well documented supporting the argument that the members of this suite are related through fractional crystallization of observed phenycrystic phases part 1 fujinawa 1988 mineralogical observations in the calc alkaline suite are compatible with the views based on their whole rock chemical compositions although fractional crystallization process is dominant in the evolution of the calc alkaline magma intermittent mixing of small amounts of magnesian high temperature magmas operates concurrently temperature estimates with the two pyroxenes geothermometer range from 1080 c to 970 c in the tholeiitic suite whereas from 990 to 850 c in the calc alkaline suite temperature oxygen fugacity trends indicate that the low alkali tholeiitic magma evolves under conditions of higher temperature and lower oxygen fugacity than the calc alkaline magma in view of the fact that the hyperstene and augite phenocryst cores in the calc alkaline samples are compositionally distinguishable from the relevant phases in the tholeiitic samples it is unlikely that magma mixing played an essential role in the generation of the calc alkaline suite at adatara volcano

petrology of arc volcanic rocks and their origin by mantle diapirs abstract calc alkalic chemical trends characteristic of arc volcanic rocks mainly result from three mechanisms which act additively 1 fractional crystallization involving separation of titanomagnetite 2 selective concentration of plagioclase phenocrysts and selective depletion of titanomagnetite phenocryst compared with the actually fractionated proportion and 3 mixing of magmas on continuous fractionation trends the association of calc alkalic and tholeiitic trends in a single composite volcano may not represent different fractional crystallization processes or different chemistries of primary magmas but the calc alkalic chemical trend can be considered as a mixing trend resulting from mixing of various magmas on associated tholeiitic chemical trends chemical variations of most arc volcanic rocks including calc alkalic ones can accordingly be essentially accounted for by the low pressure fractional crystallization of phenocrystic phases from primary basaltic magmas crystallization sequences of arc magmas which are strongly dependent on water content in magmas are deduced from the phenocryst assemblages the crystallization sequence changes laterally across arc suggesting increasing water contents in magmas toward the back arc side as is also seen for other incompatible elements such as k and rb systematic differences in the characteristic crystallization sequence are also observed among arcs roughly correlating with the crustal thickness water content in magma like other incompatible elements tends to increase with increasing crustal thickness the variation of incompatible elements including water roughly represents that of the degree of partial melting of the upper mantle which is broadly controlled by the crustal thickness the variation of water content indicates that arc magmas are not saturated with water during differentiation to late differentiates such as dacite or rhyolite this strongly constrains the maximum water contents in primary basaltic magma at most 2 5 wt this value suggests that magma generation beneath arcs is dependent on dry solidus of peridotite diapiric uprise of the hot deeper mantle and associated adiabatic decompression would be necessary for mantle peridotite to attain the temperature as high as dry solidus diapirs that begin to rise from the subduction zone may stop at or near the crust mantle boundary because of the surrounding density change and their degree of partial melting is roughly controlled by their stopped depth assuming their similar temperature across arc variation is also explained by the stopped depth of diapirs but is not controlled by crustal thickness

the onset of convection in the madden julian oscillation abstract an observational study of the onset of convection in the madden julian oscillation mjo was performed composites of radiosonde data from the comprehensive aerological reference data set were constructed for an ensemble of tropical stations located in the indian ocean maritime continent and western pacific ocean the composites suggest that for the off equatorial stations used in this study the mjo period may be set by the buildup and discharge of the low level moist static energy this result supports the discharge recharge hypothesis of blade and hartmann mjo events appear to begin when the off equatorial atmosphere has been destabilized through a combination of low level moist static energy buildup and concurrent drying of the middle troposphere by subsidence in the wake of the previous cycle of mjo convection the low level moist static energy buildup is controlled by a corresponding increase in low level moisture the increase in low level moisture is not caused by the 1000 mb convergen

the shallow structures of the guinea and ivory coast ghana transform margins their bearing on the equatorial atlantic mesozoic evolution abstract recent geological and geophysical data from a marine survey conducted off guinea and the ivory coast ghana region equatorial atlantic are discussed we interpret the southern guinean margin firstly as an area linked to the central jurassic atlantic and secondly as an area affected by the progressive transform opening of the equatorial atlantic in the early cretaceous off the ivory coast and ghana we have demonstrated distinct structural domains interpreted as consequences of the progressive transform contact between the west african and brazilian cratons also during the early cretaceous on both margins we observe a clear tectonic unconformity of probable late albian early cenomanian age we propose that this unconformity results from the final disruption between the two continental west african and brazilian crusts the geological structures generated along transform margins can be considered as good geological tools to constrain early phases of plate motions particularly around the equatorial atlantic

late quaternary volcanic record from lakes of michoacan central mexico this paper describes the initial stages of the development of a tephrochronology for the region of the michoacan guanajuato volcanic field mgvf in central mexico there are two elements to this the geochemical characterisation of volcanic glass and the linkage of tephra deposits to eruptions of known age the mgvf is dominated by cinder cones and shield volcanoes which erupt only once there are only two stratovolcanoes multiple eruptions which are common elsewhere in the trans mexican volcanic belt tephras were sampled from sub aerial sites close to cones of known age and from lake sediment cores from the zirahuen patzcuaro and zacapu basins in the state of michoacan multiple samples were collected to ensure that each tephra was well represented the glass was analysed by electron microprobe and found to be calc alkaline in composition sio2 abundances varied from 52 to 75 full results are available at http www geo ed ac uk tephra the ages of the dated cones ranged from the 20th century ad to ca 17 000 14c years bp tephras from eruptions of el jabali 3840 14c years bp jorullo 1759 1774 and paricutin 1943 1952 have been identified in lake cores these provide a means of correlating between basins and have the potential to provide a basis for understanding the volcanic history of this area and for dating a wider range of sediment sequences

experimental constraints on the effects of pressure and h 2 o on the fractional crystallization of high mg island arc basalt h2o undersaturated melting experiments of synthesized basalt sio2 50 7 wt mgo 8 3 wt mg 60 were conducted at fo2 corresponding to nno 1 and nno 1 to clarify the effects of pressure 2 7 kbar and h2o on fractional crystallization in island arcs h2o content was ranged from nominally anhydrous to 4 4 wt differentiation trends namely the liquid lines of descent change sensitively according to pressure h2o relations tholeiitic differentiation trends are reproduced with h2o 2 wt in primary magma with such quantities of h2o fractional crystallization is controlled by olivine plagioclase at 2 kbar increasing the pressure from 2 to 4 kbar induces early crystallization of orthopyroxene instead of olivine and therefore sio2 enrichment in the residual melts is suppressed increasing h2o 2 wt in primary magma stabilizes clinopyroxene relative to orthopyroxene and or magnetite although the phase relations and proportions strongly depend on fo2 and h2o content differentiation trends are always calc alkaline

structures and mechanisms of the northward propagating boreal summer intraseasonal oscillation abstract the spatial and temporal structures of the northward propagating boreal summer intraseasonal oscillation bsiso are revealed based on the analysis of both the echam4 model simulation and the ncep ncar reanalysis the bsiso structure and evolution characteristics simulated by the model bear many similarities to those derived from the ncep ncar reanalysis the most notable features are the remarkable meridional asymmetries relative to the bsiso convection in the vorticity and specific humidity fields a positive vorticity perturbation with an equivalent barotropic structure appears a few latitude degrees north of the convection center the maximum specific humidity also shows a clear northward shift in the lower troposphere two internal atmospheric dynamics mechanisms are proposed to understand the cause of the northward propagation of the bsiso the first is the vertical shear mechanism the key process associated with this mechanism is the generation of barotropic vorticity due to the couplin

mixed magma pyroclastic surge deposits associated with debris avalanche deposits at colima volcanoes mexico the transition between the terminal cones and the ancestral edifices of nevado de colima and fuego de colima volcanoes is marked by the deposits of gigantic volcanic debris avalanches of the mount st helens msh or bezymianny type unusual mafic juvenile fragments and cauliflower bombs as well as juvenile fragments of mixed and more evolved composition are abundant in dune bedded pyroclastic surge deposits directly associated with these catastrophic events at both volcanoes at nevado these mafic juvenile fragments represent the most primitive magma ever erupted by the volcano sio2 52 50 the lavas directly preceding and following the debris avalanche event are silicic andesites sio2 59 at fuego these juvenile fregments have 56 sio2 the lavas from the upper parts of the caldera wall are dacites 65 sio2 whereas the terminal cone is composed of andesites 57 to 62 sio2 at nevado petrologic evidence for interaction of mafic magma with andesitic or dacitic magma in a high level magma chamber just before the eruption of pyroclastic surge deposits consists of 1 banded juvenile bombs of intermediate composition 2 the range of composition of these bombs from sio2 52 to 58 3 the presence of highly magnesian olivine with reaction rims 4 inverse zoning in clinopyroxene with strong mg enrichment towards the rim 5 resorption of plagioclase and 6 significant compositional heterogeneity in the vitric phase volcanic debris avalanche events at nevado and fuego de colima may thus correspond with major breaks in the petrological evolution of the volcanoes and the start of a new magmatic cycle injection of mafic magma into the presently perched viscous surface dome of the active fuego cone as occurred in 1818 and 1913 could enhance the likelihood of southward collapse of the flank of an already unstable edifice and it must be considered in future hazard assessment of this active volcano risk to life and property for the entire colima region associated with such catastrophic phenomena would be immeasurably greater in comparison with hazards related to the last explosive outburst in 1913 which resulted in emplacement of pyroclastic flows over uninhabited areas of the upper flanks of the volcano

pressure temperature dependent compositional variation of phlogopitic micas in upper mantle peridotites compositional variation of phlogopitic micas in upper mantle peridotites is reviewed phlogopitic micas in garnet peridotites are systematically lower in al or eastonite component than those in spinel peridotites the core of phlogopite megacryst and phenocryst of kimberlite is always lower in al than the rim it is apparent that al al si ratio or eastonite component in phlogopitic micas in ultramafic rocks is controlled by the equilibrium pressure and temperature in the upper mantle peridotites containing garnet or spinel the al al si ratio of phlogopitic mica decreases with increasing pressure at constant temperature phlogopitic mica is a potential thermo barometer in both garnet and spinel peridotite facies

distribution of deep sea black shales of cretaceous age in the eastern equatorial atlantic from seismic profiling abstract the presence of thick 200 m carbonaceous shales of cretaceous age in the deep waters of the eastern equatorial atlantic was first revealed by drilling at dsdp sites 367 and 368 near the cape verde islands more recent seismic investigations show that black shales are widely distributed in this region in the gambia basin they are associated with a group of seismic reflectors lying at depths of 400 600 m below the sea floor these can be traced southwards from the drill sites into the sierra leone basin and eastwards beneath the african continental rise to depths at which ambient temperatures fall within the oil generation window in the gambia basin the reflectors continue westwards from the cape verde archipelago before terminating on turonian oceanic basement 92 ma south of the guinea fracture zone near 10 n extension of the same seismic sequence on to younger oceanic crust indicates that deep sea carbonaceous deposits formed close to the present equator as late as the early campanian 80 ma long after a series of oceanic anoxic events in the aptian albian oae1 and over the cenomanian turonian boundary oae2 led to widespread accumulation of black shales this late period of deposition may correspond to the more locally developed oae3 high surface productivity in equatorial upwelling zones during the late cretaceous probably played an important role in delivering large amounts of organic carbon to the sea floor when meridional movement of deep water between the south and north atlantic was restricted by closely spaced east west fracture zones in some areas seismic data show that the black shales level out rather than drape pre existing topography implying significant downslope transport of organic rich material from upwelling cells near the african margin

primary magmas at the volcanic front of the ne japan arc coeval eruption of crustal low k tholeiitic and mantle derived medium k calc alkaline basalts at azuma volcano it has long been thought that the low k tholeiitic th and medium k calc alkaline ca lavas in the nejapan arc were produced by fractional crystallization from mantle derived basalt magmas and that the latter formed from mixing of mafic and felsic magmas both derived from a common primaryth basalt through fractionation an alternative view was recently proposed on the basis of sr isotope microanalysis of plagioclase phenocrysts from the zao volcano suggesting that 1 the low k th basaltic andesites formed by melting of lower crustal amphibolite and that 2 the medium k ca basalts to andesites formed by mixing of mantle derived basalt and crustal th melts to investigate further the origin of the primary low k th and medium k ca basalts we investigated basalts and andesites from azuma volcano azuma is a quaternary eruption center at the volcanic front in the ne japan arc that has erupted two types of basalt 1 radiogenic sr sr sr1 4 0 7058 0 7062 low k th basalt lavas without evidence of magma mixing and assimilation 2 unradiogenic sr sr sr1 4 0 7039 0 7041 medium k ca basalt lavas with subtle evidence for magma mixing associated intermediate lavas are voluminous and are all 3 mildly radiogenic sr sr sr1 4 0 7044 0 7055 medium k andesites all of which have ca affinities with evidence for rigorous magma mixing but no crustal assimilation the low k th basalt has an isotopic composition similar to that of crustal granitoids beneath azuma and has a composition indicating that it potentially formed from a high degree lower crustal amphibolite melt the medium kca basalt has a basaltic groundmass with mg rich olivine fo89 and calcic plagioclase phenocrysts an90 and the most unradiogenic sr sr sr1 4 0 7037 0 7038 suggesting that it originated from a primary mantle melt major and trace element microanalysis of the basaltic groundmass indicates that the primary magma composition is close to high k we conclude that the mantle derived basalt at azuma is the result of a highto medium k magma that was later mixed with a low k th basalt melt from the amphibolitic lower crust to form medium k ca basalts and andesites this supports the view of a lower crustal origin of the low kth basalts and simultaneously requires a reappraisal of the origin of the across arc variation in k contents of the mantle derived primary arc basalts as the highto medium k ca basalt is geochemically fairly similar to the high k rear arc basalt in the ne japan arc

strong modulations on the bay of bengal monsoon onset vortex by the first northward propagating intra seasonal oscillation monsoon onset vortex ov in the form of tropical cyclone is often observed in the pre monsoon period and contributes to the subsequent abrupt establishment of summer monsoon over the bay of bengal bob it is identified here that all historical ovs occurred during the convection enhanced phase of the first northward propagating intra seasonal oscillation fniso the individual contributions from the four large scale environmental fields associated with the intra seasonal variations to the cyclone genesis are diagnosed with the aid of the genesis potential index the significant moistening of mid level atmosphere which is embedded in the fniso convection enhanced phase is shown to be the primary factor leading to the cyclone genesis the water vapor budget analysis is further done to understand the governing process for the mid level humidity increase it is clearly seen that the vertical advection process dominated by the anomalous vertical advection of the mean vertical water vapor gradient plays the critical role hence the ovs are shown to be strongly modulated by fnisos both of which are important elements of the complex story of the bob monsoon onset

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

the structure of the guinean continental margin implications for the connection between the central and the south atlantic oceans the results of a recent geological geophysical survey conducted off guinea are combined with previous data to establish a preliminary stratigraphy and provide a structural sketch of this portion of the west african continental margin three sectors are distinguished a northwestern portion of the margin comprises a wide and deeply submerged plateau the guinean marginal plateau underlain by a thick sedimentary sequence and facing westward toward the gambia abyssal plain scismic stratigraphy and structures show clear analogies to the jurassic margins of the central atlantic including the presence of a cretaceous paleoslope covered by cenozoic deposits a southern area of the margin comprises a series of aligned w e trending acoustic basement features extending along the slope and bounding the guinean plateau to the south these features basement ridges and volcanic piles are related to a fracture zone system also documented by magnetic anomalies and gravity data the bordering deep sierra leone abyssal plain also dissected by e w trending oceanic fracture zones contains a sedimentary cover apparently not older than middle cretaceous between both sectors and between two nw se trending scarps lies an intermediate area the seismic profiles show that here the margin is dissected by faults creating a series of asymmetric horst and graben features progressively narrowing towards the s e and covered by untectonized but partly eroded upper cretaceous to cenozoic sediments

boreal summer quasi monthly oscillation in the global tropics the boreal summer intraseasonal oscillation iso in the global tropics is documented here using a 7 year suite 1998 2004 of satellite measurements a composite scenario was made of 28 selected events with reference to the oscillation in the eastern equatorial indian ocean eio where the oscillation is most regular and its intensity is indicative of the strength of the subsequent northward propagation the average oscillation period is about 32 days and this quasi monthly oscillation qmo is primarily confined to the tropical indian and pacific oceans topics that were investigated are the partition of convective versus stratiform clouds the vertical structure of precipitation rates and the evolution of cloud types during the initial organization and the development of intraseasonal convective anomalies in the central indian ocean during the initiation of the convective anomalies the stratiform and convective rains have comparable rates the prevailing cloud type experiences a trimodal evolution from shallow to deep convection and finally to anvil and extended stratiform clouds a major northwest southeast slanted rainband forms as the equatorial rainfall anomalies reach sumatra and the rainband subsequently propagates northeastward into the west pacific ocean the enhanced precipitation in the west pacific then rapidly traverses the pacific along the intertropical convergence zone meanwhile migrating northward to the philippine sea a seesaw teleconnection in rainfall anomalies is found between the southern bay of bengal 5 15 n 80 100 e and the eastern pacific 5 15 n 85 105 w local sea surface temperature sst rainfall anomalies display a negative simultaneous correlation in the off equatorial regions but a zero correlation quadrature phase relationship near the equator we propose that atmosphere ocean interaction and the vertical monsoon easterly shear are important contributors to the northeastward propagation component of the intraseasonal rainband the observed evidence presented here provides critical information for validating the numerical models and it supports the self induction mechanism theory for maintenance of the boreal summer iso

particle sizes of andesitic ash fallout from vertical eruptions and co pyroclastic flow clouds volcan de colima mexico we report particle size distributions for andesitic ash fall from vertical eruptions and clouds above block and ash fl ows at volcan de colima mexico during 2004 2006 we analyzed 17 samples using laser diffraction 11 from vertical eruptions and 6 from block and ash fl ows copyroclastic fl ow ash vertical eruptions produce well sorted fall deposits whereas co pyroclastic fall deposits are poorly sorted with high proportions of very fi ne grained ash 30 m statistical analysis shows particle size distributions of vertical eruptions are more leptokurtic peaked than co pyroclastic samples deconvolution of grain size histograms shows that copyroclastic samples have at least one subpopulation with a mode of 8 3 8 7 2 4 3 1 m estimates of the number of particles in different size ranges show that co pyroclastic samples contain much greater numbers of very fi ne particles than vertical eruption samples our results provide no direct evidence that milling or comminution produces hazardous fi ne ash particles in pyroclastic fl ows but are consistent with that interpretation

elevated primary productivity of calcareous nannoplankton associated with ocean anoxic event 1b during the aptian albian transition early cretaceous 1 previous isotopic investigations of aptian albian oceanic anoxic event oae 1b from the western north atlantic blake nose posited that increased sea surface temperatures and decreased salinity led to stratification of the upper water column resulting in lowered dissolved oxygen and enhanced organic matter preservation we examined calcareous nannofossils from the same site in the western north atlantic blake nose to evaluate changes in surface water conditions prior to during and after the aptian albian oae1b the results of our analysis conflict somewhat with the previous interpretation that oae1b at blake nose was linked to water column stratification the signal from calcareous nannofossils indicates that in the late aptian prior to oae1b oligotrophic conditions in the surface waters were replaced by mesotrophic conditions that persisted throughout the oae event and into the early albian we speculate that increased surface water productivity facilitated to some degree the development of oae1b at blake nose

northward propagation mechanisms of the boreal summer intraseasonal oscillation in the era interim and sp ccsm abstractmechanisms for the northward propagation np of the boreal summer intraseasonal oscillation bsiso and associated asian summer monsoon asm are investigated using data from the interim ecmwf re analysis era interim herein called erai and the superparameterized community climate system model sp ccsm analyzed mechanisms are 1 destabilization of the lower troposphere by sea surface temperature anomalies 2 boundary layer moisture advection and boundary layer convergence associated with 3 sst gradients and 4 barotropic vorticity anomalies mechanism indices are regressed onto filtered olr anomaly time series to study their relationships to the intraseasonal oscillation iso and to equatorial rossby er waves northward propagation in erai and sp ccsm is promoted by several mechanisms but is dominated by boundary layer moisture advection and the barotropic vorticity effect sst linked mechanisms are of secondary importance but are nonnegligible the magnitudes of np mechanisms vary from

the boreal summer intraseasonal oscillation relationship between northward and eastward movement of convection the summertime intraseasonal oscillation iso is an important component of the south asian monsoon lagged regressions of intraseasonally filtered 25 80 days outgoing longwave radiation olr reveal that centers of convection move both northward and eastward from the central equatorial indian ocean subsequent to the initiation of an iso eastward movement of convection is also seen at indian subcontinent latitudes 10 8 208n based on the regression results the summertime iso convection signal appears as a band tilting northwestward with latitude and stretching from the equator to about 208n viewed along any meridian convection appears to propagate northward while equatorial convection propagates to the east to examine the robustness of the connection between eastward and northward movement individual isos are categorized and composited relative to the strength of the large scale eastward component of convection in the central equatorial indian ocean it is found that the majority of isos that exhibit northward movement onto the indian subcontinent 42 out of 54 isos or 78 also exhibit eastward movement into the western pacific ocean it is also found that when convection in the central indian ocean is not followed within 10 20 days by convection in the western pacific ocean 12 out of 54 isos or 22 the independent northward movement of convection in the indian ocean region is somewhat stunted the link between the eastward and northward movement of convection is consistent with an interpretation of the summertime iso in terms of propagating equatorial modes the northward moving portion of convection is forced by surface frictional convergence into the low pressure center of the rossby cell that is excited by equatorial iso convection a similar convergence pattern is seen for the northern winter iso but it does not generate poleward movement due to relatively cool ssts underlying the surface convergence

dynamics of breaks in the indian summer monsoon abstract in this paper the authors present results of diagnostic analysis of observations and complementary experiments with a simple numerical model that enable them to synthesize the morphology and dynamics of breaks in the indian summer monsoon ism almost one week ahead of the onset of a break spell over india a monotonically decreasing trend in convective activity is found to occur over the bay of bengal in response to a steady eastward spreading of dry convectively stable anomalies from the equatorial indian ocean a major intensification of the convectively stable anomalies over the bay of bengal is seen about 2 3 days prior to commencement of a monsoon break both observations and modeling experiments reveal that rapid northwest propagating rossby waves are triggered in response to such a large strengthening of the convectively stable anomalies it is shown that an abrupt movement of anomalous rossby waves from the bay of bengal into northwest and central india marks the initiation of a break

the guinea continental margin an example of a structurally complex transform margin abstract the guinean continental margin corresponds to a wide hinge zone which developed between the jurassic central atlantic and the cretaceous equatorial atlantic the guinea marginal plateau that characterizes the southern border of this margin is a wide triangular morphological feature bounded to the west by a typical rifted margin slope segment while its southern edge corresponds to a narrow and morphologically complex continental slope probably resulting from intracontinental transcurrent motion during the early cretaceous on the basis of seismic profiles recorded in two selected areas located along the southern guinea margin a seismic stratigraphy is established at depth the plateau is underlain by a thick mesozoic and cenozoic sedimentary wedge made of two main ensembles a lower ensemble or sequence 1 consists of early cretaceous clastics an upper ensemble includes a series of four distinct sequences attributed to albian to tertiary deposits eastward the sedimentary cover of the southern plateau edge is locally pierced by volcanic bodies of paleocene age beneath the southern margin of the guinea plateau the sedimentary sequences appear deformed by faults and folds resulting from at least two main tectonic episodes a first event only affected sequence 1 which is cut by a set of extensional faults a second tectonic phase including folding reverse faulting and transcurrent faulting is responsible for the deformation of sequences 1 and 2 during this tectonic episode former normal faults have been reactivated as reverse faults resulting in a structural inversion a third tectonic phase was responsible for structures transverse to the general e w trend of the southern guinea margin and consisting of n s trending normal faults and associated volcanoes the spatial distribution of the tectonic trends clearly substantiates a polyphase tectonic activity that can be related to plate motion changes during early stages of the continental separation a first stage is characterized by a divergent rifting leading to n s trending synsedimentary normal faults during the cretaceous the tectonic activity increased as a general transform motion occurred between the guinea plateau and its american conjugate margin the demerara plateau local tensional stresses were responsible for a splay fault system active near the tip of the guinea plateau in late albian times a slight adjustment of the africa plate with respect to the south american one resulted in local squeezing of the whole sedimentary wedge after this short compressive event final continental separation occurred in an extensional regime characterized by a general collapse along two major scarps and volcanic activity

characterization of low temperature pyroclastic surges that occurred in the northeastern japan arc during the late 19th century abstract three eruption events occurring in the central part of the northeastern japan arc were investigated and compared adatara ad1900 zao ad1895 and bandai ad1888 producing low temperature lt pyroclastic surges these events are characterized by steam eruptions ejecting no juvenile material these eruptions well preserved eruptive deposits and facies facilitated granulometric analyses of the beds which revealed the transport and deposition mechanisms of lt surges combining these results with those of investigations of documents reporting the events we correlated each eruption to the relevant individual bed and reconstructed the lt surge development sequence important findings related to the transport and deposition modes are the following 1 bed sets consisting of thin laminated ash and its overlying thick massive tuff were recognized in the adatara 1900 proximal deposits the bed set was probably produced by a strong wind that discharged and propagated quickly from the vent leading wind and a gravitationally segregated highly concentrated flow originated from the eruption column within a discrete eruption episode a similar combination might have occurred during the first surge of the bandai 1888 event 2 comparison of the proximal and distal facies for the largest eruption of adatara 1900 event indicates that the initial turbulence of the eruption cloud decreased rapidly transforming into a density stratified surge with a highly concentrated part near the base similar surges occurred in the climatic stage of zao 1895 3 bandai 1888 ejecta indicate massive beds deposited preferentially at topographic lows co occurring planar beds showed no topographic affection as indicated by the topographic blocking of a stratified surge the observed facies massive tuffs crudely stratified tuffs and thin bedded tuffs are compatible with those for high temperature surges at bandai absence of dune bedded tuffs and commonly poorer sorting in the lt surge deposits might be attributable to poor thermally induced turbulence of eruption columns condensation of vapor in the surges might have contributed to the poor sorting the estimated explosion energies were 6 10 13 j for adatara ad1900 6 5 10 10 j for zao ad1895 and 6 5 10 15 j for bandai ad1888 implying that the three events were hydrothermal eruptions with distinctive eruptive mechanisms regarding eruption sources the adatara 1900 event was caused solely by thermal energy of the hydrothermal fluid although magma intrusion likely triggered evolution of hydrothermal systems at zao in 1895 steam eruptions in the bandai 1888 event occurred simultaneously with sudden exposure of the hydrothermal system whose triggers require no internal energy

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

sector collapse of the sw flank of volcan de colima mexico the 3600 yr bp la lumbre los ganchos debris avalanche and associated debris flows abstract some 3600 years ago the sw flank of volcan de colima collapsed generating a debris avalanche that travelled 25 km to the armeria river and stopped against the cerro grande limestone escarpment the debris avalanche deposit covers an area of 48 km2 has an average thickness of 35 m and a volume of 1 7 km3 with an h l ratio of 0 13 along the la lumbre ravine the deposit forms a series of flat terraces up to 65 m thick a pyroclastic flow deposit overlies the debris avalanche deposit suggesting that the colima collapse was accompanied by a magmatic component the debris avalanche deposit dammed the armeria river forming a temporary lake that accumulated a water volume of 0 19 km3 the subsequent breaking of the dam formed a debris flow that travelled at least 20 km along the armeria river emplacing 15 70 m thick deposits the debris flow deposits cover a surface of 6 4 km2 and have a volume of 0 2 km3 the avalanche deposit contains fragments of carbonized wood that where dated at 3600 120 yr bp by komorowski et al 1997 this deposit underlies a younger debris avalanche 2505 45 yr bp and a debris flow deposit with pottery shards and charcoal dated at 1940 90 yr bp these pottery shards likely related to the capacha people 820 300 yr bc indicating that the region has been inhabited during the last 3000 years a future collapse of volcan de colima in the same direction may affect several towns for a total population of circa 40 000

origin of silicic magma and magma feeding system of the shirataka volcano ne japan abstract stratovolcanoes in ne japan produce mixed andesites dacites with a silicic end member the petrologic features of this end member remained uncertain and its origin has long been debated here we suggest how the silicic magma is generated and interpret the magma feeding system for one such stratovolcano the shirataka 0 9 0 7 ma the rocks of the shirataka are divided into six petrologic groups g1 6 mafic inclusions are observed in g1 g2 g5 and g6 silica poor inclusions have tschermakitic hornblende in the groundmass while the others are hornblende free the detailed petrologic study revealed that all products were formed by magma mixing mingling between mafic and silicic end members mafic end members 1150 c 47 52 wt sio2 have magnesian olivine magnesian clinopyroxene and an rich plagioclase phenocrysts whereas silicic end members 750 900 c 63 68 wt sio2 have an poor plagioclase low mg pyroxenes hornblende and quartz the phenocryst compositions an mg and temperature of silicic end members have changed among the groups over time the trends defined by such mafic and silicic end members of each group are systematically parallel except for g3 and g4 which have distinct specific bulk compositions suggesting that the two end members for each group have co genetic relationships the trace element model calculations indicate that each estimated silicic end member can be produced through partial remelting of the corresponding solidified mafic end member leaving a gabbroic residue the presence of groundmass hornblende in silica poor mafic inclusions suggests that volatile rich mafic magmas ascended settled and solidified as gabbroic bodies in the lower or middle crust silicic magmas were then produced by partial remelting of these bodies and ascended to the shallower part of the crust to form chambers leaving the gabbro finally wet mafic magmas newly supplied along the marginal part of the gabbroic bodies and injected into the chambers triggered the eruption and the mixing mingling took place during ascent to the surface two distinct silicic magmas were simultaneously formed by this mechanism and they mixed with corresponding mafic magmas in g3 and g4 giving rise to two discrete mixed magmas finally these magmas erupted contemporaneously mingling with each other and forming pyroclastic flows

mechanism for northward propagation of boreal summer intraseasonal oscillation convective momentum transport 1 this study demonstrates that the momentum transport by cumulus convection plays a significant role in the organization and northward propagation of intraseasonal iso convection anomalies over the indian and western pacific regions during boreal summer a version of seoul national university s atmosphere ocean coupled general circulation model simulates northward propagation when convective momentum transport cmt is implemented the northward propagation disappears when cmt is disabled an axially symmetric shallowwatermodel withaparameterized cmtis used to understand the role of cmt in the northward propagation of iso the basic mechanism of northward propagation is the lower level convergence to the north of convection which is induced by the secondary meridional circulation associated with large momentum mixing by convection in the region of large mean vertical shear a large meanverticalshearexistsinsouthasianregionduringboreal summer citation kang i s d kim and j s kug 2010 mechanism for northward propagation of boreal summer intraseasonal oscillation convective momentum transport geophys res lett 37 l24804 doi 10 1029 2010gl045072

the 30 50 day mode at 850 mb during monex abstract the analyzed wind field at 850 mb during the summer monsoon experiment monex is subjected to a time series analysis to confirm the existence of a peak in the time range of 30 50 days having established that this study presents a mapping of the motion field for this time scale in this short paper we illustrate the steady meridional propagation of a train of troughs and ridges that seem to form near the equator and dissipate near the himalayas the meridional scale of this mode is around 300 km and its meridional speed of propagation is 0 75 latitude per day the amplitude of the wind for this mode is around 3 6 m s 1 the salient contributions here are the mapping and the demonstration of a very regular behavior of this mode its existence is here noted from a period well before the onset of monsoon i e from early may to late july three major storms during monex were noted to form within the trough line of this system and the period of major cessation of rains over the indian subconti

the mt edgecumbe volcanic field alaska an example of tholeiitic and calc alkaline volcanism major element trace element and isotopic composition data for lava samples have been collected from a holocene stratovolcano in southeastern alaska results indicate that a wide variation in chemical compositions exist 48 71 sio 2 10 14 16 mgo and the suite may be divided into two series on the basis of chemical mineralogical and isotopic characteristics the tholeiitic series lavas consist of generally phenocryst poor olivine basalts whose chemical and isotopic compositions are transitional between normal morb and lree enriched lavas of atypical areas such as the azores platform cao is high mgo feo 1 fe 2 o 3 k 2 o rb and sr are low while mgo and ni are high data show that the basalts are lree depleted and sr 87 sr 86 70304 the basalts compose 65 of the volcanic field and crystal fractionation appears to have produced the compositions of the more silicic basalts the calc alkaline series lavas are composed of basaltic andesites andesites and dacites of

the 30 60 day convection seesaw between the tropical indian and western pacific oceans abstract the tropical indian and western pacific oceans are two prominent action centers for tropical 30 60 day convective variability when convection is enhanced over the equatorial indian ocean the tropical western pacific often experiences an abnormal dry condition phase i whereas the development of the convection over the tropical western pacific tends to be accompanied by suppressed convection in the equatorial indian ocean phase ii this convection seesaw is a fundamental characteristic of the tropical 30 60 day oscillation the seesaw is intimately associated with the activity of propagating low frequency convective systems lfcss its formation process is season dependent typical boreal summer seesaw results from a time lagged development of two systems a western system that originates in the equatorial indian ocean and moves eastward and or northward and an eastern system that develops in the western pacific monsoon region and moves westward and or northward the boreal winter seesaw o

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

mechanism of the northward propagating intraseasonal oscillation insights from a zonally symmetric model abstract the propagation and initiation mechanisms of the boreal summer intraseasonal oscillation bsiso in the south asian summer monsoon are examined with a zonally symmetric atmospheric model in the axially symmetric model the effects of zonally propagating atmospheric waves are intentionally excluded the model specifies mean flows and depicts the lowest baroclinic mode and a barotropic mode in the free troposphere the two vertical modes are coupled by the time mean vertical wind shear the model atmosphere produces a 15 20 day oscillation which is characterized by northward propagation of convection from south of the equator to the indian monsoon trough region and a reinitiation of convection in the region between 10 s and the equator the northward propagation in the model is produced by the free troposphere barotropic divergence which leads convection by about a quarter of a cycle the vertical advection of summer mean easterly vertical wind shear by perturbation vertical motion inside the con

a model for the boreal summer intraseasonal oscillation abstract the tropical intraseasonal oscillation iso exhibits pronounced seasonality the boreal summer iso is more complex than its winter counterpart due to the coexistence of equatorial eastward off equatorial westward and northward propagating low frequency modes and their interactions based on observational evidence and results obtained from numerical experiments a mechanism is proposed for the boreal summer iso in which the northern hemisphere summer monsoon nhsm circulation and moist static energy distribution play essential roles with a climatological july mean basic state the life cycle of model low frequency waves consists of four processes an equatorial eastward propagation of a coupled kelvin rossby wave packet an emanation of moist rossby waves in the western pacific a westward propagation and amplification of the rossby waves in south asian monsoon regions and a reinitiation of the equatorial disturbances over the central indian ocean the life cycle spans about one month and p

tectonics and volcanism of sierra chichinautzin extension at the front of the central trans mexican volcanic belt abstract because of its recent activity and position at the southern magmatic front of the trans mexican volcanic belt tmvb the sierra chichinautzin volcanic field scn is a key area for the understanding of this controversial volcanic province volcanic activity has built more than 220 monogenetic volcanoes shields scoria cones thick lava flows and hydromagmatic structures during the last 40 000 years for a total volume of about 470 km 3 the scn basalts are geochemically similar to oibs while the intermediate and felsic volcanic rocks show a calc alkaline trend and abundant evidence for magma mixing the structural analysis of this volcanic field and surrounding areas has been based on field data satellite images and a method for detecting volcanic center alignments the tectonic data together with geophysical evidence confirm active general n s extensional conditions with a strike slip component for the scn area the same structural setting that prevails in the rest of the central tmvb extensional tectonics a negative regional bouger gravity anomaly a low velocity mantle high heat flow and shallow seismicity suggest a rift type setting involving the upwelling of anomalous mantle beneath the central tmvb the combined petrological structural and geophysical arguments support that the scn volcanism is rift related and rule out processes involving the subduction of the cocos plate which casts further doubts on the standard subduction model for the tmvb volcanism

biweekly and 21 30 day variations of the subtropical summer monsoon rainfall over the lower reach of the yangtze river basin abstract the lower reach of the yangtze river basin lyrb is located at the central region of the mei yu and baiu front which represents the subtropical east asian ea summer monsoon based on the newly released daily rainfall data two dominant intraseasonal variation isv modes are identified over the lyrb during boreal summer may august with spectral peaks occurring on day 15 the biweekly mode and day 24 the 21 30 day mode these two modes have comparable intensities and together they account for above about 57 of the total intraseasonal variance both isv modes exhibit baroclinic structures over the lyrb at their extreme phases however the genesis and evolutions associated with the two modes are different considering the genesis of their extreme wet phases over the lyrb the biweekly mode is initiated by a midlatitude jet stream vorticity anomaly moving southeastward while the 21 30 day mode is primarily associated with a low level westward propagation of an anticyclonic anomaly from

biogeography and diversity of south atlantic cretaceous echinoids implications for circulation patterns abstract this survey of echinoid species living on the african and south american margins of the south atlantic ocean in cretaceous times is based on a review of published species and abundant unpublished material the synthetic approach is used to ascertain how the south atlantic was colonised by echinoids from the time it first opened in the early cretaceous until communications were established with the north atlantic and mediterranean tethys in the late cretaceous the survey focuses on the biogeographical origin of immigrant species and their relations with indo madagascan north atlantic and mediterranean tethyan faunas qualitative variations in species diversity are also studied where available fossil material permits together with the in situ evolution of certain lineages and their endemism finally echinoid colonisation of the south atlantic during the cretaceous is viewed against palaeocurrent patterns the survey shows within the limits of material available that no echinoid fauna attests to the existence of the narrow marine trough separating south america and africa in the barremian in the aptian west african echinoids display a mediterranean tethyan influence indicating a probable link between south east and west of africa via the strait opened between africa and madagascar echinoid fauna on the brazilian margin includes species found in what is now continental north america and on the pacific seaboard of south america during the albian south america separated completely from africa and normal marine communications were established with the western tethys to the north and the southern ocean domain to the south the echinoid fauna of the south atlantic during the albian and early cenomanian was particularly rich and diversified with the earliest ubiquitous species on both margins of the ocean in the late cenomanian early turonian a tethys south atlantic ocean link probably formed in west africa through the sahara and nigeria favouring the influx of mediterranean tethyan species to the south atlantic during the mid turonian coniacian the trans saharan channel closed slowing down colonisation of the south atlantic by mediterranean tethyan echinoids as the only connection was via the central atlantic this period was marked by the stability of a number of lineages that survived without major morphological changes in areas colonised during cenomanian times in the late senonian the south atlantic was largely open to the neighbouring ocean domains and a new trans saharan channel linked the mediterranean tethys and south atlantic directly this allowed substantial faunal exchanges and most senonian echinoids found on the south atlantic rim are common to both its eastern and western margins