erosion by pyroclastic flows on lascar volcano chile pyroclastic flows generated in the 19 20 april 1993 eruption of lascar volcano chile produced spectacular erosion features scree and talus were stripped from the channels and steep slopes on the flanks of the volcano exposed bedrock and boulders suffered severe abrasion producing smoothed surfaces on coarse breccias and striations and percussion marks on bedrock and large boulders erosional furrows developed with wavelengths of 0 5 2 m and depths of 0 1 0 3 m furrows commonly nucleated downstream of large boulders or blocks which are striated on the upstream side and thereby produced crag and tail structures erosive features were produced where flows accelerated through topographic restrictions or where they moved over steep slopes the pyroclastic flows are inferred to have segregated during movement into lithic rich and pumice rich parts lithic rich deposits occur on slopes up to 14 whereas pumice rich deposits occur only on slopes less than 4 and mainly at the margins and distal parts of the 1993 fan the lithic rich deposits contain large up to 1 m lithic clasts eroded from the substrate and transported from the vent whereas pumice rich deposits contain only small typically 2 cm lithic clasts these observations suggest that lithic clasts segregated to the base of the flows and were responsible for much of the erosive phenomena the erosive features distribution of lithic clasts and deposit morphology indicate that the 1993 flows were highly concentrated avalanches dominated by particle interactions in some places the flows slid over the bedrock causing abrasion and long striations which imply that large blocks were locked in fixed positions for periods of about 1 s however shorter striae at different angles impact marks segregation of the deposits into pumice and lithic rich parts and mixing of bedrock derived lithic clasts throughout the deposits indicate that clasts often had some freedom of movement and that jostling of particles allowed internal mixing and density segregation to occur within the flows

correlation of eruptive products volcan azufral colombia implications for rapid emplacement of domes and pyroclastic flow units abstract the eruptive history and morphology of azufral volcano colombia is explored and analyzed to provide a more complete picture of past eruptions as well as to infer what eruption styles may occur in the future through the use of principal component analysis on fe ti oxides domes can be correlated to the pyroclastic deposits enabling the identification of a full eruptive sequence the findings suggest that eruptive activity at azufral volcano is largely explosive experiencing long periods of quiescence punctuated by short periods of pyroclastic activity and volcanic debris avalanches geomorphology of the dome complex is reinterpreted to better understand the sequence of dome growth this reinterpretation along with geochemical analysis and comparison via pca allows for reclassification of a major deposit originally thought to be a juvenile block and ash flow as a volcanic debris avalanche

transport capacity of pyroclastic density currents experiments and models of substrate flow interaction 1 one of the more distinctive features of many ignimbrites is the presence of large lithics some greater than meter scale and pumices that have been transported great distances 10 km from the eruptive vent sometimes over steep terrain and expanses of water in many cases these particles have been transported much further than can be explained by aerodynamic forces and ballistic trajectories we examine the forces responsible for transport of large clasts and examine in detail the momentum transfer occurring when particles interact with their boundaries we performed a suite of experiments and numerical simulations to quantify the mass and momentum transfer that occurs when particles interact with a pumice bed substrate and with water substrate two geologically motivated flow end members we find that clasts transported in dilute currents are particularly sensitive to the nature of the boundary and while large particles can skip several times on a water substrate they travel less far than particles that impact pumice bed substrates all else being equal large particles in dense pyroclastic density currents are themselves relatively insensitive to the details of their boundaries however one of the most important ways boundary conditions influence large particles is not through direct interaction but by changing the local concentration of fine particles momentum transfer from fine particles to large particles appears to be required to transport large clasts great distances if initially dense flows become dilute during transport then the transport capacity of large particles in the flow is substantially decreased

latest pleistocene and holocene geomagnetic paleointensity on hawaii geomagnetic paleointensity determinations from radiocarbon dated lava flows on the island of hawaii provide an estimate of broad trends in paleointensity for holocene time and offer a glimpse of intensity variations near the end of the last glacial period when the data from hawaii are compared with others worldwide the intensity of the geomagnetic field seems to have been reduced from the holocene average by about 35 percent between 45 000 and 10 000 years ago a long term reduction of this magnitude is compatible with reported increases in the production rate of cosmogenic nuclides during the same interval

broad trends in geomagnetic paleointensity on hawaii during holocene time paleointensity determinations have been obtained from 22 basaltic lava flows on the island of hawaii using the thelliers method radiocarbon dating indicates that these flows erupted at intervals ranging from about 200 to 1000 years and results of the experiments provide an estimate of broad trends in geomagnetic paleointensity during holocene time in the vicinity of hawaii most of the samples were obtained from quickly cooled flow margins and as a consequence typically contain two titanomagnetite populations only two of the 79 samples analyzed failed to yield an estimate of the paleofield demonstrating that paleointensities can be obtained from such samples if carefully selected virtual dipole moments calculated for the flows are compared with a published curve of dipole field intensity that was constructed using worldwide archeomagnetic data the large nondipole fields that were previously postulated for the vicinity of hawaii are confirmed and the present data indicate that they were present from about 5000 years b p to perhaps as recently as the past 200 years the data indicate however that these nondipole sources must have been virtually absent between about 12 000 and 5000 years b p as they are at the present time

relative changes of the geomagnetic field intensity during the last 280 kyear from piston cores in the acores area abstract relative changes in geomagnetic field intensity over the last 280 kyears have been recovered from the study of three marine cores from the acores area one core was precisely dated by oxygen isotope study and the other two records were linked to it using light reflectance analysis which allowed precise correlation rock magnetic analysis shows that the main magnetic mineral is magnetite with a very homogeneous grain size distribution in the pseudo single domain range for the three cores changes in the amount of magnetite do not exceed a factor of 10 therefore these cores appear to be suitable for relative palaeointensity determinations two mineral magnetic components with periodicities of 23 and 18 kyears are present in the records of bulk magnetic parameters and natural remanent magnetisation nrm but the power at these frequencies is not significant at the 95 level for the normalised remanence records for each core normalisation of the nrm using different normalising parameters yields virtually identical results using saturation isothermal remanent magnetisation sirm as the normalising parameter the results of the three cores were then combined into a stacked curve squared coherence analysis between this stacked curve and bulk mineral magnetic parameters reveals that the 18 kyear component is still present in the record but that the power spectrum is barely above noise level at this frequency some of the features of this north atlantic record are consistent with the main characteristics already documented in other sedimentary or volcanic records for instance distinct periods of low intensity are observed around 40 120 and 190 kyears and periods of high intensity at 50 and 80 kyears because of the uniformity of the mineral magnetic characteristics of the three cores we suggest that this record may be a suitable palaeointensity reference curve for the central north atlantic ocean region

in situ accretion mechanism of concavo convex lacustrine oncoids swallow nests from the oligocene of the mainz basin rhineland frg oligocene oncoids from the lacustrine su wasserschichten of the mainz basin exhibit a pustular surface and a characteristic cavity on their undersides which gave rise to their designation as swallow nests oncoids grew in situ under low energy conditions during a pause in sedimentation three features indicate that they were never overturned 1 oncoids can be placed in a stable position on a smooth surface 2 laminae which are mostly concentric around the entire oncoids display polar thickenings on the upper side during all growth stages of oncoids 3 the depression is always situated on the stable side of oncoids development of this cavity is favoured by unionid shells in the stable position serving as oncoid nuclei but occurs also under flat nuclei it is interpreted as a product of reduced growth of cyanobacteria due to oligophotic conditions on a fine grained substrate and is considered as diagnostic of the in situ growth of oncoids oncoid cortices are mainly formed by a scytonema like cyanobacterium whose carbonaceous organic remnants are occasionally preserved growth stages of individual oncoids were image processed and compared by means of shape parameters due to optimum growth conditions cyanobacteria try to occupy the entire available space hence oncoids tend to achieve a spherical form even during in situ growth renewed background sedimentation triggered the formation of pustular surfaces and finally caused the cessation of cyanobacterial growth

characteristics and consequences of lava dome collapse at ruawahia taupo volcanic zone new zealand the eruption of large rhyolitic lava domes may be accompanied by the formation of large block and ash flows this may be linked to the style of dome extrusion whether it forms a series of individual lobes flows or spines exogenous or grows by internal inflation endogenous lava domes can transition from one extrusive style to another as a result of a change in extrusion rate or the formation of facilitating structures such as shear zones how this change can affect large rhyolitic lava domes is unclear as there are few historically recorded rhyolitic dome eruptions here we present structures at ruawahia lava dome a well exposed 700 year old lava dome how these facilitating structures enable exogenous extrusion at ruawahia dome and link this to collapse episodes along the fringes of the dome during growth ruawahia dome is part of the tarawera dome complex a chain of domes running parallel to regional structures across the okataina caldera complex in the taupo volcanic zone new zealand ruawahia dome consists of 1 a high porosity 44 52 crystalline 65 dre locally brecciated carapace facies with rare bread crusting and ropey flow textures 2 a core facies of dominantly low to moderate porosity 20 25 with elongate vesicles that mark weak flow bands and 3 thin 5 m thick interior breccia zones flow bands at ruawahia are complex and do not fit with hypothesised flow band orientations attributed to a single phase of exogenous or endogenous dome growth inward dipping flow bands on ramp structures on the flow surface suggest a flow like coulee morphology however steeply dipping and multidirectional flow bands on the edges of the dome challenge this hypothesis widespread block and ash flow deposits have been sourced from the leading dome fronts to the nw and se these collapse events left behind inflated and bread crusted outcrops on these dome fronts suggesting syn eruption collapse events that led to expansion of a hot pressurised dome interior we consider ruawahia erupted from multiple aligned vents either as lobes confined within the crater of a pyroclastic cone formed during the initial plinian phase of the eruption or those able to flow down the cone flank the confined lobes formed steep internal breccia zones as individual dome lobes extruded past one another lobes that were able to overcome the pyroclastic cone rim or where the vent was outside the crater were able to flow down the flanks as bulldozing thickening flows with dominantly ductile interiors and brittle exteriors these flows collapsed as the front thickened possibly due to a decrease in gradient producing widespread block and ash flows the removal of lava associated with collapse generated a decompression event which resulted in fragmentation cracking and vesiculation in the hot interior of the lava flows these events left behind a re vesiculated and bread crusted lava flow front and produced block and ash flows with abundant breadcrust bombs that reached the base of tarawera

pseudotachylite on impact marks of block surfaces in block and ash flows at merapi volcano central java indonesia surfaces of meter sized blocks in the 1998 block and ash flow deposits of merapi volcano are partially covered by centimeter sized randomly orientated impact marks which consist of an outer glassy pseudotachylite underlain by a cataclastic layer whole rock pseudotachylite melt inclusion in plagioclase and host rock groundmass compositions indicate that the pseudotachylite was generated by remelting of bulk rock on block impact the occurrence and distribution of this new type of collision related pseudotachylite on volcanic block surfaces demonstrate that blocks were transported by chaotic rotation saltation and tumbling the random orientation of impact marks suggests grain flow as the dominant process rather than any other currently discussed pyroclastic flow mechanism in addition the chaotic orientation of striations is interpreted as reflecting momentum transfer having been dominated by short lived intergranular collisions the blocks have apparently been transported in the collisional regime of grain flow

prolonged carbonate diagenesis under an evolving late cenozoic climate nullarbor plain southern australia abstract the nullarbor plain in southern australia the largest areal karst on the globe is a 240 000 km2 uplifted succession of cenozoic marine carbonates whose surface has been exposed for 14 to 15 m y the middle miocene nullarbor limestone forms the upper surface of the plain and hosts a complex and prolonged record of meteoric diagenesis such a complete record offers unique insights into the effects of climate tectonics sea level topography and hydrology on the style and placement of numerous diagenetic events in flat low lying carbonate plains alteration took place during three broad phases comprising eight stages that are interpreted to have formed against a background of dramatic climate change middle miocene phase one diagenesis took place under a humid climate and resulted in rapid mineral equilibration calcite cementation extensive karst development and finally widespread lacustrine and palustrine sedimentation resultant palustrine sediments especially terrestrial ooids are now preserved at the surface and in underlying karst cavities latest middle miocene to middle pliocene phase two diagenesis occurred during a prolonged period 8 m y of temperate climate and resulted in initial deep cave dissolution during low sea levels and later shallow cave development in the course of a high sea level onset of a somewhat more arid climate in the latest pliocene led to the development of the modern desolate landscape of the plain this final phase of diagenesis involved creation of solution pits filled with black limestone pebbles open and closed dolines with associated colluvium fill and pervasive pedogenic calcrete the nullarbor plain demonstrates that low lying carbonate plains can have low surficial erosion rates precisely record relative sea level positions be able to have extensive caves with extended periods of arrested calcite precipitation and finally host extensive terrestrial ooid deposits the importance of this comprehensive paragenetic record is its applicability to not only recognize unconformities in the rock record but to better appreciate the climate in which they formed

the steens mountain oregon geomagnetic polarity transition 2 field intensity variations and discussion of reversal models we carried out an extensive paleointensity study of the 15 5 0 3 m y miocene reversed to normal polarity transition recorded in lava flows from steens mountain south central oregon one hundred eighty five samples from the collection whose paleodirectional study is reported by mankinen et al this issue were chosen for paleointensity investigations because of their low viscosity index high curie point and reversibility or near reversibility of the strong field magnetization curve versus temperature application of the thellier stepwise double heating method was very successful yielding 157 usable paleointensity estimates corresponding to 73 distinct lava flows after grouping successive lava flows that did not differ significantly in direction and intensity we obtained 51 distinguishable complete field vectors of which 10 are reversed 28 are transitional and 13 are normal the record is complex quite unlike that predicted by simple flooding or standing nondipole field models it begins with an estimated several thousand years of reversed polarity with an average intensity of 31 5 8 5 t about one third lower than the expected miocene intensity this difference is interpreted as a long term reduction of the dipole moment prior to the reversal when site directions and intensities are considered truly transitional directions and intensities appear almost at the same time at the beginning of the transition and they disappear simultaneously at the end of the reversal large deviations in declination occur during this approximately 4500 1000 year transition period that are compatible with roughly similar average magnitudes of zonal and nonzonal field components at the site the transitional intensity is generally low with an average of 10 9 4 9 t for directions more than 45 away from the dipole field and a minimum of about 5 t the root mean square of the three field components x y and z are of the same order of magnitude for the transitional field and the historical nondipole field at the site latitude however a field intensity increase to pretransitional values occurs when the field temporarily reaches normal directions which suggests that dipolar structure could have been briefly regenerated during the transition in an aborted attempt to reestablish a stationary field changes in the field vector are progressive but jerky with at least two and possibly three large swings at astonishingly high rates each of those transitional geomagnetic impulses occurs when the field intensity is low less than 10 t and is followed by an interval of directional stasis during which the magnitude of the field increases greatly for the best documented geomagnetic impulse the rapid directional change corresponds to a vectorial intensity change of 6700 2700 nt yr 1 which is about 15 50 times larger than the maximum rate of change of the nondipole field observed during the last centuries the occurrence of geomagnetic impulses seems to support reversal models assuming an increase in the level of turbulence within the liquid core during transitions the record closes with an estimated several thousand years of normal polarity with an average intensity of 46 7 20 1 t agreeing with the expected miocene value however the occurrence of rather large and apparently rapid intensity fluctuations accompanied by little change in direction suggests that the newly reestablished dipole was still somewhat unstable

in situ production of ash in pyroclastic flows 1 abrasion and comminution of pumice clasts during the propagation of pyroclastic flows have long been recognized as a potential source for the enhanced production of volcanic ash however their relative importance has eluded quantification the amount of ash produced in situ can potentially affect runout distance deposit sorting the volume of ash introduced in the upper atmosphere and internal pore pressure we conduct a series of laboratory experiments on the collisional and frictional production of ash that may occur during different regimes of pyroclastic flow transport ash produced in these experiments is predominately 10 100 microns in size and has similar morphology to tephra fall ash from plinian events we find that collisional ash production rates are proportional to the square of impact velocity frictional ash production rates are a linear function of the velocity of the basal particle enriched bed load region of these flows using these laboratory experiments we develop a subgrid model for ash production that can be included in analytical and multiphase numerical procedures to estimate the total volume of ash produced during transport we find that for most flow conditions 10 20 of the initial clasts comminute into ash with the percentage increasing as a function of initial flow energy most of the ash is produced in the high energy regions near the flow inlet although flow acceleration on steep slopes can produce ash far from the vent on level terrain collisionally and frictionally produced ash generates gravity currents that detach from the main flow and can more than double the effective runout distance of these flows ash produced at the frictional base of the flow and in the collisional upper regions of the flow can be redistributed through the entirety of the flow although frictionally produced ash accumulates preferentially near its source in the bed load flows that descend steep slopes produce the majority of their ash in the collisonally dominant flow head and flow snouts likely develop subangular to rounded pumice during this process

paleointensity of the earth s magnetic field during the laschamp excursion and its geomagnetic implications the reversed paleomagnetic direction of the laschamp and olby flows represents a specific feature of the geomagnetic field this is supported by paleomagnetic evidence showing that the same anomalous direction was recorded at several distinct sites including scoria of the laschamp volcano to examine this anomalous geomagnetic fluctuation we studied the paleointensity of the laschamp and olby flows using the thellier method twenty five samples were selected for the paleointensity experiments and from seven we obtained reliable results because the paleointensity results of the olby and laschamp flows as well as laschamp scoria are very similar they can be represented by a single mean paleointensity f 7 7 t considering that this low paleointensity is less than 1 6 of the present geomagnetic field and is more characteristic of transitional behavior our results suggest that the paleomagnetic directions of the laschamp and olby flows were not acquired during a stable reversed polarity interval a more likely explanation is that the laschamp excursion represents an unsuccessful or aborted reversal

response of a bubble bearing viscoelastic fluid to rapid decompression implications for explosive volcanic eruptions we conducted rapid decompression experiments using bubble bearing viscoelastic fluid in a vertical shock tube we varied vesicularity and pressure difference between the inside pg and the outside po of the bubbles dp pgpo to understand the behavior of bubbly magmas under rapid decompression we find that the potential energy which depends on the initial vesicularity pg and po determines the expansion velocity of the bubbly fluid during rapid decompression higher potential energy caused by a higher and a larger dp leads to faster expansion the expansion style also depends on the vesicularity and on the pressures pg and po we observe five different styles of expansion during the rapid decompression that depend on and dp when both and dp are small bnothingq occurs as and dp increase the bubbly fluid reacts more violently first the surface of the bubbly fluid bdeformsq and the fluid elongates in the vertical direction for sufficient elongation the fluid can bdetachq from the tube wall as and dp continue to increase bubble walls can break a process we refer to as bpartial ruptureq finally for still larger dp and both bubble walls and plateau borders break allowing the fluid to bfragmentq into discrete pieces and erupt explosively our experiments show that a larger potential energy which results from higher and larger dp causes a faster expansion of magma which in turn promotes fragmentation and thus explosive eruption if we assume that the pressure inside bubbles pg scales with the depth of bubbly magma measuring the magma vesicularity in conduits or domes as a function of the depth before eruption would help assess volcanic hazard d 2005 elsevier b v all rights reserved

mt pelee martinique may 8 and 20 1902 pyroclastic flows and surges abstract the distribution stratigraphic relationships and fragmental components of the may 8 and 20 1902 pyroclastic flows from mt pelee martinique together with eyewitness accounts suggest the following explanation for those eruptions the eruptions were vertically directed magmatic perhaps initiated phreatically and contained abundant juvenile lithics from congealed magma of the dome and neck this resulted in a two part eruption column having 1 a dense lithic charged part which collapsed into the crater and flowed out of a pre existing notch in its side giving rise to pyrochlastic flows and 2 a magmatically derived column containing gases juvenile vitric material and crystals which largely by passed the neck and dome and escaped into the atmosphere all of the energy of the flows was apparently focused through the notch they emerged fully turbulent and flowed down riviere blanche gravity segregation of large and abundant fragments soon resulted in a dense high concentration poorly fluidized block and ash flow confined to the valley leaving above a fully turbulent high energy ash cloud surge as the ash cloud surge moved down the mountain it continued to expand outward the process of gravity segregation continued as the ash cloud surge expanded resulting in secondary block and ash underflows toward st pierre the secondary block and ash flows developed on a gently sloping upland surface 100 m or more above the valley of riviere blanche the turbulent fragment depleted surges above the secondary block and ash flows maintained sufficient energy to devastate the landscape outward to about 3000 m including st pierre the surges refracted around obstacles and in one place moved up a small valley in a direction opposite to the main flows

inclination shallowing in deep sea sediments from the north atlantic paleomagnetic data from miocene to recent deep sea sediments recovered during leg 94 of the deep sea drilling project dsdp in the north atlantic exhibit a systematic shallowing of inclinations with depth at some sites this shallowing is coincident with a downhole decrease in water content and is observed only in sediments with a carbonate content consistently greater than 80 compaction due to overburden pressure is believed to be responsible for the shallow inclinations observed in these unconsolidated sediments at sites where the carbonate content of the sediments is consistently below about 80 no significant downcore decreasing trends in the water content or inclination record are observed

grand sarcoui volcano chaine des puys massif central france a case study for monogenetic trachytic lava domes abstract the grand sarcoui is a prominent trachytic volcano of the intraplate quaternary volcanic field of chaine des puys massif central france which fulfills basic requirements for being qualified as monogenetic grand sarcoui looks like a simple axisymmetric lava dome but close observation reveals a complex and dissymmetric structure and composition the construction of the dome about 12 5 ka ago combined both endogenous and exogenous growth which resulted in variable modes of emplacement and textures of the lava one of its most interesting features is a large 0 29 106 m2 fan of deposits bearing hummocks and secondary hydro eruption craters cross sections of these deposits demonstrate that they originated from a sector collapse accompanied by a blast like event the dome is covered by a thin layer of lapilli and ash attributed to a delayed summit eruption which occurred about 10 6 ka ago surprisingly late after its construction so this volcano has at a reduced scale features that are more usually observed in large composite volcanoes however some of these features differ slightly from those that have been documented to date and they remain partly unexplained this shows that monogenetic well preserved trachytic lava domes are uncommon and poorly known unlike rhyolitic andesitic and dacitic domes

preliminary determinations of geomagnetic field intensity for the last 400 kyr from the hawaii scientific drilling project core big island hawaii preliminary paleointensity determinations using the thelliers method have been carried out on 141 samples of core hsdp hawaii scientific drilling project drilled into the mauna loa and mauna kea volcanoes and spanning the last 400 kyr rock magnetic investigations identify pseudo single domain magnetite as the main carrier of the natural remanent magnetization nrm all along the core in the mauna kea sequence 75 of the specimens yielded exploitable results with linear nrm trm thermoremanent magnetization plots and positive partial trm ptrm checks generally up to the highest temperatures 550 c a lower percentage of successful experiments 50 was observed for the mauna loa samples and the nrm trm diagrams are linear over a shorter temperature interval than for the mauna kea samples these preliminary results based on only one sample per flow document a marked geomagnetic field intensity low at about 40 kyr probably related to the laschamp event they also suggest that the geomagnetic field at hawaii might be characterized by large sharp intensity fluctuations spaced by a few thousand years the average value of the field intensity suggested by these results for the last 400 000 years is slightly higher than the present field at hawaii

origin and depositional environment of clastic deposits in the hilo drill hole hawaii volcaniclastic units cored at depths of about 87 164 178 226 and 246 m below sea level and carbonate units located between depths of 27 and 53 m below sea level in the hilo drill core were found to be deposited at or near sea level four of these units are hydroclastic deposits formed when subaerially erupted mauna loa lava flows entered the ocean and fragmented to produce quenched glassy fragments during hydrovolcanic explosions ash units 24 and 26 at 178 m depth accumulated at sea level in a freshwater bog they contain pyroxenes crystallized from tholeiitic magma that we infer erupted explosively at the summit of kilauea volcano two carbon rich layers from these ashes have a weighted average radiocarbon age of 38 6 0 9 ka the ashes probably correlate with the oldest and thickest part of the pahala ash ash unit 44 at the transition from mauna kea to mauna loa lava flows was probably nearly 3 2 m thick and is inferred to be equivalent to the lower thick part of the composite homelani ash mapped in hilo and on the flanks of mauna kea the age of this part of homelani ash is between 128 33 and 200 10 ka it may have erupted subglacially during the pohakuloa glacial maxima on mauna kea beach sand units 12 and 22 were derived from nearby mauna loa and mauna kea lava flows the middle of beach sand unit 38 was derived mainly from lava erupted near the distal end of the subaerial east rift zone of kilauea volcano these sands were transported about 33 km northwest to hilo bay by prevailing longshore currents combined age depth and sea level markers in the core allow us to determine that lava flow recurrence intervals averaged one flow every 4 kyr during the past 86 kyr and one flow every 16 kyr between 86 and 200 ka at the drill site and that major explosive eruptions that deposit thick ash in hilo have occurred only twice in the last 400 kyr these recurrence intervals support the moderate lava flow hazard zonation zone 3 for coastal hilo previously determined from surficial mapping

absolute paleointensity from hawaiian lavas younger than 35 ka abstract paleointensity studies have been conducted in air and in argon atmosphere on nine lava flows with radiocarbon ages distributed between 3 3 and 28 2 ka from the mauna loa volcano in the big island of hawaii determinations of paleointensity obtained at eight sites depict the same overall pattern as the previous results for the same period in hawaii although the overall average field intensity appears to be lower since the present results were determined at higher temperatures than in the previous studies this discrepancy raises questions regarding the selection of low versus high temperature segments that are usually made for absolute paleointensity the virtual dipole moments are similar to those displayed by the worldwide data set obtained from dated lava flows when averaged within finite time intervals the worldwide values match nicely the variations of the sint 200 synthetic record of relative paleointensity and confirm the overall decrease of the dipole field intensity during most of this period the convergence between the existing records at hawaii and the rest of the world does not favour the presence of persistent strong non dipole components beneath hawaii for this period

geomagnetic field inclinations for the past 400 kyr from the 1 km core of the hawaii scientific drilling project a volcanic record of geomagnetic field inclination for the past 400 kyr at hilo hawaii has been obtained from the 941 5 m of core recovered by the hawaii scientific drilling project the analysis of 195 lava flows reveals six instances of near zero inclination and two instances of fully negative inclination reverse polarity within an otherwise normal polarity core in particular flow unit 23 178 m depth records a horizontal inclination and may be associated with the laschamp event flow units 40 and 42 260 m depth record negative inclinations and are close in age to the blake event and flow unit 55 320 m depth records a negative inclination with a relative declination change of 75 with respect to the overlying flow and is probably the jamaica biwa i pringle falls event the five instances of shallow inclination found below 400 m depth appear to have resulted from long term secular variation as they are part of inclination swings between 0 and 60 with a periodicity of 10 50 kyr in contrast the inclination shifts at 178 m and 320 m depths significantly deviate from long term trends suggesting the existence of at least two independent processes producing time variations of the geomagnetic field the secular variation has a mean of 30 9 95 2 27 which is significantly shallower than the expected dipole mean of 36 the dispersion 12 5 agrees with global paleosecular variation data for 0 5 ma and secular variation models

paleomagnetic constraints on fault motion in the hilina fault system south flank of kilauea volcano hawaii movement of the south flank of kilauea volcano in hawaii has been associated with catastrophic landslide events the surface expression of this former movement is the hilina fault system with fault scarps as high as 500 m paleomagnetic directions for lava flows exposed in the hilina fault scarps at puu kapukapu and keana bihopa on the hilina pali are used to determine the average rate of movement along faults slip surfaces separating the two sections this paper reports results from two independent paleomagnetic studies within the hilina pali area twenty one paleomagnetic sites 143 cores were sampled by the michigan technological university group from lava flows between the mo o ash and middle pohakaa ash at keana bihopa in the footwall block of the 500 m high hilina pali fault scarp thirty paleomagnetic sites 152 cores were collected by the california institute of technology group from lava flows between the mo o ash and middle pohakaa ash in the 300 m high puu kapukapu section the hanging wall block a comparison of site mean directions show that lava flows in the lower part of the puu kapukapu section have been tilted more than lava flows in the upper part with respect to the keana bihopa section the systematic steepening of remanent directions downsection at puu kapukapu indicates that slippage of this block occurs along listric normal faults the average amount of backward tilt of the puu kapukapu block based on a comparison of mean directions from the two sections is 7 8 7 7 using slope stability methods the average rate of movement of the puu kapukapu block since deposition of the middle pohakaa ash is 1 7 2 4 cm yr and the average displacement subsidence is 680 740 m assuming that the average displacement resulted from a series of earthquakes producing subsidence equal to that observed in the 1975 kalapana earthquake 3 m of subsidence along a 40 km segment of coastline on the south flank of kilauea volcano one kalapana size earthquake occurring every 200 years would account for this displacement lastly overall mean directions for the two sections indicate that puu kapukapu has rotated counterclockwise with respect to the hilina pali by 14 8 8 5 about a nearby vertical axis this also suggests that slippage between the two blocks occurs along listric normal faults

geological constraints on the emplacement mechanism of the parinacota debris avalanche northern chile the holocene parinacota volcanic debris avalanche ca 8 000 years b p is located in the central andes of northern chile the avalanche formed by the sector collapse of a major stratovolcano adjacent to a lake basin in a single catastrophic event the deposit has an estimated volume of ca 6 km3 a run out of over 22 km and covers more than 140 km2 of the surrounding terrain the values of the heim coefficient 0 08 and the ratio a v 2 3 ca 50 where a is the area covered and v the volume of the deposit indicate high mobility of the avalanche debris in transport two avalanche units can be distinguished the lower unit consists mainly of blocks of rhyodacitic lavas and domes and pyroclastic flow deposits and glacial fluvial and lacustrine sediments the upper unit consists of a coarse grained breccia with little matrix largely composed of andesite blocks which are angular with little or no rounding by abrasion the avalanche displays pronounced hummocky topography in which hummock volume and amplitude as well as maximum block size within individual hummocks tends to decrease with transport distance and towards the lateral margins of the avalanche deposit some surfaces of individual breccia blocks are covered by tens to thousands of small scale impact marks indicating that neighbouring blocks were vibrating and colliding without significant shearing motion most of the deformation and shearing was instead accommodated in a basal layer of wet structureless sediments incorporated into the avalanche debris from the inundated lake basin we propose that the ancestral parinacota stratovolcano collapsed because of loading of underlying fluvioglacial and lacustrine sediments the edifice disintegrated during collapse along existing fractures into large rock domains volumes from 10 to greater than 1 106 m3 which were transported with little internal deformation and then fragmented into hummocks of breccia as they were deposited the decrease of hummock volume with distance suggests that material that travelled further broke up and had an initial greater kinetic energy

reconciling pyroclastic flow and surge the multiphase physics of pyroclastic density currents abstract two end member types of pyroclastic density current are commonly recognized pyroclastic surges are dilute currents in which particles are carried in turbulent suspension and pyroclastic flows are highly concentrated flows we provide scaling relations that unify these end members and derive a segregation mechanism into basal concentrated flow and overriding dilute cloud based on the stokes number s t the stability factor t and the dense dilute condition d d we recognize five types of particle behaviors within a fluid eddy as a function of s t and t 1 particles sediment from the eddy 2 particles are preferentially settled out during the downward motion of the eddy but can be carried during its upward motion 3 particles concentrate on the periphery of the eddy 4 particles settling can be delayed or fast tracked as a function of the eddy spatial distribution and 5 particles remain homogeneously distributed within the eddy we extend these concepts to a fully turbulent flow by using a prototype of kinetic energy distribution within a full eddy spectrum and demonstrate that the presence of different particle sizes leads to the density stratification of the current this stratification may favor particle interactions in the basal part of the flow and d d determines whether the flow is dense or dilute using only intrinsic characteristics of the current our model explains the discontinuous features between pyroclastic flows and surges while conserving the concept of a continuous spectrum of density currents

dome collapse mechanisms and block and ash flow emplacement dynamics inferred from deposit and impact mark analysis mono craters ca abstract characteristics of the panum block and ash flow baf deposit mono craters ca were analyzed to determine the mechanisms of collapse of the parent dome and dynamics of emplacement of the baf granulometry componentry and obsidian water content data were used to define distinct facies of the panum baf deposit these suggest a sequential three stage collapse model for the ancestral dome of the panum vent with destabilization first of its cold brittle outer margins and then of its hot ductile interior to an estimated depth of 260 m below the free air surface impact marks on clast faces that resulted from clast to clast interactions recorded details of the impacting mechanism we analyzed 15 000 marks and report for the first time their size shape and sorting to infer the flow regime of the baf during emplacement to a distance of 2 6 km from the vent clast to clast interactions occurred over a broad spectrum from purely collisional normal to clast faces to purely frictional clasts shearing in a unidirectional flow and recorded a transition to a more frictional flow regime in distal reaches as the sliding head of the baf decelerated and halted finally we briefly differentiate a similar but previously undescribed older deposit that predates the 1325 1350 a d north mono eruptive episode

the 40ar 39ar and k ar dating of lavas from the hilo 1 km core hole hawaii scientific drilling project mauna kea lava flows cored in the hilo hole range in age from 200 ka to about 400 ka based on 40ar 39ar incremental heating and k ar analyses of 16 groundmass samples and one coexisting plagioclase the lavas all subaerially deposited include a lower section consisting only of tholeiitic basalts and an upper section of interbedded alkalic transitional tholeiitic and tholeiitic basalts the lower section has yielded predominantly complex discordant 40ar 39ar age spectra that result from mobility of 40ar and perhaps k the presence of excess 40ar and redistribution of 39ar by recoil comparison of k ar ages with 40ar 39ar integrated ages indicates that some of these samples have also lost 39ar nevertheless two plateau ages of 391 40 and 400 26 ka from deep in the hole combined with data from the upper section show that the tholeiitic section accumulated at an average rate of about 7 to 8 m kyr and has a mean recurrence interval of 0 5 kyr flow unit samples from the upper section yield relatively precise 40ar 39ar plateau and isotope correlation ages of 326 23 241 5 232 4 and 199 9 ka for depths of 415 7 m to 299 2 m within their uncertainty these ages define a linear relationship with depth with an average accumulation rate of 0 9 m kyr and an average recurrence interval of 4 8 kyr flow unit the top of the mauna kea sequence at 280 m must be older than the plateau age of 132 32 ka obtained for the basal mauna loa flow in the corehole the upward decrease in lava accumulation rate is a consequence of the decreasing magma supply available to mauna kea as it rode the pacific plate away from its magma source the hawaiian mantle plume the age depth relation in the core hole may be used to test and refine models that relate the growth of mauna kea to the thermal and compositional structure of the mantle plume

the 17 july 1999 block and ash flow baf at colima volcano new insights on volcanic granular flows from textural analysis abstract on july 17 1999 a strong explosion occurred at colima volcano mexico that produced a 10 km high eruptive column the partial column collapse originated a block and ash flow baf that flowed to the south along the san antonio and montegrande ravines travelling 3 3 km from the volcano summit the flow filled the ravines with a volume estimated at 7 9 105 m3 the erosion of these deposits occurred between 1999 and 2002 time of sampling providing excellent longitudinal outcrops that allowed their detailed textural study the study was carried out by means of quantitative textural analysis 1 rosiwal intersections for carrying out vertical granulometric profiles 2 total grain size analysis from 11 to 9 and 3 fourier and fractal analysis of the particle morphology grain size and morphometric parameters obtained with these methods were used to identify vertical and longitudinal variation patterns in the baf deposit the grain size variations allowed to infer the main particle segregation mechanisms that acted during transport and deposition of the studied bafs the two methods used for studying the particle shape morphologies yielded results with different accuracy and reliability in particular fractal analyses have been found to be the most effective in describing the particle support mechanisms that acted during transport and deposition of the studied bafs the results highlight the importance of the information obtained by means of these techniques and provide new insights in transportation and deposition mechanisms of bafs

oncolites from lacustrine sediments in the cretaceous of north eastern spain compact micritic oncolites up to 8 cm in maximum diameter occur within maestrichtian upper cretaceous garumniense continental marls of sierra del montsec lerida province of north eastern spain synsedimentary development is documented by patches of terrigenous quartz that occur among oncolitic protuberances soluble nuclei limestone fragments and bivalves further suggest an origin through accretion rather than that of soil pisolite similarities between the petrography and isotopic compositions of the oncolites and those of interbedded garumniense limestones suggest similar sedimentary origins however these lacustrine oncolites like modern counterparts described by others probably developed through in situ metabolic precipitation of calcium carbonate evidence of this origin is their high degree of concentricity which is unlikely to have developed through sedimentary accretion inasmuch as the oncolites float in quiet water marls

absolute paleointensity between 60 and 400 ka from the kohala mountain hawaii abstract magnetic experiments including thermal demagnetization of the natural remanent magnetization nrm mineralogical studies and paleointensity measurements have been conducted on ten lava flows with ages between 60 ka and 400 ka from the kohala mountain hawaii of the samples 62 were subjected to double heating paleointensity experiments under vacuum in total 35 of the specimens did not exhibit significant magnetomineralogical changes during heating and met all the criteria for successful determinations of absolute paleointensity a technique of corrections 1 was attempted for samples that exhibited changes in their ability to acquire partial thermoremanent magnetization ptrm during heating but did not show acquisition of chemical remanence crm this procedure doubled the rate of success with consistent results between the uncorrected and the corrected data from within the same flows the successful paleointensity estimates obtained for 8 lava flows are found to be in good agreement with previous absolute paleointensities obtained from other areas the results are also consistent with the synthetic curve sint 200 of relative paleointensity obtained for the past 200 000 years from deep sea sediment cores 2 there is thus no reason to infer the presence of large non dipole fields in the vicinity of hawaii overall the geomagnetic intensity appears to have the same variability for at least the past 200 ka

geomagnetic field intensity at hawaii for the last 420 kyr from the hawaii scientific drilling project core big island hawaii four hundred twenty five new paleointensity thellier thellier determinations out of 545 analyzed samples have been obtained from core hsdp which penetrates about 1000 meters 208 flows of the mauna loa and mauna kea volcanic series encompassing the last 420 kyr rock magnetic investigations identify pseudo single domain magnetite as the main magnetic mineral inclinations are shallower than expected from a geocentric dipole field but are consistent with data from other geographical regions at the same latitude the inclination record reveals three episodes of negative inclination whose interpolated age correlates well with that of known geomagnetic events the paleointensity record from the mauna loa sequence is not very detailed and does not allow precise comparison with other data in the 0 50 kyr interval the record from the mauna kea sequence on the contrary is very detailed and documents relatively short lived episodes of low and high field strength from 15 to 60 t the average virtual dipole moment 8 7 3 0 10 22 a m 2 is not significantly different from the value reported by kono and tanaka 1995 for the last 2 5 myr a comparison with other data from hawaii and other geographical regions is described in detail there are no drastic changes in paleointensity with the inclination anomaly in agreement with previous results from hawaii but in contrast with most published results which however consider data from polarity transition spectral analysis of a particularly detailed portion of the record between 420 and 326 kyr documents significant periodicities at 36 8 5 and 4 ka in the inclination record but not in the intensity record suggesting that changes in time of the inclination are to a certain extent independent from those of the intensity

paleointensity of the geomagnetic field during the last 80 000 years high resolution records of the relative paleointensity of the geomagnetic field have been obtained from five marine cores three duplicate records were used to estimate the regional coherency of the data within a single area tyrrhrenian sea while the two others document the field variations in the eastern mediterranean and the southern indian ocean careful investigations of distinct rock magnetic parameters have established the downcore uniformity of the sediments in terms of magnetic mineralogy and grain sizes the time depth control was provided by oxygen isotopes and small scale variations in the deposition rates were constrained by means of tephrachronology the synthetic curve calculated from the mediterranean records provides a continuous record of the intensity variations during the last 80 000 years 80 kyr which correlates well with the sparse volcanic data available for the period 0 40 kyr the fact that identical behavior is seen in both data sets and that they also compare quite well with results from a core collected in the pacific ocean establishes the truly dipolar character of these variations the dipole field moment is characterized by large scale changes as shown by the existence of pronounced drops at 39 and 60 kyr alternating with periods of higher intensity the record suggests a periodic nature for these intensity variations however the period studied is not sufficiently long to state this conclusively these results demonstrate the potential of sediments for such studies and constitute a first step towards obtaining a global paleointensity record over a long period of time

continental sedimentation in an evolving rift basin the lower cretaceous of the western cameros basin northern spain abstract the lower cretaceous of the western cameros basin comprises up to 2 5 km of clastic and carbonate continental sediments deposited in a variety of fluvial and lacustrine settings kimmeridgian berriasian red beds and lacustrine limestones are overlain by the valanginian aptian pedroso group which is dominated by sheet flood sandstones and distal alluvial mudstones with vertebrate footprints mudcracks and intercalated carbonate paleosols lacustrine limestones are also present commonly containing spongiostromate oncoids the upper part of the sequence the salas group and utrillas formation aptian lower cenomanian comprises quartzose pebbly sands and conglomerates with rare muddy paleosols these sediments are interpreted as braided stream deposits deposition of this thick continental sequence may be correlated with the extensional history of the cameros basin and of the evolving bay of biscay rift to the north clastic supply was dominantly controlled by tectonic factors facies evolution may also have reflected climatic influence

field observations and surface characteristics of pristine block and ash flow deposits from the 2006 eruption of merapi volcano java indonesia abstract a series of pristine block and ash flow deposits from the may june 2006 eruption of merapi represent an exceptional record of small volume pyroclastic flows generated by gravitational lava dome collapses over a period of about two months the deposits form nine overlapping lobes reaching 7 km from the summit in the gendol river valley on the volcano s southern flank which were produced by successive flows generated during and after the major dome collapse event on june 14 both single pulse post june 14 events and multiple pulse pyroclastic flows generated by sustained dome collapses on june 14 are recognised and three types of deposits spread over an area of 4 7 km are distinguished totalling 13 3 10 6 m 3 1 valley confined basal avalanche deposits 11 7 10 6 m 3 in the gendol river valley 2 overbank pyroclastic flow and associated surge deposits 1 4 10 6 m 3 where parts of the basal avalanche spread laterally onto interfluves and were subsequently channeled into the surrounding river valleys and 3 dilute ash cloud surge deposits 0 2 10 6 m 3 along valley margins variations in the distribution surface morphology and lithology of the deposits are related to the source materials involved in individual pyroclastic flow forming events and varying modes of transport and deposition of the different flows inferred flow velocities of the largest block and ash flows generated on june 14 vary from 43 8 13 5 m s for the basal avalanche and from 62 6 24 2 m s for the ash cloud surge the minimum temperatures range from 400 c for the basal avalanche to 165 c for the overlying ash cloud due to the potential of being re channeled into adjacent river valleys and flowing laterally away from the main river channel the overbank pyroclastic flows are considered the most hazardous part of the block and ash flow system the conditions that lead to their development during flow transport and deposition must be taken into account when assessing future pyroclastic flow hazards at merapi and similar volcanoes elsewhere

geomagnetic field intensity over the last 42 000 years from core soh 4 big island hawaii a record of the absolute geomagnetic field intensity spanning the last 42 kyr has been obtained from 100 lava flows 187 successful thellier experiments recovered in the top 468 m of core from the soh 4 well on the island of hawaii assuming a linear extrusion rate between the present and the 42 kyr date obtained with a refined k ar technique this corresponds to an average of one flow every 420 years rock magnetic analysis identifies low ti content and high ti content magnetites in 75 and 25 of the samples respectively as the main carriers of magnetization the low ti magnetites are very stable while some transformations occur upon heating in the other samples but both groups allow reliable paleointensity determinations the directional record documents inclinations consistent with a geocentric dipole field for the last 16 kyr and between 38 and 42 kyr between 16 and 38 kyr shallower and in some cases negative inclinations were observed because of the high extrusion rate and the exceptional percentage of reliable intensity determinations the record of the geomagnetic field intensity over the last 42 kyr has a resolution comparable to the best sedimentary records the geomagnetic field intensity at hawaii appears to have varied between 18 and 79 t and is characterized by large fluctuation with peak to peak amplitudes of 20 25 t the observed values are quite consistent with other values obtained from hawaii for the last 31 kyr mankinen and champion 1993a b tanaka and kono 1991 coe et al 1978 the mean intensity over the explored time interval 45 t is slightly higher than the present value of the geomagnetic field in hawaii

merapi volcano java indonesia and merapi type nuee ardente a particular nuee ardente type merapi type avalanche nuee has been defined at the merapi volcano because of its prominent role in the recent activity of the volcano gravity plays a significant role during the eruption

possible mechanisms causing failure of thellier palaeointensity experiments in some basalts summary the normally magnetized zone of the jurassic lesotho basalts although providing apparently quite reliable palaeofield directions kosterov perrin 1996 shows anomalous behaviour when studied in vacuum using the thellier palaeointensity method typically the slope of the natural remanent magnetization thermoremanent magnetization nrm trm curves is very steep at intermediate temperatures 200 to 400 460 c in order to elucidate the reasons for such an anomalous behaviour six representative samples from a total of 74 studied using this method were subjected to a variety of analyses these experiments indicate that the magnetic properties are dominated by pseudo single domain psd magnetite grains some 1 mm in size resulting from high temperature oxidation of titanomagnetite laboratory heatings in vacuum up to the curie point do not change significantly the roomtemperature hysteresis characteristics or the initial susceptibility k similarly the k t curves in vacuum are with a single exception rather reproducible since the laboratory trms yield almost ideal nrm trm plots the anomalous nrm trm plot is presumably due to some peculiarity of the natural trm the partial trm ptrm acquisition capacity in the moderate temperature range cooling from 200 to 20 c is generally very strongly reduced after heating to 270 c which indicates that some magnetic alteration has already occurred at these temperatures hysteresis measurements between room temperature and the curie temperature t c show that some small less than 10 per cent but significant irreversible changes in hysteresis characteristics also occur during heating in particular the coercive force h c0 at room temperature is typically reduced after heating at a moderate temperature 175 c but increases after treatments at 475 c and more pronouncedly at 580 c the saturation magnetization j s0 remains unchanged except for a very small decrease less than 5 per cent occurring in some samples after the two latter treatments these changes are most clearly seen on h c t j s t bilogarithmic plots which show that the moderate temperature change in coercivity can extend up to 200 250 c thus hysteresis measurements as a function of temperature over a promising tool for sample pre selection for thellier experiments alternating field demagnetization and cycling of ptrms at liquid nitrogen temperature suggest that the blocking mechanism is largely multidomain like near room temperature but becomes less so as the curie point is approached the main reason for the failure of the thellier experiments is the loss of a fraction of the nrm natural trm at temperatures apparently lower than the blocking temperatures in nature it is suggested that this anomalous behaviour results from the reorganization of the domain structure of the psd grains during heating this transformation which seems to be triggered by the coercivity decrease observed at very moderate temperatures can reduce the nrm intensity without requiring any correlated ptrm acquisition

high resolution geochemical stratigraphy of mauna kea flows from the hawaii scientific drilling project core trace elements have been measured in the deepest 101 lava flows from the hawaii scientific drilling project hsdp by inductively coupled plasma mass spectrometry in order to compensate for the lack of a precise timescale for the hsdp samples and in spite of some probable long stratigraphic hiatuses a constant eruption rate is assumed that provides a timescale for the geochemical dynamics of mauna kea periods of nearly continuous variations in incompatible element ratios suggest that geochemical variability is controlled by reservoirs larger the shallow magma chambers inferred from seismicity and deflation data the principal component analysis of the differenced data clearly separates three components associated with 1 olivine spinel phenocryst abundance 2 weathering and 3 melting conditions and source mineralogy the picritic character of some lavas does not seem associated with a particular mantle source or melting process the weathering component appears to discriminate lateral from vertical transfer a recurrence of the melting conditions or source heterogeneities on a scale time of 30 kyr is inferred from the segmented record of incompatible element distributions

friction marks on blocks from pyroclastic flows at the soufriere hills volcano montserrat implications for flow mechanisms friction marks on surfaces of blocks 1 to 10 m in pyroclastic flow deposits generated by lava dome collapse in the 1995 1999 eruption of the soufriere hills volcano montserrat consist of a thin 5 to 10 mm layer of cataclasite with a polished slickensided surface in some examples thin surface layers and veins of pseudotachylyte are found the friction marks occur in several orientations on all block surfaces they are typically 0 5 to 10 cm wide and up to 1 m long the marks are similar to tectonic structures on faults they indicate that the blocks were tumbling and sliding within the flows large transient stresses result when the mass of a large sliding block is applied to a small contact area on a neighboring block or the ground compositional data indicate that the pseudotachylytes involved melting of the flow matrix these observations support models of pyroclastic flows as rapid granular avalanches

el escondido a newly identified silicic quaternary volcano in the ne region of the northern volcanic segment central cordillera of colombia abstract el escondido 1650 m a s l is a silicic quaternary volcano recently identified on the central cordillera of colombian andes it is located about 50 km ne from the northern segment of active volcanoes it has been recognized as a polygenetic pyroclastic dome ring complex structure whose eruptive products derived from a crater like semi circular depression of 1 7 km in diameter this crater formed at the contact of the granitic florencia stock 54 9 1 8 my with the paleozoic metamorphic basement and at the intersection of the nnessw trending palestina fault system with an ew trending fault the preserved volcano morphology and the volcanic succession associated with it were defined as the el escondido volcanic center the eruptive products are alternating silicic mostly dacitic pyroclastic density current deposits pumice fall deposits and ballistic projectiles which suggest high explosiveness two 14c radiogenic dates obtained on carbonized wood fragments of the middle part of the eruptive sequence displayed ages of 36 030 380 bp and 33 550 280 bp suggesting two major eruptive episodes of the volcano about 2500 years apart the latest eruptive activity is inferred to be the emplacement of lava domes to the south of the crater like structure which are responsible for the partial destruction of the pyroclastic ring in that sector the products emitted by el escondido correspond to medium k andesites to dacites of the calc alkaline series and petrographically to amphibolic andesites towards the western and eastern sector of the volcano there are chlorinated thermal waters with temperatures up to 45 c the first monitoring of the volcanic activity carried out by the colombian geological survey cgs through the volcanological and seismological observatory of manizales vsom has not detected so far unusual volcano seismic activity the previously unreported el escondido volcano represents one of the northernmost occurrence of quaternary volcanism in the whole andean cordillera thus its study may have wide implications regarding the geodynamics of the region and subduction zone the correct assessment of geologic hazards and the identification of natural resources like geothermal reservoirs and mineral deposits of economic interest