drilling deep into young oceanic crust hole 504b costa rica rift hole 504b is by far the deepest hole yet drilled into the oceanic crust in situ and it therefore provides the most complete ground truth now available to test our models of the structure and evolution of the upper oceanic crust cored in the eastern equatorial pacific ocean in 5 9 m y old crust that formed at the costa rica rift hole 504b now extends to a total depth of 1562 3 m below seafloor penetrating 274 5 m of sediments and 1287 8 m of basalts the site was located where the rapidly accumulating sediments impede active hydrothermal circulation in the crust as a result the conductive heat flow approaches the value of about 200 mw m predicted by plate tectonic theory and the in situ temperature at the total depth of the hole is about 165 c the igneous section was continuously cored but recovery was poor averaging about 20 the recovered core indicates that this section includes about 575 m of extrusive lavas underlain by about 200 m of transition into over 500 m of intrusive sheeted dikes the latter have been sampled in situ only in hole 504b the igneous section is composed predominantly of magnesium rich olivine tholeiites with marked depletions in incompatible trace elements nearly all of the basalts have been altered to some degree but the geochemistry of the freshest basalts is remarkably uniform throughout the hole successive stages of on axis and off axis alteration have produced three depth zones characterized by different assemblages of secondary minerals 1 the upper 310 m of extrusives characterized by oxidative seafloor weathering 2 the lower extrusive section characterized by smectite and pyrite and 3 the combined transition zone and sheeted dikes characterized by greenschist facies minerals a comprehensive suite of logs and downhole measurements generally indicate that the basalt section can be divided on the basis of lithology alteration and porosity into three zones that are analogous to layers 2a 2b and 2c described by marine seismologists on the basis of characteristic seismic velocities many of the logs and experiments suggest the presence of a 100 to 200 m thick layer 2a comprising the uppermost rubbly pillow lavas which is the only significantly permeable interval in the entire cored section layer 2b apparently corresponds to the lower section of extrusive lavas in which original porosity is partially sealed as a result of alteration nearly all of the logs and experiments showed significant changes in in situ physical properties at about 900 1000 m below seafloor within the transition between extrusives and sheeted dikes indicating that this lithostratigraphic transition corresponds closely to that between seismic layers 2b and 2c and confirming that layer 2c consists of intrusive sheeted dikes a vertical seismic profile conducted during leg 111 indicates that the next major transition deeper than the hole now extends that between the sheeted dikes of seismic layer 2c and the gabbros of seismic layer 3 which has never been sampled in situ may be within reach of the next drilling expedition to hole 504b therefore despite recent drilling problems deep in the hole current plans now include revisiting hole 504b for further drilling and experiments when the ocean drilling program returns to the eastern pacific in 1991

a multichannel seismic study of lithospheric flexure across the hawaiian emperor seamount chain existing models assume that the thickened crust beneath seamounts is the result of a surface volcanic load flexing an elastic plate new results suggest that flexed oceanic crust beneath the hawaiian emperor seamount chain is underlain by a 4 km thick deep crustal body we intepret the body as a deep crustal sill complex associated with the tholeiitic stage of volcano building along the chain

on geoid heights and flexure of the lithosphere at seamounts the sea surface height has now been mapped to an accuracy of better than 1 m by using radar altimeters on board orbiting satellites the major influence on the mean sea surface height is the marine geoid which is an equipotential surface we have carried out preliminary studies of how oceanic volcanoes which rise above the ocean floor as isolated seamounts and oceanic islands or linear ridges contribute to the marine geoid simple one and two dimensional models have been constructed in which it is assumed that the oceanic lithosphere responds to volcanic loads as a thin elastic plate overlying a weak fluid substratum previous studies based on gravity and bathymetry data and uplift subsidence patterns show that the effective flexural rigidity of oceanic lithosphere and the equivalent elastic thickness te increase with the age of the lithosphere at the time of loading the models predict that isolated seamounts emplaced on relatively young lithosphere on or near a mid ocean ridge crest will be associated with relatively low amplitude geoid anomalies about 0 4 0 5 m km of height while seamounts formed on relatively old lithosphere on ridge flanks will be associated with much higher amplitude anomalies 1 4 1 5 m km studies of the seasat altimetric geoid prepared by nasa s jet propulsion laboratory support these model predictions geoid amplitudes are relatively low over the mid pacific mountains and line islands which formed on or near a mid ocean ridge crest and relatively high over the magellan seamounts and wake guyots which formed off ridge direct modeling of the altimetric geoid over these features is complicated however by the wide spacing of the satellite tracks which can exceed 100 km and poor bathymetric control beneath individual satellite tracks in regions where multibeam bathymetric surveys are available models can be constructed that fit the altimetric geoid to better than 1 m studies of geoid anomalies over the emperor seamount chain for example suggest that these seamounts formed on 20 30 ma old oceanic lithosphere while anomalies over the louisville ridge and the hawaiian ridge suggest that these seamounts formed on lithosphere older than about 80 ma this indicates that geoid anomalies over bathymetrie features depend not only on the crustal structure prior to loading and the overall shape and density of the features but also on their tectonic setting we examine the implications of these results to studies which attempt 1 to separate temporal variations of the sea surface due to oceanographic effects 2 to predict seamounts directly from satellite derived gravity and geoid data and 3 to isolate the geoid effects of deep processes occurring beneath the lithosphere such as those due to mantle convection

shallow crustal structure beneath the juan de fuca ridge from 2 d seismic refraction tomography summary the formation of oceanic lithosphere along ocean ridges and the role that crustal magma chambers play in the accretionary process continue to be fundamental issues in plate tectonics to address these issues a multireceiver airgun oceanbottom seismograph refraction line was shot across the endeavour segment of the juan de fuca ridge near 48 n 129 w 2 d traveltime tomography applied to the data results in a three layer model for the upper crust layer 1 is 250 650m thick with u1 2 5 km s and vzvl 0 5 s layer 2 is 800 m thick u2 4 8 km s and vzuz 1 0 layer 1 and layer 2 probably represent the sequence of extrusives and the transition to layer 3 u3 5 8 km s l vzv3 0 5 s is associated with the extrusives dike complex transition an abrupt velocity transition between layer 1 and layer 2 may be a metamorphic front within the basalt pillows or it may be the depth to which surface fissures penetrate a low velocity anomaly velocities decreased by 0 45 km s exists beneath the ridge in layer 2 and upper layer 3 it is interpreted as a zone of increased fracture porosity and or permeability associated with axial hydrothermal circulation and correlates reasonably well with a sub axial reflector no evidence is found for the existence of a crustal magma chamber in the depth range of 1 5 3 5 km sub bottom however the velocity anomalies observed in layer 3 suggest that crustal temperatures in this layer are elevated by 150 250 c beneath and to the east of the ridge relative to temperatures west of the ridge

crosshole seismic tomography many tomographic interpretations of crosshole seismic traveltimes have approximated the raypaths with straight lines connecting the source and receiver this approximation is valid where the velocity does not vary greatly but in many regions of interest velocity variations of 10 20 percent or more are observed causing significant ray curvature other work has taken this nonlinear effect into account but there do not appear to be many cases of demonstrated success in its application to the crosshole seismic problem we present here an iterative inversion scheme based on two dimensional ray tracing and its successful application to field data the interpretation method iteratively ray traces and then updates the velocity model within each iteration the differences between the data and the current model traveltimes obtained by ray tracing are related to the unknown velocity perturbations through a system of linear equations a damped least squares method solves for the velocity perturbations which updat

a linear gradient crustal model for south hawaii a new crustal model with linear velocity gradients within layers does as good a job of locating earthquakes on south hawaii as any model yet published incorporating linear gradients means the model can be simpler and free of artificial velocity discontinuities using travel time residuals from local earthquakes and consistency of focal mechanism solutions as tests it is seen that a low velocity zone at the base of the crust is not required

contribution of induced magnetization to magnetization of seamounts a fundamental assumption in modelling seamount magnetic anomalies is that the contribution of induced magnetization is negligible the general consistency of seamount and non seamount palaeopoles scarcity of poles skewed toward the present field direction and the high ratio of remanent to induced magnetization koenigsberger ratio of many oceanic basalts have been cited as evidence supporting this assumption1 2 recent discussions concerning the dominance of normally magnetized seamounts have focused attention on the possible role of viscous and induced magnetization in seamount anomalies3 6 here we determine natural remanent magnetization initial volume susceptibility and the koenigsberger ratio for more than 2 000 samples from a subaerially exposed seamount section on la palma canary islands table 1 by contrast to results from the oceanic crust and ophiolites these data indicate that a variety of rock types are potential magnetic sources the significant induced component of intrusives underscores the importance of the lithological distribution in determining the character of seamount magnetic anomalies the la palma data together with a plausible lithological distribution indicate that induced magnetization may account for one sixth of seamount magnetization

seamount loading and stress in the ocean lithosphere 2 viscoelastic and elastic viscoelastic models analytical solutions are presented for the deformation and stress state of a horizontally stratified earth subject to normal loads of size and wavelength that are characteristic of seamounts the models investigated include layered elastic plates homogeneous viscoelastic plates and elastic over viscoelastic models in all cases the composite models overlie an inviscid half space the principal advantage of the laminated elastic models over homogeneous elastic plates is that they can result in a substantial reduction in the maximum stress differences in those parts of the plate least capable of supporting large stress differences authors

tectonic deformation by rotating parallel faults the bookshelf mechanism abstract in various tectonic environments extension of crustal parts and shear distortion has been accommodated by the rotation of an array of parallel faults because of its kinematic resemblance to the tilting of a row of books on a shelf the tectonic process may be referred to as a bookshelf mechanism its most important manifestions are cross faulting between normal faults or parallel wrench faults the extension of deltaic slope deposits and all forms of tilted block tectonics kinematically there are two basic modes of the mechanism a the dilational mode where rotating faults tend to open up and possibly become paths for fluid migration and b the domino style where reclining faults remain closed in both cases formation and rotation of the faults may take place in one tectonic event fault displacements may therefore often remain small and difficult to detect from the mechanical point of view bookshelf operations that accommodate shearing between parallel and predominantly parallel moving rock boundaries bookshelf mechanisms proper have to be differentiated from those operations responding to an imposed crustal extension or shortening after a review of geological evidence the process is addressed from a geomechanical point of view in order to determine the geological operating conditions and controlling parameters the crucial mechanical factor in the bookshelf mechanism is the component of normal stress that acts in the direction of the imposed displacement component parallel to the driving boundary plates in contrast to the usual assumption the driving shear stress on the boundary plates not only depends on the normal stress on the plates but it also varies with the value of the normal stress that acts inside the sheared material in the direction of the plate parallel displacement component it is the relationship between the two normal stresses that also determines the type of rotating parallel faults large scale rotation of parallel normal faults in domino style tilted block tectonics is primarily associated with the extension of ductile substrata from mechanical arguments and sand box experiments it has been inferred how the process and in particular the dip direction of the parallel faults is controlled by the way substratal extension progresses by means of the direction of a substratal squeeze flow the presence of a surface slope the strength anisotropy associated with fore set beds and by the configuration of competent boundaries of the extending overburden

gravity inversion using seminorm minimization density modeling of jasper seamount a gravity inversion algorithm for modeling discrete bodies with nonuniform density distributions is presented the algorithm selects the maximally uniform model from the family of models which fit the data ensuring a conservative and unprejudiced estimate of the density variation within the body the only inputs required by the inversion are the gravity anomaly field and the body shape tests using gravity anomalies generated from synthetic bodies confirm that seminorm minimizing inversions successfully represent mass distribution trends but do not reconstruct sharp discontinuities we apply the algorithm to model the density structure of seamounts inversion of the seasurface gravity field observed over jasper seamount suggests the edifice has a low average density of 2 38g cm3 and contains a dense body within its western flank these results are consistent with seismic magnetic and petrologic studies of jasper seamount

seamount statistics in the pacific ocean we apply the wide beam sampling technique of jordan et al 1983 to approximately 157 000 km of wide beam profiles to obtain seamount population statistics for eight regions in the eastern and southern pacific ocean population statistics derived from wide beam echograms are compared with seamount counts from sea beam swaths and with counts from bathymetric maps we find that the average number of seamounts with summit heights h h is well approximated by the exponential frequency size distribution h oe h the exponential model for seamount sizes characterized by the single scale parameter 1 is found to be superior to a power law self similar model which has no intrinsic scale in describing the average distribution of pacific seamounts and it appears to be valid over a size spectrum spanning 5 orders of magnitude in abundance large scale regional variations in seamount populations are documented we observe significant differences in seamount densities across the murray fracture zone in the north pacific and the eltanin fracture zone system in the south pacific the eltanin discontinuity is equally evident on both sides of the pacific antarctic ridge in the south pacific regions symmetrically disposed about the ridge axis have very similar seamount densities despite the large difference between pacific plate and antarctic plate absolute velocities evidently any differences in the shear flows at the base of the pacific and antarctic plates do not affect seamount emplacement systematic variations in o and are observed as a function of lithospheric age with the number of large seamounts increasing more rapidly than small seamounts these observations have been used to develop a simple model for seamount production under the assumptions that 1 an exponential size frequency distribution is maintained 2 production is steady state and 3 most small seamounts are formed on or near the ridge axis the limited data available from this study appear to be consistent with the model but they are insufficient to provide a rigorous test of the assumptions or determine accurately the model parameters however the data from the south pacific indicate that the off axis production of large seamounts probably accounts for the majority of seamounts with summit heights greater than 1000 m

a seafloor and sea surface gravity survey of axial volcano seafloor and sea surface gravity measurements are used to model the internal density structure of axial volcano seafloor measurements made at 53 sites within and adjacent to the axial volcano summit caldera provide constraints on the fine scale density structure shipboard gravity measurements made along 540 km of track line above axial volcano and adjacent portions of the juan de fuca ridge provide constraints on the density over a broader region and on the isostatic compensation the seafloor gravity anomalies give an average density of 2 7 g cm 3 for the uppermost portion of axial volcano the sea surface gravity anomalies yield a local compensation parameter of 23 significantly less than expected for a volcanic edifice built on zero age lithosphere three dimensional ideal body models of the seafloor gravity measurements suggest that low density material with a density contrast of at least 0 15 g cm 3 may be located underneath the summit caldera the data are consistent with low density material at shallow depths near the southern portion of the caldera dipping downward to the north the correlation of shallow low density material and surface expressions of recent volcanic activity fresh lavas and high temperature hydrothermal venting suggests a zone of highly porous crust seminorm minimization modeling of the surface gravity measurements also suggest a low density region under the central portion of axial volcano the presence of low density material beneath axial caldera suggests a partially molten magma chamber at depth

crustal structure of the madeira tore rise eastern north atlantic results of a dobs wide angle and normal incidence seismic experiment in the josephine seamount region summary during november december 1988 an extensive geophysical data set was collected over the josephine seamount which is located at the northeasterly end of the madeira tore rise in the eastern north atlantic the josephine seamount lies at the intersection of the madeira tore rise and azores gibraltar fracture zone the latter representing the africa eurasia plate boundary in this part of the eastern north atlantic from this data set a 275 km long explosive refraction line has been modelled together with wide angle airgun profiles seismic reflection and gravity data the velocity depth model shows that the crust either side of the seamount is typically oceanic in character however beneath the seamount there exists a region of anomalously high velocity and crustal thickening to a depth of about 17 18 km gravity modelling also suggests that the josephine seamount is compensated by a crustal root and that the josephine seamount madeira tore rise system is in local isostatic equilibrium calculations of the flexural rigidity and effective elastic thickness of the lithosphere in this region suggest that the madeira tore rise formed contemporaneously with the lithosphere on which it lies this age of crustal loading is consistent with the proposal that the madeira tore rise is an aseismic ridge which formed at or near the mid atlantic ridge

nonuniform magnetization of jasper seamount paleopoles derived from seamounts have been used to reconstruct the tectonic history of ocean basins however the interpretation of seamount magnetization models and the validity of seamount paleopoles may be affected by inhomogeneous magnetization multibeam bathymetric data sea surface and deep tow magnetic field data and paleomagnetic analyses of dredged samples were used to examine the origin of nonuniform magnetization within jasper seamount 30 27 n 122 44 w models indicate that the seamount is predominantly reversely magnetized with local zones of normal polarity as corroborated by deep tow measurements lithologies likely to be volumetrically important in a seamount edifice show highly variable magnetic properties basalts have high intensities 0 5 27 0 a m high koenigsberger ratios q and low viscous remanence vrm acquisition low q ratios and high vrm acquisition coefficients of coarse grained material and volcaniclastics suggest that they may have substantial viscous and induced components models for jasper are characterized by low uniform intensities and far sided paleopoles the shallow model inclinations may be attributed to nondipolar components in the time averaged geomagnetic field the low intensities of the uniform models and the large nonuniform component in the seminorm solutions imply a complex distribution of magnetization sources within jasper this nonuniformity may result from either lithological variability or construction of the seamount spanning two or more polarity intervals

the three dimensional seismic velocity structure of the east pacific rise near latitude 9 30 n three dimensional images of crustal seismic structure beneath the east pacific rise show pronounced axial heterogeneity over distances of a few kilometres a linear high velocity anomaly approximately 1 2 km in width and restricted to the uppermost 1 km of the crust is centred on the rise axis an axial low velocity anomaly at depths of 1 3 km varies in amplitude along the axis consistent with a zone of higher crustal temperatures midway between two discontinuities in the morphology of the rise axis this apparent thermal segmentation along axis is consistent with injection of mantle derived melt midway along a locally linear 12 km long segment of the rise

abundances distribution and sizes of volcanoes in the pacific ocean and implications for the origin of non hotspot volcanoes abstract in this paper i present data on the abundances sizes and crustal age for all volcanoes volcanic islands and seamounts which appear on published bathymetric charts of the pacific ocean these new data shed light on the origin of non hotspot volcanoes and are important in combination with data on the chemical compositions of seamounts and volcanic islands for estimates of the bulk composition of ocean crust these data also provide firm constraints on off ridge oceanic volcanism models results of this study show that the size frequency distribution of pacific volcanoes is poisson like and that the smallest volcanoes are much more abundant than large ones this study shows clearly that the most abundant volcanoes on the earth are the submerged oceanic volcanoes which comprise 5 25 of the oceanic volcanic layer on pacific crust of eocene age and younger the abundance of volcanoes number of volcanoes per unit area increases monotonically with increasing age assuming steady state the production rate of new off ridge volcanoes number of volcanoes per unit area per unit time is inversely proportional to the square root of the lithosphere age 1 on crust older than eocene the number of volcanoes per unit area of crust decreases monotonically with increasing age however the total volume of lava represented by these edifices increases with increasing age size frequency distributions of volcanoes on swaths of successively older crust indicate that these abundance patterns are partly due to the effect of sediment burial of small edifices on old pacific crust as well as the effect of increased lithosphere thickness on seamount size these general patterns are not appreciably changed by omitting from consideration known hotspot volcanoes 2 and volcanoes built at fossil constructional plate margins 3

geology and petrology of jasper seamount fifteen dredges on the summit and upper flanks of jasper seamount 122 44 w 30 27 n recovered a wide variety of lithologies including pillow lavas vesicular lapillistones from shallow submarine explosive volcanism and a range of xenoliths on the basis of dredge locations geochemical characteristics and 40ar 39ar age data three distinct phases of volcanism can be distinguished a shield building tholeiitic transitional phase flank transitional series fts followed by a flank alkalic series fas and a late stage summit alkalic series sas all three series consist exclusively of differentiated mg 54 to 21 mg mg2 mg2 fe2 compositions the fts represents a low pressure differentiation trend from tholeiitic transitional basalts to quartz normative residual liquids and probably accounts for more than 90 of the volume of jasper 40ar 39ar age data the dominant reversed polarity of jasper and a plausible duration 1 m y for shield construction suggest fts volcanism began about 11 ma and ended about 10 ma fts lavas probably erupted from a nw trending hotspot track parallel rift system the intermediate alkalinity fas lavas which probably comprise 3 8 of the volume of jasper erupted from 8 7 to 7 5 ma possibly after a brief volcanic hiatus or period of reduced eruptive activity normative projections suggest the fas lavas are the product of fractionation or equilibration at elevated pressures the hawaiites and mugearites of the sas erupted between 4 8 and 4 1 ma after a probable 2 7 m y period of volcanic quiescence and probably constitute 1 of the seamount volume a suite of xenoliths incorporated in sas lavas includes 1 tholeiitic basalt fragments from either the ocean crust or seamount interior 2 a range of differentiated gabbros largely derived from the ocean crust 3 residual mantle spinel lherzolites and 4 pyroxenite and peridotite cumulates the abundance of crustal gabbro and spinel lherzolite xenoliths in evolved lavas of the sas suggests that these lavas probably fractionated in a magma chamber at the crust mantle boundary the occurrence of orthopyroxene bearing alkalic cumulate xenoliths in these lavas however is enigmatic and may reflect complexities such as magma mixing or the inappropriateness of pressure estimates the sas vents of jasper define a ne sw volcanic trend which is orthogonal to the fts rift the pattern of volcanic activity including periods of volcanic quiescence and the general increase in alkalinity as well as the structural reorganization of magmatic feeder systems of jasper seamount is strikingly similar to the patterns observed on hawaiian volcanoes thus our data from jasper 690 km3 extend the concepts of structural and petrological evolution of hotspot volcanoes based on hawaii to moderate sized seamounts

eruption styles of basalt in oceanic spreading ridges and seamounts effect of magma temperature and viscosity basalts erupted along oceanic zones of spreading and on intraplate seamounts at depths where hydroexplosions are not likely follow various styles of eruption quiet eruptions produce a variety of forms including lava lakes and sheet pillow and pavingstone lavas reactive eruptions occur instead when the lava is thermally shattered as in the formation of hyaloclastites the stresses induced in a rapidly cooling lava flow are important in determining the different lava forms with lava lakes and sheet flows resulting from low stresses pillows from higher stresses and hyaloclastites from even higher stresses which cause rupture calculations show that if we consider basaltic lavas of the same composition the intensity of the stresses induced in the cooling lava increases proportionally to the initial viscosity and inversely to the temperature and rate of discharge of the lava at eruption the viscosity is strongly dependent on the crystal liquid ratio of the magma at eruption the temperature and viscosity of the lava at eruption are related ultimately to the thermal regime of the lithosphere below viscosity tends to increase going from fast to slow spreading ridges and to intraplate seamounts within a given ridge segment temperature tends to decrease and viscosity to increase approaching a transform or overlapping boundary thus the model predicts that going from fast to slow spreading ridges i e from east pacific rise to mid atlantic ridge the probability of forming lava lakes and sheet flows decreases and the probability of forming pillow flows increases moreover within a given ridge segment the areal sheet to pillow ratio should decrease as a transform or overlapping boundary is approached sheet flows should be significant only in the earliest stages of the growth of large intraplate seamounts pillows and hyaloclastites in later stages the probability of forming hyaloclastites should be low in spreading ridges higher in intraplate seamounts recent high resolution observations on the east pacific rise the galapagos rift the mid atlantic ridge and the red sea are consistent with the model because they show that 1 the areal ratio of lava lakes and sheet flows over pillows decreases going from fast to slow spreading ridges 2 within a given ridge segment it decreases approaching a transform and 3 hyaloclastites are important on seamounts only

jasper seamount seven million years of volcanism jasper seamount is a young mid sized 690 km sup 3 oceanic intraplate volcano located about 500 km west southwest of san diego california reliable sup 40 ar sup 39 ar age data were obtained for several milligram sized samples of 4 to 10 ma plagioclase by using a defocused laser beam to clean the samples before fusion gee and staudigel suggested that jasper seamount consists of a transitional to tholeiitic shield volcano formed by flank transitional series lavas overlain by flank alkalic series lavas and summit alkalic series lavas twenty nine individual sup 40 ar sup 39 ar laser fusion analyses on nine samples confirm the stratigraphy 10 3 10 0 ma for the flank transitonal series 8 7 7 5 ma for the flank alkalic series and 4 8 4 1 ma for the summit alkalic series the alkalinity of the lavas clearly increases with time and there appear to be 1 to 3 m y hiatuses between each series the age data are consistent with the complex magnetic anomaly of jasper however the dominant reversed polarity inferred from the anomaly suggests that most of the seamount formed at ca 11 ma prior to the onset of chron c5n the duration of volcanism of jasper seamount is slightly longer than the duration of volcanism at hawaiian volcanoes suggesting thatmore individual age data from seamounts may constrain the age of a seamount only to within about 7 m y unless the stage of volcanism can be unambiguously determined extrapolating from the results of our study similar precision in age determinations should be possible on 50 mg of 1 ma plagioclase from mid ocean ridge basalt opening new possibilities in the geochronology of young low potassium volcanic rocks less

the pliocene seamount series of la palma canary islands a pliocene submarine series of alkali basaltic pillow lavas hyaloclastites and breccias a a sheeted dike swarm b and a basal suite of gabbro and ultramafic rocks c from la palma canary islands is interpreted as a cross section through an uplifted seamount this series has been tilted to its present orientation of 50 230 plunge and azimuth probably by upwarping due to intrusions in the central portion of the island the basal plutonic complex c also includes intrusives coeval with up to 2000 m of younger subaerial alkali basaltic lavas unconformably overlying the submarine series the plutonic suite c is overlain abruptly by more than 1800 m of sills b 0 4 1 m thick on average with minor screens of lavas and breccias extrusives a form a 1750 m thick sequence of pillow lavas breccias and hyaloclastites the clastic rocks increase in abundance upward and are of four main types 1 breccias consisting of partly broken pillows formed nearly in situ 2 heterolithologic pillow fragment breccias 3 hyaloclastites composed dominantly of highly vesicular lapilli and ash sized shards the latter thought to have formed by near surface explosive eruptions and been subsequently transported downslope by mass flows 2 and 3 being interpreted to have been resedimented and 4 pillow scoria breccias from the upper 700 m of the extrusive section consisting of amoeboidal highly vesicular pillows and lava stringers and local bombs probably formed by cracking and bleeding of gas rich expanding pillow lava and some shallow submarine subaerial lava fountaining the extrusive series is chemically and mineralogically crudely zoned with the most differentiated rocks metatrachytes and mugearites at the base and most picritic lavas occurring near the top of the series subsequent to emplacement the entire extrusive and intrusive series has been hydrothermally altered the lower part to greenschist and the upper part to smectite zeolite facies mineral assemblages the la palma succession combined with evidence from surface studies of seamounts suggests that seamounts are formed by intrusive and extrusive processes in approximately equal portions the nature of eruptive clastic and depositional mechanisms changes drastically during growth of a seamount if the critical depth for major magmatic degassing is surpassed and especially if magmatic explosive processes can occur at very shallow water depth the critical depth depending on magma and thus volatile composition changes in slopes of a seamount influence depositional processes based on these factors at least three major depositional sites develop as a seamount grows summit flank and apron facies nonexplosive extrusive processes prevail in the deep water stage dominantly producing pillow lavas 75 these consist of individual pillow volcanoes up to 200 m high with large pillows near the base and decreasing pillow size toward the top of a volcano pillow breccias and pillow fragment breccias comprise approximately 20 of this facies the deep water flank and apron facies are characterized by debris flow deposits with possibjy rather dense matrix material the shallow water shoaling stage is reached when the seamount top reaches the critical depth for drastic increase in exsolution of magmatic volatiles about 800 m for the alkali basaltic seamount of la palma resulting in formation of mainly clastic rocks 70 in situ pillow rind breccias and scoriaceous amoeboidal breccias and pillows are formed in the summit regions of the seamount by repeated expansion and leaking of frothy pillow lava possibly by lava fountaining resedimented heterolithologic pillow fragment breccias lapilli breccias and hyaloclastites are deposited on the flanks and aprons of the seamount pillow lavas comprise 30 of these deposits

ocean acoustic tomography from ships mesoscale mapping of the ocean sound speed field in a 1000 1000 km area by means of ocean acoustic tomography is greatly enhanced by augmenting a few acoustic moorings with a movable ship based receiver computer simulations based on realistic noise levels in the measured acoustic travel times give 5 1 residual variance in c x y z for four six acoustic source moorings in an ocean perturbed in the gravest baroclinic mode for comparison objective mapping based on traditional vertical profiles requires 3 times the steaming distance to yield equivalent residual error detailed results depend on many parameters the assumed mesoscale spectrum and vertical mode structure the number of observed multipaths the mooring configuration the number of ship stations and the travel time signal level due to mesoscale eddies and noise level due to internal waves and position keeping errors these parameters have critical values below which there is distinct deterioration and beyond which there is little gain we believe that the critical values can be attained in practice so the ultimate limit on mesoscale mapping is imposed by the internal wave induced travel time error this assumes that position keeping of the submerged acoustic sources and receiver by a combination of satellite navigation and high frequency acoustics can be achieved with 10 m accuracy the present study assumes a stationary ocean a second paper will deal with reciprocal transmissions yielding currents and hence the barotropic mode this is required in a dynamic ocean model for estimating c x y z t all this is preparatory to a tomography experiment in the greenland sea in 1988 1989

diverse basalt types from loihi seamount hawaii loihi seamount is the southeasternmost active volcano in the hawaiian emperor volcanic chain the seamount is considered representative of the early phase of hawaiian volcanism because of its youth small size and location near the melting anomaly seventeen dredge stations recovered transitional basalt alkalic basalt and basanite in addition to the expected tholeiitic basalt four flows of alkalic basalt contain common small dunite xenoliths the recovered samples have thin palagonite rinds and almost no manganese on the glassy surfaces we estimate that the lavas are less than about 4 000 yr old and many are less than 1 000 yr old loihi seamount is apparently in a transitional growth phase between the early eruption of alkalic lavas and the commonly observed subaerial tholeiitic eruptive phase previously thought to dominate hawaiian volcanism from inception until the postcaldera collapse alkalic stage

densities and porosities in the oceanic crust and their variations with depth and age we have used drilling and downhole logging results from deep sea drilling project holes 395a and 418a measured properties of core samples from the seafloor and from ophiolites and seismic velocity structure models of the oceanic crust to constrain empirical statistical relationships between in situ densities porosities and seismic velocities and to estimate densities and porosities in the oceanic crust with the following principal results our review of drilling results and studies of ophiolites suggests that the velocity structure of the oceanic crust is more closely related to changes in porosity and alteration or metamorphic grade than to its igneous structure the transition from layer 2a 3 7 0 4 km s 1 to 2b 5 2 0 4 km s 1 occurs within the extrusive pile the top of layer 2c 6 2 0 2 km s 1 is associated with the top of the sheeted dike complex and with the first appearance of greenschist facies alteration products ophiolite studies have shown that the top of layer 3 occurs within the sheeted dikes where the metamorphic grade of alteration products increases from greenschist to amphibolite facies a comparison of reprocessed and corrected downhole logs from holes 395a and 418a indicates that in situ densities and fractional porosities differ by 0 2 mg m 3 and 0 07 respectively these differences are consistent with prevailing views of the progressive alteration of the upper oceanic crust statistical relationships between in situ density porosity and slowness are highly linear and indicate that the density slowness relation estimated from laboratory samples pb 3 81 0 02 6 0 0 1 s applies to in situ properties as well we obtained a similar empirical relationship for porosities from the hole 418a log data f 0 35 0 03 2 37 0 15 s r2 0 84 s e 0 015 applying these empirical models to seismic velocity structure models yields the following crustal densities and porosities layer 2 2 62 2 69 mg m 3 0 10 0 12 and layer 3 2 92 2 97 mg m 3 the estimated average density of the oceanic crust is 2 86 0 03 mg m 3 densities and porosities estimated from a seismic crustal structure model for old atlantic crust agree very well with the logging data from hole 418a suggesting that in situ densities and porosities can be estimated from seismic velocities high density and porosity gradients are characteristic of the upper oceanic crust increasing densities reflect both a progressive decrease of porosity and increasing grain density with depth based on the seismic properties of layer 2a we estimate that the layer thins at an approximate rate 20 to 45 m m y 1 while the velocity increases by 0 040 km s 1 m y 1 the corresponding rates of change of density and porosity are 0 016 0 009 mg m 3 m y 1 and 0 005 0 003 m y 1 respectively

jasper seamount structure seafloor seismic refraction tomography the velocity structure of jasper seamount was modeled using one and three dimensional inversions of p wave travel times the results represent the first detailed seismic images of a submerged intraplate volcano two seismic refraction experiments were completed on jasper seamount incorporating ocean bottom seismometers and navigated seafloor shots the p wave travel times were first used to compute a one dimensional velocity profile which served as a starting model for a three dimensional tomographic inversion the seamount p velocities are significantly slower than those observed in typical oceanic crust at equivalent subbasement depths this suggests that jasper seamount is constructed predominantly of extrusive lavas with high average porosity the velocity models confirm morphological predictions jasper seamount is a shield volcano with rift zone development high seismic velocities were detected beneath the large radial ridges while low velocities characterize the shallow summit and flanks comparisons between p velocity models of jasper seamount and the island of hawaii reveal that these two shield volcanoes are not structurally proportional jasper seamount is far smaller than hawaii yet both volcanoes exhibit an outer extrusive layer of similar thickness this suggests that seamount size influences the intrusive extrusive proportions density equilibrium between melt and country rock may explain this behavior

the snow mountain volcanic complex an on land seamount in the franciscan terrain california the snow mountain volcanic complex in the franciscan terrain of northern california is a 1 1 2 to 2 km thick sequence of altered submarine pillow lavas pillow breccias diabase and interbedded sediments forming a klippe that structurally overlies nearby rocks of the franciscan assemblage the coast range ophiolite and great valley sequence all of the snow mountain rocks bear a weak overprint of blue schist facies metamorphism the volcanic rocks were deposited on top of 200 300 m of interbedded turbiditic greywacke shale and radiolarian chert which are interpreted as an interfingering abyssal fan and pelagic sequence the volcanic pile is therefore supracrustal and is not an ophiolitic fragment formed at a mid ocean spreading center rather it is probably the remnant of a former seamount the lavas are mostly titanium rich transitional basalts minor associated rocks include rhyolite and peralkaline rhyolite relict clinopyroxenes in the basaltic rocks are all titanium aluminum rich augites thes

hydrochemical characteristic and interaction process of surface and groundwater in mid lower reach of hanjiang river china hanjiang river the largest tributary of yangtze faces environmental pressures from south to north water transfer project and anthropogenic influences in order to reveal the chemical characteristics of river and contribution of groundwater to the river along mid lower reaches of hanjiang river field works have been conducted in wet and dry seasons the major ion compositions are characterized by the dominance of ca2 and hco3 in surface water and groundwater as a result calcite weathering is a dominant process for surface water and groundwater additionally dolomite weathering also contributes to hydrochemical evolution in the groundwater of the basin stable isotopes 18o d show that both surface and groundwater most probably originate from present day precipitation isotopic of both surface water and groundwater can provide greater confidence in identifying the weathering and extent of interaction processes it was found that river water was mainly recharged by tributaries in wet season while recharged by groundwater in dry season dissolved solute flux from tributaries and groundwater to the river was estimated by water balance budget model and environmental tracer mass balance approach tributaries and groundwater contribute to mid lower reach of hanjiang river in wet season and dry season respectively from section h1 to h9 there were 455 4 kg s of total dissolved solute tds transferred from tributaries to the main channel and 99 8 kg s of tds fluxed from groundwater to river water in dry season

distribution of seamounts in the north atlantic the paucity of multi narrow beam bathymetric data in the ocean basins has meant that previous studies of seamount distribution and size have relied on wide beam echo sounder data and small1 8 amounts of seabeam multi narrow beam data since 1967 however the us naval oceanographic office has been collecting multi narrow beam bathymetric data9 14 here we use an extensive collection of such data to locate over 800 seamounts in the north atlantic we find that ridge crest generated seamounts are formed only between iceland and the hayes fracture zone where there are contributions of plume material to the mid ocean ridge plumbing system it appears that in the slow spreading north atlantic an injection of plume material into the ridge plumbing system is necessary for the formation of seamounts at the ridge crest

the evolution of craters and calderas on young seamounts insights from sea marc i and sea beam sonar surveys of a small seamount group near the axis of the east pacific rise at 10 n a northwest trending group of seamounts located just west of the east pacific rise epr axis has been mapped using high resolution sea marc i side scan sonar and sea beam multibeam bathymetry these seamounts have 9 13 km basal diameters and the shapes of the volcanos change from conical domes near the epr axis to truncated cones and larger elliptical truncated cones farther away from the rise axis the detailed resolution of constructional volcanic features on these seamounts and the progressive development of summit depressions craters to large complex calderas and summit plateaus suggest to us a succession of evolutionary stages through which seamounts grow our evolutionary scheme for constructing these volcanos calls for the development of satellite extrusive centers around the base of a growing seamount which serve to drain off magmas and precipitate summit collapse the creation of a summit depression on a seamount will structurally constrain the style of eruptive volcanism to be principally along ring fractures which localize vents around the margin of a crater or caldera the concentration and channelization of volcanic eruptions around the top of the volcano lead to a progradation of the summit area and the establishment of a summit plateau the steep sides of a seamount reflect growth of the summit area out over the upper flanks of the volcano caused by continued eruption of lavas around the margin of the caldera the steep constructional slopes are sites for vigorous mass wasting that shapes and modifies flank terrain as a seamount evolves the crestal topography is the product of many volcanic cycles characterized by filling and subsequent draining of the caldera and expansion of caldera area through progressive collapse and migration of primary magmatic conduits the final phase of volcanism may be a calderafilling episode that builds a rough conical mount over the summit plateau

tomographic inversion of local earthquake data from the hengill grensdalur central volcano complex iceland parameters is generally low however a quantitative analysis of resolution provides an unambiguous assessment of well resolved volumes within the better resolved regions of the model the averaging volumes are 1 2 km and 2 4 km in vertical and horizontal extent respectively results of tomographic inversion image three distinct bodies of anomalously high velocity two of these extend from near the surface to a depth of about 3 km these high velocity volumes are located directly beneath the surface expressions of the extinct grensdalur volcano and the extinct husmuli basalt shield the third high velocity structure occurs in the depth range of 3 4 km but does not extend to the surface these three high velocity bodies are interpreted to be solidified magmatic intrusions relatively low velocities underlay limited portions of the trace of the present accretionary axis and a low velocity body is imaged in the roots of the active hengill volcano the volume of lower velocities located beneath the surface expression of the hengill volcano is interpreted to be a region of partial melt

hawaiian xenolith populations magma supply rates and development of magma chambers hawaiian volcanoes pass through a sequence of four eruptive stages characterized by distinct lava types magma supply rates and xenolith populations magma supply rates are low in the earliest and two latest alkalic stages and high in the tholeiitic second stage magma storage reservoirs develop at shallow and intermediate depths as the magma supply rate increases during the earliest stage magma in these reservoirs solidifies as the supply rate declines during the alkalic third stage these magma storage reservoirs function as hydraulic filters and remove dense xenoliths that the ascending magma has entrained during the earliest and latest stages no magma storage zone exists and mantle xenoliths of lherzolite are carried to the surface in primitive alkalic lava during the tholeiitic second stage magma storage reservoirs develop and persist both at the base of the ocean crust and 3 7 km below the caldera only xenoliths of shallow origin are carried to the surface by differentiated lava during the alkalic third stage magma in the shallow subcaldera reservoir solidifies and crustal xenoliths including oceanic crustal rocks are carried to the surface in lava that fractionates in an intermediate depth reservoir worldwide xenolith populations in tholeiitic and alkalic lava may reflect the presence or absence of subvolcanic magma storage reservoirs

structure of young oceanic crust at 13 n on the east pacific rise from expanding spread profiles we present the results of the analysis of expanding spread profiles esps collected on and near the axis of the east pacific rise at 13 n these profiles were collected at 0 1 1 2 1 3 6 and 9 5 km from the rise axis and all but the most distant profile show a distinct low velocity zone lvz located within layer 3 of the oceanic crust at the ridge crest the top of the magma chamber is at the base of layer 2 while 3 6 km off axis the roof of the lvz is 1 1 km below the top of layer 3 the profile farthest from the ridge could possibly have a residual lvz confined to the lower 1 5 km of the crust the total width of the lvz as determined from the esp data is at least 6 km and possibly much greater this wide lvz apparently contradicts multichannel seismic data which show cross axis reflections from the magma chamber with a width of 5 km we suggest that a resolution of this apparent contradiction lies in a model of the rise axis with a small and transient central magma chamber of high partial melt fraction surrounded by a much larger and permanent region of hot rock with only isolated pockets of partial melt the esp data are sensitive to this larger region while the reflection data accurately map the presence or absence of the central magma chamber with its high impedance contrast we identify the presence of a layer at the top of the oceanic crust with initial p wave velocities between 2 35 and 2 6 km s while the s wave velocity is estimated as being 0 8 km s the layer thickness lies between 100 and 200 m these velocities are consistent with previous estimates for the pacific and recent results for the atlantic the thickness of this layer is consistent with that of layer 2a determined from geophysical measurements at deep sea drilling project hole 504b

seismic tomography of jasper seamount a vertical section of the interior structure of jasper seamount was modeled using a spectral tomographic inversion of p wave travel times an array of ocean bottom seismographs obss deployed over the seamount detected the arrivals from a series of ocean bottom shots a reference velocity model reveals that average compressional velocities within the seamount are similar to those found within kilauea and are consistently slower than velocities at equivalent depths in typical oceanic crust this suggests jasper seamount has a high average porosity perturbations from the reference model were imaged by tomographic inversion a high velocity zone within the northwest flank of the seamount may result from dikes associated with a radial rift or from a shallow solidified magma reservoir a low velocity summit may result from shallow explosive eruptions the tomographic model is consistent with the results of gravity magnetic and dredging analyses

seismic detection of the summit magma complex of kilauea volcano hawaii application of simultaneous inversion of seismic p wave arrival time data to the investigation of the crust beneath kilauea volcano yields a detailed picture of the volcano s heterogeneous structure zones of anomalously high seismic velocity are found associated with the volcano s rift zones a low velocity zone at shallow depth directly beneath the caldera coincides with an aseismic region interpreted as being the locus of kilauea s summit magma complex