weathering rates of historic sandstone structures in semiarid environments ebro basin ne spain abstract long term weathering rates of sandstones were obtained for well dated from 400 to 1100 years bp historic structures wall sections of the alberuela castle in a semiarid environment of the central ebro basin ne spain using an exclusive movable contour plotting frame this sandstone has a low durability controlled mainly by textural properties differences observed in weathering rates are clearly controlled by the age of building as well as the orientation of the studied sandstone wall sections weathering processes related to insolation control the asymmetric spatial pattern of weathering rates in surfaces exposed for since 1100 bp which are higher in southeast facing sandstone wall sections 8 4 mm 100 years than in southwestern 4 4 mm 100 years and northern faces 3 9 mm 100 years in addition exponential increase in weathering rates with time from 1 6 2 2 mm 100 years in surfaces exposed for since 800 400 bp to 8 4 mm 100 years in surfaces exposed for since 1100 bp in southeastern faces may be probably related to environmental changes during the last millennium

coral ages and island subsidence hilo drill hole a 25 8 m thick sedimentary section containing coral fragments occurs directly below a surface lava flow the 1340 year old panaewa lava flow at the hilo drill hole ten coral samples from this section dated by accelerator mass spectrometry ams radiocarbon and five by thermal infrared multispectral scanner tims 230th u methods show good agreement the calcareous unit is 9790 years old at the bottom and 1690 years old at the top and was deposited in a shallow lagoon behind an actively growing reef this sedimentary unit is underlain by a 34 m thick lava flow which in turn overlies a thin volcaniclastic silt with coral fragments that yield a single 14c date of 10 340 years the age depth relations of the dated samples can be compared with proposed eustatic sea level curves after allowance for island subsidence is taken island subsidence averages 2 2 mm yr for the last 47 years based on measurements from a tide gage near the drill hole or 2 5 2 6 mm yr for the last 500 000 years based on the ages and depths of a series of drowned coral reefs offshore from west hawaii the age depth measurements of coral fragments are more consistent with eustatic sea levels as determined by coral dating at barbados and albrolhos islands than those based on oxygen isotopic data from deep sea cores the panaewa lava flow entered a lagoon underlain by coral debris and covered the drill site with 30 9 m of lava of which 11 m was above sea level this surface has now subsided to 4 2 m above sea level but it demonstrates how a modern lava flow entering hilo bay would not only change the coastline but could extensively modify the offshore shelf

paleointensity of the geomagnetic field from dated lavas of the chaine des puys france 1 7 12 thousand years before present paleointensities were determined by the thellier method for six dated lava flows from the chaine des puys france with ages from 7 to 12 ka ka kiloannum before present the three younger flows overlap the archaeomagnetic period for europe and the older flows extend the record to more ancient times our results support the quasi sinusoidal variation of the geomagnetic field intensity since 10 ka for europe punctuated by a peak virtual dipole moment of about 11 1022 a m2 between 8 and 10 ka the data from 10 to 12 ka show considerably lower virtual dipole moments between 4 and 5 1022 a m2 about 60 of the global average since 10 ka when compared with available worldwide data the maximum in the virtual dipole moment between 8 and 10 ka is supported by measurements from japan and possibly hawaii lower than average virtual dipole moments between 10 and 12 ka have also been observed in north america and hawaii the existing data suggest that from 8 to 10 ka the geomagnetic field supported a dipole moment up to 30 above and from 10 to 12 ka about 30 below the average value since 10 ka

the time averaged geomagnetic field global and regional biases for 0 5 ma summary palaeodirectional data from lava flows and marine sediments provide information about the long term structure and variability in the geomagnetic field we present a detailed analysis of the internal consistency and reliability of global compilations of sediment and lava flow data time averaged field models are constructed for normal and reverse polarity periods for the past 5 ma using the combined data sets nonzonal models are required to satisfy the lava flow data but not those from sediments alone this is in part because the sediment data are much noisier than those from lavas but is also a consequence of the site distributions and the way that inclination data sample the geomagnetic field generated in the earth s core different average field configurations for normal and reverse polarity periods are consistent with the palaeomagnetic directions however the differences are insignificant relative to the uncertainty in the average field models thus previous inferences of non antipodal normal and reverse polarity field geometries will need to be re examined using recently collected high quality palaeomagnetic data our new models indicate that current global sediment and lava flow data sets combined do not permit the unambiguous detection of northern hemisphere flux lobes in the 0 5 ma time averaged field highlighting the need for the collection of additional high latitude palaeomagnetic data anomalous time averaged field structure is seen in the pacific hemisphere centred just south of hawaii the location of the anomaly coincides with heterogeneities in the lower mantle inferred from seismological data the seismic observations can be partly explained by lateral temperature variations however they also suggest the presence of lateral compositional variations and or the presence of partial melt the role of such heterogeneities in influencing the geomagnetic field observed at the earth s surface remains an unresolved issue requiring higher resolution time averaged geomagnetic field models along with the integration of future results from seismology mineral physics and numerical simulations

geomagnetic field intensity between 70 000 and 130 000 years b p from a volcanic sequence on la reunion indian ocean abstract a detailed paleointensity study was made of a sequence of 70 successive lava flows of the piton des neiges volcano on the island of la reunion indian ocean radiometric dating brackets the age of this sequence between 130 3 ka and 72 3 ka rock magnetic investigations show that titanomagnetites in the pseudo single domain range are the main magnetic carrier of the natural remanent magnetisation nrm over 350 samples were used for paleointensity determinations carried out with the thellier method in vacuum or in an argon atmosphere of these 89 samples yielded reliable results with within flow scatter often lower than 20 these results indicate that the geomagnetic field intensity has varied at la reunion between 13 and 65 t during the period of time explored the average value 42 t is higher than the present field at la reunion 35 t the results from the upper part of the section are consistent with previous results obtained for the 82 98 ka period also at la reunion 1 and document a broad low around 95 ka not associated with large directional changes on a larger geographic scale the paleointensity values from la reunion are significantly higher than those obtained from mount etna 2 precise comparison is however difficult because of the lack of detail in the etna results in the lower part of the section a marked intensity low coinciding with significant deviation from the dipole field direction is observed at 115 ka and could correspond to the end of the blake event

preliminary paleointensity measurements and detailed magnetic analyses of basalts from the skalamaelifell excursion southwest iceland among the many excursions reported in the late brunhes the skalamaelifell excursion recorded in 22 flows in three hills of assumed late wisconsin basalts in the active rift zone of the reykjanes peninsula is one of the most reliable it is characterized by a single well defined transitional direction d 260 i 16 paleointensity pi measurements using the thellier method as modified by coe in a vacuum with partial thermoremanent magnetization checks were made on six samples from the excursion flows for comparison 10 samples from the adjacent and probably nearly contemporaneous normally magnetized flows were also measured because of sample scarcity four of the excursion samples and seven of the normal samples had been already af demagnetized to 20 mt in iceland the samples however were chosen because of their high coercivity and lack of viscous remanent magnetization they retained 70 95 of their natural remanent magnetization nrm after 20 mt demagnetization and their remanence directions were stable during the 20 mt demagnetization and during 2 6 years of storage following it saturation magnetization js measurements suggest that the reason for the low nrm of the excursion samples was a paleofield one fourth to one tenth that of the normal basalts the mean pi from four excursion samples is 4 3 0 6 t whereas the mean pi of the five normal samples considered reliable is 30 9 t the quality of the nrm thermoremanent magnetization trm data correlated well with the reversibility of the js t curves the pi differences agree in general with those predicted by the nrm js data especially when comparing the means of the two groups which average the effects of different grain sizes a prior partial alternating field af demagnetization of one sample in each of two pairs from the same cores caused the pi to be underestimated by only 10 continuous af demagnetization during the nrm trm measurements of one sample in another pair that had nonlinear trm trm plots even in the undemagnetized sample caused the pi to be overestimated by 40 these results combined with other studies suggest that the effect of partial af demagnetization on high coercivity samples depends on when the demagnetization is done i e only prior to or during the nrm trm measurements and probably more importantly the thermochemical stability of the sample if correct this increases the usefulness for pi determinations of old hard to recollect partially demagnetized collections the very low pi of 4 t is similar to that found for excursions and transitions in iceland and other places however o its virtual geomagnetic pole lies 90 from a north south vertical plane through the sites which rules out a purely axisymmetric model for the excursion the low normal intensities are similar to those found in the chaine des puys france and elsewhere for the period 10 0000 50 000 years b p given its very low pi and a direction only 70 from complete reversal the skalamaelifell excursion is most likely the result of nondipole fields and an unusually weak dipole field such as that observed in france for the 10 000 year interval 35 000 45 000 years b p surrounding the laschamp excursion

simulating the growth of tafoni throughout the world large caves in rocks tafoni are found which originate from salt weathering the mechanisms that control their development are poorly understood the growth of tafoni has been studied with a model that describes how a rock surface containing a small pit disintegrates by salt crystallization during wetting drying cycles in the model the rock is mapped on a grid the migration and crystallization of salts are simulated explicitly in the drying phase of a cycle at the end of each wetting drying cycle the amount of salt deposited at the grid nodes is evaluated and the shape of the rock surface is adjusted by removing nodes the length of the drying period in a single cycle proved to be the key parameter for short drying periods the amount of crystallized salt at the surface is proportional to the drying rate therefore for short drying periods most salts are deposited at the more exterior parts of the rock surface as a result most damage will develop at these parts of the surface which results in smooth surfaces due to the characteristics of the drying process for long drying periods most salts accumulate at regions with low evaporation rates which are the sheltered parts of the rock surface these parts are not exposed to the wind or the sun as a result the pit grows and a tafone develops copyright 2004 john wiley sons ltd

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

origin and development of tafoni in tunnel spring tuff crystal peak utah usa bedding parallel tafoni are well developed over much of the surface of the tunnel spring tuff oligocene exposed in 300 m high crystal peak an inselberg the tunnel spring tuff is a crudely stratified non welded rhyolite ash flow tuff with 30 per cent porosity clasts of palaeozoic dolomite limestone and quartzite make up 10 per cent of the tuff the tafoni are remarkable because of their size up to 20 m wide but rarely wider than 4 m shape of the openings spherical arch like or crescent shaped and abundance up to 50 per cent of an outcrop face they are actively forming today calcite is abundant 10 to 40 per cent by weight in tafoni as an efflorescence in spalling flakes of tuff on their roofs and walls halite and gypsum generally make up less than 0 01 per cent of the efflorescence the absence of corroded quartz and feldspar grains in spall fragments indicates that chemical weathering is unimportant in development of the tafoni calcite aragonite halite and gypsum dust from modern salt pans less than 20 km from crystal peak are potential sources of salt in the tuff but the prevailing winds are in the wrong direction for significant amounts of these evaporite minerals to reach the inselberg calcite is the only evaporite mineral present in the tafoni in more than trace amounts and this mineral is readily available within the tuff itself as a result of rock weathering we propose that meteoric water containing carbonic acid infiltrates the tuff dissolves carbonate clasts and migrates to the steep flanks 20 of the peak through abundant megapores and micropores there it evaporates and precipitates calcite crystallization pressure spalls off grains and sheets as the physical manifestation of salt weathering the quasi uniform spacing of tafoni suggests that a self organization process is active in the water flow copyright 2000 john wiley sons ltd

a morphological study of greenschist weathering on dated coastal structures south devon uk three dated structures up to 450 years in age display the effects of coastal weathering of the greenschist of which they are constructed a variety of weathering forms is present the various topographic surfaces of the structures create variation in weathering environments and consequent weathering processes and rates weathering is enhanced by direct exposure to salt bearing spray and by humid conditions and apparently limited by direct exposure to solar radiation the maximum rates of weathering on the three surfaces approximate to 0 6 mm a 1 over this period consistent with measured contemporary weathering rates for a natural surface formed by this rock type in a nearby coastal location 1997 by john wiley sons ltd

geomagnetic field intensity during the last 5 myr ltd dht shaw palaeointensities from volcanic rocks of the society islands french polynesia summary on the basis of the thellier palaeointensities reported so far it has been thought that the time averaged virtual dipole moment vdm and virtual axial dipole moment vadm for the last few million years are almost the same as the present geomagnetic dipole moment 8 1022 a m2 this estimate has been called into question however because recent studies have revealed that the thellier method occasionally overestimates palaeointensities by as much as twice the true values in contrast a recently developed palaeointensity technique namely the double heating technique of the shaw method combined with low temperature demagnetization the ltd dht shaw method can yield expected field intensities from samples which give unreliable palaeointensities using the thellier method therefore we have measured absolute palaeointensities from 0 5 4 6 ma volcanic rocks from the society islands french polynesia mainly using the ltd dht shaw method as a result 195 out of 361 samples passed the selection criteria some of which are compared with additional results obtained with coe s version of the thellier method in the thellier experiments 18 out of 40 samples passed the criteria giving palaeointensities both consistent and inconsistent with the ltd dht shaw results these samples are characterized mostly by two segmented arai diagrams if we take the ltd dht shaw palaeointensities 24 reliable site means are available and give a mean vadm of 3 64 2 10 1022 a m2 this is nearly half of the mean of the 0 5 ma thellier data selected from the latest palaeointensity database 7 46 3 10 1022 a m2 n 458 as well as the present dipole moment the ltd dht shaw palaeointensities newly determined in this study suggest that the present day field is so strong that it may not be typical of the past geomagnetic field

the variation of intensity of earth s magnetic field with time abstract a new and larger database is established to assess the variation of paleointensity with time it is shown that submarine basaltic glass probably contains a grain growth chemical remanent magnetization in magnetite intensity estimates from these samples are used to establish a lower bound estimate on intensities statistical tests on various subsets of the database are used to establish a more reliable database for finding long term trends of the paleointensity we can obtain a reasonable estimate for intensity versus time for the cenozoic but not for the mesozoic or late paleozoic the distributions of data as a function of time also provide valuable information in particular the two well documented superchrons exhibit different intensity properties which suggests that there is not a simple correlation between reversal rate and intensity

the gilsa excursion and the matuyama brunhes transition recorded in 40ar 39ar dated lavas from lanai and maui hawaiian islands summary directional results thellier type palaeointensity determinations and ar ar ages are presented from volcanic units of pleistocene age from the hawaiian islands of maui and lanai twenty nine non contiguous lava flows were sampled on lanai the lava flows formed during the late matuyama polarity chron about 1 6 ma and recorded the gilsa geomagnetic polarity excursion the obtained ages of 1 6 ma for the lanai lavas are significantly older than ages previously obtained for this island these new ages are however in agreement with the age progression of the hawaiian hotspot the palaeomagnetic record over the sampled succession is characterized by reversed and intermediate directions palaeointensities are generally very low about half the present day field intensity dropping to values of 5 t during transitional field states transitional virtual geomagnetic poles vgps from lanai are situated first near the west coast of south america and then switch to offshore western australia normal polarity is never reached in the sampled succession transitional vgps located close to australia accompanied by relatively low palaeointensities are found in most records of short geomagnetic excursions during the matuyama polarity chron e g the cobb mountain subchron the punaru event and the matuyama brunhes precursor suggesting very similar physical processes likely related to dipolar dominance during the onset of these excursions and influences of lower mantle heterogeneities approximately 800 kyr after the gilsa excursion lava flows on maui recorded the last reversal the matuyama brunhes transition fourteen lava flows were sampled which recorded the pre and post transitional behaviour of the earth s magnetic field the geomagnetic field intensity is low prior to the reversal approximately 8 t and increases strongly afterwards up to 63 t these values of about twice the present field intensity of hawaii after the transition emphasize a strong asymmetry between pre and post transitional fields which is particularly strong in the region of hawaii

paleomagnetic field intensity recorded in submarine basaltic glass from the east pacific rise the last 69 ka paleointensity determinations were obtained from basaltic glass recovered from the northern east pacific rise using the thelliers 1959 method as applied by pick and tauxe 1993a the samples are up to a69 ka old based on their distance from the ridge axis and u series disequilibria measurements the results show a general increase in paleointensity of the field from 69 ka to the present with values ranging from 50 higher than the present field to as low as 26 of the present dipole field the average dipole moment is 8 32 1022 am which is similar to the present axial dipole moment the results are in general agreement with other absolute and relative paleointensity records

cavernous weathering in the capitol reef desert utah tafoni and honeycomb weathering are abundant in sandstones at capitol reef national park cavities are particularly common in talus blocks resting on alkali rich soil in vertical walls of desert washes and in sandstones containing calcareous cement chemical analyses x ray diffraction data and field observations suggest that salt weathering is the most important cause of disintegration but dissolution of calcite cement also occurs in calcareous sandstones cavernous weathering at capitol reef produces cavities similar in appearance to features found in arkose outcrops along the coast of northwest washington even though the environmental conditions at these two locations are very different

paleointensity study of early miocene lavas from pingzhuang inner mongolia china 1 a paleomagnetic and geochronologic investigation has been conducted on 17 lava flows from inner mongolia north china radiometric k ar dating experiments give an age between 23 and 22 ma for these volcanic units rock magnetic measurements of thermomagnetic curves and hysteresis loops show the primary carrier of remanence to be pseudo single domain titanomagnetite stepwise thermal demagnetization isolated a well defined direction of characteristic remanent magnetization chrm in all studied lava flows with a mean direction of d i 359 8 62 2 95 3 8 fifty samples from 11 lavas were pre selected on the basis of thermomagnetic analyses to carry out thellier thellier paleointensity experiments with systematic partial thermoremanent magnetization ptrm checks twenty three samples from 7 lavas yield reliable paleointensity estimates with a mean virtual dipole moment vdm of 3 9 0 9 1022 am2 about half the strength of the modern day field

paleointensity high at 9000 years ago from volcanic rocks in japan six paleointensities were determined from young lava flows of fuji and oshima volcanoes whose ages are around 8000 to 15000 years b p two lava flows which have well defined ages at 8970 390 years b p and 9260 430 years b p give large paleointensities of 67 12 and 65 6 t respectively another lava flow of less accurate age also gives large paleointensity of 63 6 t the new paleointensity data are consistent with the trend of averaged paleointensity variation in japan for the last 9000 years they form a peak of paleointensity high at around 9000 years b p comparable to the well documented one at 2000 years b p this suggests that there exists a dominant component with a characteristic period of about 7000 years in paleointensity variation large paleointensity at about 8000 10000 years ago is also reported in france and iceland paleointensity variation averaged every 1000 years for the last 14000 years is quite similar in europe and japan the sine curve like variation of paleointensity observed in both europe and japan probably reflects the nature of the dipole moment if we consider much difference of longitude between the two regions

paleointensities for 10 22 ka from volcanic rocks in japan and new zealand abstract eight volcanic rocks from japan and new zealand in the age range 10 50 ka b p were studied using the thellier method paleointensity experiment and yielded five successful results for 10 22 ka samples from the shikotsu kuttara and daisen volcanoes in japan were taken from pyroclastic flows both welded and non welded and a pumice fall deposit one of the two new zealand rocks is a rhyolite lava from the taupo volcanic zone four sites out of seven located on rocks that are not usually subjected to thellier s experiment yielded successful results which indicates that volcanic products such as pyroclastic flows and rhyolite lavas are as good a material as basalt and andesite lavas for application of the paleointensity experiment one paleointensity from new zealand at 9 9 ka is very large at about 100 t and provides support for the paleointensity high found in europe and japan 1 2 the other four paleointensities for 14 22 ka are smaller than the present day value and this agrees with the idea of a broad paleointensity minimum for the period 10 50 ka that was suggested by mcelhinny and senanayake 3 it is also noted that the variation curve of absolute paleointensity from the volcanic rocks is reasonably similar to the relative paleointensity curves from marine and lake sediments which were summarized by tauxe 4

salt weathering influence of evaporation rate supersaturation and crystallization pattern micro and macroscale experiments which document the dynamics of salt damage to porous stone have yielded data which expose weaknesses in earlier interpretations previously unexplained differences are found in crystal morphology crystallization patterns kinetics and substrate damage when comparing the growth of mirabilite na2so4 10h2o and thenardite na2so4 versus halite nacl the crystallization pattern of sodium sulphate was strongly affected by relative humidity rh while a lesser rh effect was observed for sodium chloride macroscale experiments confirmed that mirabilite crystallizing at rh 50 per cent and thenardite crystallizing at rh 50 per cent tend to form subflorescence in highly localized areas under conditions of constant rh and temperature this crystallization pattern was more damaging than that of halite since halite tended to grow as efflorescence or by filling the smallest pores of the stone in a homogeneous fashion a result which contradicts wellman and wilson s theoretical model of salt damage low rh promoted rapid evaporation of saline solutions and higher supersaturation levels resulting in the greatest damage to the stone in the case of both sodium sulphate and sodium chloride crystallization at any particular crystallization condition sodium chloride tended to reach lower supersaturation levels resulting in the crystallization of isometric crystals and created negligible damage while sodium sulphate reached higher supersaturation ratios resulting in non equilibrium crystal shapes resulting in significant damage esem showed no damage from sodium sulphate due to hydration instead after water condensation on thenardite crystals rapid dissolution followed by precipitation of mirabilite took place resulting in stone damage by means of crystallization pressure generation it is concluded that salt damage due to crystallization pressure appears to be largely a function of solution supersaturation ratio and location of crystallization these key factors are related to solution properties and evaporation rates which are constrained by solution composition environmental conditions substrate properties and salt crystallization growth patterns when combined with a critical review of salt damage literature these experiments allow the development of a model which explains variations in damage related to combinations of different salts substrates and environmental conditions

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

weathering of coastal defensive structures in southwest england a 500 year stone durability trial historic structures can be viewed as exposure trials of the stone of which they are constructed as such they represent a geomorphological weathering experiment several structures of henrician sixteenth century and greater age on the coast of southwest england have been exposed to coastal salt weathering for 500 600 years long term weathering rates on five different rock groups are derived from careful study of weathering depths and forms there is significant variation in weathering rate between five major rock groups rank ordering of weathering rate values reveals a durability order of these rock groups which is confirmed by local juxtapositions controls on rock durability in the coastal weathering environment include both mechanical and mineralogical characteristics specific density and combined quartz and muscovite content are positively related to durability high feldspar and chlorite content are associated with low durability copyright 2000 john wiley sons ltd

rates of tafoni weathering on uplifted shore platforms in nojima zaki boso peninsula japan many tafoni occur on the faces of marine cliffs on three uplifted shore platforms with different altitudes and with known ages of emergence the mean value of the ten largest depths of tafoni d cm was obtained and the period of their formation t years was estimated the relationship between the two variables was found to be d 20 3 1 e 0 005t

salt as an agent in the development of cavernous weathering a well recognized spatial association exists between cavernous weathering and the presence of soluble salts disaggregation of rock in caverns is attributed to mechanical pressures exerted by crystallization and or hydration of salts however scanning electron micrographic analysis demonstrates that solutional etching is the major weathering process in caverns in near coastal locations in western australia and new south wales australia it is suggested here that the breakdown of the caverned sandstone is caused by sodium chloride which increases the rate of quartz dissolution

the cretaceous superchron geodynamo observations near the tangent cylinder abstract if relationships exist between the frequency of geomagnetic reversals and the morphology secular variation and intensity of earth s magnetic field they should be best expressed during superchrons intervals tens of millions of years long lacking reversals here we report paleomagnetic and paleointensity data from lavas of the cretaceous normal polarity superchron that formed at high latitudes near the tangent cylinder that surrounds the solid inner core the time averaged field recorded by these lavas is remarkably strong and stable when combined with global results available from lower latitudes these data define a time averaged field that is overwhelmingly dominated by the axial dipole octupole components are insignificant these observations suggest that the basic features of the geomagnetic field are intrinsically related superchrons may reflect times when the nature of core mantle boundary heat flux allows the geodynamo to operate at peak efficiency

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

the origin of honeycomb weathering honeycomb weathering occurs throughout the world but the origin remains a matter of controversy wind erosion exfoliation frost shattering and salt weathering have been proposed as explanations although few attempts have been made to substantiate these hypotheses with chemical or mineralogical studies chemical analyses and field observations indicate that honeycomb weathering in coastal exposures of arkosic sandstone near bellingham washington results from evaporation of salt water deposited by wave splash microscopic examination of weathered surfaces show that erosion results from disaggregation of mineral grains rather than from chemical decomposition thin walls separating adjacent cavities seem to be due to protective effects of organic coatings produced by microscopic algae inhabiting the rock surface cavity walls are not reinforced by precipitation of elements released by weathering as has often been suggested at other locations honeycomb weathering develops rapidly and can be observed on surfaces that were planar less than a century ago

cavernous weathering dynamical instability and self organization cavernous weathering may be conceptualized as a self reinforcing process characterized by positive feedback within the weathering system a morphometric study of caverns in the valley of fire state park nevada usa demonstrates the applicability of a dynamically unstable or conditionally unstable model of cavernous weathering systems outcrop surfaces displaying caverns tend to show increasing fragmentation of the surface in the early stages of cavernous weathering succeeded by convergent evolution of the surface in which caverns tend to grow and coalesce a paradoxical relationship exists between the weathering system output at the scale of individual forms and the outcrop scale caverns tend toward minimum interior surface area by developing a spheroidal form yet the outcrop surface tends toward maximum exposed surface area by increasing the degree of fragmentation of the surface copyright 2004 john wiley sons ltd

a three dimensional representation of geomagnetic reversal records representations of the reversing geomagnetic field are generally based on paleomagnetic directions or virtual geomagnetic poles i e one coordinate related to field intensity is missing interpretations based on these representations have led to conflicting models of reversals we have assembled records of reversals for which an intensity determination is available either absolute volcanic sequences or relative sedimentary sequences and we have chosen to represent them in terms of directions in local cartesian coordinates this representation is identical to the zijderveld representation which is traditional in paleomagnetism the 38 selected records correspond to matuyama brunhes jaramillo olduvai and some older reversals up to 5 m y old a total of 22 are from volcanic sequences and the rest from sedimentary ones vector behavior falls into one of five categories a the reversing field follows the local trend of the geocentric axial dipole gad passing through the origin with noise added b same as a but offset by a constant component from the origin c the reversing field follows a linear trend going through the origin in cartesian coordinates but the direction is different from that of the gad d same as c but offset from the origin by a constant component e large noise no linear or other simple bahavior over half 55 of the records fall in either category a or b 18 in category e the c type data often consist of dipole parallel segments separated by jumps the significance of which is as yet unclear the number of cases with a linear behavior is remarkable 82 large directional swings in the traditional representations often correspond to far less conspicuous changes on the vector plots at points close to the origin behavior there is dominated by noise one source of which may simply be secular variation of the nondipole field the constant offsets sometimes observed 34 of the cases are as yet unexplained but may be due to rock magnetic problems and therefore may have no global significance we doubt whether the present data allow us to extract resolvable terms such as g20 or h31 which have been inferred from the directional representation

oxygen isotope ratios in olivine from the hawaii scientific drilling project oxygen isotope ratios of olivine in 23 tholeiites from the hawaii scientific drilling project hsdp core 15 from mauna kea 8 from mauna loa and three samples of outcropping subaerial or dredged submarine mauna kea lavas have been measured by laser fluorination the 18 o values are 4 6 5 4 confirming previous observations that some hawaiian lavas are derived from sources with 18 o values lower than typical upper mantle 18 oolivine 5 2 0 2 the mauna kea mauna loa transition marks a shift from 18 o values lower than the mantle average in mauna kea olivines 4 8 to more typical mantle values in mauna loa olivines lavas containing olivines with 18 o values similar to the typical upper mantle are associated with more primitive or less depleted radiogenic isotope characteristics i e with higher 3he 4he 13 ra higher 87 sr 86 sr 0 7036 and lower nd 6 5 and with 206 pb 204 pb ratios less than 18 3 these relationships indicate that the 18 o values of the relatively enriched source components of the hawaiian plume sampled by mauna loa lavas are comparable to or greater than the mantle average this conclusion is supported by 18 o values of olivine from other high 3he 4he islands which are also comparable to the upper mantle average the low 18 o values in hawaiian lavas are derived from a source having more morb like or depleted he nd and sr isotope ratios but more radiogenic pb than is seen in the mauna loa lavas assimilation of 18 o depleted lower oceanic crust from the underlying pacific crust by hot mgo rich parental magmas or melting of older recycled oceanic crust entrained in the hawaiian plume are both possible sources of this 18 o depleted morb like component in hawaiian magmas

the myth of the pacific dipole window the angular dispersion of virtual geomagnetic poles vgp from lava flows is often cited as having been anomalously low in the pacific during the brunhes epoch because the dispersion from hawaiian lavas is said to be much lower than measured elsewhere this led to the concept of the pacific dipole window or pacific non dipole low because lavas tend to be erupted in bursts of activity many of the hawaiian data are serially correlated and thus cannot necessarily be treated as independent observations geochronological controls as are available for hawaii have been used in a previous analysis to thin the data to try to avoid repeated sampling of the same geomagnetic field unfortunately in that analysis the angular dispersion of vgps was calculated about the mean vgp instead of about the spin axis and thus underestimated the dispersion we have therefore reanalyzed the relevant data using the following criteria 1 only those lavas with 95 10 have been considered 2 a latitude dependent cut off angle is used to eliminate those vectors that are not part of the normal secular variation 3 we have developed a statistical method to rationalise those flows that have repeatedly sampled the same geomagnetic field vector because suitable geochronological controls are rarely available application of this new method to the hawaiian data shows excellent agreement with the results from using geochronological controls where possible we have therefore included this method in our overall analysis as expected the resulting global data are compatible with a fisher distribution about the spin axis brunhes age data from hawaii n 96 independent measurements give a vgp angular dispersion about the spin axis of sf 12 4 and for the pacific region as a whole sf 12 5 n 190 between latitudes 15 and 30 north or south these values are the same as sf 12 4 n 160 calculated for the rest of the world in the same latitude range this clearly demonstrates that the hypothesis of the pacific dipole window may confidently be rejected

palaeointensity study of quaternary east eifel phonolitic rocks germany summary compared to the other volcanic rocks of the quaternary east eifel volcanic field germany the phonolites stand out because of very low susceptibilities and intensities of natural remanent magnetization geomagnetic palaeointensities have been studied by means of the thellier method for seven of these rock units while five of them are lava flows or domes the other two have been deposited at temperatures of about 500 c as zeolitized tuffs results of rock magnetic investigations indicate that the magnetic carriers are ti poor titanomagnetites or magnetite heating above 200 300 c often leads to an alteration of the magnetic carriers of the rocks in the low temperature range well defined lineari segments in the succession of the thellier experiment data points are found for most of the lava samples as well as for the zeolitized tuffs because of inconsistencies of within unit results and the presence of viscous magnetization components the palaeointensities of only two rock units are regarded as reliable the obtained values are about 30pt as compared to the present day field of 48 pt

surface deformation due to a strike slip fault in an elastic gravitational layer overlying a viscoelastic gravitational half space we extend a technique previously used to model surface displacements resulting from thrust faulting in an elastic gravitational layer over a viscoelastic gravitational half space to the case of strike slip faulting the method involves the calculation of the green s functions for a strike slip point source contained in an elastic gravitational layer over a viscoelastic gravitational half space the correspondence principle of linear viscoelasticity is applied to introduce time dependence the resulting green s functions are then integrated over the source region to obtain the near field displacements several sample calculations are presented involving 90 o and 30 o dipping faults and ruptures completely and partially through the elastic layer we also illustrate the time dependent deformation due to a buried fault results show that the use of a viscoelastic half space underlying an elastic layer introduces a long wavelength component into the deformation field cohen and kramer 1984 even in cases of non vertical strike slip fault and inclusion of gravitational effect that cannot be modeled by purely elastic techniques calculations have shown that vertical postseismic displacement is insignificant and that the horizontal movement is about the same magnitude as the coseismic strike slip displacement the inclusion of gravity affects the horizontal displacement due to vertical strike slip faulting in far field and the vertical displacement for dipping strike slip faulting in near field the computed results have been fit to the global positioning system measurements of the landers earthquake taken shortly after the main shock assuming a relaxation time of the order of days this relaxation time is considerably shorter than times of the order of years to decades found in previous studies the major differences between this detailed three dimensional and simplified two dimensional model are the decay of magnitude in displacement field and the distinct displacement pattern in the regions beyond the fault tip the displacement field due to the cyclic earthquakes was constructed by considering the finite fault length and inclusion of gravity it is found that the displacement field is dominated by the plate motion in the case of short recurrence time on the other hand a looping and migrating pattern in the displacement field is found in the case of very long recurrence time which is not seen in those 2d simplified models

changes of the geomagnetic field vector obtained from lava sequences on the island of vulcano aeolian islands sicily abstract a combined geochronologic k ar and palaeomagnetic study has been conducted on five volcanic sections 70 flows on the island of vulcano the sections span the interval from 15 2 to 135 4 ka with large hiatuses rock magnetic investigations indicate that the natural remanent magnetisation nrm is carried by titanomagnetites in the four oldest sections with a contribution from high coercivity magnetic minerals in the youngest section palaeomagnetic directions obtained by thermal demagnetisation document a large magnetic excursion at around 110 ka with virtual geomagnetic poles vgps that lie over eastern siberia these directions may represent a volcanic record of the termination of the blake event only about 15 of the samples from 19 flows proved suitable for palaeointensity thellier and thellier determinations these measurements indicate that the geomagnetic field has varied at vulcano between 16 and 53 t in the explored time interval when combined with previous results obtained at mt etna for the period from 140 to 60 ka tric et al 1994 phys earth planet inter 85 113 129 these results document a broad low at around 115 ka the palaeointensity values from vulcano and etna are consistent with global values suggesting the presence of a dominant dipole field contribution these values on the other hand are significantly smaller than those recently obtained from la reunion in the southern hemisphere this suggests that large long lived non dipolar components of the geomagnetic field may be present at la reunion in the interval from 130 to 90 ka these observations are consistent with the fact that the vgp angular dispersion calculated from non excursional data is significantly smaller at vulcano than at la reunion

holocene paleointensity of the island of hawai i from glassy volcanics this study presents new high quality paleointensity records and 14c radiocarbon age determinations from the island of hawai i during the holocene previous studies on hawai i use experimental methods and statistical selection criteria that may produce inaccurate geomagnetic field strength estimates additional high quality paleointensity results can be used to evaluate the existing hawaiian dataset and investigate holocene geomagnetic field behavior new paleointensity sites from 22 lava flows were calculated using the izzi thellier laboratory technique and a strict set of selection criteria rapidly cooled glassy volcanic material was collected for all sites isotopic age determinations range from 270 10 000 years before present nine new 14c ages are also presented as part of this study the median intensity for the 22 flows is 47 5 t with a median absolute deviation uncertainty of 5 6 t substantially greater than the present day field strength at hawai i 36 t these new results are comparable to previously published data from this location and are consistent with global paleointensity models there is no evidence of an intensity spike at 3 000 years before present as seen in the levant and elsewhere previously published data vary in intensity by experimental technique relative to data using glassy material and strict selection criteria non thellier type data are biased low a result of these techniques estimating intensity from possibly non single domain magnetic carriers thellier thellier data are biased high the reasons for which remain unclear as no cooling rate effect was demonstrated and we were unable to reproduce the high bias with different selection criteria

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

a link between geomagnetic reversals and events and glaciations abstract the apparent duration of geomagnetic polarity events in arctic ocean sediments is much longer than in sediments from lower latitudes in fact while the remanence of brunhes age sediment cores from the yermak plateau at 82 n is fully reversed for 30 of their lengths 1 the events often evade detection in many other continuously deposited sediments for example the laschamp event is absent in an otherwise high resolution record of secular variation from lac du bouchet 2 which is located near the laschamp volcanics where the event was first detected very short event durations of a few hundred years at the most have been suggested before 2 3 because sedimentation rates in the arctic ocean were increased during glaciations the exaggerated proportion of reverse polarities in sediments from high latitudes suggests a link between glaciation and field reversals this suggestion is supported by magnetostratigraphic results obtained from thick loess paleosol sequences in china 4 these demonstrate that all polarity boundaries separating chrons and subchrons since the gauss matuyama field reversal have been recorded in loess and thus during periods of cold climate although conflicting evidence exists for some boundaries furthermore the ages of 22 events and chron boundaries have been compared with the oxygen isotope record 5 thought to represent global ice volume all events and reversals younger than 2 6 ma may have occurred during periods of global cooling or during cold stages however some ages are still too poorly dated for a definite correlation climatic signals also exist in the two longest relative paleointensity records 6 7 but these are suspected to be caused by climatically driven variations in the rock magnetic parameters a mechanism for field reversals may be the acceleration of the earth s rotation caused by lowering of the sea level during glaciations the short duration of events also implies that the geomagnetic field can reverse an order of magnitude faster than commonly assumed

absolute paleointensity and magnetomineralogical changes determinations of absolute paleointensity are often hampered by magnetomineralogical changes produced within samples of lava flows during successive heatings such changes which directly affect the capacity for a sample to acquire a partial thermoremanent magnetization ptrm are reflected by negative ptrm checks a deviation between the original ptrm and a repeated measurement after heating at a higher temperature because the deviations indicated by the ptrm checks can provide a direct estimate of the effects to the ptrms we suggest that they can be used to correct ptrms the correction method we present requires additional measurements in particular ptrm checks performed after each ptrm and specific diagrams to detect acquisition of chemical remanence performing multiple successive ptrm checks can also provide information on the blocking temperatures of the alteration product these experiments conducted on lava flows from different localities show that the magnetomineralogical changes mainly involved grains with blocking temperatures lower than the last heating step the corrections have been tested on recent and historic lava flows from several localities we have also compared results on samples from the same lava flows that were heated in air and in vacuum the mean paleointensity values were found to lie within less than 10 from the expected field intensity at each site the corrections resulted in the recovery of reliable paleointensity determinations for about twice as many specimens heated in vacuum and 65 additional data for specimens heated in air successive heatings at the same temperature in air are often accompanied by a time dependent oxidation of the natural remanent magnetization which is responsible for paleointensity values lower than the expected field a problem that is not related to the corrections we conclude that corrections can be used with fairly high confidence to the ptrms performed in vacuum and in air

paleointensity of the earth s magnetic field recorded by two late quaternary volcanic sequences at the island of la reunion indian ocean a paleomagnetic study has been undertaken on two late quaternary volcanic sequences from the piton de la fournaise volcano island of la reunion in order to provide absolute paleointensities of the earth s magnetic field from a site in the southern hemisphere the pattern of secular variation recorded by both sequences clearly indicates that the extrusion rate of the lava flows was not constant with time a detailed investigation of the magnetic properties of the samples was carried out in parallel with paleointensity experiments which were performed with the thellier method samples with minimum overprint composed by single domain or pseudo single domain magnetite showing good thermal stability and a narrow blocking temperature spectrum were the most successful during thellier s experiments values of the paleofield strength from 19 to 55 mt have been obtained on 23 flows among the 30 which were sampled few variations 7 5 9 9 1022 a m2 are observed in the virtual dipole moments vdms determined on the youngest sequence 5 12 ka those values are within the range of fluctuations of the worldwide variations of vdms for this period on the other hand data from the oldest sequence 82 98 ka are the first paleointensity values obtained in that time period and the observed vdms extend from 4 1 to 8 8 1022 a m2 these results suggest that for this time interval the field did not depart from its average behavior

geomagnetic paleointensities at hawaii between 3 9 and 2 1 ma preliminary results abstract thellier thellier experiments have been conducted on 454 samples from different sequences of the waianae and koolau volcanoes in oahu hawaii the sampled interval as determined by k ar dating spans from 3 9 to 2 1 ma results from 89 samples from 41 flows meet restrictive reliability criteria and provide the first paleointensity estimates from hawaii for the sampled period with good temporal resolution on this time interval the vadm shows ample fluctuations from a minimum of 2 3 1022 am2 to a maximum of 11 5 1022 am2 about an average value of 7 0 2 3 1022 am2 suggesting that fluctuations of the geomagnetic field strength comparable in amplitude to the time averaged value may be a characteristic of the field during both the reverse mammoth chron and the normal chron between the mammoth and the kaena the field strength appears to increase with time

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

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

patterns of seismicity associated with asperities in the central new hebrides island arc in the central part of the new hebrides island arc efate malekula region 16 s 18 6 s a sequence of moderate sized earthquakes their aftershock sequences and other clusters of small earthquakes together form an intricate but coherent time space pattern that probably reveals a major asperity complex along the interplate boundary of the subduction zone this pattern is determined by study of data from a local network the sequences including one event with magnitude mw 7 1 eight events with magnitudes mw between 5 8 and 6 3 and nearly 13 000 smaller events occurred during the six year period 1978 1984 the seismicity is very unevenly distributed in space a sharply defined east west line near 17 2 s separates the very active efate region to the south where nearly 10 500 earthquakes occurred from the less active malekula region to the north where less than 2 500 earthquakes occurred during the six year period in the efate region several spatial patterns are highlighted first the seismic regimes of the updip and the downdip part of the interplate boundary are different the updip part is characterized by a low background activity and main shocks with large aftershock zones while the downdip part is characterized by a very high level of background activity and main shocks with much smaller aftershock zones or no aftershocks this difference in seismic regimes suggests that asperities are located in the updip part of the interplate boundary while the downdip part may slip predominantly by creep a creep episode may have been responsible for the tilt signal observed by periodic relevelings of a l km aperture network of benchmarks on efate island second specific locations along the interplate boubdary are identified either as sites of very intense and repeatedly activated concentrations of hypocenters or as sharp boundaries limiting the spatial development of aftershock zones two areas of concentrated activity particularly stand out one located in the updip part of the interplate boundary and the other in the downdip part most of the clustered activity that occurred during the six years of observation is concentrated in these two areas four boundaries limiting the spatial development of aftershock zones are located in the updip part of the interplate boundary one of these boundaries coincides with one of the areas of intense activity epiceniers of moderately large events that occurred in the region since 1960 are also concentrated along the boundaries the zones of concentrated activity and the boundaries limiting the development of aftershock zones can be interpreted either as locations of penties or as edges of asperities an alternative interpretation is that a giant asperity is located in the ylekula region with the different spatial patterns observed in the efate region being interpreted as the southern edge of that asperity a transition zone between a locked part to the north and normal i subduction to the south study of the temporal patterns of the seismicity shows that the main shocks in tbe downdip part of the interplate boundary are not preceded by obvious long term months to year nor short term days to week precursory activity in the updip part of the interplate boundary short term fipreshocks preceded each main shock but only one of these foreshock sequences is outstanding although no clear cyclic pattern related to the main shocks is evident clustered activity that occurred in the two zones of concentrated activity and two other zones located in the back arc region are correlated ih time with the occurrence of the major sequences located in the updip part of the interplate boundary activation of these four zones always follows the sequences in the updip part of the interplate boundary qith varying time intervals days to months these four zones are also sometimes activated before sequences in the updip part of the interplate boundary in particular one of the zones of concentrated a ctivity shows possibly repeated long term few months precursory activity this phenomenon could b e an indicator of increasing probability of occurrence of main events in the updip part of the interplate b oundary i tuk ukiis lmcui l ilu tl6aj2 qfiztqf

tafoni on coastal slopes south devon u k cavernous weathering tafoni development occurs on coastal slopes in greenschist bedrock at elevations up to 40 m above sea level the freshly weathered surfaces of the cavern interiors are irregular in morphology discordant to major rock structure formed by substantially weakened rock and associated with granular weathering debris the weathering debris contains soluble elements in proportions similar to those present in seawater and the penetration of elements associated with sea salts into the weathering surface to estimated depths of at least 0 1 0 2 m is indicated by the presence of chlorine scanning electron microscopy and microprobe analyses suggest that rock breakdown occurs principally through limited chemical weathering at grain boundaries the mechanism for the emplacement of marine salts within sheltered rock surfaces in the tafoni is postulated to be a combination of dry deposition under turbulent atmospheric conditions and wetting by coastal fog

origins of honeycomb weathering the role of salts and wind honeycomb weathering is a common surface phenomenon affecting a variety of rocks in a range of environments it develops on building stones and it shapes ocean cliffs rocks in hot deserts and arctic landscapes honeycomb weathering may also help alter rocks on other planets such as mars although first noted in the nineteenth century its origins are still not well understood and a dearth of laboratory experiments testing the many theories proposed for its development has added to the ambiguity incipient honeycomb weathering in a homogeneous limestone has been experimentally reproduced by wind exposure and salt crystallization our experiments show that heterogeneous wind flow over a stone surface is important in the development of this weathering pattern wind promotes evaporative salt growth between grains on a stone surface resulting in the development of small randomly distributed cavities a reduction in air pressure within the cavities results in increased wind speed and rapid evaporation a high evaporation rate and evaporative cooling of the saline solution in the cavity leads to more rapid and greater granular disintegration than in the surrounding areas it seems that this local supersaturation and subsequent buildup of salt crystallization pressure ultimately result in the formation of honeycomb features for the first time these experimental results demonstrate the close relationship between salts wind and honeycomb weathering they also offer new ways to understand the genesis of this striking and sometimes harmful weathering pattern

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

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

absolute paleointensities between 60 and 160 kyear bp from mount etna sicily abstract paleointensity measurements were conducted on 17 distinct lava flows from mount etna sicily in order to obtain records of absolute paleointensities for the period covering the last 60 160 kyear a complementary aspect of this study was to improve and extend the calibration of relative paleointensities obtained for the same interval from sedimentary sequences based on absolute values of virtual axial dipole moments seventy three samples have been subjected to double heating thellier coe experiments under vacuum and care was taken to perform additional studies dealing with the magnetic mineralogy and to scrutinize the effects of heating upon magnetic minerals potassium argon k ar datings provided precise ages in most cases absolute paleointensities were obtained with success for ten lavas flows the succession of the virtual axial geomagnetic dipoles vadm oscillate between extrema centered around a mean value of 6 3 2 6 10 22 a m 2 this result is in good agreement with the sedimentary record published for the same period and confirms that there has been an appreciable decrease in geomagnetic field intensity between 80 and 150 kyear

a new technique for non destructive field measurement of rock surface strength an application of the equotip hardness tester to weathering studies tafone like depressions have developed on the aoshima sandstone blocks used for a masonry bridge pier in the coastal spray zone a thin layer of partial granular disintegration was found on the surface in depressions to evaluate quantitatively the strength of the thin weathered layer the hardness was measured at the surface of the sandstone blocks using both an equotip hardness tester and an l type schmidt hammer comparison of the two testing results indicates that the equotip hardness value is more sensitive in evaluating the strength of a thin layer of weathered surface rock than the schmidt hardness value by applying two methods i e both the repeated impact method and the single impact method the equotip tester can evaluate the strengths of fresh internal and weathered surficial portions of rocks having a thin weathering layer comparison of the two strengths enables evaluation of strength reduction due to weathering copyright 2007 john wiley sons ltd

rates of development of tafoni in the moenkopi and kaibab formations in meteor crater and on the colorado plateau northeastern arizona tafoni are pits formed by non uniform weathering in otherwise uniform rock two equations have been proposed for the rate of development of tafoni both based on 2000 year old outcrops from the coast of japan we have taken tafoni measurements from the meteor crater arizona and vicinity that extend the equations back at least 50 000 years as reported in earlier studies we found pit depth to be the best tafone parameter to measure the size of the pit decreases significantly with increasing inclination of the rock surface however the size of the pit can vary greatly for other reasons in some cases the measurements are statistically significantly different between two stations taken from contiguous areas of similar inclination and aspect in an apparently homogeneous bed it is clear however that over tens of thousands of years tafoni enlarge significantly our data are generally log normal and all are markedly heteroscedastic the 1991 equation proposed by matsukura and matsuoka does not fit our data the 1996 equation proposed by sunamura provides a better fit we propose a sigmoidal equation d b1 e b2 b3 t where d is the depth t is the age and b1 b2 and b3 vary with lithology this new equation fits our data far better than the earlier published equations copyright 2002 john wiley sons ltd

late pleistocene geomagnetic excursion in icelandic lavas confirmation of the laschamp excursion in 1980 kristjansson and gudmundsson 1 reported a late glacial geomagnetic excursion in three hills in the reykjanes peninsula iceland with shallow negative inclinations and westerly declinations they named it the skalamaelifell excursion more extensive field work has identified the same excursional paleomagnetic direction declination 258 inclination 15 at four additional outcrops in a 10 10 km area in the reykjanes peninsula the excursion lavas are olivine tholeiites with similar petrography and chemical compositions paleointensity determinations by the thellier method average 4 2 0 2 t for 8 samples more than an order of magnitude weaker than the present geomagnetic field in iceland together these results suggest extrusion of the excursion lavas in a very brief span of time probably less than a few hundred years kar dating of the excursion lavas gives a mean age for 19 determinations of 42 9 7 8ka 2 compilation of thirty kar ages of the laschamp and olby flows by three laboratories yield a new age for the laschamp excursion in france of 46 6 2 4ka 2 the age of the excursion in southwestern iceland is statistically indistinguishable from the laschamp excursion at the 95 confidence level and both have very low paleointensities therefore we suggest that the laschamp and olby flows in france and the skalamaelifell units of iceland recorded essentially the same geomagnetic excursion differences in the virtual paleomagnetic poles vgps of these excursions may be due to 1 the probable non dipole character of the geomagnetic field during the excursion 2 rapid geomagnetic secular variation and possible small age differences of the extrusive rocks in france and iceland and or 3 crustal magnetic anomalies which might dominate the local geomagnetic field during the excursion at either or both locations

a physical model for the rate of coastal tafoni development to explain the temporal change in growth rates of tafoni on homogeneous insoluble rocks modeling is carried out based on a formulated relationship for weathering induced reduction of rock strength the model is expressed by z z c 1 sqrt t ast t where z is the depth of tafoni z c is the ultimate depth t is the time and t is the time lag application of the model to the pacific coast of japan reveals that 1 the model accurately describes tafoni development 2 average growth rates presented by most previous studies are underestimated and 3 an exponential decrease in growth rates stems from strength reduction by inherent rock weathering

evidence for a new geomagnetic reversal from lava flows in idaho discussion of short polarity reversals in the brunhes and late matuyama polarity chrons k ar ages and paleomagnetic data for basalt samples from a new core hole site e at the idaho national engineering laboratory inel indicate that the age of the reversed polarity event recorded in snake river plain lavas is older than 465 50 ka 1000 years before present reported previously by champion et al 1981 nine basalt flows eight with normal polarity and one with reversed polarity were recognized in the site e core hole the flows above and below the reversed flow have ages of 491 80 ka and 580 93 ka respectively the inclination of the paleomagnetic field direction of the reversed flow at site e agrees with the inclination of reversed flows elsewhere at inel which have an age of 565 14 ka these reversed flows were previously thought to be correlative with the emperor event we suggest that this polarity event is an older event which we name the big lost reversed polarity subchronozone and subchron a review of data documenting short reversal records from volcanic and sedimentary rocks shows that there is evidence for eight polarity subchrons in the brunhes and two besides the jaramillo in the late matuyama these 10 short subchrons begin to indicate the many short events that cox 1968 hypothesized must exist if polarity interval lengths have a poisson distribution these events are true subchrons not excursions and may or may not have low associated paleointensities although low field strengths might explain why the reversal process aborts the mean sustained polarity interval length since late matuyama chron time is 90 000 years the similarity of this number with the 105 year period of the earth s orbital eccentricity suggests anew that linkage between geomagnetic paleoclimatic and possible underlying earth orbital parameters should be evaluated

a simple statistical model for geomagnetic reversals the diversity of paleomagnetic records of geomagnetic reversals now available indicate that the field configuration during transitions cannot be adequtely described by simple zonal or standing field models a new model described here is based on statistical properties inferred from the present field and is capable of simulating field transitions like those observed some insight is obtained into what one can hope to learn from paleomagnetic records in particular it is crucial that the effects of smoothing in the remanence acquisition process be separated from true geomagnetic field behavior this might enable us to determine the time constants associated with the dominant field configuration during a reversal

a preliminary scanning electron microscope study of honeycomb weathering of sandstone in a coastal environment honeycomb weathering has been observed in a carboniferous sandstone at a coastal location near ballycastle on the north coast of northern ireland specimens of this sandstone have been analysed by x ray diffraction scanning electron microscopy and energy dispersive spectrometry results reveal that calcium sulphate gypsum is the only salt present and is found only at and immediately below the rock surface sem observations suggest that crystallization of salts in pores could easily dislodge quartz grains to promote granular disintegration whilst etching of quartz grain surfaces attests to chemical weathering activity within the rock however the reason for the development of the honeycomb pattern is not known

biogenic origin of coastal honeycomb weathering honeycomb weathering occurs in two environments in late cretaceous and eocene sandstone outcrops along the coastlines of south west oregon and north west washington usa and south west british columbia canada at these sites honeycomb weathering is found on subhorizontal rock surfaces in the intertidal zone and on steep faces in the salt spray zone above the mean high tide level in both environments cavity development is initiated by salt weathering in the intertidal zone cavity shapes and sizes are primarily controlled by wetting drying cycles and the rate of development greatly diminishes when cavities reach a critical size where the amount of seawater left by receding tides is so great that evaporation no longer produces saturated solutions encrustations of algae or barnacles may also inhibit cavity enlargement in the supratidal spray zone honeycomb weathering results from a dynamic balance between the corrosive action of salt and the protective effects of endolithic microbes subtle environmental shifts may cause honeycomb cavity patterns to continue to develop to become stable or to coalesce to produce a barren surface cavity patterns produced by complex interactions between inorganic processes and biologic activity provide a geological model of self organization surface hardening is not a factor in honeycomb formation at these study sites salt weathering in coastal environments is an intermittently active process that requires particular wind and tidal conditions to provide a supply of salt water and temperature and humidity conditions that cause evaporation under these conditions salt residues may be detectable in honeycomb weathered rock but absent at other times honeycomb weathering can form in only a few decades but erosion rates are retarded in areas of the rock that contain cavity patterns relative to adjacent non honeycombed surfaces copyright 2010 john wiley sons ltd

paleointensity of the earth s magnetic field and k ar dating of the louchadiere volcanic flow central france new evidence for the laschamp excursion we report paleointensity results of the earth s magnetic field from a late pleistocene lava flow louchadiere central france which recorded an intermediate geomagnetic field direction 5 sites mean declination 114 1 inclination 58 2 k 130 95 6 7 new k ar age determinations confirm that this flow is contemporaneous with the laschamp and olby flows and that this excursion occurred around 45ka ago using the thellier double heating method reliable paleointensities have been obtained for ten samples from three different sites providing an average field strength of 12 9 t 3 3 this low value and previous results of the laschamp excursion from france and iceland confirm that the earth s magnetic field was in an intermediate state during the laschamp excursion

combined paleointensity and 40ar 39ar age spectrum data from volcanic rocks of the west eifel field germany evidence for an early brunhes geomagnetic excursion results of virtual geomagnetic pole position paleointensity and isotopic age determinations have been derived from lavas of the quaternary west eifel volcanic field germany the characteristic remanent magnetization carm directions and corresponding positions of virtual geomagnetic poles vgp have mainly been determined by bohnel et al they are exclusively of normal polarity the vgps form a strongly non fisherian distribution characterized by about 30 pole positions with latitudes below 60 n which are mainly confined to the longitude sector from 20 w to 120 e eight independent poles are clustered at 40 n and 45 e geomagnetic paleointensity values from 37 volcanic units of the west eifel have been determined paleointensities range from 7 to 62 t with an unexpectedly large number of paleointensity values below 30 t about half of the low paleointensity values are associated with intermediate vgp latitudes isotopic age determinations from west eifel volcanic rocks have been published fuhrmann and lippolt additional isotopic ages are presented here the west eifel volcanic field is usually devided into an older part in the nw and a younger one in the se based on petrological models and geological evidence all available isotopic ages of the west eifel volcanic rocks which are exclusively from the northwestern part of the field lie in the interval between 460 and 660 kyr for seven of the clustered low latitude vgps it was possible to determine paleointensities and 40ar 39ar ages within their error limits six poles have identical virtual dipole moments vdm at 3 5 1022a m2 furthermore the individual isotopic age intervals of the 40ar 39ar data overlap the assigned age of 510 30 kyr is considered to be the age of a geomagnetic excursion for higher latitude vgp positions 45 n vdm values range from 2 to 10 1022a m2 the mean vdm value about 7 5 1022a m2 of the younger volcanoes ages estimated to be below 100 kyr is not remarkably different from the present dipole moment of the earth in contrast the older volcanoes with higher latitude vgp positions yielded a mean value of only 5 3 1022a m2 and a bimodal frequency distribution of the vdm values isotopic ages determined for two of these volcanic units are close to the inferred excursion age suggesting a decrease of the geomagnetic dipole moment during a time interval which surpassed the duration of the excursion

rates of weathering and temporal changes in strength of bedrock of marine terraces in boso peninsula japan the weathering rates of bedrock of the dated erosional marine terraces were examined for two definitions 1 the rates at which the thicknesses of weathered zones with different strengths increase with weathering time and 2 the rates at which the strength of weathered materials at an arbitrary depth decrease with time the bedrock of terraces in the study area consists mainly of tertiary sandstone and mudstone the weathering profiles were observed from drill cores on three levels of terrace surfaces i e 2850 290 and 70 years b p in the emergence age of terraces which is regarded as the weathering time of bedrock the degree of weathering grade was described by the residual strength ratio which is defined as the ratio of the needle penetration hardness of weathered part to that of fresh part of bedrock the weathering rates of the first definition do not decrease monotonically but decelerate logarithmically with time zones with lower weathering grades have faster weathering rates than those with higher weathering grades with respect to the weathering rates of the second definition mudstone starts to weather earlier than sandstone but is exceeded by the latter after a certain time has elapsed because mudstone has a higher slaking susceptibility

a statistical model of the fluctuations in the geomagnetic field from paleosecular variation to reversal the statistical characteristics of the local magnetic field of earth during paleosecular variation excursions and reversals are described on the basis of a database that gathers the cleaned mean direction and average remanent intensity of 2741 lava flows that have erupted over the last 20 million years a model consisting of a normally distributed axial dipole component plus an independent isotropic set of vectors with a maxwellian distribution that simulates secular variation fits the range of geomagnetic fluctuations in terms of both direction and intensity this result suggests that the magnitude of secular variation vectors is independent of the magnitude of earth s axial dipole moment and that the amplitude of secular variation is unchanged during reversals

evidence of honeycomb weathering on mars features similar to honeycomb weathering are observed in some martian rocks at the pathfinder landing site honeycomb weathering on earth develops in a range of environments including extremely arid and cold areas in antarctica and on many types of rocks e g basalts and andesites subjected to physical and or chemical weathering salt weathering in combination with frost shattering and chemical weathering provides a viable explanation for the development of honeycomb features on mars under current environmental conditions data supporting this theory are presented and discussed these findings are important in understanding the present climate geology and in particular the geomorphology of mars they also suggest new paths for research and experimentation in future missions to mars

models of hawaiian volcano growth and plume structure implications of results from the hawaii scientific drilling project the shapes of typical hawaiian volcanoes are simply parameterized and a relationship is derived for the dependence of lava accumulation rates on volcano volume and volumetric growth rate the dependence of lava accumulation rate on time is derived by estimating the eruption rate of a volcano as it traverses the hawaiian plume with the eruption rate determined from a specified radial dependence of magma generation in the plume and assuming that a volcano captures melt from a circular area centered on the volcano summit the timescale of volcano growth is t 2 r plate where r is the radius of the melting zone of the circular plume and plate is the velocity of the pacific plate the growth progress of a volcano can be described by a dimensionless time t t plate 2r where t 0 is chosen to be the start of volcano growth and t 1 approximates the end of shield growth using a melt generation rate for the whole plume of 0 2 km 3 yr a plume diameter of 50 km and a plate velocity of 10 cm yr we calculate that the lifetime of a typical volcano is 1000 kyr for a volcano that traverses the axis of the plume the standard dimensions are a volume of 57 000 km 3 a summit thickness of 18 km a summit elevation of 3 6 km and a basal radius of 60 km the volcano first breaches the sea surface at t 0 22 when it has attained only 5 of its eventual volume 80 of the volume accumulates between t 0 3 and t 0 7 typical lava accumulation rates start out over 50 m kyr in the earliest stages of growth from the seafloor and level out at 35 m kyr from t 0 05 until t 0 4 from t 0 4 to t 0 9 the submarine lava accumulation rates decrease almost linearly from 35 m kyr to 0 subaerial accumulation rates are about 30 lower the lava accumulation rate is a good indicator of volcano age a volcano that passes over the plume at a distance 0 4r off to the side of the plume axis is predicted to have a volume of about 60 of the standard volcano a lifetime about 8 shorter and lava accumulation rates about 15 20 smaller the depth age data for mauna kea lavas cored by the hawaii scientific drilling project are a good fit to the model parameters used given that mauna kea appears to have crossed the plume about 15 20 km off axis the lifetime of mauna kea is estimated to be 920 kyr mauna loa is predicted to be at a stage corresponding to t 0 8 kilauea is at t 0 6 and loihi is at t 0 16 the model also allows the subsurface structure of the volcanoes the interfaces between lavas from different volcanoes to be modeled radial geochemical structure in the plume may be blurred in the lavas because the volcanoes capture magma from a sizeable cross sectional area of the plume this inference is qualitatively born out by available isotopic data the model predicts that new hawaiian volcanoes are typically initiated on the seafloor near the base of the next older volcano but generally off the older volcano s flank

application of an s shaped curve model to the temporal development of tafoni of salt weathering origin to describe temporal change in tafone development an s shaped curve equation is proposed z zc 1 n 1 exp t n exp 1 1 n t where z is observed tafone depth zc is ultimate tafone depth t is time and n and are constants the applicability of this model is examined using tafone data selected from seven sites which are categorized into three different salt weathering environments a spray splash dominant occasionally wave affected supra tidal zone aerosol affected coastal regions and inland desert areas the results indicate that the equation can well describe tafone development in each of these environments an investigation based on the values of n and determined through a best fit of the equation to the data suggests that n characterizes site specific environmental conditions and reflects the magnitude of factors controlling the recession mechanism of tafone surfaces it is found that 1 the maximum rate of tafone growth dramatically decreases from supra tidal through coastal to desert environments and 2 the growing mode of tafoni is different depending on the environmental settings the erosional force to facilitate the development of tafoni at supra tidal sites is estimated to be about 400 times greater than that in the general coastal area copyright 2011 john wiley sons ltd

mono lake excursion recorded in sediment of the santa clara valley california 1 two intervals recording anomalous paleomagnetic inclinations were encountered in the top 40 meters of research drill hole ccoc in the santa clara valley california the younger of these two intervals has an age of 28 090 330 radiocarbon years b p calibrated age 32 8 ka this age is in excellent agreement with the latest estimate for the mono lake excursion at the type locality and confirms that the excursion has been recorded by sediment in the san francisco bay region the age of an anomalous inclination change below the mono lake excursion was not directly determined but estimates of sedimentation rates indicate that the geomagnetic behavior it represents most likely occurred during the mono lake laschamp time interval 45 28 ka if true it may represent one of several recurring fluctuations of magnetic inclination during an interval of a weak geomagnetic dipole behavior noted in other studies in the region

geomagnetic paleointensities derived from volcanic rocks of the quaternary east eifel volcanic field germany geomagnetic paleointensities have been determined for 18 rock units of basanitic thephritic or nephelinitic composition from the quaternary east eifel volcanic field germany in general the rocks are well suited for paleointensity determination and gave consistent results of paleointensity for each rock unit at least five individual values have been measured with the thellier method compared with the present day field of 48 t the unweighted mean paleointensity values range from 29 to 62 t with relative standard errors mostly below 10 paleointensity values are concentrated at 30 and 50 t with a gap at 40 t the distribution is bimodal rather than gaussian although all paleopoles lie close to the geographic pole this cannot be explained by the very discontinuous distribution of volcanic activity in time which took place mainly around 200 and 400 ka in each age group both low and high paleointensity values are found however a further temporal separation within each age group cannot be excluded furthermore the paleointensity values of the west eifel volcanic field show a similar bimodal distribution which indicates that this case seems to be connected with a geomagnetic excursion recorded in those rocks therefore it might be considered that the east eifel rocks recorded a characteristic feature of the earth s magnetic field namely a quasi cyclic variation of paleointensity on the other hand the now available global data set of 128 virtual dipole moment values from the brunhes chron and older than 30 ka shows an almost gaussian distribution with a mean value of 7 4 3 4 1022 am2 if 14 values with transitional pole positions are excluded a quasi cyclic variation of the dipole moment therefore does not seem to be a persistent global characteristic of the earth s magnetic field

holocene paleointensities thellier experiments on submarine basaltic glass from the east pacific rise a complete description of the geomagnetic field requires both paleodirectional and paleointensity data although the paleointensity data base has grown steadily over the last three decades it remains limited in time and space the majority of data are of holocene age and come from europe furthermore it has been difficult to assess the reliability of the paleointensity determinations here we present paleointensity determinations on precisely dated holocene 0 to 3500 years old submarine basaltic glass from the east pacific rise 15 s to 22 s although hysteresis measurements and low temperature isothermal remanent magnetization irm acquisition experiments document a significant contribution of superparamagnetic grains high blocking temperatures above 400 c and curie temperatures between 490 c and 550 c indicate a single domain low ti magnetite as the carrier of the remanent magnetization this notion is further supported by the fact that saturation of remanence is achieved in moderate fields of about 200 300 mt submarine basaltic glass proves to be nearly ideal for paleointensity determinations in that it produces a high success rate for thellier experiments twenty six out of 30 samples resulted in acceptable paleointenisty determinations multiple experiments on splits from the same sample show good reproducibility the paleointensities for zero age glasses correspond precisely with the present field intensity at the site of recovery the results of the remaining samples range from 16 7 to 53 9 t with corresponding virtual axial dipole moments vadm of 3 61 1022 to 11 9 1022 a m2 the intensities vary rapidly with time excluding a westward drifting nondipole component as the source for these fluctuations basaltic glass is frequently recovered in both dredged and drilled material from the ocean floor the availability of submarine basaltic glass throughout the world oceans therefore holds great potential for a better distribution of paleointensity data through time and space