Summit Collapse Research Articles

How summit calderas collapse on basaltic volcanoes: New insights from the April 2007 caldera collapse of Piton de la Fournaise volcano

In April 2007, Piton de la Fournaise volcano experienced a caldera collapse during its largest historical eruption. We present here a structural analysis both of the caldera and the surrounding area, and precise GPS data recorded with a dense GPS network specifically dedicated to the analysis of deformation related to the summit collapse structures. Despite a collapse of more than 300 m in the central zone, the geometry of the new caldera is similar in map view to that of the pre-existing collapsed structure, which was formed from the coalescence of several pit craters. The caldera shows an asymmetric inner geometry with sub-vertical walls in the NW quadrant and steep scarps composed of inward tilted blocks in the southern half. The presence of preserved polished surfaces on the lower part of the sub-vertical scarp indicates that it corresponds to the caldera north-western ring fault. The April 2007 caldera collapse led to the development and the reactivation of concentric fractures on the caldera rim, mostly along the southern limit of the caldera. GPS data show that fractures result from radial extensional stresses that are restricted within the first tens of meters of the caldera edge. GPS data also reveal that the caldera collapse was coeval with a centripetal deflation, whose magnitude is largest along the southern half of the caldera. The displacements recorded by GPS result from both a general deflation, due to magma withdrawal from Piton de la Fournaise's summit magma chamber, and additional local effects related to the caldera collapse. Comparison of the caldera collapses at Piton de la Fournaise, Miyakejima and Fernandina reveals striking similarities, with cyclic seismic signals accompanying small-scale deflation–inflation cycles. This strongly suggests a common mode of collapse. Hence, we propose a unifying model of caldera collapse in basaltic setting, in which the inward deflation due to magma withdrawal from the magma chamber prevents the collapse of the caldera roof until the gravitational stress acting on the rock column above the magma chamber exceeds the shear strength along pre-existing ring faults. The downward displacement stops when the pressure increase into the magma chamber is able to again sustain the rock column. The succession of (1) inward deflation that prevents the collapse, (2) collapse due to gravitational stress and (3) stopping of the downward motion is repeated many times. The frequency of the cycles is influenced by the rate of magma withdrawal and by the amount of intrusion of magma along the ring faults.

Edifice growth, deformation and rift zone development in basaltic setting: Insights from Piton de la Fournaise shield volcano (Réunion Island)

The overall morphology of basaltic volcanoes mainly depends on their eruptive activity (effusive vs. explosive), the geometry of the rift zones and the characteristics of both endogenous and exogenous growth processes. The origin of the steep geometry of the central cone of Piton de la Fournaise volcano, which is unusual for a basaltic effusive volcano, and its deformation are examined with a combination of a detailed morphological analysis, field observations, GPS data from the Piton de la Fournaise Volcano Observatory and numerical models. The new caldera walls formed during the April 2007 summit collapse reveal that the steep cone is composed of a pyroclastic core, inherited from an earlier explosive phase, overlapped by a pile of thin lava flows. This suggests that exogenous processes played a major role in the building of the steep central cone. Magma injections into the cone, which mainly occur along the N25–30 and N120 rift zones, lead to an asymmetric outward inflation concentrated in the cone's eastern half. This endogenous growth progressively tilts the southeast and east flanks of the cone, and induces the development of a dense network of flank fractures. Finally, it is proposed that intrusions along the N120 rift zone are encouraged by stresses induced by magma injections along the N25–30 rift zone.

Size and volume evaluation of the caldera collapse on Piton de la Fournaise volcano during the April 2007 eruption using ASTER stereo imagery

The scale of summit collapse during the April 2007 eruption of Piton de la Fournaise volcano, Réunion Island, western Indian Ocean, was evaluated using before‐and‐after differential digital elevation models (DEMs) derived from nadir and backward‐looking images from the Advanced Spaceborne Emission and Reflection radiometer (ASTER) instrument. The dimensions of horizontal length and width, volume and depth of the depression were estimated as 1,100 × 800 m, 9.6 × 107 m3 and 320 m, respectively. These estimates show excellent agreement with field survey data. A ring shaped thermal anomaly (diameter of about 500 m) was found at Dolomieu Crater during or just after the collapse on nighttime ASTER SWIR image. It may correspond to the high temperature areas associated with the cross section of a hydrothermal zone at a constant altitude inside the crater. Our analysis of ASTER orbital data documents topographic and related thermal changes as a result of the 2007 eruption and demonstrates the power of ASTER as a volcanological tool.

Oxygen Isotope Evidence for Chemical Interaction of Ki lauea Historical Magmas with Basement Rocks

Kīlauea historical summit lavas have a wide range in matrix δ18OVSMOW values (4·9–5·6‰) with lower values in rocks erupted following a major summit collapse or eruptive hiatus. In contrast, δ18O values for olivines in most of these lavas are nearly constant (5·1 ± 0·1‰). The disequilibrium between matrix and olivine δ18O values in many samples indicates that the lower matrix values were acquired by the magma after olivine growth, probably just before or during eruption. Both Mauna Loa and Kīlauea basement rocks are the likely sources of the contamination, based on O, Pb and Sr isotope data. However, the extent of crustal contamination of Kīlauea historical magmas is probably minor (< 12%, depending on the assumed contaminant) and it is superimposed on a longer-term, cyclic geochemical variation that reflects source heterogeneity. Kīlauea's heterogeneous source, which is well represented by the historical summit lavas, probably has magma δ18O values within the normal mid-ocean ridge basalt mantle range (5·4–5·8‰) based on the new olivine δ18O values.

A decade of exploring a submarine intraplate volcano: Hydrothermal manganese and iron at Lō'ihi volcano, Hawai'i

Decadal time series observations of hydrothermal fluid emissions from Lō'ihi volcano were initiated in 1992 using a combination of submersible and shipboard sampling strategies. Magmatic‐tectonic processes associated with a spectacular seismic event in July–August 1996 led to the collapse of Pele's Vents (31°C) near the volcano summit into a new pit crater (Pele's Pit) inundated by high‐temperature (Tmax = 198°C) focused and lower‐temperature (&lt;95°C) diffuse fluid discharge. Variations in the concentrations and relative abundances of vented Fe and Mn characterize the precollapse, summit collapse, and postcollapse periods. The precollapse plume emanating from Pele's Vents had a low rise height (∼35 m) and concentration intensity (&lt;5 nmol/L TDMn, 70 nmol/L TDFe, ∼1.8 km distant) and dispersed to the southwest. Plumes coeval with the 1996 summit collapse were characterized by extreme metal concentrations within Pele's Pit (up to ∼10,000 nmol/L DMn and 400,000 nmol/L TDFe) and nearby East Pit (∼34,000 nmol/L DMn and 120,000 nmol/L TDFe). An intense hydrothermal plume dispersed predominantly to the southwest of the several hundred meter deep pits with concentration anomalies as high as 236 nmol/L DMn and 3,800 nmol/L TDFe measured 9 km distant. Iron and Mn concentrations within the pits decreased &gt;30‐ to 200‐fold ∼1.5 months postcollapse and, during the following year, decreased a further twofold to threefold at Pele's Pit and ∼30‐fold at East Pit. While a steady concentration of ∼400 nmol/L TDFe prevailed throughout the remaining years of this study at Pele's Pit, a gradual and threefold decrease in the concentration of DMn to about 15 nmol/L was observed. High‐temperature fluids (128–198°C 1997–1999, ∼90°C 2001) venting simultaneously from different orifices within Pele's Pit had distinguishable Fe/Mn ratios that can be attributed to different subseafloor origins. Fe/Mn ratios characteristic of fluids moderated by high‐temperature water‐rock reaction had low values in 1997–1998 (1.6 ± 0.7), increasing to about 7 in 1999. Fluids moderated by magmatic degassing of CO2 had much higher Fe/Mn ratios, increasing from 24 ± 15 in 1997–1998 to 50 in 1999 and 63–87 in 2001. Fe/Mn values of dispersing plumes at Lō'ihi reflect an admixture of these sources and a relative Fe abundance that is consistently high compared to mid‐ocean ridge systems. The pulsed injection of Mn and Fe into the surrounding ocean associated with the 1996 tectonic‐magmatic event at Lō'ihi was massive. Our decadal observations confirm that Mn and Fe are useful markers of the magnitude and evolution of the effects of magmatic perturbation on hydrothermal systems influenced by chronic magmatic degassing.

Magma plumbing system of the 2000 eruption of Miyakejima Volcano, Japan

During the 2000 eruption at Miyakejima Volcano, two magmas with different compositions erupted successively from different craters. Magma erupted as spatter from the submarine craters on 27 June is aphyric basaltic andesite (<5 vol% phenocrysts, 51.4–52.2 wt% SiO2), whereas magma issued as volcanic bombs from the summit caldera on 18 August is plagioclase-phyric basalt (20 vol% phenocrysts, 50.8–51.3 wt% SiO2). The submarine spatter contains two types of crystal-clots, A-type and A′-type (andesitic type). The phenocryst assemblages (plagioclase, pyroxenes and magnetite) and compositions of clinopyroxene in these clots are nearly the same, but only A′-type clots contain Ca-poor plagioclase (An < 70). We consider that the A′-type clots could have crystallized from a more differentiated andesitic magma than the A-type clots, because FeO*/MgO is not strongly influenced during shallow andesitic differentiation. The summit bombs contain only B-type (basaltic type) crystal-clots of Ca-rich plagioclase, olivine and clinopyroxene. The A-type and B-type clots have often coexisted in Miyakejima lavas of the period 1469–1983, suggesting that the magma storage system consists of independent batches of andesitic and basaltic magmas. According to the temporal variations of mineral compositions in crystal-clots, the andesitic magma became less evolved, and the basaltic magma more evolved, over the past 500 years. We conclude that gradually differentiating basaltic magma has been repeatedly injected into the shallower andesitic magma over this period, causing the andesitic magma to become less evolved with time. The mineral chemistries in crystal-clots of the submarine spatter and 18 August summit bombs of the 2000 eruption fall on the evolution trends of the A-type and B-type clots respectively, suggesting that the shallow andesitic and deeper basaltic magmas existing since 1469 had successively erupted from different craters. The 2000 summit collapse occurred due to drainage of the andesitic magma from the shallower chamber; as the collapse occurred, it may have caused disruption of crustal cumulates which then contaminated the ascending, deeper basalt. Thus, porphyritic basaltic magma could erupt alone without mixing with the andesitic magma from the summit caldera. The historical magma plumbing system of Miyakejima was probably destroyed during the 2000 eruption, and a new one may now form.

Gigantic SO2 emission from Miyakejima volcano, Japan, caused by caldera collapse

Research Article| May 01, 2004 Gigantic SO2 emission from Miyakejima volcano, Japan, caused by caldera collapse Kohei Kazahaya; Kohei Kazahaya 1Geological Survey of Japan, Institute of Advanced Science and Technology, Tsukuba, Ibaraki 305-8567, Japan Search for other works by this author on: GSW Google Scholar Hiroshi Shinohara; Hiroshi Shinohara 1Geological Survey of Japan, Institute of Advanced Science and Technology, Tsukuba, Ibaraki 305-8567, Japan Search for other works by this author on: GSW Google Scholar Kozo Uto; Kozo Uto 1Geological Survey of Japan, Institute of Advanced Science and Technology, Tsukuba, Ibaraki 305-8567, Japan Search for other works by this author on: GSW Google Scholar Masanobu Odai; Masanobu Odai 2Japan Meteorological Agency, Chiyoda-ku, Tokyo 100-8122, Japan Search for other works by this author on: GSW Google Scholar Yasuhiro Nakahori; Yasuhiro Nakahori 2Japan Meteorological Agency, Chiyoda-ku, Tokyo 100-8122, Japan Search for other works by this author on: GSW Google Scholar Hirokazu Mori; Hirokazu Mori 2Japan Meteorological Agency, Chiyoda-ku, Tokyo 100-8122, Japan Search for other works by this author on: GSW Google Scholar Hideki Iino; Hideki Iino 2Japan Meteorological Agency, Chiyoda-ku, Tokyo 100-8122, Japan Search for other works by this author on: GSW Google Scholar Makoto Miyashita; Makoto Miyashita 2Japan Meteorological Agency, Chiyoda-ku, Tokyo 100-8122, Japan Search for other works by this author on: GSW Google Scholar Junichi Hirabayashi Junichi Hirabayashi 3Volcano Fluid Research Centre, Tokyo Institute of Technology, Meguro-ku, Tokyo 152-0033, Japan Search for other works by this author on: GSW Google Scholar Geology (2004) 32 (5): 425–428. https://doi.org/10.1130/G20399.1 Article history received: 10 Dec 2003 rev-recd: 10 Jan 2004 accepted: 12 Jan 2004 first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Kohei Kazahaya, Hiroshi Shinohara, Kozo Uto, Masanobu Odai, Yasuhiro Nakahori, Hirokazu Mori, Hideki Iino, Makoto Miyashita, Junichi Hirabayashi; Gigantic SO2 emission from Miyakejima volcano, Japan, caused by caldera collapse. Geology 2004;; 32 (5): 425–428. doi: https://doi.org/10.1130/G20399.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract An extremely large amount of volcanic gas has been released since mid-August 2000 from the volcanic island of Miyakejima, Japan, after formation of a summit caldera of 1.6 km diameter. The volcanic gas emission was continuous with very little extrusive magma activity. Variation of the SO2 emission rate was monitored by repeated measurements with an airborne correlation spectrometer. In December 2000, the SO2 emission rate averaged for the month peaked at 54 kt/d, which is twice the global SO2 emission rate from nonerupting volcanoes evaluated before this activity. The SO2 emission rate gradually decreased, almost linearly when plotted on a log scale, to 7 kt/d by the end of 2002, and then remained constant until at least December 2003. The total SO2 emission amounts to 18 Mt, comparable to the emission of a large explosive eruption such as Pinatubo in 1991. A theoretical evaluation, based on the model of magma convection in a conduit, suggests that extremely large volcanic gas emissions can be caused by formation of a magma pathway with a slightly larger diameter than exists in common systems, because the magma-transport rate is proportional to the fourth power of the conduit radius. Because volcanic gas emissions were initiated by formation of a summit collapse caldera of 1.6 km diameter, the creation of a large magma-conduit system through fractures formed during caldera collapse is likely the underlying cause of the extremely large volcanic gas emissions from the volcano. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Resistivity and self-potential changes associated with volcanic activity: The July 8, 2000 Miyake-jima eruption (Japan)

The Miyake-jima volcano abruptly erupted on July 8, 2000 after 17 years of quiet and gave birth to a crater, 1 km in diameter and 250 m deep. This expected unrest was monitored during the years 1995–2000 by electromagnetic methods including DC resistivity measurements and self-potential (SP) surveys. Beneath the 2500 yr old Hatcho-Taira summit caldera audio-magnetotelluric soundings made in 1997–98 identified a conductive medium, 200–500 m thick (within the 50 Ω m isoline) located at a few hundred metres depth. It was associated with the active steady-state hydrothermal system centred close to the 1940 cone and extending southward. A DC resistivity meter set in a Schlumberger array with 600, 1000 and 1400 m long injection lines evidenced strong resistivity changes between September 1999 and July 3, 2000 in the vicinity of the newly formed crater. The apparent resistivity has reached about three times its initial values on the 1400 m long line and has lowered to about 20% on the 600 m line. Just prior to the July 8, 2000 eruption SP mapping made inside the summit Hatcho-Taira caldera revealed negative anomalies where positive ones had occurred during the previous tens of years. The largest negative anomaly, −225 mV in amplitude, mainly took place above the 1940 cone which collapsed in the crater formation. A permanent 1 km long SP line across the caldera suggests accelerating changes during the 3 months preceding the eruption. On a larger scale, the comparison between 1995 and 2000 surveys has shown a global increase of the hydrothermal activity beneath the volcano. Its source could have been 250 m to the south of the crater. These observations suggest that the hydrothermal system was slowly disturbed in the months preceding the eruption while drastic changes have occurred during the 2 weeks before the summit collapse when tectonic and volcanic swarms have appeared.

A28 三宅島火山の山頂陥没・水蒸気爆発・脱ガス過程から推定したマグマの動き

A28 三宅島火山の山頂陥没・水蒸気爆発・脱ガス過程から推定したマグマの動き

Magmatic processes and segmentation at a fast spreading mid‐ocean ridge: Detailed investigation of an axial discontinuity on the East Pacific Rise crest at 9°37′N

Geophysical and petrological boundaries on mid‐ocean ridges provide ideal locations to study the relationships between magmatic, tectonic, and hydrothermal processes. Alvin‐based observational data and geochemical data for basalts and hydrothermal fluids are used to investigate these relationships at an axial discontinuity on the East Pacific Rise (EPR) crest between ∼9°36′N and 9°38′N. This ridge‐crest discontinuity is morphologically expressed by the overlap of an eastern and western axial summit collapse trough (ASCT) that delimits the primary volcanic and hydrothermal loci along the ridge crest in this area. The ASCTs overlap by ∼3 km and are offset in a right‐lateral sense by 0.45 km. Near‐bottom imaging of this area in 1989 and 1991 shows changes in volcanic morphology and increases in hydrothermal and biological activity consistent with the occurrence of a magmatic event during that time interval. When combined with the inferred age and structure of the seafloor, basalt geochemistry, and hydrothermal fluid chemistry, these temporal changes suggest active southward propagation of the eastern ASCT and show that the western ASCT was unaffected by the recent magmatic event. Numerous extinct hydrothermal vents and older‐looking lava flow surfaces suggest waning of magmatic activity in the western ASCT.Young‐looking lava flows within or proximal to the eastern ASCT have anomalously high Mg numbers relative to the regional trend and are chemically similar to lava erupted in 1991 along the 9°46′–52′N EPR region. We propose that the young‐looking lava flows in the eastern ASCT are related to the 1991 eruption. Data show that the 9°37′N axial discontinuity marks a magmatic and hydrothermal boundary along the EPR ridge‐crest, and we argue that it be classified as a third‐order discontinuity. This result is consistent with geophysical evidence suggesting fundamental differences in the crust and upper mantle north and south of the 9°37′N discontinuity.

三宅島２０００年噴火活動に伴う長周期地震のメカニズムとその解釈

A sudden collapse of the summit of Miyake-jima occurred on July 8, 2000, together with intermittent eruptions. This collapse generated long-period seismic waves with a dominant period of about 10s. Following this event, very long-period seismic pulses (VLP pulses) with a duration of about 50s were observed a few times a day until they ceased at the largest summit eruption on August 18. We analyzed these seismic pulses using waveform data recorded at several domestic stations for broadband seismographs and strong motion seismometers on Miyake-jima. The July 8 event is well characterized by a single-force directed initially upward and later downward during 12 sec. The single-force is interpreted as an abrupt collapse of massive rock. The total mass is estimated to be about 5 × 1010 kg with fall of about 300 m. On the other hand, VLP pulses are modeled by moment-tensors with an isotropic component. They are located about 1 km southwest from the summit and 2 to 3 km deep. All three principal values are positive. The largest one is horizontal and the smallest one is near vertical. The total volume change due to 39 VLP pulses is 2.6× 108m3, amounting to nearly one half of the total volume of the summit collapse. Based on theresults, we propose a buried geyser model. A large reservoir of hot water was formed just after the summit collapse on July 8. The ground water poured into the reservoir, being rapidly heated by hot rock underneath, and evaporated to form a highly pressurized steam, which pushed a lower conduit piston into the magma reservoir to generate VLP pulses. Non-isotropic expansion of the VLP pulses may be ascribed to the shape of the magma reservoir.

Caldera Formation Process during the Miyakejima 2000 Volcanic Activity Detected by Spatio-temporal Gravity Changes

Employing both absolute and relative gravimeters, we carried out a hybrid microgravity survey at Miyakejima volcano, Japan. We detected significant gravity changes, e.g., exceeding 1000 At gal, during the course of our measurements since June 1998. The spatio-temporal gravity changes provide us with strong evidence of preexisting vacant space beneath the island before the summit collapse of the volcano. Also, our data are totally in conflict with the so-called magma drainback model, suggesting that the magma flowed out laterally.

Cyclical summit collapse events at Ascraeus Mons, Mars

The sequential development of multiple summit calderas of shield volcanoes gives clues about how the spatial location of shallow magma chambers evolves with time and how the magma supply into these chambers evolves. We re-interpret the sequence in which caldera collapse events took place at the Martian shield volcano Ascraeus Mons and argue that the formation of the central caldera, usually accepted to be the most recent, was followed by two further collapse events. We also present structural evidence for an early, large central caldera and, therefore, also an early large magma reservoir. This challenges the current hypothesis for Ascraeus Mons that successive magma reservoirs have increased in size as this volcano matured and that a reservoir occupied a central location within the volcano only at the end of its active lifespan. We propose that there has been a cyclical oscillation in the site of shallow storage of magma at Ascraeus Mons between central and peripheral magma reservoirs. The earliest reservoir formed centrally, but magma was subsequently redirected into peripheral reservoirs, over which the peripheral calderas formed. Later, the central location was utilized again, only to be followed once more by a development of peripheral storage reservoirs. This alternating pattern implies substantial variations with position and time in both the magma supply rate from the Martian mantle and the distribution of stresses within the volcanic edifice.

Volcanic Morphology of the East Pacific Rise Crest 9°49′–52′: Implications for volcanic emplacement processes at fast-spreading mid-ocean ridges

Deep sea photographs were collected for several camera-tow transects along and across the axis at the East Pacific Rise crest between 9°49′ and 9°52′ N, covering terrain out to 2 km from the ridge axis. The objective of the surveys was to utilize fine-scale morphology and imagery of seafloor volcanic terrain to aid in interpreting eruptive history and lava emplacement processes along this fast-spreading mid-ocean ridge. The area surveyed corresponds to the region over which seismic layer 2A, believed to correspond to the extrusive oceanic layer, attains full thickness (Christeson et al., 1994a, b, 1996; Hooft et al., 1996; Carbotte et al., 1997). The photographic data are used to identify the different eruptive styles occurring along the ridge crest, map the distribution of the different morphologies, constrain the relative proportions of the three main morphologies and discuss the implications of these results. Morphologic distributions of lava for the area investigated are 66% lobate lava, 20% sheet lava, 10% pillow lava, and 4% transitional morphologies between the other three main types. There are variations in inferred relative lava ages among the different morphological types that do not conform to a simple increase in age versus distance relationship from the spreading axis, suggesting a model in which off-axis transport and volcanism contribute to the accumulation of the extrusive layer. Analysis of the data suggests this ridge crest has experienced three distinctly different types of volcanic emplacement processes: (1) axial summit eruptions within a ∼1 km wide zone centered on the axial summit collapse trough (ASCT); (2) off-axis transport of lava erupted at or near the ASCT through channelized surface flows; and (3) off-axis eruptions and local constructional volcanism at distances of ∼0.5-1.5 km from the axis. Major element analyses of basaltic glasses from lavas collected by Alvin, rock corer and dredging in this area indicate that the most recent magmatic event associated with the present ASCT erupted relatively homogeneous and mafic (>8.25 weight percent wt.% MgO) basalts compared to older, off-axis lavas which tend to be more chemically evolved (Perfit and Chadwick, 1998; Perfit and Fornari, unpublished data). The more primitive lavas have a more extensive distribution within and east of the ASCT. More evolved basalts (MgO ∼500 m from the ASCT suggests that regions of off-axis volcanism may be sourced from older and cooler sections of the axial magma lens. Analysis of these data suggests that this portion of the EPR has not experienced large scale volcanic overprinting in the past ∼30 ka. The predominance of lobate flows (66%) throughout much of the crestal region, and subtle variations in sediment cover and apparent age between flows, suggest that eruptive volumes and effusion rates of individual eruptions have been similar over much of the last 30 ka and that most of the eruptions have been small, probably similar in volume to the 1991 EPR flow which had an estimated volume of ∼1×106 m3 (Gregg et al., 1996).

A past giant lateral collapse and present-day flank instability of Fogo, Cape Verde Islands

Fogo island is a large and extremely steepsided oceanic island volcano in the Cape Verde archipelago. It has a large (ca. 9 km across) east facing summit collapse structure, the Monte Amarelo collapse, with a probable volume of at least 150–200 km3. For most of its history the Monte Amarelo volcano had a small but productive central vent complex and radial rift zones fed by laterally propagating dykes. Shortly before the collapse the latter were replaced by north–south-trending arrays of en echelon, vertically propagating dykes. Since the Monte Amarelo collapse the scar has partly filled with a new volcano, the Cha das Caldeiras volcano. The summit cone of this volcano, the Pico do Fogo, is a very young feature but has been abandoned in the most recent phase of activity, from the 18th century onwards. The same period has also seen the abandonment of earlier radial rift zones with laterally propagating dykes and their replacement with en echelon arrays of vents fed by vertically propagating dykes. These form an N–S-trending array within the older collapse structure and are associated with seaward displacement of the eastern flank of the volcano within the old collapse structure. The most recent eruptions, those of 1951 and 1995, appear to be associated with episodes of flank instability manifested in N–S surface fissuring and east facing normal faults. These recent structural changes in the volcano parallel those which took place in the Monte Amarelo volcano prior to its collapse.

Widespread assimilation of a seawater-derived component at Loihi Seamount, Hawaii

Many tholeiitic and transitional pillow-rim and fragmental glasses from Loihi seamount, Hawaii, have high Cl contents and Cl/K 2O ratios (and ratios of Cl to other incompatible components, such as P 2O 5, H 2O, etc.) relative to other Hawaiian subaerial volcanoes (e.g., Mauna Loa, Mauna Kea, and Kilauea). We suggest that this results from widespread contamination of Loihi magmas by a Cl-rich, seawater-derived component. Assimilation of high-Cl phases such as saline brine or Cl-rich minerals (halite or iron–hydroxychlorides) with high Cl/H 2O ratios can explain the range and magnitude of Cl contents in Loihi glasses, as well as the variations in the ratios of Cl to other incompatible elements. Brines and Cl-rich minerals are thought to form from seawater within the hydrothermal systems associated with submarine volcanoes, and Loihi magmas could plausibly have assimilated such materials from the hydrothermal envelope adjacent to the magma chamber. Our model can also explain semiquantitatively the observed contamination of Loihi glasses with atmospheric-derived noble gases, provided the assimilant has concentrations of Ne and Ar comparable to or slightly less than seawater. This is more likely for brines than for Cl-rich minerals, leading us to favor brines as the major assimilant. Cl/Br ratios for a limited number of Loihi samples are also seawater-like, and show no indication of the higher values expected to be associated with the assimilation of Cl-rich hydrothermal minerals. Although Cl enrichment is a common feature of lavas from Loihi, submarine glasses from other Hawaiian volcanoes show little (Kilauea) or no (Mauna Loa, Mauna Kea) evidence of this process, suggesting that assimilation of seawater-derived components is more likely to occur in the early stages of growth of oceanic volcanoes. Summit collapse events such as the one observed at Loihi in October 1996 provide a ready mechanism for depositing brine-bearing rocks from the volcanic edifice into the top of a submarine summit magma chamber.

Continuous near‐bottom gravity measurements made with a BGM‐3 gravimeter in DSV Alvin on the East Pacific Rise crest near 9°31′N and 9°50′N

A Bell BGM‐3 gravimeter has been used to collect continuous, underway, nearbottom (3‐ to 10‐m altitude) gravity measurements from the deep‐diving submersible DSV Alvin during surveys on the East Pacific Rise (EPR) crest near 9°31′N and 9°50′N. Closely spaced (20‐to 30‐m) gravity measurements were made along transects up to 8 km long in both regions. Repeatability of measurements made at the same location on different dives is ∼0.3 mGal. Along‐track spatial resolution of anomalies is ∼130–160 m, with the limiting factors being precision and sampling rate of the pressure gauge depth data used to calculate vertical accelerations of the submersible. The average upper crustal density of the ridge crest determined from the relationship between depth and free‐water gravity anomalies varies greatly between 9°31′N and 9°50′N. Average upper crustal densities of 2410 kg/m3 for the 9°50′N area and 2690 kg/m3 for the 9°31′N area were calculated. The different densities are not due to differing geometry of the Layer 2A‐2B boundary or a regional cross‐axis gravity gradient. Differences in porosity of the shallow crustal rocks, or a difference in the proportion of low density extrusives to higher‐density dikes and sills within Layer 2A in these two areas, are the likely causes of the different upper crustal densities. Bouguer gravity anomalies near the EPR axis are primarily small amplitude (0.5–2 mGal), are a few hundred meters across, and appear to be lineated parallel to the axis. Larger‐amplitude Bouguer anomalies of up to 4 mGal were found at a few locations across the crestal plateau and are associated with pillow ridges composed of lavas which are clearly younger than the surrounding seafloor. These ridges have distinct chemical compositions compared to lavas from the axial summit collapse trough (ASCT) at the same latitude. Probable sources of the 0.5‐ to 2‐mGal anomalies observed on the summit plateau include areas of collapsed and fissured terrain and dike swarms feeding melt through Layer 2A to the surface. A grid survey of the ridge axis near 9°50′N shows Bouguer anomalies lineated along the axis, suggesting that dike swarms do contribute to the observed Bouguer anomalies. The along‐axis continuity of the gravity anomalies is disrupted at a 75‐m offset of the ASCT, suggesting that shallow feeders of lava to the surface may be segmented on a finer scale than the deeper crustal magmatic system. This initial study confirms the ability to conduct high‐resolution, near‐bottom, continuous gravity measurements from Alvin. It also provides important information on how the shallow crustal structure of a fast spreading mid‐ocean ridge develops and how it varies with the surface morphology.

Axial summit trough of the East Pacific Rice 9°–10°N: Geological characteristics and evolution of the axial zone on fast spreading mid‐ocean ridge

The nature and morphological characteristics of axial summit troughs on fast (∼90–130 mm/yr−1 full spreading rate) and superfast spreading (&gt;130 mm/yr−1) mid‐ocean ridge crests reflect the time‐integrated effects of long‐term magmatic cycles, short‐term volcanic episodicity, and the tensional stress regime imposed on young ocean crust. Two principal types of axial trough morphology have been identified and associated with distinct volcanic and tectonic processes occurring at fast and superfast spreading mid‐ocean ridge crests. (1) Narrow axial troughs, &lt;∼200 m wide and &lt;∼15 m deep, are termed axial summit collapse troughs and are interpreted to be generated by shallow collapse of lava flow surfaces over zones of primary eruptive fissures and subsurface lava conduits (such as channels and tubes). Axial summit collapse trough position, physical characteristics, and associations with hydrothermal venting on the ridge crest strongly suggest a primary volcanic/magmatic origin for the trough. (2) Wider, fault‐bounded axial troughs (&gt;∼300–2000 m wide and ∼30–100 m deep) on the East Pacific Rise crest are classified as axial summit graben. The dimensions of axial summit graben, as well as the morphological and structural character of their walls and floors, suggest a primary tectonic origin. An axial summit graben may contain a nested axial summit collapse trough, implying that processes responsible for these endemic features may be linked. Near‐bottom, side‐looking sonar and observational data collected using the towed vehicle Argo I and submersible Alvin have been used to characterize the axial summit trough of the fast spreading East Pacific Rise between 9° and 10°N. A four‐stage model is presented for the evolution of this axial summit collapse trough, as well as for other well‐studied portions of the East Pacific Rise crest from 21°N to ∼20°S. We propose that the transition from a narrow, surface collapse‐dominated axial trough to a broader, fault‐bounded graben is controlled by the relative importance of diking, volcanism, hydrothermal cooling, and tectonism along a ridge segment over time periods &lt;104 years.

High‐resolution bathymetry of the East Pacific Rise axial summit trough 9° 49′ ‐ 51′N: A compilation of Alvin scanning sonar and altimetry data from 1991–1995

Bathymetric data acquired during six cruises to the East Pacific Rise crest 9° 49′‐51′N are compiled into a single bathymetric database and used to create high‐resolution bathymetry maps of two geologically and biologically active areas. Scanning sonar and Alvin altimetry were collected during 23 Alvin dives to the P‐vent/Bio9 and Biomarker‐141 areas during 1991–95. Bathymetry data were filtered, edited, and processed as individual dives then combined into a single database for each cruise and statistically evaluated. Cruise data sets were numerically and visually compared and cross‐correlated to known bottom features and fixed seafloor reference markers, then horizontally shifted to establish a consistent database with 3 m vertical and 5 m horizontal resolutions. The compiled and corrected bathymetry was used to produce bathymetric maps of two areas within the axial summit collapse trough; a 2 × 104 m² area around P and Bio9 vents, and a 1.1 × 10³ m² area around Biomarker 141. The resulting detailed bathymetry maps reveal topography dominated by low‐relief volcanic constructional features and post‐eruptive collapse of lobate and sheet lava surfaces.

An integrated shipboard ocean floor research system using a SeaBeam 210 ocean survey system installed on the R/V Ka'imikai‐o‐Kanaloa

An integrated shipboard research system capable of operating in waters of up to2,000 m has been designed and implemented by the Hawai'i Undersea Research Laboratory (HURL), University of Hawai'i. It uses a remotely operated ocean‐bottom survey camera system, a remotely operated vehicle (ROV), the Pisces V submersible, and a multibeam mapping system aboard the 225‐ft RIV Ka'imikai‐o‐Kanaloa. These systems are operated from the stern of Ka'imikai. The Pisces V submersible is deployed and recovered with a mounted Caley telearm and articulated A‐frame system. The integrated shipboard system was designed for a multipurpose use of the R/V Ka'imikai‐o‐Kanaloa for Pisces V daytime operations or nighttime operations with an electro‐optical cable using either the HURL ocean‐floor sidescan or photo and video systems operated at 10 m above the ocean floor. The 1.14‐in. diameter cable is also used to operate the RCV‐150 ROV. In order to obtain a reliable bathymetric database for potential dive sites, a SeaBeam system was installed on the vessel. The installation consists of a hybrid system with classic shipboard electronics and ceramic SeaBeam 210 underhull arrays. The first test results with the ship's bow thruster tunnel closed showed that the SeaBeam 210 system peformed continuously with a normal central beam‐crossing error of less than 10 m. The first field study with the system was conducted during September 1996 over the submarine volcano Loihi, Hawai'i. to resurvey the edifice after an unprecedented collapse of the summit had taken place. The use of both SeaBeam surveys and visual observations with the submersible Pisces V allowed the accurate mapping of newly formed features, such as the 300‐m‐deep. 1,000‐m‐side summit crater, to take place. Almost 300 million tons of rock collapsed into the summit during this event. Magma withdrawal within the magmatic plumbing of Loihi is probably the mechanism by which the summit collapse took place.