Submarine Volcano Research Articles

Environmental monitoring of El Hierro Island submarine volcano, by combining low and high resolution satellite imagery

El Hierro Island, located at the Canary Islands Archipelago in the Atlantic coast of North Africa, has been rocked by thousands of tremors and earthquakes since July 2011. Finally, an underwater volcanic eruption started 300m below sea level on October 10, 2011. Since then, regular multidisciplinary monitoring has been carried out in order to quantify the environmental impacts caused by the submarine eruption. Thanks to this natural tracer release, multisensorial satellite imagery obtained from MODIS and MERIS sensors have been processed to monitor the volcano activity and to provide information on the concentration of biological, chemical and physical marine parameters. Specifically, low resolution satellite estimations of optimal diffuse attenuation coefficient (Kd) and chlorophyll-a (Chl-a) concentration under these abnormal conditions have been assessed. These remote sensing data have played a fundamental role during field campaigns guiding the oceanographic vessel to the appropriate sampling areas. In addition, to analyze El Hierro submarine volcano area, WorldView-2 high resolution satellite spectral bands were atmospherically and deglinted processed prior to obtain a high-resolution optimal diffuse attenuation coefficient model. This novel algorithm was developed using a matchup data set with MERIS and MODIS data, in situ transmittances measurements and a seawater radiative transfer model. Multisensor and multitemporal imagery processed from satellite remote sensing sensors have demonstrated to be a powerful tool for monitoring the submarine volcanic activities, such as discolored seawater, floating material and volcanic plume, having shown the capabilities to improve the understanding of submarine volcanic processes.

Shallow submarine volcano group in the early stage of island arc development: Geology and petrology of small islands south off Hahajima main island, the Ogasawara Islands

Small Islands south off Hahajima, the southernmost of the Ogasawara Archipelago, consist of primitive basalts (<12wt.% MgO) to dacite erupted during the transitional stage immediately following boninite volcanism on the incipient arc to sustained typical oceanic arc. Strombolian to Hawaiian fissure eruptions occurring on independent volcanic centers for the individual islands under a shallow sea produced magnesian basalt to dacite fall-out tephras, hyaloclastite and a small volume of pillow lava, which were intruded by NE-trending dikes. These volcanic strata are correlated to the upper part (<40Ma) of the Hahajima main island. Volcanic rock samples have slightly lower FeO*/MgO ratios than the present volcanic front lavas, and are divided into three types with high, medium and low La/Yb ratios. Basalt to dacite of high- and medium-La/Yb types show both tholeiitic (TH) and calc-alkaline (CA) differentiation trends. Low-La/Yb type belongs only to TH basalt. The multiple magma types are coexistence on the each island. TH basalts have phenocrysts of olivine, clinopyroxene and plagioclase, while CA basalts are free from plagioclase phenocrysts.

1P1-F05 全方向移動可能なホバリング型AUVの開発 : 行動決定メインモジュールの開発(水中ロボット・メカトロニクス(2))

Ocean development has actively progressed in the sea near Japan. However, in marine research, it is impossible for people to work in deep sea because of low temperature, high pressure and so on. Therefore, AUVs (Autonomous Underwater Vehicles) are expected as applications such as investigating and mapping a submarine volcano and so on. In previous researches, many AUVs have been developed, however they have to change the attitude when the robot change the direction of movement. In this research, we developed an AUV which is able to move all directions in a small space in the same as a quad-rotor helicopter.

통가열도 해저화산 표층 퇴적물 내 열수변질의 화학적 특성

통가열도 내 우리나라 해저열수광상 독점 탐사지역에 위치하는 5개 해저화산(TA12, TA19, TA22, TA25, TA26) 주변 표층 퇴적물의 열수변질 여부를 규명하고, 열수변질 특성을 확인하기 위하여 각 해저화산에서 채취된 29개의 표층 퇴적물에 대해 미량 원소와 희토류 원소 분석을 실시하였다. 분석 결과 TA12, TA19, TA22 해저화산에서는 열수에 의한 퇴적물의 변질작용이 없거나 매우 적은 것으로 나타났으며, TA25와 TA26 해저화산의 일부 지역은 현재에도 열수의 영향을 받고 있는 것으로 나타났다. 열수의 영향이 있는 해저화산 사이에도 부화된 미량 원소의 종류에 차이가 있는데, TA25 해저화산의 경우 주로 Ni, Cu, Sn, Zn, Pb, Cr, Cd, Sb, W, Ba, Ta, Rb, Sr, As가, TA26 해저화산에서는 주로 Cu, Sn, Zn, Pb, Cd, Sb, Ba, Rb, Sr이 부화되어 있다. 미량 원소의 부화가 확인된 지역은 희토류 원소의 부화도 관찰된다. 상부지각 희토류 원소 농도로 표준화된 희토류 원소의 분포형태는 가벼운 희토류 원소(LREE)가 매우 낮고 중간 희토류 원소(MREE)에서 무거운 희토류 원소(HREE)로 갈수록 증가하는 형태를 보이며, TA25와 TA26 해저화산의 일부 정점에서 Eu의 부화가 확인된다. 또한 TA26 해저화산의 일부 정점에서 Ce이 부화가 관찰되었는데, 이는 주변지역에서 채취된 열수 내 희토류 원소의 높은 Ce분포형태와 매우 유사하다. 또한 TA25와 TA26 해저화산은 부화된 원소의 특징이 다르게 나타나는데, TA25 해저화산에서는 Cu를 포함한 미량 원소가, TA26 해저화산에서는 Ce와 Eu를 포함한 희토류 원소가 우세하게 부화되어 있는 것으로 나타났다. 조사결과 나타난 각 해저화산 표층 퇴적물 내 부화된 미량 원소 및 희토류 원소의 종류와 농도는 차후 통가열도 지역의 열수광상 탐사시에 지역적인 열수변질의 지시자로써 유용하게 활용될 것으로 판단된다. We analyzed 29 surface sediment samples in five submarine volcanoes (TA12, TA19, TA22, TA25, and TA26) located in the southern part of the Tonga arc for trace elements and rare earth elements to investigate characteristics of the hydrothermal alteration of surface sediments. Based on analytical results of trace element and rare earth element (REE), surface sediments of TA12, TA19, and TA22 submarine volcanoes, which are located in the northern part of the study area, were very little or not influenced by hydrothermal fluids. In contrast, some stations of TA25 and TA26 submarine volcanoes were strongly affected by hydrothermal fluids. However, these two submarine volcanoes showed different features in element concentration in the sediments. Some stations of TA25 submarine volcano showed enrichment of Ni, Cu, Sn, Zn, Pb, Cr, Cd, Sb, W, Ba, Ta, Rb, Sr, and As, however, those of TA26 submarine volcano showed enrichment of Sn, Zn, Pb, Cd, Sb, Ba, Rb, and Sr. Stations which enriched trace elements were observed, enriched REEs were also observed. Average upper continental crust (UCC)-normalized REE patterns of the surface sediments generally showed low light REE (LREE) abundances and increased heavy REE (HREE) abundances. Eu enrichment was identified at several stations of TA25 and TA26 submarine volcanoes. In addition, enrichment of Ce was found at some stations of TA26 submarine volcano and these enrichment patterns were similar with hydrothermal fluid of near stations. Furthermore, TA25 and TA26 submarine volcanoes showed different enrichment characteristics of trace elements and REE. Trace elements were concentrated at TA25 submarine volcano. TA26 submarine volcano, on the other hand, observed highly enrichment of REE especially, Eu and Ce. As a result of the investigation, the characteristics and concentrations of REEs and trace elements in the surface sediments of each submarine volcano can be applied to identify hydrothermal alteration of sediments during exploration for hydrothermal deposits.

Acoustic response of submarine volcanoes in the Tofua Arc and northern Lau Basin to two great earthquakes

Acoustic response of submarine volcanoes in the Tofua Arc and northern Lau Basin to two great earthquakes

Dynamic Overshoot Near Trench Caused by Large Asperity Break at Depth

In an attempt to explain the large shallow slip that occurred near the trench during the 2011 Tohoku-Oki earthquake, numerical simulations of earthquake dynamic rupture were carried out using a fault model with a subduction interface containing a bump-shaped asperity, which might result from subduction of an old submarine volcano or seamount. It was assumed that during the interseismic period, slip only occurs outside the bump area and that stress accumulates inside the bump, creating a seismogenic asperity. We roughly evaluated the amount of slip outside the bump during the interseismic period, assuming a constant long-term subduction rate. Then we could estimate the accumulated stress inside the bump. We constructed the initial stress distribution based on the stress change caused by the slip-deficit distribution. A constitutive relation was constructed based on a slip-weakening friction law and was used to compute spontaneous ruptures. The results indicate that a large slip can occur between the trench and the bump, even though a very small amount of stress is accumulated there before the rupture. This is due to an interaction between the free surface and the fault that causes slip overshoot. On the region of the fault below the bump, such overshoot cannot occur because the fault is pinned by the deeper un-slipped zone. However, on the shallower side, the edge of the fault becomes free when the rupture approaches the free surface. In this region, such a large slip can occur without releasing a large amount of stress.

Earthquake occurrence reveals magma ascent beneath volcanoes and seamounts in the Banda region

Characteristic seismicity patterns beneath the volcanic arcs of the Banda region, SE Asia, suggest that magmatic processes have recently occurred beneath submarine portions of the arcs, forming yet-unrecognised submarine volcanoes. We have found that almost 50 % of earthquakes spatially associated with the Banda and Ambon volcanic arcs occurred in sequences with epicenters often concentrated in a small area and foci distributed in vertically elongated domains. The most pronounced occurrence of such earthquake series and swarms was observed in the area of the Manipa submarine basin (latitude 3.75°S, longitude 127.5°E, ESE of Buru Island), the remarkable morphology of which resembles a huge caldera (60 km in diameter) with a distinct cone seamount in its center, reaching almost 3000 m above seafloor. Another candidate for an unrecognised submarine volcano is an area between volcanoes Banda Api and Manuk, with a huge 1973/74 earthquake swarm. We assume that such a specific occurrence of earthquakes is induced by magma ascent and migration along faults above the subducting slab, with magma possibly occasionally reaching the sea floor. Utilization of teleseismic data can thus reveal activation of plumbing systems of submarine volcanoes, and highlight areas with the potential of near-future volcanic events.

Leakage of magmatic–hydrothermal volatiles from a crater bottom formed by a submarine eruption in 1989 at Teishi Knoll, Japan

A submarine eruption occurred off the Izu Peninsula of Japan on 13 July 1989, forming Teishi Knoll, which has a diameter of 450m and a height of ca. 10m above the surrounding 90–100m deep seafloor. Immediately after the eruption, intense gas release was observed from two vents in the crater. The gas bubbling gradually decreased and apparently ceased in 1990. Given that no survey has been undertaken to examine volatile release from the crater of Teishi Knoll, we collected seawater samples at three different sites from just above the crater bottom on 17 July 2012, in order to detect signs of magmatic volatile release. Seawater samples from the crater bottom have dissolved CH4 contents and δ13C values higher than those of shallower (50–100m deep) seawater samples. Total inorganic carbon contents from the bottom seawater samples are also higher, and δ13C and Δ14C values lower than those of shallower seawater samples. These data indicate the addition of minor CH4 and CO2 of hydrothermal or magmatic origin to the bottom seawater from the crater. 3He/4He ratios and total organic carbon data are also consistent with the leakage of magmatic fluids. The most prominent CH4 and CO2 anomalies were observed at the site located closest to one of the bubbling gas sites of the 1989 eruption. As such, volcanic gas emissions still continue today at extremely low levels, 23years after eruption of this monogenetic volcano. The monitoring of ultra-trace amounts of chemical components in seawater is a prospective method to monitor temporal changes in magmatic activity at such submarine volcanoes.

Time series analysis of discolored seawater reflectance observed by Advanced Visible and Near Infrared Radiometer type 2 (AVNIR-2) at Fukutoku-Okonaba submarine volcano, Japan

Monitoring submarine volcanoes is not an easy task compared to land volcanoes because they are covered by seawater and located in remote areas. Satellite remote sensing is a powerful tool for monitoring underwater volcanic activities such as discolored seawater, floating material and volcanic plumes. Discolored seawater is a good indicator of submarine volcanic activities. Advanced Visible and Near Infrared Radiometer type 2 (AVNIR-2) made extensive observations from 2006 to 2011 of the Fukutoku-Okanoba submarine volcano, which is located 1300km south of Tokyo, and is one of the most active submarine volcanoes in Japan. The high discolored seawater brightness coincides with relatively high activity of Fukutoku-Okanoba. No discolored seawater was observed for 6months before the 2010 Fukutoku-Okanoba submarine eruption, meaning that Fukutoku-Okanoba was quiescent before the eruption. Both high brightness and apparent color change of discolored seawater derived from AVNIR-2 mean emergence of large amount of hot spring water, implying that the submarine volcano is highly active. This study demonstrates that satellite remote sensing is an effective tool for monitoring activities of inaccessible submarine volcanoes.

Observation and modeling of three-dimensional basin scale acoustics

In this paper, an overview of some of the basin-scale recordings of the International Monitoring System of the Comprehensive Test Ban Treaty will be presenting, including observations of distant earthquakes, under-sea volcanoes and cracking ice-sheets. Long range experiments were conducted in 1960 when nearly antipodal receptions were made at Bermuda from SuS charges deployed off the coast of Australia. Three-dimensional propagation effects was an important part of the propagation. For propagation ranges of thousands of kilometers, interaction with islands, ridges and seamounts is expected to influence propagation. Previous modeling approaches have been either a fully 3D ray approach, a hybrid adiabatic mode-ray approach and a hybrid adiabatic mode—PE approach. In this paper, we present the a global scale three-dimensional split-step Pade Parabolic Equation. Model results are compared with several direct observations of arrivals that are clearly in the 2D propagation shadow. The impact of 3D propagation on the coverage maps of the CTBTO are significant, relative to current 2D predictions.

Seismicity Associated with the May 2010 Eruption of South Sarigan Seamount, Northern Mariana Islands

On 29 May 2010, at approximately 11:48 UTC, an undersea volcano south of Sarigan in the Northern Mariana Islands (Figs. 1 and 2) erupted sending a cloud of volcanic ash and water vapor to 40,000 feet (12 km; Washington Volcanic Ash Advisory Center). Bathymetric data (Stern and Smoot, 1998; Embley et al. , 2007) indicate an undersea vent exists at approximately 16.582° N and 145.821° E or about 12 km southwest of Sarigan and about 30 km northeast of Anatahan (Fig. 3). The vent depth is estimated to be 184 m below sea level. Based on physical observations of water discoloration (Juan Camacho, personal comm., 2010) and satellite imagery, the eruption plume is attributed to this undersea seamount. Figure 1. Map showing locations of islands and seismic stations in the Northern Mariana Island (NMI) chain. The names of active volcanoes are underlined. The location of the South Sarigan seamount is marked with an X. Inset shows the location of the NMI relative to Japan, Asia, the Philippine Islands, and New Guinea. Figure 2. Map showing the location of undersea vent that was the site of the 29 May 2010 eruption relative to the islands of Anatahan and Sarigan. Triangles, locations of seismic stations SARN, ANNE, and ANA2. Figure 3. Bathymetry of South Sarigan seamount and vicinity. Note data gap over part of summit. Individual minimum sounding are noted for some peaks. Contour interval is 200 m with 60 m grid‐cell size. Multibeam bathymetry data sets compiled in the area of Sarigan include: National Oceanographic and Atmospheric Administration (NOAA) Coral Reef Ecosystem Division EM300 and Reson 8101 surveys (AHI‐07‐02, AHI‐07‐03, HI‐07‐02, HI‐07‐03); NOAA Vents Program EM300 (TN153 and TN167); SeaBeam 2100 data (MGLN02MV, Onnuri); Japan Agency for Marine‐Earth Science and Technology (JAMSTEC) Seabat data (NT0518); National Geophysical Data Center (NGDC) Seabeam data (1 line north of data gap). Figure and caption courtesy of Robert Embley, …

Integrated volcanologic and petrologic analysis of the 1650 AD eruption of Kolumbo submarine volcano, Greece

Kolumbo submarine volcano, located 7km northeast of Santorini, Greece in the Aegean Sea, last erupted in 1650AD. Submarine and subaerial explosive activity lasted for a period of about four months and led to the formation of thick (~250m) highly stratified pumice deposits on the upper crater walls as well as extensive pumice rafts that were dispersed throughout the southern Aegean Sea. Subaerial tephra fallout from eruption columns that breached the surface occurred as far east as Turkey.Remotely operated vehicle (ROV) video observations from transects of the northern and southwestern crater walls have been used to create stratigraphic sequences of the 1650AD deposits. Petrographic and geochemical analyses reveal that the bulk of the deposits consists of white, highly vesicular, sparsely porphyritic, biotite-bearing rhyolite pumice. Pre-eruption volatile contents determined from plagioclase-hosted melt inclusions yield a median value of 6.5wt.%. This corresponds to a pre-eruption storage pressure of ~200MPa, or a depth of ~6km assuming a H2O-saturated magma. Comparison of the natural glass compositions and mineral assemblage of the Kolumbo samples with experimental results on other rhyolite magmas of similar composition in the modified haplogranite system supports the pressure and total volatile estimates. Pre-eruption temperature was calculated as 750°C based on plagioclase–melt geothermometry.The high volatile content of the Kolumbo magma and historical accounts of substantial subaerial eruption plumes suggest that the explosive eruption was driven by primary volatile degassing. Three phases of activity can be inferred from historical accounts and deposit stratigraphy; an initial submarine eruption during which time the vent became shallower as pyroclastic material accumulated, a transitional phase with substantial fallout from a hybrid submarine/subaerial eruption plume, and a final subaqueous phase as the eruption waned. Depositional processes included a complex interplay of submarine fallout, pyroclastic density currents, and clast segregation associated with the transition from submarine to subaerial plumes, producing a highly stratified pumiceous sequence. This is the first study of the in situ pyroclastic deposits of a well-documented silicic submarine explosive eruption in the relatively shallow (<500m below sea level) marine environment.

Bathymetry of a new-born submarine volcano: El Hierro. Canary Islands

We present here a bathymetric map of a new underwater volcano which began its growth on 10 October 2011 to the south of El Hierro Island (Canary Is., Spain). The map scale is 1:25,000 and the map covers 210.9 sq. km. In July 2011, the seismic monitoring network of Spanish National Geographic Institute (IGN), began to detect an increase in low-intensity earthquakes around El Hierro Island along with ground deformation. This seismic crisis culminated on 10th October with a submarine eruption 2 km south of the small port of La Restinga, and lasted until March 2012 when IGN determined the end of the eruption process. After eight surveys monitoring the morphological and bathymetric evolution during the eruptive phase that ended in March 2012, Spanish Oceanographic Institute and the Hydrographic Institute of the Navy, carried out a survey from the 6th to the 8th of December 2012 to map the bathymetric and morphologic situation after the 2011–2012 eruptive period. The map presented here is based on full seafloor coverage by multibeam swath data echosounder carried out when the submarine volcano was in a quiet phase, using a grid mesh size of 12 × 12 meters.

Geophysical modeling of collapse‐prone zones at Rumble III seamount, southern Pacific Ocean, New Zealand

Catastrophic collapses of submarine volcanoes have the potential to generate major tsunami, threatening many coastal populations. Recognizing the difficulties surrounding anticipations of these events, quantitative assessment of collapse‐prone regions based on detailed morphological, geological, and geophysical mapping can still provide important information about the hazards associated with these collapses. Rumble III is one of the shallowest, and largest, submarine volcanoes found along the Kermadec arc, and is both volcanically and hydrothermally active. Previous surveys have delineated major collapse features at Rumble III; based on time‐lapse bathymetry, dramatic changes in the volcano morphology have been shown to have occurred over the interval 2007–2009. Furthermore, this volcano is located just ∼300 km from the east coast of the North Island of New Zealand. Here, we present a geophysical model for Rumble III that provides the locations and sizes of potential weak regions of this volcano. Shipborne and near‐seafloor geological and geophysical data collected by the AUV Sentry are used to determine the subsurface distribution of weak and unstable volcanic rocks. The resulting model provides evidence for potentially unstable areas located in the Southeastern flank of this volcano which should be included in future hazard predictions.

The 1998 eruption of Axial Seamount: New insights on submarine lava flow emplacement from high‐resolution mapping

Axial Seamount, an active submarine volcano on the Juan de Fuca Ridge at 46°N, 130°W, erupted in January 1998 along 11 km of its upper south rift zone. We use ship‐based multibeam sonar, high‐resolution (1 m) bathymetry, sidescan sonar imagery, and submersible dive observations to map four separate 1998 lava flows that were fed from 11 eruptive fissures. These new mapping results give an eruption volume of 31 × 106 m3, 70% of which was in the northern‐most flow, 23% in the southern‐most flow, and 7% in two smaller flows in between. We introduce the concept of map‐scale submarine lava flow morphology (observed at a scale of hundreds of meters, as revealed by the high‐resolution bathymetry), and an interpretive model in which two map‐scale morphologies are produced by high effusion‐rate eruptions: “inflated lobate flows” are formed near eruptive vents, and where they drain downslope more than 0.5–1.0 km, they transition to “inflated pillow flows.” These two morphologies are observed on the 1998 lava flows at Axial. A third map‐scale flow morphology that was not produced during this eruption, “pillow mounds,” is formed by low effusion‐rate eruptions in which pillow lava piles up directly over the eruptive vents. Axial Seamount erupted again in April 2011 and there are remarkable similarities between the 1998 and 2011 eruptions, particularly the locations of eruptive vents and lava flow morphologies. Because the 2011 eruption reused most of the same eruptive fissures, 58% of the area of the 1998 lava flows is now covered by 2011 lava.

Long range acoustic measurements of an undersea volcano

A seamount 8 km southeast of Sarigan Island erupted on 29 May 2010 and was visually observed. The recordings on two sets of hydrophones, operated by International Monitoring System (IMS) of the Comprehensive Test Ban Treaty Organization (CTBTO) are analyzed. Each array is a triplet of axial single hydrophones deployed as a 2 km triangle. Measurements of acoustic intensity for the path to the southern triplet are on the order of 6 dB lower than those received on the northern triplet. Temporal cross-correlation beamforming estimation is performed and the estimated arrival angles for the two arrays, 265° and 267° were consistent with the predicted geodesic arrival of 264.6° and 267.8°, respectively. Cross-correlation between single phones on the northern and southern arrays reveals a peak at 266°, with a cross-correlation of 0.1. Nx2D parabolic equation modeling predicts complete blockage due to seamount interaction along the geodesic path. Overprediction of the seamount blockage indicates that the 2D approximation is incorrect, and three-dimensional propagation must be used to explain the observations. This is demonstrated by the computation of the Adiabatic Mode Parabolic Equation Transmission Loss, which predicts a 5-10 dB lower reception at the southern site.

A different ocean acidification hazard—The Kolumbo submarine volcano example

1039 Detailed knowledge of the geochemistry of CO 2 , the signature molecule of the 21 st century, is a modern day requirement for almost all geochemists. Concerns over CO 2 driven contemporary climate change, its relationship to past climates in Earth history, skills required for geologic CO 2 sequestration, and the rapid emergence of ocean acidifi cation as an environmental threat are all prime subject matter for the literate geoscientist today. In this issue of Geology, Carey et al. (2013, p. 1035) describe a new, interesting, and quite powerful natural example of the intersection of these concerns in describing the build-up of a large body of acidic, dense CO 2 rich sea water in the shallow crater of the Kolumbo volcano close to the Mediterranean island of Santorini. They present this fi nding in the context of a geochemical hazard to humans and as a natural test bed for CO 2 sequestration leakage from shallow injection. How real are these concerns, how similar are the situations to known threats, what strategies could be taken, and how useful an analog is this fi nding to the broad discussion over world-wide ocean acidifi cation or the specifi c concerns over leakage from geologic CO 2 sequestration? Kolumbo last explosively erupted in A.D. 1650, and ~70 people were killed by volcanic gases (Fouque, 1879) so plainly the potential is real. The build-up of CO 2 in the crater at the time of this earlier eruption is not known, but some signifi cant concentration must surely have been present. The population of Santorini has likely increased since then, and thus some concern is clearly justifi ed. But what would it take to destabilize the CO 2 -rich waters, how large is the chemical reservoir compared to known hazards, and how might this threat be detected? The fi nding of submarine volcanos close to populated areas, and discussion of hazards from their eruptions, is not new, with the 2011‐ 2012 eruption of El Hierro, Canary Islands (see the Geology Research Focus by Schmincke and Sumita [2013]) a prominent recent example. But Carey et al. focus not only on the sporadic eruptive events, but also on the physical stability of the large (2.0 ◊ 10 5 m 3 STP; 395 metric tons) pool of excess CO 2 accumulated within the crater only 500 m below sea level, and draw attention to the similarity with the deadly abrupt release of CO 2 gas from the volcanic lakes Monoun and Nyos in Cameroon, Africa (Sigurdsson et al., 1987). Clearly there are some similarities: in both locations there is a crater-contained pool of CO 2 -enriched water made dense, and thus held in place, by the dissolution of the gas under hydrostatic pressure. This arises simply due to the fact that the partial molal volume of CO 2 in sea water (~30 ml/mol) is much less than its molecular weight (44 g/mol), so that dissolution creates a dense fl uid. The effect is well known, and dense plumes of CO 2 -enriched water have been readily created in experimental investigations of deep seafl oor CO 2 sequestration scenarios (Brewer et al., 2005).

CO2 degassing from hydrothermal vents at Kolumbo submarine volcano, Greece, and the accumulation of acidic crater water

Discharge of volcanic gases in the marine environment can lead to local perturbations in ocean acidity, with consequences for biological communities and the potential for hazards related to depressurization and release of gases at the surface. Numerous hydrothermal vents in the crater of Kolumbo submarine volcano (Aegean Sea) are discharging virtually pure gaseous CO 2 together with clear fluids at temperatures up to 220 °C. Acoustic imaging of the ascending bubbles suggests that the gas is being dissolved into seawater within ∼10 m above the crater floor (500 m below sea level). Dissolution of the gas likely causes local increases in water density that result in sequestration of CO 2 within the enclosed crater, and the accumulation of acidic seawater. Lack of macrofauna at the Kolumbo hydrothermal vents, occurrence of carbonate-poor sediment in the crater, and pH values as low as 5.0 in recovered water samples point to acidic conditions within the crater. Buildup of CO 2 -rich water in the bowl-shaped crater of Kolumbo may be producing conditions analogous to some African volcanic lakes (Lake Monoun and Lake Nyos, Cameroon) where overturn of gas-rich bottom waters led to abrupt releases of CO 2 at the surface. A minimum estimate of 2.0 × 10 5 m 3 (STP) of excess CO 2 may currently exist in the bottom 10 m of the Kolumbo crater.

Global volcanic unrest in the 21st century: An analysis of the first decade

We define volcanic unrest as the deviation from the background or baseline behaviour of a volcano towards a behaviour which is a cause for concern in the short-term because it might prelude an eruption. When unrest is preceded by periods of quiescence over centuries or millennia it is particularly difficult to foresee how a volcano might behave in the short-term. As a consequence, one of the most important problems is to assess whether unrest will culminate in an eruption or not. Here, we review and evaluate global unrest reports of the Smithsonian Institution Global Volcanism Program (GVP) between January 2000 and July 2011. The aim of the evaluation is to establish the nature and length of unrest activity to test whether there are common temporal patterns in unrest indicators and whether there is a link between the length of inter-eruptive periods and unrest duration across different volcano types. A database is created from the reported information on unrest at 228 volcanoes. The data is categorised into pre-eruptive or non-eruptive unrest indicators at four different subaerial volcano types and submarine volcanoes as defined by the GVP. Unrest timelines demonstrate how unrest evolved over time and highlight different classes of unrest including reawakening, pulsatory, prolonged, sporadic and intra-eruptive unrest. Statistical tests indicate that pre-eruptive unrest duration was different across different volcano types. 50% of stratovolcanoes erupted after about one month of reported unrest. At large calderas this median average duration of pre-eruptive unrest was about twice as long. At almost five months, shield volcanoes had a significantly longer unrest period before the onset of eruption, compared to both large calderas and stratovolcanoes. At complex volcanoes, eruptive unrest was short lived with only a median average duration of two days. We find that there is only a poor correlation between the length of the inter-eruptive period and unrest duration in the data; statistical significance was only detected for the pair-wise comparison of non-eruptive unrest at large calderas and stratovolcanoes. Results indicate that volcanoes with long periods of quiescence between eruptions will not necessarily undergo prolonged periods of unrest before their next eruption.Our findings may have implications for hazard assessment, risk mitigation and scenario planning during future unrest crises.

Caldera structure of submarine Volcano #1 on the Tonga Arc at 21°09′S, southwestern Pacific: Analysis of multichannel seismic profiling

Volcano #1 is a large submarine stratovolcano with a summit caldera in the south central part of the Tonga Arc. We collected and analyzed multichannel seismic profiles in conjunction with magnetic data from Volcano #1 to investigate the structure of the intracaldera fill and processes of caldera formation. The intracaldera fill, exhibiting stratified units with a maximum thickness of 2 km, consists of at least four seismic units and a thick wedge of landslide debris derived from the caldera wall. The structural caldera floor, deepening toward the northwestern rim, suggests asymmetric collapse in the initial stage, which, in turn, appears to have contributed to the creation of a caldera elongated to the northwest by enhancing gravitational instability along the northwestern caldera boundary. Occasional, but repeated, eruptions resulted in a thick accumulation of the intracaldera fill and further subsidence in the mode of piston collapse. Magnetization lows are well-defined along the structural rim of the caldera that is interpreted as the inner principal ring fault. The magnetization lows indicate sites of submarine hydrothermal vents that caused an alteration of magnetic minerals. Faults recognized on the outer slope of the volcano are interpreted to be involved in hydrothermal fluid circulation.