Vent Formation Research Articles

Fault‐controlled fluid flow inferred from hydrothermal vents imaged in 3D seismic reflection data, offshore NW Australia

AbstractFluid migration pathways in the subsurface are heavily influenced by pre‐existing faults. Although studies of active fluid‐escape structures can provide insights into the relationships between faults and fluid flow, they cannot fully constrain the geometry of and controls on the contemporaneous subsurface fluid flow pathways. We use 3D seismic reflection data from offshore NW Australia to map 121 ancient hydrothermal vents, likely related to magmatic activity, and a normal fault array considered to form fluid pathways. The buried vents consist of craters up to 264 m deep, which host a mound of disaggregated sedimentary material up to 518 m thick. There is a correlation between vent alignment and underlying fault traces. Seismic‐stratigraphic observations and fault kinematic analyses reveal that the vents were emplaced on an intra‐Tithonian seabed in response to the explosive release of fluids hosted within the fault array. We speculate that during the Late Jurassic the convex‐upwards morphology of the upper tip‐lines of individual faults acted to channelize ascending fluids and control where fluid expulsion and vent formation occurred. This contribution highlights the usefulness of 3D seismic reflection data to constraining normal fault‐controlled subsurface fluid flow.

Endogenous and exogenous growth of the monogenetic Lemptégy volcano, Chaîne des Puys, France

The monogenetic Lemptegy volcano in the Chaine des Puys (Auvergne, France) was quarried from 1946 to 2007 and offers the possibility to study scoria cone architecture and evolution. This volcano was originally 50–80 m high, but scoria excavation has resulted in a 50-m-deep hole. Beginning in the 1980s, extraction was carried out with the advice of volcanologists so that Lemptegy’s shallow plumbing system and three-dimensional stratigraphy have been preserved. Detailed mapping enabled key stratigraphic units to be distinguished and the constructional phases to be reconstructed. The emplacement and evolution of the shallow plumbing system have also been unraveled. The growth of this monogenetic scoria cone included two temporally well-separated eruptions from closely spaced vents. The activity included Hawaiian, Strombolian and Vulcanian explosions, lava effusion, cryptodome and dome formation, partial collapse, satellite vent formation, eruptive pauses, and intrusion emplacement with consequent uplift. The cone shape, structure, and hence the local stress field, plumbing system, and thermal state were continuously changing, which in turn influenced the eruptive style and location. The plumbing system morphology and microtectonic structures both record local stress field and magmatic flow direction changes. Lemptegy volcano’s internal architecture, stratigraphy, and evolution show how complex a monogenetic volcano can be.

The diverging volcanic rift system

Abstract Eruptions and volcano internal growth are mostly fed by dykes. The comprehension of the control factors on dyke paths is fundamental for the assessment of areas prone to vent formation and to the general understanding of how volcanoes work. We analyse an understudied magma path system; field data of nine volcanoes show they have a rectilinear rift zone in the central part passing into fan-arranged dykes at the two opposite volcano flanks. The geological, geomorphological and structural characteristics of these volcanoes and their substrate suggest that the formation of these “diverging rifts” is not specifically linked to substrate lithology and mechanical behaviour. The studied volcanoes have elongation V > 10 km 3 (mostly > 300 km 3 ). Eight volcanoes have the central rift that is normal to the regional tectonic least principal stress (σ 3reg ) and in one case it is sub-perpendicular. Field data have been combined with scaled analogue modelling, suggesting that if the σ 3reg is oblique to the volcano elongation axis, dyke geometry in the edifice axial zone is controlled by elongation and thus by local gravity σ 3 , but dyke strike becomes perpendicular to σ 3reg when dykes intrude the more external areas of the volcano. If a dyke is injected under the volcano flanks with slope inclination > 50°, it attains a geometry parallel to the slope. At lower slope inclinations at the edifice terminations, magma paths diverge outwards and crosscut slopes at high angle. Our data are in agreement with the assumption that regional tectonic stresses can affect large volcanoes up to the summit area guiding the development of a rectilinear thoroughgoing rift, both in extensional and transtensional regimes. The diverging pattern takes place due to reorientation of the local stress field guided by topography only when dyke inception localizes laterally respect to the edifice axis.

A first hazard analysis of the Quaternary Harrat Al-Madinah volcanic field, Saudi Arabia

The northern portion of the 20,000km2 Harrat Rahat basaltic field in NW Saudi Arabia (Harrat Al-Madinah) has hosted some of the most recent volcanic eruptions in the country. Rapid growth of the city of Al-Madinah has spread out onto the lava flows and scoria cones of the Harrat, increasing exposure to any potential renewed volcanism. We present here a first-order probabilistic hazard analysis related to new vent formation and subsequent lava flow from this volcanic field. The 501 visible eruption vent sites were integrated with aeromagnetic survey data (as representative of potential regions of buried volcanic vents) to develop a probability density function for new eruption sites using Gaussian kernel smoothing. This revealed a NNW striking zone of high spatial hazard terminating <10km south of the city. Using the properties of the AD1256 eruption lava flows and the spatial PDF, an analysis of lava hazard was carried out. Assuming a future lava-producing eruption, around 25% of the city of Al-Madinah is exposed to a probability of 0.001 to 0.005 of lava inundation. The temporal eruption recurrence rate is estimated at approximately one vent per 3300years, but the temporal record of the field is so poorly constrained that the lower and upper bounds for the recurrence interval are 13,300yrs and 70yrs, respectively. Applying a Poisson temporal model, this results in a worst-case lava inundation recurrence interval of approximately 14,300years.

Hazard assessment at San Martín volcano based on geological record, numerical modeling, and spatial analysis

The San Martin shield volcano, located in the Los Tuxtlas Volcanic Field, has experienced effusive shield-building activity, as well as explosive eruptions, as evidenced by direct observations during the last eruption in 1793. The threat to the surrounding villages consists principally of lahars, especially because of the tropical climate in the region. Ash fallout and lava flows represent additional hazards. In addition, the surrounding Quaternary monogenetic field includes more than 300 scoria cones and about 40 explosion craters (mainly maars) that also represent a hazard source. In the present study we constructed hazard maps using field data, orthophotos, spatial analysis, and specialized software (LAHARZ and HAZMAP) to deliminate lahar inundation zones, areas that could potentially be affected by ash fallout (including the evaluation of houses prone to roof collapse due to ash load), and the most susceptible areas for hosting future monogenetic vent formation.

A3-05 複成火山における寄生火山の形成(火山地殻変動・地震,口頭発表)

A3-05 複成火山における寄生火山の形成(火山地殻変動・地震,口頭発表)

A3-02 ひずみ記録による噴火・火道生成メカニズムの推定(火山地殻変動・地震,口頭発表)

A3-02 ひずみ記録による噴火・火道生成メカニズムの推定(火山地殻変動・地震,口頭発表)

Sandstone dikes in dolerite sills: Evidence for high-pressure gradients and sediment mobilization during solidification of magmatic sheet intrusions in sedimentary basins

Sediment dikes are common within dolerite sill intrusions in the Karoo Basin in South Africa. The dikes are subvertical and as much as 2 m wide, sometimes with abundant fragments of sedimentary rocks and dolerite. The matrix consists of contact-metamorphic sandstone. There is no petrographic evidence for melting within the sediment dikes. The maximum temperature during heating is restricted to the plagioclase and biotite stability field, or above ∼350 °C. Thermal modeling of a sandstone dike in a dolerite sill shows that a temperature of 350–450 °C is reached in the dike after a few hundred years of sill cooling. The calculated pressure history of a cooling sill and its contact aureole shows that substantial fluid pressure anomalies develop on a short time scale (1–15 yr) and are maintained for more than 100 yr. Calculated pressure anomalies in the sill (-7 to -22 MPa) and the aureole (4–22 MPa) are significant and may explain sill fracturing and sediment mobilization from the aureole into the sill. We conclude that sediment dikes represent common features of sedimentary basins with sill intrusions in which fluid pressure gradients have been high. Sediment dikes thus signify that pore fluids may escape from the aureoles on a short time scale, representing an intermediate situation between fluid loss during formation of microfractures and fluid loss during violent vent formation.

The length of channelized lava flows: Insight from the 1859 eruption of Mauna Loa Volcano, Hawai‘i

The 1859 eruption of Mauna Loa Volcano, Hawai'i, produced paired 'a'ā and pāhoehoe flows of exceptional length (51 km). The 'a'ā flow field is distinguished by a long (> 36 km) and well-defined pāhoehoe-lined channel, indicating that channelized lava remained fluid to great distances from the vent. The 1859 eruption was further unusual in initiating at a radial vent on the volcano's northwest flank, instead of along the well-defined rift zone that has been the source of most historic activity. As such, it presents an opportunity both to examine controls on the emplacement of long lava channels and to assess hazards posed by future flank eruptions of Mauna Loa. Here we combine evidence from historical chronicles with analysis of bulk compositions, glass geothermometry, and microlite textures of samples collected along the 1859 lava flows to constrain eruption and flow emplacement conditions. The bulk compositions of samples from the 'a'ā and pāhoehoe flow fields are bimodally distributed and indicate tapping of two discrete magma bodies during eruption. Samples from the pāhoehoe flow field have bulk compositions similar to those of historically-erupted lavas (< 8 wt.% MgO); lava that fed the 'a'ā channel is more primitive (> 8 wt.% MgO), nearly aphyric, and was erupted at high temperatures (1194–1216 °C). We suggest that the physical properties of proximal channel-fed lava (i.e., high-temperature, low crystallinity, and low bulk viscosity) promoted both rapid flow advance and development of long pāhoehoe-lined channels. Critical for the latter was the large temperature decrease (~ 50 °C) required to reach the point at which plagioclase and pyroxene started to crystallize; the importance of phase constraints are emphasized by our difficulty in replicating patterns of cooling and crystallization recorded by high-temperature field samples using common models of flow emplacement. Placement of the 1859 eruption within the context of historic activity at Mauna Loa suggests that the formation of radial vents and eruptions of high-temperature magma may not only be linked, but may also be a consequence of periods of high magma supply (e.g., 1843–1877). Flank eruptions could therefore warrant special consideration in models and hazard mitigation efforts.

Topographic control on lava flow paths at Mount Etna, Italy: Implications for hazard assessment

Assessment of the hazard from lava flow inundation at the active volcano of Mount Etna, Italy, was performed by calculating the probability of lava flow inundation at each position on the volcano. A probability distribution for the formation of new vents was calculated using geological and volcanological data from past eruptions. The simulated lava flows from these vents were emplaced using a maximum expected flow length derived from geological data on previous lava flows. Simulations were run using DOWNFLOW, a digital‐elevation‐model‐based model designed to predict lava flow paths. Different eruptive scenarios were simulated by varying the elevation and probability distribution of eruptive points. Inundation maps show that the city of Catania and the coastal zone may only be impacted by flows erupted from low‐altitude vents (&lt;1500 m elevation) and that flank eruptions at elevations &gt;2000 m preferentially inundate the northeast and southern sectors of the volcano as well as the Valle del Bove. Eruptions occurring in the summit area (&gt;3000 m elevation) pose no threat to the local population. Discrepancies between the results of simple, hydrological models and those of the DOWNFLOW model show that hydrological approaches are inappropriate when dealing with Etnean lava flows. Because hydrological approaches are not designed to reproduce the full complexity of lava flow spreading, they underestimate the catchment basins when the fluid has a complex rheology.

Dike propagation in volcanic edifices: Overview and possible developments

Eruptions are fed by dikes; therefore, better knowledge of dike propagation is necessary to improve our understanding of how magma is transferred and extruded at volcanoes. This study presents an overview of dike patterns and the factors controlling dike propagation within volcanic edifices. Largely based on published data, three main types of dikes (regional, circumferential and radial) are illustrated and discussed. Dike pattern data from 25 volcanic edifices in different settings are compared to derive semi-quantitative relationships between the topography (relief, shape, height, and presence of sector collapses) of the volcano, tectonic setting (presence of a regional stress field), and mean composition (SiO 2 content). The overview demonstrates how dike propagation in a volcano is not a random process; rather, it depends from the following factors (listed in order of importance): the presence of relief, the shape of the edifice and regional tectonic control. We find that taller volcanoes develop longer radial dikes, whose (mainly lateral) propagation is independent of the composition of magma or the aspect ratio of the edifice. Future research, starting from these preliminary evaluations, should be devoted to identifying dike propagation paths and likely locations of vent formation at specific volcanoes, to better aid hazards assessment.

Mechanically disrupted and chemically weakened zones in segmented dike systems cause vent localization: Evidence from kimberlite volcanic systems

Deformation and alteration zones along kimberlite dikes hold clues as to how point-source volcanic vents can localize along sheet-like intrusions. Brittle deformation zones occur in host rock adjacent to kimberlite intrusions of the Swartruggens Kimberlite Dike Swarm, South Africa. Deformation includes local fracturing and brecciation and is associated with relay zones between offset dike segments. Breccia zones indicate dilation and hydraulic fracturing, and some zones along the dikes were affected by chemical corrosion, forming fresh cores surrounded by onion-skin concentric shells of altered rock. The alteration was caused by either exsolved magmatic volatiles moving in advance of the magma through the fracture, or by hydrothermal fluids. Consideration of the time scales needed for chemical corrosion of host rock requires intrusions to stall at depth prior to transport to higher crustal levels. Highly disrupted offsets could be preferred locations for explosive activity and initial vent formation as dikes approach the surface. A kimberlite pipe forms after magma breaks through to the surface; the altered zones are reamed out and fresh cores in spheroidally altered rock are incorporated into the pipe fill along with more angular country-rock material, as observed in layered volcanic breccias in kimberlite pipes at the Venetia Mine, South Africa. This model may have wider implications for the localization of conduits along dikes in other volcanic systems. Dike segmentation provides weak zones where hydrothermal fluids and magmatic volatiles can be preferentially channeled. Chemical corrosion can further weaken these zones, which may then become the locus for initial phreatic and phreatomagmatic explosions, creating shallow vents that can then channel magma to the surface during eruption.

Modeling of the 2001 lava flow at Etna volcano by a Cellular Automata approach

The 2D Cellular Automata model, MAGFLOW, simulates lava flows and an algorithm based on the Monte Carlo approach solves the anisotropic flow direction problem. The model was applied to reproduce a lava flow formed during the 2001 Etna eruption. This eruption provided the opportunity to verify the ability of MAGFLOW to simulate the path of lava flows which was made possible due to the availability of the necessary data for both modeling and subsequent validation. MAGFLOW reproduced quite accurately the spread of flow. A good agreement was highlighted between the simulated and observed length on steep slopes, whereas the area covered by the lava flow tends to be overestimated. The major inconsistencies found in the comparison between simulated and observed lava flow due to neglecting the effects of ephemeral vent formation.

Formation of hydrothermal vents in the North Fiji Basin: Sulfur and lead isotope constraints

Hydrothermal sulfides were recovered from the 16°50′S triple junction area in the North Fiji Basin, at a water depth of ca. 1900 m. The chimney samples can be divided into three groups based on their major metal contents: 1) type 1 (Fe–Cu-rich), 2) type 2 (Fe–Cu-rich with minor Zn), and 3) type 3 (Zn-rich) chimneys. Type 1 chimneys are mainly composed of chalcopyrite and pyrite, and are enriched in elements commonly precipitated under high temperature conditions (> 300 °C), such as Cu, Co, Mo, and Se. Type 3 chimneys consist dominantly of sphalerite and marcasite with traces of pyrite and chalcopyrite, and are enriched in elements commonly associated with relatively low temperatures (150 to 250 °C), such as Zn, Cd, Pb, As, and Ga. Type 2 chimneys have a mineralogy similar to that of type 1 chimneys although their trace metal contents range between those of type 1 and type 3 samples. Type 2 chimneys appear to be the products of low-temperature replacement, which might be related to formation of type 3 chimneys, of type 1 chimneys. The trace element composition of basaltic rocks indicates that magma generation in the triple junction area was influenced by two different sources: N-MORB and E-MORB. Sulfur and lead isotope patterns of the hydrothermal chimneys show distinct differences between the type 1 and type 3 chimneys. The type 1 sulfides ( δ 34S = 2.8 ± 1.2‰; 206Pb/ 204Pb = 18.082 to 18.132; 207Pb/ 204Pb = 15.440 to 15.481; 208Pb/ 204Pb = 37.764 to 37.916) are depleted in 34S and have lower Pb isotope ratios compared to types 2 and 3 chimneys ( δ 34S = 4.1 ± 0.9‰; 206Pb/ 204Pb = 18.122 to 18.193; 207Pb/ 204Pb = 15.475 to 15.554; 208Pb/ 204Pb = 37.882 to 38.150). The higher δ 34S values for types 2 and 3 sulfides than type 1 sulfides can be explained by mixing of hydrothermal fluids with ambient seawater. The more radiogenic Pb isotope composition of types 2 and 3 chimneys compared to type 1 chimneys suggest that the formation of types 2 and 3 chimneys were possibly related to E-MORB volcanism that shows higher Pb isotope composition than N-MORB rocks. However, ferromanganese crust samples have radiogenic Pb isotope compositions that are identical to those of E-MORB. Therefore, input of hydrogenous Pb is also a possible source for radiogenic Pb isotopes in the types 2 and 3 chimneys.

Degassing activity from Iwodake rhyolitic cone, Satsuma-Iwojima volcano, Japan: Formation of a new degassing vent, 1990–1999

Large changes in the surface manifestation of degassing activity were observed from 1990 to 1999 at the summit crater of Iwodake cone of Satsuma-Iwojima volcano. During this period, a new high-temperature fumarolic area formed in the center of the crater floor and became a degassing vent with a diameter of 40 m. Altered volcanic rocks were ejected during the course of vent formation. Although glass fragments were observed in the ejected ash, the glass comes from altered Iwodake rhyolite that covers the crater floor. The highest fumarolic temperature and equilibrium temperatures of volcanic gases had a maximum of about 900°C at the beginning of the vent formation. The flux of SO2, measured by COSPEC, varied from 300 to 700 ton/day and correlated directly with maximum fumarole temperature. During this period, open fractures formed along the southern rim of the crater almost contemporaneously with the vent formation and changes in the nature of fumarolic discharges. The continuous and intense degassing at Satsuma-Iwojima is likely caused by volatile transport from a deep magma chamber through a convecting magma column. An increase in the magma convection rate might have caused these large changes in surface manifestations, including increase in the SO2 flux and fumarolic temperatures, ground deformation, and the vent formation.

Venting Insert Chambers Increases Fluid Production

This article is a synopsis of paper SPE 56664, "Increasing Fluid Production by Properly Venting Formation Gas in Insert-Chamber Installations," Ali Hernandez, SPE, PDVSA Intevep, and Cesar Perez, Ulbio Navarro, and Williams Lobo, PDVSA E&amp;P, originally presented at the 1999 SPE Annual Technical Conference and Exhibition, Houston, 3-6 October.

Lava tube morphology on Etna and evidence for lava flow emplacement mechanisms

Lava tubes play a pivotal role in the formation of many lava flow fields. A detailed examination of several compound `a`a lava flow fields on Etna confirmed that a complex network of tubes forms at successively higher levels within the flow field, and that tubes generally advance by processes that include flow inflation and tube coalescence. Flow inflation is commonly followed by the formation of major, first-order ephemeral vents which, in turn, form an arterial tube network. Tube coalescence occurs when lava breaks through the roof or wall of an older lava tube; this can result in the unexpected appearance of vents several kilometers downstream. A close examination of underground features allowed us to distinguish between ephemeral vent formation and tube coalescence, both of which are responsible for abrupt changes in level or flow direction of lava within tubes on Etna. Ephemeral vent formation on the surface is frequently recorded underground by a marked increase in size of the tube immediately upstream of these vents. When the lining of an inflated tube has collapsed, `a`a clinker is commonly seen in the roof and walls of the tube, and this is used to infer that inflation has taken place in the distal part of an `a`a lava flow. Tube coalescence is recognised either from the compound shape of tube sections, or from breached levees, lava falls, inclined grooves or other structures on the walls and roof. Our observations confirm the importance of lava tubes in the evolution of extensive pahoehoe and `a`a flow fields on Etna.

Volcano geodesy: The search for magma reservoirs and the formation of eruptive vents

Routine geodetic measurements are made at only a few dozen of the world's 600 or so active volcanoes, even though these measurements have proven to be a reliable precursor of eruptions. The pattern and rate of surface displacement reveal the depth and rate of pressure increase within shallow magma reservoirs. This process has been demonstrated clearly at Kilauea and Mauna Loa, Hawaii; Long Valley caldera, California; Campi Flegrei caldera, Italy; Rabaul caldera, Papua New Guinea; and Aira caldera and nearby Sakurajima, Japan. Slower and lesser amounts of surface displacement at Yellowstone caldera, Wyoming, are attributed to changes in a hydrothermal system that overlies a crustal magma body. The vertical and horizontal dimensions of eruptive fissures, as well as the amount of widening, have been determined at Kilauea, Hawaii; Etna, Italy; Tolbachik, Kamchatka; Krafla, Iceland; and Asal‐Ghoubbet, Djibouti, the last a segment of the East Africa Rift Zone. Continuously recording instruments, such as tiltmeters, extensometers, and dilatometers, have recorded horizontal and upward growth of eruptive fissures, which grew at rates of hundreds of meters per hour, at Kilauea; Izu‐Oshima, Japan; Teishi Knoll seamount, Japan; and Piton de la Fournaise, Réunion Island. In addition, such instruments have recorded the hour or less of slight ground movement that preceded small explosive eruptions at Sakurajima and presumed sudden gas emissions at Galeras, Colombia. The use of satellite geodesy, in particular the Global Positioning System, offers the possibility of revealing changes in surface strain both local to a volcano and over a broad region that includes the volcano.

In vivo studies with antisense oligonucleotides

The ability of short, synthetic, single- stranded DNA or RNA oligonu- cleotides to interdict individual gene expression in a sequence-specific manner is the basis for antisense oligonucleotide-based therapies (for recent reviews see Refs 1,2). Unlike traditional drugs which typically interact with proteins, these oligonu- cleotide molecules are designed to selectively bind, by Watson-Crick base pairing, to complementary sequences in the target messenger RNA to interfere with and/or pre- vent formation of the gene product. In principle, antisense oligonu- cleotides can bind to and inhibit the translation of a given messenger RNA by several putative mecha- nisms including simply blocking ribosomal reading or by activating RNaseH, an endogenous enzyme which selectively degrades the message. The fact that very specific Watson-Crick binding of oligo- nucleotides to target sequences may be possible suggests that antisense- based therapies have the potential to be orders of magnitude more specific than conventional therapies. Antisense therapeutics, therefore, represents an exciting new tech- nology for manipulating gene expression in the treatment of human viral diseases such as AIDS, inflammatory disorders and cancers. The purpose of this article is to review the current literature on in vioo pre-clinical and clinical studies with a view to assessing the extent to which antisense oligonucleotides have ‘lived up’ to their initial thera- peutic potential and expectations. Pre-clinical efficacy studies The fact that antisense oligonu- cleotides may be useful therapeutic agents, particularly as antiviral agents, was demonstrated almost a decade ag+. However, it was quickly realized that unmodified phosphodiester oligonucleotides were rapidly degraded in biological fluid&-7 and in viao following intra- venous (i.v.) administration”. Hence, for therapeutic purposes biologically stable analogues would be desirable. The most widely studied of these analogues at present are the phosphorothioate oligonucleotides (S-oligos) where one of the non- bridging oxygens in the phospho- diester backbone is replaced with a sulphur. The initial ‘proof-of concept’ dis- coveries were made with such first- generation oligonucleotides in cell culture (for review see Ref. 9) but rapid development to in viva models followed. Table 1 (see Refs 10-23) highlights some of the in viva studies that have formed the basis for evalu- ating oligonucleotide-based thera- pies in clinical trials (see below). These and the pharmacokinetic dis- tribution studies (Table 2; see Refs 24-36) suggest that the organs of the reticuloendothelial system (RES) such as liver, spleen, kidney and lungs may be good sites for oligonucleotide efficacy as much of the oligonucleotide administered, by almost any route, appears to end up in these tissues (see below). For example, the study of Dean and McKay17 reported a dose-dependent, antisense-mediated downregulation (up to 90%) of pro- tein kinase C-a mRNA in the livers of hairless mice following intraperi- toneal (i.p.) administration. In another study, an oligonucleotide complementary to angiotensinogen that was administered to the liver via the portal vein showed a measurable reduction in angiotensinogen levels in plasma and a lowering of mean blood pressure for three to five days20. However, preferential accu- mulation of oligonucleotides within RES organs may also precipitate adverse effects (see below).

Seismic facies and specific character of the bottom simulating reflector on the western margin of Paramushir Island, Sea of Okhotsk

Seismic profiles from a venting area on the western margin of Paramushir Island (Sea of Okhotsk) reveal a local complex structure and an interesting, unusual pattern of the bottom simulating reflector (BSR). The BSR is gradual rising towards the venting area. The geothermal gradient and the bottom temperature confirmed the methane hydrate. The temperature appears to be the most important factor controlling the hydrate stability. A locally higher heat flow caused the upward migration of the hydrate stability field and the subsequent degradation of the hydrated sediments, causing gas vent formation and the flux of methane gas into the water column.