Igneous Rock Associations 6. Modelling of Deep Submarine Pyroclastic Volcanism: A Review and New Results
Bibliographic record
Abstract
Deep submarine explosive volcanism has been a topic of controversy for over 20 years. The role seawater pressure plays in inhibiting volatile phase expansion and thereby the depth of submarine explosive eruptions has been the topic of rigorous debate. Until now, the water-vapour curve has been interpreted to mean that the pressure exerted by the overlying seawater column is significant enough to inhibit explosive volcanism at depth. This interpretation assumes that pyroclastic eruptions cannot occur below the critical point of seawater (31.5 MPa or 3.15 km water depth) in the region of the two phase liquid-vapour fields. In fact, most eruptions are interpreted to occur at depths much shallower than 3.15 km, i.e., 0.5 to 1.0 km. What has been overlooked, however, is that volatile phase expansion (specific volume changes in P-T space) plays an important, if not dominant, role in explosive eruptions at depths greater than this critical point. This controversy has led to debate on the environment of formation of volcanic massive sulfide deposits (VMS), because pyroclastics are recognized in both the footwall and (or) hangingwall sequences of many of them and are commonly interpreted as reworked, mass-flow deposits from shallow water rather than of deep-water origin, i.e., they have no genetic relationship with the formation and distribution of VMS deposits. To evaluate the possibility that submarine eruptions can occur at depths greater than 1 km, the 1-D numerical model CONFLOW was used. This program uses a specified melt composition, conduit diameter and length, and the initial temperature and pressure at the base of the conduit to calculate the pressure gradient in a conduit of constant cross-sectional area, the enthalpy of the magma, the viscosity of the volatile-magma mixture at specified P-T conditions, the fragmentation depth where the volume fraction gas is 75% (vg ≌ 0.75), and the exit velocity of the volatile-magma mixture. Results of the CONFLOW modelling support our hypothesis that magmatic volatile phase expansion is alone capable of providing enough energy and high enough melt/gas ratio, to initiate submarine pyroclastic eruptions in silicic magmasto the water depths typically associated with VMS genesis, i.e., below the two-phase (liquid-vapour) region for seawater. RESUME Le volcanisme sous-marin explosif a ete l'objet de controverse pendant plus de vingt ans. Le role inhibiteur de la pression de l'eau de mer, et donc de la profondeur d'eau, sur l'expansion de la phase volatile des eruptions sous-marines explosives a ete l'objet d'un rigoureux debat. Jusqu'a maintenant, on a suppose que l'interpretation de la courbe de pression de vapeur d'eau permettait de croire qu'a partir d'une certaine profondeur, la pression de la colonne d'eau de mer etait suffisamment importante pour inhiber le volcanisme explosif sous cette profondeur. Cette interpretation implique qu'il ne peut y avoir d'eruptions pyroclastiques en mera partir d'une profondeur critique (31,5 MPa ou 3,15 km de profondeur) dans la region de la courbe ou coexistent les phases liquides et gazeuses. De fait, dans la plupart des cas, on suppose que les eruptions se produisent a des profondeurs bien inferieures a 3,15 km, soit entre 0,5 et 1,0 km. Cependant, on a neglige le fait que l'expansion de la phase gazeuse (le volume specifique change dans le domaine P-T) joue un role important, voire determinant, dans le phenomene des eruptions explosives aux profondeurs depassant la profondeur critique. Cette controverse a entraine un debat sur milieu de formation des gisements de sulfures massifs volcanogeniques (SMV), etant donne qu'on retrouve des les sequences de roches pyroclastiques de l'eponte inferieure et/ou de l'eponte superieure de nombreux gisements SMV, l'interpretation generale voulant qu'il s'agisse de gisements de mouvement de masse remanies en milieux peu profonds, plutot que de milieux profonds - une interpretation qui exclue toute relation genetique concernant la formation et la distribution des gisements SMV. Dans le but d'evaluer la possibilite que des eruptions sous-marines puis-sent se produire a des profondeurs depassant 1 km, on a eu recours au programme de modelisation numerique 1D CONFLOW. Ce programme permet de tenir compte de la composition magmatique, du diametre et de la longueur du conduit ainsi que de la temperature et de la pression initiales a la base du conduit, dans le calcul du gradient de pression dans un conduit de lumiere constante,de l'enthalpie du magma, de la viscosite du melange des composantes magmavolatiles sous des conditions P-T definies, de la profondeur de fragmentation ou le volume du gaz fractionne atteint 75 % (vg ≌0.75), de meme que de la velocite a la sortie du melange des composantes magmatiques-gazeux. Les resultats de notre etude de modelisation par le programme CONFLOW appuient notre hypothese selon laquelle la seule expansion de la phase volatile pourrait etre suffisamment energique et avoir un taux magma/gaz assez eleve pour permettre des eruptions pyroclastiques sous-marines au sein de magmas siliceux a des profondeurs d'eau typiques des milieux de genese des gisements deSMV, soit sous les zones diphasiques (liquides-vapeurs) en eaux de mer.
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How this classification was reachedexpand
Full frame distilled prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. Learned from the 10,348 direct Codex labels and 10,348 direct Gemma labels. Candidate is the union of thresholded teacher heads; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels or direct frontier model labels.
Codex and Gemma teacher scores by category
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.001 | 0.000 |
| Bibliometrics | 0.000 | 0.001 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.001 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one teacher head, not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".