Note on a paper by D. R. Caldwell,C. W. Van Atta And K. N. Helland

Abstract

In comparing their laboratory findings with those of other investigators, the authors incorrectly ascribed an eddy coefficient and mixing length too large by a factor of 2.2 to the results from a numerical model. An explanation is offered for the apparently small magnitude of dimensionless velocity deficit from the laboratory study.     

Petrochemistry of quaternary pumiceous pyroclastic products in southern Guadeloupe (F.W.I.)

Pumiceous pyroclastic products are present as flows and falls at several stages of the evolution of the southern Guadeloupe volcanic island. An understanding of this volcanism had to rely on detailed petrochemical data of these products to complement similar data for effusive rocks so as to yield complete stratigraphical coverage. On the other hand most pumiceous rocks are more or less conspicuousily banded suggesting that mixing phenomena occurred to different degrees in their genesis. Three major classes of pumiceous products are found: (1) the Axial Chain deposits (2.0–0.5 My) are characterized by An90, 75-55 + En55 + Wo42En37 + Usp35-37 ± Fo68 ± Hble, SiO₂ 60%, SiO₂/Th 35.6, and La/Th 3.9. Banded samples have components that differ in evolution indices by about 50%; (2) the Bouillante Chaine pyroclastics (0.3–0.1 My) consist of scattered deposits with variable mineralogical and geochemical compositions that seem to have erupted from a number of small eruptive centers. Qz-dacitic pumice is common with An90, 70-45 + En66-56 + Usp32-38 + Ilm94 + Hble ± Wo40-42En40-42, SiO₂ 62%, SiO₂/Th 22.9, and La/Th >4. Mixed pumice samples have highly contrasted evolution indices differing by up to 120%; (3) the Pintade pumice flows and falls correspond to the major pyroclastic event (approx. 10 km³) in the southern Basse Terre area. They are characterized by An85-70 + En66-56 + Usp32-37 ± Wo42En42, SiO₂/Th 18.7-22?6, and La/Th 3.0-4.0. Banded pumice lumps are scarce and show slight compositional contrasts; differences in evolution indices do not exceed 38%. Axial and Bouillante chain pyroclastics and Grande Découverte volcano pumice respectively, form two different families in trace element plots. Minor elements in pyroxenes also are distinctive. These trends are similar to those obtained for effusive rocks and define comagmatic series. Major and trace element data for the separated components of inhomogeneous pumice in each formation always plot in the corresponding series. These chemical discriminants can be used to attribute samples of unknown provenance to a given volcanic ensemble. An inverse relationship between differences in evolution indices in inhomegeneous pumice and the volume of any single eruptive sequence is noted. This is an indication that pumiceous pyroclastic rocks were erupted from a zoned magma chamber. We favor an interpretation where zonation is produced by influx of less envolved magma in superficial differentiated chambers which is a direct cause for eruption.     

Phreatic eruption clouds: the activity of la Soufrière de Guadeloupe,F.W.I., August — October, 1976

From August to October, 1976, La Soufrière de Guadeloupe was observed, and recorded with an automated sequence camera and numerous handheld cameras. During the period of observation, the nature of volcanic activity ranged from mild steam emission to moderately energetic phreatic eruptions. Background fumarolic activity (steam emission) was characterized by the emission of generally tephra-free steam clouds 50 to 150 m above the summit. The clouds rose buoyantly above the vent and were blown downwind at prevailing wind velocities. Phreatic eruptions were well-documented on September 22, October 2, and October 4. In the latter two eruptions, small bursts of tephra-laden steam erupted at intervals of 30 to 45 min, and rose from 350 to 500 m above the summit. In the largest observed eruption, that of October 2, the steam and tephra cloud rose to a maximum height of 600 to 650 m in 20 min. A white vapor cloud and a medium gray, tephra-laden cloud were erupted simultaneously from the summit vent and both were surrounded by a vapor collar: the clouds were thoroughly mixed within 1 km downwind of the summit. The concurrent growth of clouds from separate vents (summit and flank) implies a common source. Simultaneous eruption of tephra-free and tephra-laden clouds from the same vent is puzzling and implies: (i) lateral changes in the degree of alteration of dome rocks along the elongate vent, hence erodability of the dome lavas, or (ii) differences in the gas velocities. These «mixed» clouds moved westward, downwind and downslope as a density current, along the watersheds of the R. Noire and R. des Pères with an approximate velocity of 10 to 25 m/sec. Upon reaching the sea the clouds continued to move forward, but at a decreased velocity, and spread laterally, having left behind the restrictions of valley walls. A thin gray veneer of moist tephra, ranging from several cm thick near the dome to less than 1 mm thick several km downwind, was deposited along a narrow corridor southwest of the summit. Tephra from the phreatic eruptions consisted mostly of hydrothermally altered lithic, mineral, and glass fragments derived from dome lavas; no fresh (juvenile) pyroclasts were present in the tephra. Absence of juvenile tephra at La Soufrière supports the view that activity was due to groundwater circulating in a vapor-dominated geothermal system, probably driven by a shallow heat source. At La Soufrière, most vapor-dominated systems are located in elevated areas of groundwater recharge where groundwater movement is downward and outward. The sporadic phreatic eruptions may be related to the rate of recharge of meteoric waters within the dome, the decrease in pore pressure during fortnightly tidal minimums or both. Whatever the triggering mechanism, vapor-dominated fluids eroded vent walls during phreatic eruptions and carried out fine-grained, hydrothermally altered, pre-existing dome material as tephra.