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Notes: [Auszug] Here we report two volcanic phenomena which have been only rarely described in the literature from first hand experience, namely, the occurrence of laya tubes in the ocean and the observance of "popping rocks". To our knowledge, popping rocks have only been reporte}! twice before; once from the ...

Notes: Abstract Still photographs and video images collected along the Neovolcanic Zone of the East Pacific Rise from 10°15′N to 11°53′N show that recent volcanic sheet flows, possibly less than 100 years old, are superimposed on an older sediment-laden pillow terrane. This recent activity is restricted to a narrow zone that crosses two topographic highs at 10°55′N and 11°26′N and diminishes along-axis away from these highs. The association of recent sheet flows with older flows and collapse structures on the overlapping spreading centers at 11°45′N supports the evolutionary model for the occurrence and evolution of overlapping spreading centers by MacDonald and others (1986, 1988).

Notes: Abstract An investigation of glassy volcanics erupted within the last ten-million years along various segments of the mid-Atlantic Ridge and the East Pacific Rise has revealed major crustal compositional changes. The available data from the mid-Atlantic Ridge shows the existence of two petrological provinces: One, located between latitudes 33° and 53° N, is characterized by volcanics which have a tendency to be oversaturated ocean ridge basalts (OSORB) with respect to normative quartz; the second group of rocks, found between 25° S and 33° N, is generally composed of saturated ocean ridge basalts (SORB). In addition, the SORB volcanics have higher TiO2 (1.7±0.3%), higher Na2O (2.8±0.2%) and higher FeO*/MgO (1.36±0.2) values than do the OSORB types (with 1.1±0.2%, 2.2±0.2% and 1.22±0.2 for the TiO2, Na2O, and FeO*/MgO respectively), There is a correlation between the rate of crustal spreading and the compositional changes observed on the volcanics erupted along various segments of oceanic ridges. Slow-accreting plate boundaries having a total spreading rate of 2–3 cm/year are characterized by a low TiO2 content (1.1±0.2%), low FeO*/ MgO ratio (1.22±0.2) and a high an/an+ab ratio (0.62±0.05). Segments of fast-spreading ridges (total rate 11–13 cm/year) show a higher range of TiO2 (2.1±0.4%) and FeO*/MgO (1.6±0.4) and a lower range of the an/an + ab ratio (0.5±0.07). Ridge segments with a total spreading rate of 5–9 cm/year con sist of volcanics having intermediate values for the above parameters. Different degrees of partial melting of rising mantle material are suggested as a possible mechanism for explaining the compositional diversities encountered along oceanic ridge systems.

Notes: Abstract Eighty samples of submarine basaltic lava were sampled from an 8 km segment of the floor and walls of the inner rift valley of the Mid-Atlantic Ridge during the French American Mid-Ocean Undersea Study (project Famous). The samples were collected from outcrops and talus slopes by the three submersibles: Alvin, Archimede, and Cyana at water depths of about 2600 meters. The early formed mineral content of the pillow lavas' glassy margins enables classification of the rocks into 5 types: (1) olivine basalt, (2) picritic basalt, (3) plagioclase-olivine-pyroxene basalt, (4) aphyric basalt, and (5) plagioclase-rich basalt. Chemical and mineralogical study indicates that at least 4 types are directly interrelated and that types (1) and (2) are higher-temperature, primitive lavas, and types (3) and (4) are lower-temperature, differentiated lavas derived from the primitive ones by crystal-liquid differentiation. The plagioclase-rich basalts also have a chemical composition of their glass comparable to that of the most differentiated basalts (types 3 and 4) but they differ in their greater amount of early formed plagioclase (12–35%). In general, the mineralogical variation across the rift valley shows an assymetrical distribution of the major basalt types. Despite the mineralogical diversity of the early formed crystals, the chemistry of the basalt glasses indicates a symmetrical and a gradual compositional change across the rift valley. Based primarily on their chemistry, the rock types 1 and 2 occupy an axial zone 1.1 km wide and make up the central volcanic hills. Differentiated lavas (types 3, 4) occupy the margins and walls of the inner rift valley and also occur near the center of the rift valley between the central hills. FeO/MgO ratios of olivine and coexisting melt indicate that the average temperature of eruption was 40 ° C higher for the primitive melts (types 1 and 2). Aside from major elements trends, the higher temperature character of the primitive basalts is shown by their common content of chrome spinel. The thickness of manganese oxide and palagonite on glassy lava provide an estimate of age. In a general fashion the relative age of the various volcanic events follow the compositional zoning observed in the explored area. Most of the youngest samples are olivine basalt of the axial hills. Most older samples occur in the margins of the rift valley (West and N.E. part of explored area) but are significantly younger than the spreading age of the crust on which they are erupted. Intermediate lava types occur mainly east of the rift valley axis and in other areas where plagioclase—olivine—pyroxene basalt and aphyric basalt are present. The above relations indicate that the diverse lava types were erupted from a shallow, zoned magma chamber from fissures distributed over the width of the inner rift valley and elongate parallel to it. Differentiation was accomplished by cooling and crystallization of plagioclase, olivine, and clinopyroxene toward the margins of the chamber. The centrally located hills were built by the piling up of frequent eruption of mainly primitive lavas which also are the youngest flows. In contrast smaller and less frequent eruptions of more differentiated lavas were exposed on both sides of the rift valley axis.