Hyaloclastite and Lava Flows on Young Seamounts Examined with a Submersible
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Hyaloclastite and lava flows on young seamounts examined with a submersible PETER LONSDALE University of California, San Diego, Marine Physical Laboratory of Scripps Institution of Oceanography, La Jolla, California 92093 R. BATIZA Department of Earth and Planetary Sciences and McDonnell Center for Space Sciences, Washington University, St. Louis, Missouri 63130 ABSTRACT clastite (aquagene tuff) is produced. Flows at two of the volcanos closest to the axis of of this material are not very mobile on level the fast-spreading East Pacific Rise, Sea- Four small seamounts that rise 800 to terrain; in subglacial Pleistocene Iceland, mount Amph D1 at lat 7°50'S (Bonatti, 1,200 m above the flanks of the East Pacific hyaloclastite piled up as "moberg ridges" 1967) and East Seamount at lat 8°45'N Rise at the Pacific-Rivera plate boundary along eruptive fissures (Walker, 1965). (Lonsdale and Spiess, 1979), yielded both were examined and sampled during four Flows of hyaloclastite have not been re- tholeiitic lava and hyaloclastite, together dives of DSV-3 Turtle. They are all volcanic ported from deep submarine spreading cen- with rapidly accumulated iron and man- cones, at least two of them with summit ters, although off-axis drilling has revealed ganese oxides. craters. Samples from three of the volcanos thick beds of this rock type within the We present here geologic results of four are tholeiitic basalt that chemically resem- oceanic crust (for example, Schmincke and dives by the U.S. Navy submersible DSV-3 bles the basalt erupted at the rise crest, others, 1979). Most fissure eruptions at Turtle onto the summits of central volcanos although they are less fractionated. One of mid-ocean rises are type examples of the close to the axis of the East Pacific Rise (Fig. the cones within 15 km of the spreading "quiet effusion" mode of submarine vol- 1). The purpose was to observe the field re- axis has very young flows of sheet and pil- canism (Bonatti, 1967), forming flows of lationships and collect selected samples of low lava on its summit, including some en pillow and sheet lava whose immediate in- several types of volcanic flows and metal- echelon pillow walls. The other seamounts, teraction with bottom water is usually lim- liferous deposits. The only young, plate- farther from the plate boundary, appear ited to formation of glassy crusts. Lonsdale boundary seamounts that have previously extinci and generally have more weathered and Spiess (1980) did describe thin sheet been observed close-up are those whose lava with thicker ferromanganese crusts. flows on the crest of the East Pacific Rise summits have been built to sea level by They also have extensive flows of hyalo- that have shattered into 10-cm cubes along long-continued volcanism (such as Cobb ciastite that probably formed in deep-water regular vertical contraction cracks. How- Seamount — Schwartz and Lingbloom, phreatomagmatic eruptions. Typical ever, the intense physical and chemical in- 1973; Isla Tortuga — Batiza, 1978) or hyaloclastite landforms are stone streams of teraction of basaltic magma and seawater those that rise several hundred metres rocks that have moved down side slopes of that produces deep-sea hyaloclastite is most above the axes of aberrantly subaerial volcanos and been stabilized by precipita- characteristic of eruptions at central vol- spreading centers (Walker, 1965; Barberi tion of volcanogenic ferromanganese ce- canoes, providing many seamounts with at and Varet, 1971). The submersible Turtle ments. least a superficial cover of this rock (for has previously been used to examine appar- example, Bonatti, 1967; Barr, 1974; Nat- ently young seamounts on Cocos Ridge INTRODUCTION land, 1976). (Fornari and others, 1979), but these vol- Although fissure basalts create most of canos are in in an unusual tectonic setting A range of volcanic lithologic units and the newly accreted upper oceanic crust, on an aseismic ridge and consist of alkali landforms is created at accretionary plate large-scale volcanic cones that are the basalt. boundaries, despite the gross chemical uni- products of central volcanism are also im- formity of the tholeiitic magma generated portant components. On the East Pacific METHODS OF STUDY in this tectonic setting. Fissural volcanism at Rise, seamounts are absent from the axial subaerial spreading centers in Afar and Ice- zone of fissural volcanism (Menard, 1969), Turtle is a sister of the heavily modified land forms volcanic plains of both aa and but cones 0.75 to 1.5 km high are common DSRV Alvin (Winget and McCamis, 1967), pahoehoe lavas; although these lava types at the outer margins of the plate-boundary but less effort has been made to upgrade its differ greatly in appearance and viscosity zone of active tectonism. Most, perhaps all, scientific capabilities. During our diving (and hence build different landforms), of these young additions to the oceanic program it carried one observer, plus pilot transformations from pahoehoe to aa are crust are tholeiitic, although eruption of and copilot, to a maximum operating depth common in a single flow unit. Where axial alkali basalt becomes increasingly likely if of 2,000 m for 2 to 4 hr. Bottom photos eruptive fissures underlie shallow water or the volcanos continue to grow as they drift were taken through the observer's port with glacial ice, drastic chilling of basalt magma away from the plate boundary (Engel and a hand-held 35-mm reflex camera, and causes explosive granulation and hyalo- Engel, 1971; Batiza, 1977). Dredge stations samples of loose rock were collected with Geological Society of America Bulletin, Part I, v. 91, p. 545-554, 7 figs., 2 tables, September 1980, Doc. no. 00907. 545 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/91/9/545/3419150/i0016-7606-91-9-545.pdf by guest on 28 September 2021 546 LONSDALE AND BATIZA hydraulic manipulators. A pressure gauge ing axis. A pre-dive echo-sounder survey vehicle's lights were regularly reflected from measured depth, and a surveillance sonar improved the bathymetric map and estab- mirror-like glass (as also occurs at the very located rock outcrops. The submersible's lished that at the crest of its 20° to 30° side youngest flows on some spreading centers; launch and recovery positions were deter- slopes is a flatter 8 km2 summit area with a Lonsdale, 1977a). A few extensive flows mined by the satellite navigator and 12-kHz minimum depth of 1,825 m. Turtle's dive were traversed, but over most of the summit echo sounder of the mother ship, but navi- 278 (Batiza, observer) began on the south- area rock crops out on slight elevations in gation of the submersible while on the bot- ern margin of the summit, at a depth of 20- to 30-m patches separated by calcare- tom was by rudimentary dead-reckoning. 1,880 m. The boat moved northwest, at ous ooze that blankets half of the seafloor. Criteria for selecting dive targets were first up a slope that was never more than 5° The sediment surface is smooth, not swept that the volcano summit must rise above and then across a hummocky summit plain, into current ripples as on many other sea- 2,000 m and should be as close as possible toward a depression that might be a crater; mount summits. to the axis of spreading. The spatial pattern however, before reaching this objective, Smooth and ropy sheet flows (Figs. 2A, of seamounts on this and other segments of after traveling about 500 m in 2.5 hr, the 2B) and flattened pillows (Fig. 2D) are the the East Pacific Rise suggests that formation dive was terminated because of deteriorat- commonest lava type along most of the dive of large cones on new crust is delayed 0.2 to ing weather at the sea surface. track. At mid-ocean spreading centers, both 0.3 x 106 yr until the lithosphere is strong Throughout the dive there were abun- types are indicative of high rates of lava dis- enough to support their weight, and then it dant lava exposures of varying freshness: charge and are most often seen near the proceeds rapidly as volcanos grow to some were covered with a thin ferroman- summits of axial eruptive vents (Ballard and heights limited by an isostatic equilibrium ganese crust, while from some outcrops the Moore, 1977; Lonsdale and Spiess, 1980). between the magma column in their con- duits and the rocks of the adjacent thicken- ing lithosphere (Vogt, 1974; Lonsdale and UO'W 109° W 1 08"W 107° W Spiess, 1979). The volcano nearest to the spreading axis that is tall enough (0.85 km) to be within Turtle's diving range is at lat 20°50'N, just 15 km west of the axis on the crust with an estimated age of 0.5 x 106 yr. 2 3°N The rise crest at this latitude is the medium-spreading boundary of the Pacific and Rivera plates and has been the locale for intensive study with deeply towed in- struments (Larson, 1971; Normark, 1976), rock dredging (Moore and others, 1977), and rock-collecting submersibles (Fran- cheteau and others, 1979; Spiess and others, 1980). We hoped to complement these studies of the results of fissure erup- 2 2° N tion, tensional faulting, and hydrothermal activity by examining the products of adja- cent central volcanism which rise (just) above 2,000 m depth, but persistent bad weather allowed only a single dive in this region (dive 278). On three other dives (279, 280, 281) we studied seamounts on older crust farther north on the rise. 21 °N Samples of basaltic lava and hyaloclastite were examined petrographically, and by electron microprobe and instrumental neu- tron activation analysis.