Annotated record of the detailed examination of Mn deposits observed from bottom photography taken during the INDOMED-1 expedition

35 km either side of the Pacific-Cocos spreading axis at 846'N, on crust 0.6 x 10SUP-6 years old, a pair of small seamounts rises 1.4 km above the faulted terrain of young abyssal hills. A high-resolution traverse made with the sonars and cameras of a deeply towed vehicle shows that the seamounts are inactive volcanoes, with concave side slopes that steepen upwards to 30, and then abruptly flatten at 2-4 km wide summits. Ledges of pillow basalt outcrop on the lower slopes, and form some talus slopes, but the superficial rock of the upper slopes appear to be mainly hyaloclastite. A transponder-navigated deep-tow survey at the summit area of the eastern seamount mapped an asymmetric caldera containing 3 pit craters, which have vertical walls and flat floors. Dredging across the caldera floor recovered hyaloclastite, angular fragments of primitive tholeiite, and manganese nodules. The paired seamounts probably had a common origin at a 'hot spot' on the spreading axis; continued to grow after they were split by sea-floor spreading; and became extinct and suffered summit collapse after they spread too far from their magma source.

在北纬8°46′处的太平洋-科科斯扩张脊（Pacific-Cocos spreading axis）两侧各35 km范围内，位于年龄为0.6×10^-6年的洋壳（crust）之上，一对小型海山（seamounts）从年轻深海丘陵（abyssal hills）的断错地形之上抬升1.4 km。采用深拖装置（deeply towed vehicle）搭载的声呐（sonars）与相机开展的高分辨率测线观测显示，该海山为休眠火山，其边坡呈凹形，坡度向上逐渐变陡至30°，随后在宽度2~4 km的山顶处骤然变平缓。下坡段出露枕状玄武岩（pillow basalt）岩阶，并发育部分岩屑坡（talus slopes）；而上坡段的表层岩石则主要为玻质碎屑岩（hyaloclastite）。针对东侧海山山顶区域开展的应答器导航（transponder-navigated）深拖测绘，圈定出一处不对称破火山口（caldera），其内包含3个坑状火山口（pit craters），这些火山口均拥有直立的壁体与平坦的坑底。对破火山口底进行的拖网采样，采获了玻质碎屑岩、原始拉斑玄武岩（tholeiite）角砾以及锰结核（manganese nodules）。该成对海山大概率起源于扩张脊处的一处热点（hot spot）；在被海底扩张（sea-floor spreading）分割后仍持续生长；最终在距岩浆源（magma source）过远时停止活动，并经历了山顶坍塌。