HYDROTHERMAL VENTS

THE EASTERN PACIFIC VENTS

The existence of hydrothermal vents was first suspected in the eastern Pacific from records of hot-water ‘spikes’ and bottom photographs of giant bivalves made by the towed instrument system Deep Tow in 1976 on the Galapagos Rift (Lonsdale, 1977a,b). The presence of primordial gases, particularly helium isotopes, from water samples, provided evidence that these hot springs were heated by molten rock below the seabed. But the extraordinary nature of the associated vent fauna was revealed soon after to dumbfounded observers in the manned submersible ‘Alvin’ (Corliss & Ballard, 1977; Corliss et al., 1979). This vent field is situated on the Galapagos Rift lying almost on the Equator between the Galapagos spreading centre and nearby Galapagos Islands, off the coast of Ecuador (Fig. 15.1). Active vents with associated fauna were discovered at 2.5 km depth on the basaltic rock bottom formed from fresh lava flows running along a 1–2 km stretch of ridge formed along the Rift Valley axis of this relatively slowly separating spreading centre. This forms part of the sometimes branching but continuous, global system of mid-oceanic ridges (Fig. 15.2).

Although such vent communities were initially regarded as isolated and rare phenomena, a quickening pace of exploration has found similar communities to be associated with nearly all areas of tectonic activity that have been investigated in the deep Pacific and Atlantic (see below).