Different Formations Reflect Different Conditions

The spacing, growth rates, and mineralogical evolution of black smoker chimneys are complex and not well understood. Black smokers’ shapes vary depending on different spreading environments, even when only a few hundred meters separate them. Along the mid-ocean ridge system, one of the largest single known deposits occurs on the Mid-Atlantic Ridge, a slow-spreading environment. The entire deposit, known as TAG, is a large sulfide mound measuring 250 meters in diameter, and 50 meters high; it was probably formed by individual venting structures that combined into one deposit over time. Active venting is maintained by a black smoker complex located on the top of the mound that hosts multiple, spire-shaped chimneys up to fifteen meters high, with fluid temperatures of around 370°C (700°F). White smokers on the margins of the mound discharge fluids at 265–300°C. Their venting fluids are white because the dark sulfide minerals precipitate within the mound before the fluids exit the chimney. Scientists think that in this area of active faulting, large intersecting faults have channeled flow to the hydrothermal mound episodically over a 50, 000-year period. In contrast to the large sulfide mounds that may typify deposits on the slow-spreading Mid-Atlantic Ridge, black smokers at the fast-spreading East Pacific Rise commonly occur as small, discrete individual structures rarely more than fifteen meters tall. Individual venting sites may be spaced 200–300 meters apart, and most venting occurs within shallow, fault-bounded central valleys, called grabens, which are located at the crest of the summit. In this volcanically active area, the small size of the deposits is probably due to at least two factors. First, molten basaltic rock frequently erupts and covers the hydrothermal vents and associated biological communities. Second, magma moves up from the magma chamber into the shallow underlying crust, which disturbs the fluid flow channels that had been established to feed hydrothermal vents. This causes the vents to shut down until the channels become reestab-lished, or even to relocate on a new site. Somewhere in between these configurations is the Endeavour Segment of the Juan de Fuca Ridge, an extremely active hydrothermal area 300 kilometers off the coast of Washington State. It is one of the best-studied underwater environments. Active faulting (rather than volcanic activity) along this segment has produced a one kilometer wide, 100–200 meters deep graben in which four known vent fields are spaced 2–3 kilometers apart. The fields, which are generally 400–500 meters long, host abundant large sulfide structures on top of which stand abundant black smokers. Areas where lower-temperature fluids vent more weakly are also common. In three of the fields, the most common sulfide forms include large, irregularly-shaped structures (up to eighteen meters tall) that host vigorously venting 350–400°C black smoker chimneys on their tops (Figure 2B). Lower-temperature venting of nutrient-rich hydrothermal fluids through the porous chimney walls support rich and diverse colonies of tube, sulfide, and palm worms, galatheid crabs, and a variety of snails and limpets. On actively venting structures, these animal communities are so dense that they obscure the underlying host rock. Many structures are characterized by stair-step arrays of large sulfide ledges that form an almost tree-like structure. The most spectacular of these structures, “Godzilla” in the Endeavour Segment’s High Rise Field, was forty-five meters high and contained 15–16 tiers of ledges up to seven meters in length which trapped 330°C fluids. This structure collapsed sometime in 1996. In contrast to the steep mound and ledge shapes that typify most of the sulfide structures in the three other hydrothermal fields at Endeavour, the recently-discovered Mothra Hydrothermal Field hosts at least five sulfide clusters composed of multiple, isolated, and fused pinnacles that reach up to twenty-four meters in height (Figure 2A). Many structures are awash in cooler, slower-moving hydrothermal fluids (30–210°C) that support dense and diverse biological communities composed predominantly of a variety of worms, snails, and crabs. These animals colonize the steep outer surfaces of active chimneys. Venting fluids from some of these steep, isolated pinnacles reach 305°C. Horizontal fractures, marked by dense colonies of single-celled organisms that include filamentous bacteria and microbial mats, commonly cut the sulfide structures, and toppled structures are fairly common. Mothra is the largest hydrothermal field on the Endeavour, reaching over 500 meters in length.