Volcanic morphology and hydrothermal activity in 2011

Downslope to the west of South Crater several white smoker vents were discovered.

A cluster is marked as field #6 where fluids discharge through the scree-covered slope.

The fluids were exceptionally acidic, ranging to pH-values as low as 0.87 (Seewald et al., 2014). Fluid temperatures of up to 284°C were measured, but the most acidic fluid was 220°C. When we retracted the T-probe after sticking it into the scree next to the vent site, it was covered with native sulfur, indicating that the scree-covered slope around the vents is saturated with liquid sulfur. A massive NNE-trending volcanic outcrop featured remarkable flanges of native sulfur that appeared to have oozed out of the massive rock.

The western slopes of South Crater were generally covered with fine volcaniclastics and littered with clasts up to several 10’s of cm in diameter. The south and east slopes of South Crater were not investigated in 2006.

The area of South Crater changed dramatically between 2006 and 2011. A new volcanic cone has built up, consisting of volcaniclastic material and lava (Fig. 3.5 c, e, g, k, l, q, s) with its crest ~50 m west of South Crater center in 2006. We will refer to this structure as South Peak hereinafter. The white smoker vent site #6 of the 2006 survey was buried under > 50 m of volcanic material, as documented by ROV investigations during cruise SO-216 in 2011.

The southern edge of the North Peak crest is defined by a steep wall that was present in 2006. It is marked in Fig. 3.7 as a cliff (Fig. 3.5 n) and 2011 video observations revealed no major morphologic changes, although the abundance of hydrothermal staining and putative microbial mats had increased. No black smoker vents were observed south of that >14 m high structure. The seafloor below this cliff was scattered with scree and fine volcaniclastics, and hosts a huge white smoker vent site (#8) that extends towards west along the boundary of North and South Peak. This extensive white smoker field did not exist in 2006 though scattered white smoker activity was observed in the area (vent #6) in 2006 that are now covered by South Peak volcanics. Very poor visibility inside the white smoker fields limited the video mapping abilities, but zones of extensive altered rocks could be identified.

Another new white smoker field was discovered in 2011 and named Sulfur Candles (#8, Fig. 3.5 i; Fig. 3.7 b). Eponymous are copious amounts of sulfur that formed small chimneys or accumulated in m-sized knolls. Release of bubbles of liquid CO2 was common and CO2-clathrate formation could be confirmed visually in an improvised bubble-catching device. The white smoker hydrothermal activity was highly variable within the two weeks of survey work in 2011. For instance, a white smoker chimney situated in between North and South Peak in the easternmost part of white smoker field

#8 was nearly inactive during one dive and, 12 days later, vigorous vented white smoker fluids (Fig. 3.8). Similar changes were observed at the Sulfur Candle site, where vigorous boiling sulfur was discharged on the seafloor during dive 302 on 26.06.2011 compared to a much subdued activity 14 days later, when visual observations were made difficult, however, by intense ground-hugging white and yellow turbidity clouds. These fluctuations in activity are not expressed in the entire North Su hydrothermal area. Much rather, they appear to reflect shifts in the locations of vigorous discharge.

Figure 3.8: Left: White smoker chimney on 27.06.2011; Right: The same chimney 12 days later on 09.07.2011 illustrating the pulsating nature of the white smoker hydrothermal system.

Sulfur Candle is bordered to the east by a ridge line that already existed in 2006.

Further inspections in 2011 confirmed a complete buildup of volcaniclastics with pillar structures (Fig. 3.5 p) in absence of coherent volcanic outcrops. The clasts are 1 mm to several cm’s in diameter and blocky. Flanges of solidified native sulfur appear to have oozed between the clasts, similar to what was observed on the north slope of North Peak.

At the southern edge of Sulfur Candle, the rim of South Peak crater has the same morphology as was observed in 2006 (Fig. 3.7 b). The rim is covered by fine volcaniclastics, but in some areas the rim is broken and the interior shows abundant native sulfur deposits and volcanic clasts up to several cm in size (Fig. 3.5 f). Following the crater rim to the south, more and more native sulfur flows appear on the outside rim and stick out of the slope (Fig. 3.5 o). In some parts the flow structures of the sulfur flows are still preserved. The eastern and southern slopes below the sulfur flows are littered with scree of broken sulfur slabs as well as fresh and altered volcanic rocks.

South Crater was filled by the eruption of South Peak that emerges to the west. The slopes of South Peak expose predominantly unconsolidated, variably blocky clasts up to several 10’s of cm in diameter (Fig. 3.5 e, g). The upper 20 m of the new cone are dominated by sand- and granule-sized volcaniclastics (Fig. 3.5 k, l, q) with less pebbles- and cobble-sized clasts. Only occasional blocky volcanic clasts are up to several tens of centimeters in diameter.

On the west and southwest slopes below the uppermost 20 m of South Peak, fresh volcanic rocks occasionally crop out of the mainly scree-covered slope. Outcrops are blocky, stubby, fissured and (Fig. 3.5 c) similar to outcrops on the northeastern slope of North Peak. Only one lava lobe with a rounded surface was found that slightly breached through the scree covered slope (Fig. 3.5 e). White staining occurs mainly on the southwestern slope, whereas the western slope is almost entirely covered by an

unidentified orange coating, likely Fe oxyhydroxide. Two zones of diffuse fluid discharge, elongated in shape and oriented radial to the center of South Peak crest were observed, and the larger one is included in the map in Figure 3.7 b. The hydrothermal fluids deposit native sulfur and other unidentified phases between the loose rocks that are buried by these precipitates in places.

The morphology of the South Peak crest can be divided into two zones: a plateau (Fig. 3.5 k) and a crater group (Fig. 3.5 q). The location of the crater group on South Peak is also indicated in Figure 3.7a with a red dotted circle. The part of the South Peak crest that is connected to North Peak is a smooth plateau littered with a clasts dominantly <1 cm in diameter (Fig. 3.5 s). The seafloor shows surficial white staining with ripple textures created by currents and occasional native sulfur crusts. Area-wide diffuse seepage of shimmering water was identified.

The area at the edge of the crest features several small (max. 10 m in diameter) and shallow (<3 m) craters (Fig 6 q). The seafloor around the craters is littered with unsorted clasts of variable sizes. The dominant clast size is <10 cm, but outsized blocks in the range of 10’s of cm were observed. Diffuse venting of clear fluids was noticeable, with native sulfur crusts at some crater rims.

In the southwestern corner of Figure 3.7a is a NE striking ridge that was not visited in 2006 but clearly imaged by ABE bathymetry. In 2011, the presence of that ridge was visually confirmed by an ROV Quest dive, during which outcrops of massive volcanic rock were observed. The ROV-based bathymetry was identical to the AUV bathymetry from 2006, suggesting that the emplacement of the lava forming this ridge took place before 2006. The area north of the ridge, however, was filled with material from the South Peak eruption.