rift valley along this crossing consisted of small volcanic constructions less than
50 m high alternating with collapsed lava ponds and fissures. When approaching
close to the first western wall’s scarp, fissures and faults became more prominent
in a pillow lava type of terrain. Based on the observations made during this
crossing, it was concluded that the volcanically active portion of the rift valley is
very narrow, less than 200 m, over a total width of about 2 km wide. Thus, as was
seen on dive OT-09, the OH3 segment of the ridge is in a quieter period of
volcanism. It appeared that spreading in this part of the ridge is mainly the result of
tectonic activity, which gives rise to fissures and faults with only a small amount of
magmatism.
Volcanoes on the Mid Atlantic Ridge Walls
The OCEANAUT cruise was the first time that our group included three women
scientists (Claire Bassoulet, Christèle Guivel and Eulalia Gracia) so we named
three of the new volcanoes with their first names. Three of the explored volcanoes
are located on the eastern ridge flank, up to 25 km away from the present-day ridge
axis, and one was found on the western flank, about 20 km from the axis
(Fig. 7.8b). The edifices are not presently active and the flanks of the off-axis
volcanoes show heavy sediment cover on gentle slopes ([50 % in surface). Small
depressions are filled with pelagic sediment and tend to concentrate the debris of
sessile fauna, such as dark fragments of gorgonians, organisms encrusted with
manganese oxyhydroxides, and siliceous sponge needles. The abundance of debris
clearly increases with distance from the axis. Large communities of living sessile
organisms are only observed on top of the volcanoes. Their debris appears as dark
patches sparsely distributed along the Median Ridge (MR) and around the Claire
volcano. All the volcanoes have flat summits made up of pillows and occasional
altered sheet flows encrusted with iron and manganese coatings. Based on submersible observations, all four volcanoes consist mainly of highly vesicular pillows (up to 25–50 % vesicles). The youngest volcano, called ‘‘Claire’’ (dive OT05), is closest to the ridge axis and terminates in small haystacks (Fig. 7.8b, 7.9).
The rocks sampled from this volcano have moderately palagonatized, glassy
margins and thin Fe–Mn coatings (\0.5 mm). The ‘‘Eulalia’’, ‘‘Christèle’’ and
‘‘Fara’’ volcanoes are found further away from the ridge axis, and have an older
appearance with their abundant sediment cover ([30 %).
The summit ([900 m depth) of the oldest-looking ‘‘Eulalia’’ volcano (Dive
OT04), located at about 22 km east of the axis, shows altered basaltic rocks with
smooth surfaces, without any Fe–Mn oxyhydroxide coating nor visible chilled
margins on their top. There are thick (cm) Fe–Mn coatings in the rock interstices,
which indicate the effect of seawater alteration or hydrothermalism. The samples
contain abundant clay and iron hydroxide with soft, external alteration halos
(0.5–2 cm thick) and carbonate material fills their vacuoles.
Axial Rift Valley
195
50 m high alternating with collapsed lava ponds and fissures. When approaching
close to the first western wall’s scarp, fissures and faults became more prominent
in a pillow lava type of terrain. Based on the observations made during this
crossing, it was concluded that the volcanically active portion of the rift valley is
very narrow, less than 200 m, over a total width of about 2 km wide. Thus, as was
seen on dive OT-09, the OH3 segment of the ridge is in a quieter period of
volcanism. It appeared that spreading in this part of the ridge is mainly the result of
tectonic activity, which gives rise to fissures and faults with only a small amount of
magmatism.
Volcanoes on the Mid Atlantic Ridge Walls
The OCEANAUT cruise was the first time that our group included three women
scientists (Claire Bassoulet, Christèle Guivel and Eulalia Gracia) so we named
three of the new volcanoes with their first names. Three of the explored volcanoes
are located on the eastern ridge flank, up to 25 km away from the present-day ridge
axis, and one was found on the western flank, about 20 km from the axis
(Fig. 7.8b). The edifices are not presently active and the flanks of the off-axis
volcanoes show heavy sediment cover on gentle slopes ([50 % in surface). Small
depressions are filled with pelagic sediment and tend to concentrate the debris of
sessile fauna, such as dark fragments of gorgonians, organisms encrusted with
manganese oxyhydroxides, and siliceous sponge needles. The abundance of debris
clearly increases with distance from the axis. Large communities of living sessile
organisms are only observed on top of the volcanoes. Their debris appears as dark
patches sparsely distributed along the Median Ridge (MR) and around the Claire
volcano. All the volcanoes have flat summits made up of pillows and occasional
altered sheet flows encrusted with iron and manganese coatings. Based on submersible observations, all four volcanoes consist mainly of highly vesicular pillows (up to 25–50 % vesicles). The youngest volcano, called ‘‘Claire’’ (dive OT05), is closest to the ridge axis and terminates in small haystacks (Fig. 7.8b, 7.9).
The rocks sampled from this volcano have moderately palagonatized, glassy
margins and thin Fe–Mn coatings (\0.5 mm). The ‘‘Eulalia’’, ‘‘Christèle’’ and
‘‘Fara’’ volcanoes are found further away from the ridge axis, and have an older
appearance with their abundant sediment cover ([30 %).
The summit ([900 m depth) of the oldest-looking ‘‘Eulalia’’ volcano (Dive
OT04), located at about 22 km east of the axis, shows altered basaltic rocks with
smooth surfaces, without any Fe–Mn oxyhydroxide coating nor visible chilled
margins on their top. There are thick (cm) Fe–Mn coatings in the rock interstices,
which indicate the effect of seawater alteration or hydrothermalism. The samples
contain abundant clay and iron hydroxide with soft, external alteration halos
(0.5–2 cm thick) and carbonate material fills their vacuoles.
Axial Rift Valley
195