Both dives (# 1989 and # 1991) took place in the axial graben near 11°06
0 N and
11°29
0 N at 2,520–2,550 m depth. We landed on the first steps of the graben’s
western wall and went in a northeasterly direction until we crossed the ridge axis.
On both sides of the axial graben, the walls are made up of pillow lava but as soon
as we approached the axis of the ridge, flat lava (sheet flow) and collapsed lava
ponds were observed and the flows were younger in appearance with a lack of
interstitial sediment. This is an indication that we were approaching the most
recently active volcanic zone. Several lava ponds with depressions (20–50 m deep
holes about 15 m wide) were found aligned towards the strike of the ridge axis.
We were attentive to look for the area with the freshest lava flow. We made a stop
at the interior of a lava pond, to sample the most recent lava flow having a shiny
appearance. After sampling, we continued the dive and rose above the depression
leaving the lava pond. Progressing northward (11°07.40
0 N), the structures of the
ridge axis changed and became more chaotic with rock debris and collapsed
features due to extensive fracturing of the crust. Abundant fissures (dive # 1891)
cutting through sheet flows were filled with fragmented pillow lava suggesting that
we had come across a tectonically active area. Such type of activity is due to the
separation of two plates. Indeed, we crossed the limit where the Mexican plate at
the east is separated from the Pacific plate to the west.
It was during this expedition on the EPR axis that my colleague Bill Bryan from
the Woods Hole Oceanographic Institution and I had the opportunity and the
privilege of making the 2000th dive of ALVIN on March 22, 1988. Ralph Hollis
was our pilot during the dive and we had the pleasure of opening a bottle of wine at
2300 m depth to inaugurate the event. The submarine was launched at 9 AM at
11°50
0 05
0 ’ N at about 2 km west of the ridge axis. The purpose of the dive was to
document the eruptive cycle, the nature of the ridge-axis segmentation and the type
of faulting. The Alvin landed on a pillow lava constructional hill at about 2,700 m
depth. Before starting our exploration, Ralph, the pilot, took a plastic marker with
two flags, French and American, and laid it on top of the pillow lava mound.
Several of these volcanic constructions were crossed on our way towards the
ridge axis, however, no really fresh lava was noticed. The degree of lava freshness
is noticeable to the experienced diver by the black and ‘‘oily’’ or ‘‘shiny’’ type of
coloration on the surface of the flow. Although we did not really observe shiny
lava flow, we saw partially buried fissures and ‘‘pillars’’ of lava ponds along the
ridge axis. The fact that the delicate features such as pillars are still intact suggested that since the last eruptive events no tectonic event due to spreading had
taken place in this particular portion of the ridge segment. The spatial and temporal relationship between the pillow lava and the younger sheet flow indicates
that magmatic activity is episodic and discontinuous along the spreading axis.
Thus magmatism follows a cyclical pattern, progressing from an initial eruption of
pillow lava to the extrusion of sheet flows to a waning phase of pillow extrusion
and finally to pelagic sedimentation. If there is a rapid rate of expansion, sheet flow
will be the main type of flow extruded. If magma is depleted, or if the emplacement is slow, then pillow lava will dominate.
Off-Axial Seamounts on the EPR at 12°50
0 N and 11°20
0 N
233
0 N and
11°29
0 N at 2,520–2,550 m depth. We landed on the first steps of the graben’s
western wall and went in a northeasterly direction until we crossed the ridge axis.
On both sides of the axial graben, the walls are made up of pillow lava but as soon
as we approached the axis of the ridge, flat lava (sheet flow) and collapsed lava
ponds were observed and the flows were younger in appearance with a lack of
interstitial sediment. This is an indication that we were approaching the most
recently active volcanic zone. Several lava ponds with depressions (20–50 m deep
holes about 15 m wide) were found aligned towards the strike of the ridge axis.
We were attentive to look for the area with the freshest lava flow. We made a stop
at the interior of a lava pond, to sample the most recent lava flow having a shiny
appearance. After sampling, we continued the dive and rose above the depression
leaving the lava pond. Progressing northward (11°07.40
0 N), the structures of the
ridge axis changed and became more chaotic with rock debris and collapsed
features due to extensive fracturing of the crust. Abundant fissures (dive # 1891)
cutting through sheet flows were filled with fragmented pillow lava suggesting that
we had come across a tectonically active area. Such type of activity is due to the
separation of two plates. Indeed, we crossed the limit where the Mexican plate at
the east is separated from the Pacific plate to the west.
It was during this expedition on the EPR axis that my colleague Bill Bryan from
the Woods Hole Oceanographic Institution and I had the opportunity and the
privilege of making the 2000th dive of ALVIN on March 22, 1988. Ralph Hollis
was our pilot during the dive and we had the pleasure of opening a bottle of wine at
2300 m depth to inaugurate the event. The submarine was launched at 9 AM at
11°50
0 05
0 ’ N at about 2 km west of the ridge axis. The purpose of the dive was to
document the eruptive cycle, the nature of the ridge-axis segmentation and the type
of faulting. The Alvin landed on a pillow lava constructional hill at about 2,700 m
depth. Before starting our exploration, Ralph, the pilot, took a plastic marker with
two flags, French and American, and laid it on top of the pillow lava mound.
Several of these volcanic constructions were crossed on our way towards the
ridge axis, however, no really fresh lava was noticed. The degree of lava freshness
is noticeable to the experienced diver by the black and ‘‘oily’’ or ‘‘shiny’’ type of
coloration on the surface of the flow. Although we did not really observe shiny
lava flow, we saw partially buried fissures and ‘‘pillars’’ of lava ponds along the
ridge axis. The fact that the delicate features such as pillars are still intact suggested that since the last eruptive events no tectonic event due to spreading had
taken place in this particular portion of the ridge segment. The spatial and temporal relationship between the pillow lava and the younger sheet flow indicates
that magmatic activity is episodic and discontinuous along the spreading axis.
Thus magmatism follows a cyclical pattern, progressing from an initial eruption of
pillow lava to the extrusion of sheet flows to a waning phase of pillow extrusion
and finally to pelagic sedimentation. If there is a rapid rate of expansion, sheet flow
will be the main type of flow extruded. If magma is depleted, or if the emplacement is slow, then pillow lava will dominate.
Off-Axial Seamounts on the EPR at 12°50
0 N and 11°20
0 N
233