lenses located at relatively shallow depths (\1.5 km). High sheet flow/pillow
ratios and episodic dyke injection occur. Very little hydrothermal activity
except for shimmering of low temperature fluids with low metal content and
gaseous phases (H 2 S) takes place. The chimney constructions are in an
immature stage; they consist essentially of thin veneers of Zn-sulfides,
anhydrite and amorphous silica. For example, the domed shaped ridge segment similar to a Hawaiian ‘‘shield volcano’’ near 17°26
0 S at 2,585 m depth
has been recently in activity for a length of about 800 m along the ridge axis.
Fig. 7.31 Reading from the bottom to the top of this figure, we can see three stages of tectonic,
magmatic and hydrothermal activity along ultrafast spreading ridge segments at 17°26
0 S-21°26
0 S.
(1) Stage of intense volcanic activity shows a domed shaped structure. High thermal regime and
lack of tectonic activity is accompanied by eruption of fluidal lava and diffuse hydrothermalism.
(2) Stage of lithosphere cooling accompanied by tectonic activity. The formation of an axial
graben due to magma retrieval and extrusion of sheet flow and pillow lava. Hydrothermal fluid
circulation and metallic discharge influenced by cooling and fracturing of the lithosphere. (3)
Stage of tectonic activity and lack of volcanism accompanied by intense hydrothermal events.
When tectonic activity due to spreading increases, there could be a renewal of magma ascent
underneath the axial graben, so stage 1 of intense volcanism will repeat itself
242
7 Oceanic Spreading Ridges and Sea Floor Creation
ratios and episodic dyke injection occur. Very little hydrothermal activity
except for shimmering of low temperature fluids with low metal content and
gaseous phases (H 2 S) takes place. The chimney constructions are in an
immature stage; they consist essentially of thin veneers of Zn-sulfides,
anhydrite and amorphous silica. For example, the domed shaped ridge segment similar to a Hawaiian ‘‘shield volcano’’ near 17°26
0 S at 2,585 m depth
has been recently in activity for a length of about 800 m along the ridge axis.
Fig. 7.31 Reading from the bottom to the top of this figure, we can see three stages of tectonic,
magmatic and hydrothermal activity along ultrafast spreading ridge segments at 17°26
0 S-21°26
0 S.
(1) Stage of intense volcanic activity shows a domed shaped structure. High thermal regime and
lack of tectonic activity is accompanied by eruption of fluidal lava and diffuse hydrothermalism.
(2) Stage of lithosphere cooling accompanied by tectonic activity. The formation of an axial
graben due to magma retrieval and extrusion of sheet flow and pillow lava. Hydrothermal fluid
circulation and metallic discharge influenced by cooling and fracturing of the lithosphere. (3)
Stage of tectonic activity and lack of volcanism accompanied by intense hydrothermal events.
When tectonic activity due to spreading increases, there could be a renewal of magma ascent
underneath the axial graben, so stage 1 of intense volcanism will repeat itself
242
7 Oceanic Spreading Ridges and Sea Floor Creation