Pacific-Antarctic Ridge (PAR) spreading axis (Hekinian et al. 1999; Hekinian
et al. 2002; Stoffers et al. 2003). Most of these flows are blocky and/or form giant
lobated tubes with flattened tops. They have conchoidal fractures and elongated
vesicles (see Chap. 8). Due to their high viscosity and consequent low rate of
diffusion of ions to the potential sites of their crystal lattice, highly siliceous lavas
commonly solidify as black obsidian rather than as a crystalline rock. The viscosity of ascending magma will increase as solidification or crystallization takes
place. Therefore, near the surface of the rising lava, the flow will decrease its
velocity of ascension and form domed structures, usually enriched in silica (in the
form of obsidian). Silica-rich lavas are responsible for the formation of volcanic
‘‘pipe’’ intrusions similar to what have been observed on Mont Pelée on the Island
of Martinique.
Composition of Silica-Enriched Rocks
Several types of silica-rich rocks consisting of trachytes, trachy-andesites, trachybasalt, andesites, dacites and rhyolites occur in the different oceanic environments such as hotspot volcanoes and spreading ridges. Some are classified as being
silica and incompatible-element enriched rocks, or Trachytic rocks.
Trachytic rocks have the highest SiO 2 (59–64 %) and total alkali content
(Na 2 O ? K 2 O = 8–11 %), followed by trachy-andesites (SiO 2 = 53–59 %,
Na 2 O ? K 2 O = 4–8 %), and trachybasalts (SiO 2 = 49–53 %, Na 2 O ? K 2 O
= 4–7 %). Other silica-enriched rocks are classified as being rhyolitic rocks
because they are silica enriched and incompatible-element depleted rocks.
They have a SiO 2 content of 53–59 % and Na 2 O ? K 2 O contents of 3–5.5 %.
They could be dacite (SiO 2 = 59–66 %, Na 2 O ? K 2 O = 3–5 %), andesite
(SiO 2 = 52–59 %, Na 2 O ? K 2 O = 3–5 %) or rhyolites (SiO 2 = 70–73 %,
Na 2 O ? K 2 O = 5–6 %) which are most commonly found on spreading
ridges, island arcs and back-arc basins.
The major differences between the two types of silica-enriched rocks (trachytic
and rhyolitic) are in their incompatible element contents for approximately the
same amount of silica. However, the andesite-dacite suites from back arc basins
are known to be different from common MORBs and IOBs. Previous detailed
studies (Turner and Verhoogen 1960; Myashiro 1974) have claimed that the island
arc andesites consist essentially of calc-alkaline suites enriched in augite (a calcium-alumina-silicate mineral) and hypersthene (an iron alumina silicate mineral),
pyroxenes, plagioclase phenocrysts and with a high alkali content when compared
to the tholeiitic basalt suites found on Hawaiian and Icelandic volcanoes.
84
4 Sea Floor Rocks
et al. 2002; Stoffers et al. 2003). Most of these flows are blocky and/or form giant
lobated tubes with flattened tops. They have conchoidal fractures and elongated
vesicles (see Chap. 8). Due to their high viscosity and consequent low rate of
diffusion of ions to the potential sites of their crystal lattice, highly siliceous lavas
commonly solidify as black obsidian rather than as a crystalline rock. The viscosity of ascending magma will increase as solidification or crystallization takes
place. Therefore, near the surface of the rising lava, the flow will decrease its
velocity of ascension and form domed structures, usually enriched in silica (in the
form of obsidian). Silica-rich lavas are responsible for the formation of volcanic
‘‘pipe’’ intrusions similar to what have been observed on Mont Pelée on the Island
of Martinique.
Composition of Silica-Enriched Rocks
Several types of silica-rich rocks consisting of trachytes, trachy-andesites, trachybasalt, andesites, dacites and rhyolites occur in the different oceanic environments such as hotspot volcanoes and spreading ridges. Some are classified as being
silica and incompatible-element enriched rocks, or Trachytic rocks.
Trachytic rocks have the highest SiO 2 (59–64 %) and total alkali content
(Na 2 O ? K 2 O = 8–11 %), followed by trachy-andesites (SiO 2 = 53–59 %,
Na 2 O ? K 2 O = 4–8 %), and trachybasalts (SiO 2 = 49–53 %, Na 2 O ? K 2 O
= 4–7 %). Other silica-enriched rocks are classified as being rhyolitic rocks
because they are silica enriched and incompatible-element depleted rocks.
They have a SiO 2 content of 53–59 % and Na 2 O ? K 2 O contents of 3–5.5 %.
They could be dacite (SiO 2 = 59–66 %, Na 2 O ? K 2 O = 3–5 %), andesite
(SiO 2 = 52–59 %, Na 2 O ? K 2 O = 3–5 %) or rhyolites (SiO 2 = 70–73 %,
Na 2 O ? K 2 O = 5–6 %) which are most commonly found on spreading
ridges, island arcs and back-arc basins.
The major differences between the two types of silica-enriched rocks (trachytic
and rhyolitic) are in their incompatible element contents for approximately the
same amount of silica. However, the andesite-dacite suites from back arc basins
are known to be different from common MORBs and IOBs. Previous detailed
studies (Turner and Verhoogen 1960; Myashiro 1974) have claimed that the island
arc andesites consist essentially of calc-alkaline suites enriched in augite (a calcium-alumina-silicate mineral) and hypersthene (an iron alumina silicate mineral),
pyroxenes, plagioclase phenocrysts and with a high alkali content when compared
to the tholeiitic basalt suites found on Hawaiian and Icelandic volcanoes.
84
4 Sea Floor Rocks