(Bideau and Hekinian 1995) also involves a cyclical production of basaltic
magma, when similar processes would be repeated several times under similar
conditions. In the field, this is observed by compositional zoning, which reflects
cycles of magmatic activity. Such cyclic magmatic events have been observed
during the detailed morphological and systematic sampling of volcanic outcrops
on volcanoes from the Pitcairn hotspot (Hekinian et al. 2002), on the MAR
spreading center near 34°55
0 N (Hekinian et al. 2000) and elsewhere on the EPR
near 13°N (Hekinian et al. 1989).
Thus, the diversity of basalts found in a given area can be explained by the
presence of small-scale mantle heterogeneities. The small-scale variability
includes the mineralogical diversity of the mantle material. This newer multi-stage
mixing theory involves the partial melting of a heterogeneous composite-mantle
made up of spinel-lherzolite containing olivine (49–55 %), orthopyroxene
(25–29 %), clinopyroxene (18–21 %) and spinel (1–2 %) (Bideau and Hekinian
1995). This composite mantle, with lenses of clinopyroxenite ([80 % clinopyroxene) in a lherzolite matrix, will first give rise to an alkali enriched melt (having
the composition of enriched MORBs and alkali basalts), which accumulates in the
warm lithosphere. As melting continues, the higher temperature phases will be
extracted giving rise to a depleted melt of residual MORBs (N-MORB and
T-MORB types). The mixing of the two types of melts (alkali-rich and residual)
inside the magma chamber will subsequently give rise to the various types of
MORBs as well as to the alkali basalts mentioned above. This was also called the
‘‘3-M Theory’’, which stands for Mantle Melting and Mixing.
Silica-Rich Lava and Obsidian
It has been thought that silica-rich rocks (such as rhyodacites or felsites) are found
mainly on continents and ancient subaerial regions such as islands and continental
landmasses. In fact, silica-rich lavas are close in composition to granitic rocks
forming continental crust but they are also erupted on the sea floor associated with
various types of geological environments. They have a relatively high viscosity
and thus have greater tendency to erupt explosively than their mafic counterparts
such as basaltic flows. Hence, the emplacement of silica-rich lava is often associated with island arcs in subduction zones and with mantle plumes forming
hotspot volcanoes. Silica-rich lavas are rarely found on accreting ridge systems
except when associated with areas influenced by mantle plume upwelling.
Examples of silica-rich lavas associated with mantle plumes giving rise to hotspot
volcanism are found in Iceland, in the Azores (Faial Island), in the Galapagos
islands, as well as on the ‘‘Axial volcano’’ of the Juan de Fuca Ridge (Wanless
et al. 2010). Because of their highly viscous nature when compared to basaltic
flows, silica-rich lavas are more limited in the extent of their dispersion on the
ocean floor (see Chap. 7).
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4 Sea Floor Rocks
magma, when similar processes would be repeated several times under similar
conditions. In the field, this is observed by compositional zoning, which reflects
cycles of magmatic activity. Such cyclic magmatic events have been observed
during the detailed morphological and systematic sampling of volcanic outcrops
on volcanoes from the Pitcairn hotspot (Hekinian et al. 2002), on the MAR
spreading center near 34°55
0 N (Hekinian et al. 2000) and elsewhere on the EPR
near 13°N (Hekinian et al. 1989).
Thus, the diversity of basalts found in a given area can be explained by the
presence of small-scale mantle heterogeneities. The small-scale variability
includes the mineralogical diversity of the mantle material. This newer multi-stage
mixing theory involves the partial melting of a heterogeneous composite-mantle
made up of spinel-lherzolite containing olivine (49–55 %), orthopyroxene
(25–29 %), clinopyroxene (18–21 %) and spinel (1–2 %) (Bideau and Hekinian
1995). This composite mantle, with lenses of clinopyroxenite ([80 % clinopyroxene) in a lherzolite matrix, will first give rise to an alkali enriched melt (having
the composition of enriched MORBs and alkali basalts), which accumulates in the
warm lithosphere. As melting continues, the higher temperature phases will be
extracted giving rise to a depleted melt of residual MORBs (N-MORB and
T-MORB types). The mixing of the two types of melts (alkali-rich and residual)
inside the magma chamber will subsequently give rise to the various types of
MORBs as well as to the alkali basalts mentioned above. This was also called the
‘‘3-M Theory’’, which stands for Mantle Melting and Mixing.
Silica-Rich Lava and Obsidian
It has been thought that silica-rich rocks (such as rhyodacites or felsites) are found
mainly on continents and ancient subaerial regions such as islands and continental
landmasses. In fact, silica-rich lavas are close in composition to granitic rocks
forming continental crust but they are also erupted on the sea floor associated with
various types of geological environments. They have a relatively high viscosity
and thus have greater tendency to erupt explosively than their mafic counterparts
such as basaltic flows. Hence, the emplacement of silica-rich lava is often associated with island arcs in subduction zones and with mantle plumes forming
hotspot volcanoes. Silica-rich lavas are rarely found on accreting ridge systems
except when associated with areas influenced by mantle plume upwelling.
Examples of silica-rich lavas associated with mantle plumes giving rise to hotspot
volcanism are found in Iceland, in the Azores (Faial Island), in the Galapagos
islands, as well as on the ‘‘Axial volcano’’ of the Juan de Fuca Ridge (Wanless
et al. 2010). Because of their highly viscous nature when compared to basaltic
flows, silica-rich lavas are more limited in the extent of their dispersion on the
ocean floor (see Chap. 7).
82
4 Sea Floor Rocks