Chapter 5
Earth’s Mantle Melting and Volcanism
Abstract Convection currents inside the Earth’s asthenosphere will cause
instability at shallow depths in the mantle. Rising material and subsequent
decompression melting will form hot, upwelling mantle plumes or diapirs. This
phenomenon is more common on slow spreading ridges (total rate \5 cm/yrs),
rather than beneath fast (total rate [5 cm/yrs) spreading ridge segments with their
extensive fissural magmatism. Magma upwelling after partial melting of the
mantle will depend on the force of buoyancy and on the permeability of the
lithosphere. The effects of permeability and buoyancy will be modified by tectonic
stress-release as well as by compression due to spreading following a cooling of
the lithosphere. Instability in the melting zone is related to pressure release during
a period without magma extraction. If pressure is released during spreading, the
heat supply will generate more melting. With increased tension, the melting zone
expands laterally and deepens until enough melt aggregates and accumulates in a
confined zone to form a magma chamber. Rapid migration of melt through fissures
at shallow depths enables a release of tension in the magmatic zone as it undergoes
periods of melting and magma accumulation. When this process is repeated several times, it will trigger successive arrivals of more deep-seated magma, which
can replenish the magma reservoir.
Magma Reservoirs Underneath Spreading Ridges
After the partial melting of the mantle, molten material from about 30–60 km deep
will rise towards the sea floor and this magma can form shallow sub-crustal
reservoirs or ‘‘magma chambers’’ (Figs. 5.1 and 5.2). The location of these reservoirs is found in a transition zone where liquid has been stored prior to its escape
on the surface during volcanism. The existence of magma reservoirs has long been
inferred from land-based geology records mainly gathered from exposed sections
of ophiolite complexes. In the field, the presence of layered gabbros is considered
as being the result of crystal-liquid differentiation giving rise to stratiform
R. Hekinian, Sea Floor Exploration, Springer Oceanography,
DOI: 10.1007/978-3-319-03203-0_5,
Ó Springer International Publishing Switzerland 2014
109
Earth’s Mantle Melting and Volcanism
Abstract Convection currents inside the Earth’s asthenosphere will cause
instability at shallow depths in the mantle. Rising material and subsequent
decompression melting will form hot, upwelling mantle plumes or diapirs. This
phenomenon is more common on slow spreading ridges (total rate \5 cm/yrs),
rather than beneath fast (total rate [5 cm/yrs) spreading ridge segments with their
extensive fissural magmatism. Magma upwelling after partial melting of the
mantle will depend on the force of buoyancy and on the permeability of the
lithosphere. The effects of permeability and buoyancy will be modified by tectonic
stress-release as well as by compression due to spreading following a cooling of
the lithosphere. Instability in the melting zone is related to pressure release during
a period without magma extraction. If pressure is released during spreading, the
heat supply will generate more melting. With increased tension, the melting zone
expands laterally and deepens until enough melt aggregates and accumulates in a
confined zone to form a magma chamber. Rapid migration of melt through fissures
at shallow depths enables a release of tension in the magmatic zone as it undergoes
periods of melting and magma accumulation. When this process is repeated several times, it will trigger successive arrivals of more deep-seated magma, which
can replenish the magma reservoir.
Magma Reservoirs Underneath Spreading Ridges
After the partial melting of the mantle, molten material from about 30–60 km deep
will rise towards the sea floor and this magma can form shallow sub-crustal
reservoirs or ‘‘magma chambers’’ (Figs. 5.1 and 5.2). The location of these reservoirs is found in a transition zone where liquid has been stored prior to its escape
on the surface during volcanism. The existence of magma reservoirs has long been
inferred from land-based geology records mainly gathered from exposed sections
of ophiolite complexes. In the field, the presence of layered gabbros is considered
as being the result of crystal-liquid differentiation giving rise to stratiform
R. Hekinian, Sea Floor Exploration, Springer Oceanography,
DOI: 10.1007/978-3-319-03203-0_5,
Ó Springer International Publishing Switzerland 2014
109