Page 52 - Fundamentals of Physical Volcanology

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MAGMA GENERATION AND SEGREGATION 29

centration of melt. The point may come at which 2.5 Summary
the total buoyancy of the region in which the melt
is now concentrated is large enough that this melt- • Most volcanoes on Earth are located along the
rich region begins to accelerate ahead of the rest boundaries of tectonic plates and thus are related
of the rising plume that gave rise to it, and can be to the large-scale convection of the mantle which
thought of as a separate entity – a diapir. This term drives plate tectonics. Other volcanoes are found
can be applied to any very buoyant region contain- far from plate boundaries and mark the locations
ing melt, from the upper part of a large (150– of hot spots – mantle plumes which are thought
200 km wide) plume in the upper mantle to a to form due to thermal anomalies in the deep
much smaller body of melt only a few kilometers mantle.
in size generated in a subduction zone, or where a • The composition of the lava erupted from volca-
mantle plume rises under the base of a continent noes is related to the tectonic setting. At mid-
and causes melting of overlying crustal rocks. ocean ridges, and where hot-spot plumes occur
By definition, any diapirically rising body of rock under ocean-floor crust, melting of the mantle by
is moving from hotter into cooler surroundings, decompression produces hot, fluid basalts. Hot
and the viscosity of almost all liquids increases as spots beneath continents also produce basalt but
they get cooler. Eventually, therefore, the increas- interaction of these basalts with the continental
ing viscosity of the rocks surrounding the diapir crust generates a wide diversity of magmas. At sub-
must drastically slow and eventually stall its ascent. duction zones the presence of water carried
Around the time that happens, or earlier in some down with the descending plate lowers the melt-
cases, a new process that moves magma to shal- ing temperature, causing melting of the mantle
lower depths sets in. That new method is the exten- wedge to generate basalts. Interaction of the basalt
sive joining together of many of the veins of melt with the overlying crust, and the fractional cryst-
within the diapir to create larger fractures, through allization of the resulting magma as it ascends,
which melt moves much more efficiently. These generates a great diversity of magmas, particu-
large fractures are called dikes, and the relation- larly where oceanic crust descends beneath
ship between diapiric rise and dike formation is the continental crust. Although magma compositions
subject of the next chapter. are diverse in subduction-zone settings, the dom-
There is one tectonic setting where these cat- inant magma type is andesite.
egorizations into diapiric rise and dike formation • Although a great diversity of magmas are found

become blurred. Under some mid-ocean ridges, the on Earth these magmas all ultimately owe their
rate at which the overlying plates spread apart may generation to the production of basalts by mantle
be fast enough to allow the upwelling of mantle melting. This melting can occur by decompres-
material to proceed more or less continuously. As sion of upwelling mantle material or by melting
a result, instead of thinking of a mantle plume head induced by the release of water from subducting
stalling at some level we should think of the plume oceanic lithosphere, decompression melting being
splitting in the middle as it rises, with half of it head- the dominant mode.
ing off horizontally under each of the two plates • Melting starts in rock at favorable contacts
being formed at the ridge. It is, of course, the between mineral grains when the combination
melt separating from the plume that is forming the of slowly decreasing temperature and more
new crust. Although it is clear from ophiolite com- rapidly decreasing pressure reaches the solidus.
plexes that large numbers of dikes do form in these When enough melting has taken place, individ-
settings, the chemical compositions of magmas ual melt pockets between grains start to connect
arriving at shallow depths imply that some (probably together. The natural buoyancy of the melt then
small) fraction of the magma has traveled most of causes it to start to move by percolation through
the way from its melt source by percolating slowly the existing melt pathways. Because the melt is
through the network of narrow veins between less dense than the rocks from which it is gener-
the dikes. ated, it tries to occupy a larger volume than its