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30 CHAPTER 2

parent materials. This causes a pressure increase Niu, Y.L. (1997) Mantle melting and melt extraction
in the melt as it forms, and extra space for melt processes beneath ocean ridges: Evidence from
movement may be created by this fluid pressure abyssal peridotites. J. Petrol. 38, 1047–74.
fracturing some of the grain contacts. Perfit, M.R. & Davidson, J.P. (2000) Plate tectonics
• As melt moves upward, the mineral grains in the and volcanism. In Encyclopedia of Volcanoes (Ed.
lower part of the region where melting is taking H. Sigurdsson), pp. 89–113. Academic Press, San
place are compacted, and the melt fraction in the Diego, CA.
upper part of the region increases. Eventually the Rubin, A.M. (1998) Dike ascent in partially molten
whole upper part of the melt-rich region may rock. J. Geophys. Res. 103, 20 901–19.
Scott, D.R. & Stephenson, D.J. (1986) Magma ascent
begin to rise as a separate body called a diapir.
by porous flow. J. Geophys. Res. 91, 9283–96.
The rise of diapirs is limited by the increasing
Sleep, N.H. (1988) Tapping of melt by veins and dikes.
viscosity of the surrounding rocks, and subsequ-
J. Geophys. Res. 93, 10 255–72.
ently the melt is likely to rise through magma-
Spiegelman, M. (1993) Flow in deformable porous
filled fractures called dikes, initiated when the
media. J. Fluid. Mech. 247, 17–63.
strain rate induced in the host rocks becomes so
Winter, J.D. (2001) An Introduction to Igneous and
large that they respond in a brittle, rather than
Metamorphic Petrology. Prentice-Hall, Upper Saddle
plastic, fashion.
River, NJ.
2.6 Further reading 2.7 Questions to think about
Kelemen, P.B., Hirth, G., Shimizu, N., Spiegelman, M.
1 What are the main differences, and how do these
& Dick, H.J.B. (1997) A review of melt migration
arise, between magmas generated at mid-ocean
processes in the adiabatically upwelling mantle
ridges and hot spots on the one hand and sub-
beneath oceanic spreading ridges. Phil. Trans. Roy.
duction zones on the other?
Soc. Lond. Ser. A 355, 283–318.
2 What factors cause magmas to separate from
Maaloe, S. (2003) Melt dynamics of a partially molten
their parent rocks when partial melting occurs in
mantle with randomly oriented veins. J. Petrol.
the mantle?
44(7), 1193–210.
McKenzie, D.P. (1985) The extraction of magma

from the crust and mantle. Earth Planet. Sci. Lett.
74, 81–91.