Oceanography Vol. 20, No. 1 78 Oceanography Vol. 20, No. 1 78 SPECIAL ISSUE FEATURE Mantle Meltıng Beneath Mıd-Ocean Rıdges The plate-tectonic revolution was initially “kinematic”— a description of plate motions across Earth’s surface. Plate tectonics is now recognized as the surface manifestation of a greater process—circulation of the solid earth. Magma ascends to the surface at mid-ocean-ridge spreading centers to cool and form oceanic crust, which millions of years later returns to the mantle at subduction zones. Formation of oceanic crust is the greatest contribution of flow from our planet’s interior, as two- thirds of the earth is resurfaced about every 100 million years. Partial melting of the mantle at spreading centers is the mecha- nism by which this flow takes place, and thus is fundamental to understanding solid-earth circulation. Melting is a primary means by which the earth cools: sea- floor spreading brings hot mantle from depth to the colder sur- face. Because we normally think that melting occurs through heating (e.g., putting a slab of butter in a frying pan), it may seem paradoxical to say the earth melts while cooling down. The explanation for this paradox is that melting temperatures are dependent on pressure as well as temperature. Just as in- creased temperature excites atoms so they free themselves from their ordered, solid, crystalline state, so increased pressure squeezes atoms, making it more difficult for them to transi- tion from solid to liquid. Thus, temperature and pressure exert opposite effects on melting, and melting can occur by decreas- ing pressure at a given temperature as well as by increasing temperature at a given pressure. The reason melting by pres- sure release seems foreign to common experience is because human life on Earth is lived in an environment of almost con- stant pressure caused by the weight only of the atmosphere. The solid earth, however, is subject to huge changes in pressure, because the weight of hundreds of kilometers of rock exerts pressures equivalent to thousands of atmospheres in the inte- rior. As mantle ascends beneath the mid-ocean ridge, less and less rock lies above it, so large pressure changes occur, which leads to melting. The melt is less dense than the solid, and rises to the surface to form the oceanic crust. Figure 1 shows how rising mantle crosses the “solidus” (the transition from complete solid to partial melt) and melts pro- gressively towards the surface. Note that because the mantle is a solid consisting of many different molecules, it does not melt entirely at a single temperature, but progressively over a range of temperatures—from 0 percent melting at the solidus to 100 percent melting several hundred degrees higher at the liq- uidus. Thus, partial melting is possible. Several lines of evidence provide information about this melting process. New maps generated over the past 25 years show the variations in shape and depth of thousands of volca- noes distributed along the ridge. These maps have enabled sci- entists to sample the volcanic rocks—ocean-ridge basalt—that make up the surface pavement of the oceanic crust. About 80 percent of the 60,000-kilometer-long mid-ocean ridge has been mapped and sampled at least to 100-km spacing (see www.petdb.org). At the same time, experimental studies have led to quantitative models of how melt composition and amount vary with temperature and pressure (e.g., Jacques and Green, 1980; Kinzler and Grove, 1992; Baker and Stolper, 1994; Pickering-Witter and Johnston, 2000). And seismic studies, which are able to probe Earth’s interior directly, provide infor- mation about the “melting regime” beneath the ridge axis. This article synthesizes some of these developments, and outlines a set of major questions for future research. BY CHARLES H. LANGMUIR AND DONALD W. FORSYTH Oceanography Vol. 20, No. 1 78 is article has been published in Oceanography, Volume 20, Number 1, a quarterly journal of e Oceanography Society. Copyright 2007 by e Oceanography Society. All rights reserved. Permission is granted to copy this article for use in teaching and research. Republication, systemmatic reproduction, or collective redistirbution of any portion of this article by photocopy machine, reposting, or other means is permitted only with the approval of e Oceanography Society. Send all correspondence to: info@tos.org or  e Oceanography Society, PO Box 1931, Rockville, MD 20849-1931, USA.