Melting the Mantle

The stony part of the Earth is solid under normal conditions at the present day. Volcanism is the eruption of molten or partially molten rock, that is, magma. The first stage of any volcanic process therefore must be melting: We have to produce a liquid by partial melting before it can migrate from the source region (see Migration of Melt chapter) and subsequently erupt (Part II). The locations of volcanoes on Earth are controlled primarily by where melting takes place. The volume, frequency, and style of eruptions are dependent on many factors but the first considerations are how much magma is supplied by melting, at what depth melting takes place, and the distribution of melt production in the source region in space and time. Before roughly 1960, igneous petrology was mostly concerned with the evolution and differentiation of magma after it left its source region. In the following two decades, attention shifted to the pressure (P) and temperature (T) at which primary melts were extracted from the mantle, without detailed understanding of how the melts were produced. In the last 20 years, however, the application of ideas from thermodynamics, experimental petrology, and fluid dynamics has provided a strong basis for understanding the basic physical processes underlying the melting of Earth's mantle. This chapter explores how and why the crust and mantle of the Earth melt, which together with information about tectonic processes and chemical composition (discussed in other chapters in this part) determines where and when melting takes place