Attempts to quantify the physical and thermodynamic properties of silicate melts that influence such important igneous processes as diffusion, nucleation, crystal growth, major and trace-element crystal-liquid partitioning, or the formation of immiscible liquids, require at least an elementary understanding of the structure of these melts. It is clear, however, that the word structure has different implications for silicate minerals on the one hand and for melts on the other. Silicate minerals, in common with other crystalline solids, are characterized by systematic arrangement of atoms in a three-dimensional array. The structures of the most common silicate minerals reveal that the Si⁺⁴...

It is a pleasure to contribute to a volume honoring N. L. Bowen’sThe Evolution of the Igneous Rocks. Since it was first published fifty years ago, Bowen’s book has had a profound influence on several generations of geoscientists, who found in it a happy blend of introductory text and advanced treatise. The book continues to play an important part in the modern development of igneous petrology: even today it contains surprisingly little that is stale or badly dated. That this is so is not at all an indirect demonstration that petrologists have been idle since its publication. On the...

Viscosity and density are two properties of magma that have an important effect on igneous processes. Bowen (1934) emphasized the importance of viscosity as a dominant controlling factor during the passage of magma through the Earth, the pouring-out of lava upon the surface, and the sinking of crystals and the rise of gases within a magma body. A number of viscosity measurements have been made by both field observations of lava flows and experiments in laboratories (e.g., Kani, 1934; Kozu and Kani, 1935; Nichols, 1939; Krauskopf, 1948; Minakami, 1951; Shaw et al., 1968; Murase and McBirney, 1973; Bottinga and Weill,...

Perhaps the greatest change since Bowen’s day in our approach to magma genesis is in the fields of trace-element and isotopic geochemistry. Mass spectrometers, although first operated in 1918, did not make an appearance in the earth sciences until the classic studies of Nier (1938, 1939). The field of trace-element geochemistry was initiated in 1937 by the pioneering works of Goldschmidt (1937) and Fersmann (1934), but the major impact of trace-element studies on igneous petrogenesis was not felt until the early 1950’s, when the development of the optical spectrograph allowed useful trace element analysis of geologic specimens. The most notable...

One of the most pleasureable aspects of contributing to this volume was the stimulus it provided for the re-reading ofThe Evolution of the Igneous Rocks, something which this author is ashamed to confess to not having done for fifteen years. On re-reading Bowen’s words today, one is struck first by Bowen’s almost uncanny insight in discriminating among the plethora of hypotheses for magma generation prevalent at that time, and second, by how little our understanding of this field has changed in fifty years.

Bowen recognized the fundamental importance of basalt. He saw that it was probably generated by the...

This paper consists almost entirely of ideas about mechanics, but it is really concerned with the processes of igneous evolution. The proposed fracture mechanisms represent a viewpoint that bridges gaps in my ability to visualize a continuity of igneous processes in the earth. Although much of the discussion refers to some formal ideas about fracture mechanics, the references are made from the standpoint of a user who has some acquaintance with the history and current advances in this field, but who is far from being expert in the conceptual applications. (See Roberts (1970) for another slant on similar mechanisms of...

The problem of the transport or mobilization of magma is central not only to igneous petrogenesis but also to the evolution and dynamics of the Earth. The generation and emplacement, for instance, of roughly 15 km³per year of basaltic magma along the axes of diverging plate margins is a first-order, petro-tectonic process. A corollary of the implied mass-transfer process is the transport of large quantities of heat from the interior of the Earth. The heating (from 0°C to 1,200°C) and complete fusion of 15 km³of magma requires an amount of energy roughly equivalent to about 10% of the...

As background, it is useful first to review briefly the terminology of igneous cumulates. This is conveniently done by means of Figure 1 (after Wager, Brown and Wadsworth, 1960), which portrays three types of plagioclase cumulates from the Skaergaard intrusion. The rocks consist essentially ofcumulus crystalsof plagioclase that were, by some mechanism, fractionated from the main body of magma and accumulated in layers on the floor of the magma chamber. After accumulation, the crystals were cemented together bypostcumulus materialscrystallized from the interstitial orintercumulus liquidtrapped in the pore spaces between them. The parental liquid of...

The potential importance of diffusion in igneous processes was recognized by Bowen (1921), who noted that “the growth of crystals is accomplished largely by the diffusion of material to crystalline nuclei.” Bowen was, however, more intrigued by the possibility of diffusion-controlled mechanisms to explain larger features such as the formation of basic border phases in igneous bodies. In principle, a chemical gradient will be induced in an initially uniform medium if a temperature gradient is imposed on the system, and this (Soret) effect might generate chemical gradients in cooling magma bodies. Bowen conducted experiments on the diffusion in silicate melts...

During the first half of the twentieth century, while Bowen and his collaborators were elucidating the phase relations of igneous rocks, the theories of crystal growth and nucleation were evolving only rather slowly. It was not until after the Second World War, with the development of solid-state electronics, that experimental and theoretical effort became intense. For this reason, and because it is necessary to know the equilibrium phase relations in order to model the driving forces of kinetic processes, the understanding of the role of kinetics has lagged far behind that of phase equilibria.

Despite the recent progress in kinetic...

Experimental crystallization of melts with cooling histories simulating those in nature (dynamic crystallization) is both a very old and a very new approach to studying the crystallization features of rocks and minerals. James Hall (1805) sealed powdered basalt from Arthur’s Seat in Edinburgh in an iron pipe and placed it in a blacksmith’s forge. He cooled the lava at different rates by either cooling in air, in hot coals, or quenching in water. Visual examination of the products convinced Hall that basalts crystallized from a silicate liquid and were not deposited from solution on the ocean floor as was believed...