PART I
Origin and Transport of Magma


MagmaVolcanic eruptions are the surface expression of processes that occur deep within the Earth. Many of these processes take place just below the Earth's outer rigid shell, while some volcanic eruptions owe their origin to very deep disturbances, even at the boundary between the core and the Earth's mantle at 2890 km below the surface.

The origin and transport of magma is treated in Part I of the Encyclopedia of Volcanoes. An understanding of these processes is an essential foundation for an appreciation of the volcanism that is observed at the surface.

We set the stage with the chapter Mantle of the Earth, describing the chemical and physical properties of the source region of virtually all magmas. Although rarely seen, the mantle is the most voluminous part of the planet, and it is here that the great heat manifest in volcanic eruptions is stored. It is hot, perhaps 1300° to 1500°C, but the mantle is for the most part stony and solid. The formation of magma occurs in only certain regions of the mantle, by partial melting of the rock peridotite, as described in Melting the Mantle. This chapter explains how melting occurs either by the lowering of pressure in the mantle, or by the lowering of the melting temperature of peridotite by introduction of water to the mantle from the Earth's surface during subduction.

When melting begins in the mantle, the hot silicate liquid or magma begins to migrate upwards towards the surface, because of its low density compared to the surrounding high-density mantle rock. The migration and transport of magma within the Earth is a perplexing problem, treated in the chapter Migration of Melt. We know that magma begins deep in the mantle where it forms a small part of the peridotite rock, like water in a sponge, and we know that eventually it pools into large liquid bodies near the surface. The intermediate stage in magma transport, between the source and the shallow reservoir, is one of the major problems in the study of magma evolution, as discussed here.

How does the magma percolate upwards towards the surface, or, does it migrate upwards through the action of dikes?

Melting in the mantle determines the location of volcanoes on Earth, but the location of melting is largely determined by plate tectonics, the theory that describes the motion of the great crustal plates that make up the rigid exterior shell of the Earth. As described in the chapter Plate Tectonics and Volcanism, melting occurs beneath plate boundaries where plates are either moving apart, such as below the mid-ocean ridges, or below converging plates in subduction zones. In the former case, meting is entirely due to decompression, as the mantle rises up to fill the void between the diverging plates. In the latter case, however, decompression melting is further aided by the introduction of water and lowering of the solidus. However, a significant fraction of Earth's volcanism occurs totally independent of plate motion, creating hot spots. It is hypothesized by many geologists-although not yet proven-that hot spots are the result of great mantle plumes, which may originate from a region near the boundary of the mantle and the core.

Our most important clues about the formation and origin of magmas come from the study of the chemical and mineralogical composition of volcanic rocks, as described in the chapter Composition of Magmas. These silicate liquids, drawn from deep inside the Earth, have much to tell about their peridotite source rock as well as about the history and formation of the planet. The chemical diversity of magmas or volcanic rocks seems at first bewildering, with a plethora of names used by geologists to describe all of these rock types. The reason for this great diversity is treated in the chapter Origin of Magmas, which explains the great variety of physical and chemical processes leading to the differentiation of magmas.

Virtually all of the known chemical elements occur in magmas, although most are present in trace amounts only. A group of chemical compounds in magmas, including water, carbon dioxide, and sulfur dioxide, are normally dissolved in the magma at high temperatures and pressures, but come out of solution when the magma nears the surface and erupts. These are the volatiles in magmas, whose properties and evolution is dealt with in the chapter Volatiles in Magmas. The concentration and behavior of the volatiles is particularly important, as their abundance in the magma largely determines the explosive nature of volcanic eruptions. The distinction between the quiet effusion of lava flows and explosive ejection of volcanic ash and pumice is primarily a reflection of the original water content in the magma. However, the rheological behavior of magmas is also dictated by their physical properties, such as viscosity and temperature, as discussed in the chapter Physical Properties of Magmas. The importance of the bulk viscosity is, for example, demonstrated by the fact that volatiles can rise and escape freely to the surface as gas bubbles from a low-viscosity basaltic magma, whereas the high viscosity of andesitic or rhyolitic magmas is so high that the bubbles cannot rise, but are carried with the magma to the surface, with explosive results.

Magmas rising from the mantle may often gather in reservoirs at the base of the crust or within the Earth's crust. Reservoirs may be tens of km in dimensions and thus represent huge reserves of magma. The chapter Magma Chambers explains the behavior of these magma chambers, the geological and geophysical evidence for their size and dimensions, and the processes that occur within them. Then Rates of Magma Ascent describes how petrologic studies and geochemical research on short-lived isotopes in magma are providing information on the rates of magma ascent.

Most of the magma is transported in dikes from the the chapter Plumbing Systems. During dike flow the magma experiences extreme gradients in temperature and pressure, as it moves through the cold and rigid upper crust of the Earth. It seems at first amazing that magma may flow for several km in a 1-m wide dike without solidifying, but as explained in this chapter, the magma may retain most of its heat during dike flow and arrive at the surface in a fiery eruption.

During its ascent in a dike or a conduit, magma experiences a very important change, as the volatiles-mainly water, carbon dioxide and sulfur dioxide-exsolve from the magma to form a separate gas phase. The level of exsolution occurs when the pressure decreases below the solubility limit of these volatiles in the magma, as described in the chapter Magma Ascent at Shallow Levels. In the crust above this level, the magma behaves in the dike or conduit as two-phase flow, consisting of silicate melt on one hand and a gas phase that is continually expanding as pressure decreases. This is the most dynamic region of magma evolution and rates of motion are of the order of meters per second, whereas deeper magma flow is at rates of cm per second. The exsolving gas appears as growing bubbles that expand until they burst to tear apart the magma in the conduit, and expel it to the surface.

Haruldur Sigurdsson
University of Rhode Island

[Return to Contents]