magma and magmatism

magma and magmatism Magma is molten rock that has not yet reached the surface of the Earth. Magmatism is the set of processes that lead to the generation of magmas and their emplacement. Although volcanoes are the surface expression of magmatism, a magma, once it has reached the surface of the Earth, is called a lava.

Much progress has been made in understanding the processes that generate magmas, and models have been developed for the composition and volumes of magma whose products are observed on Earth. The development of plate-tectonic theory in the late 1960s led to the recognition that certain types of volcanic activity are associated with specific plate-tectonic environments, and this has in turn led to refined models of melt generation. Melting under these conditions produces a range of parent magmas of different chemical composition. The compositions of volcanic rocks at the surface of the Earth reflect the composition of the parent magma and the modifying effects of the processes occurring between the source region and the point of eruption. The composition of a parent magma can be modified by crystallization or by contamination by the rocks through which it passes on its way to the surface.

The outer part of the solid Earth consists of a thin (30–150 km) solid lithosphere, composed of tectonic plates, ‘floating’ on the asthenosphere, which is the upper part of the mantle. Although the mantle is a crystalline solid, over millions of years it convects, carrying heat from the Earth's core to its surface; the lithosphere can be thought of as a lid, insulating the mantle from space. Experiments in the Department of Applied Mathematics and Theoretical Physics at Cambridge University have shown that the convection in the 3000 km thick mantle can be modelled by a 3 cm thick layer of golden syrup, heated from underneath. The syrup layer has the advantage of being thinner, cooler, and less viscous than the mantle and it therefore convects much more quickly. The syrup in these experiments forms polygonal convection cells with hot, rising plumes at the centre and cold, sinking sheets at the edges. Analogous structures are observed in the mantle: the hot, rising plumes are at about 1550 °C and the cold, sinking sheets are identified with tectonic plates sinking into the mantle (i.e. subducting). Heat is continually being lost from the Earth, which is slowly cooling. Heat is continually being supplied to the mantle from two sources: the radioactive decay of elements and the heat of crystallization of the Earth's core.

Figure 1 shows the thermal structure of the lithosphere and mantle. The mantle, because it is convecting, is of roughly constant temperature, while the lithosphere acts as a conducting lid. The temperature profile across the two layers is called the geotherm. Material in the mantle that becomes unusually cold sinks; material that has been heated by the Earth's core rises as a hot plume. If the mantle is dry, it melts at the dry solidus and if volatiles (such as water and carbon dioxide) are present it melts at the wet solidus. The solidus temperature is the temperature at which the first droplets of melt are formed. Figure 1 shows that there are three main ways in which the mantle can be melted: by a reduction in pressure, by an increase in temperature, and changing from the dry solidus to the wet solidus by adding volatiles to the melt.

Pressure is reduced when the lithosphere is stretched and consequently thinned, leading to a steepening of the geothermal gradient (A in Fig. 1). The geotherm now crosses the dry solidus and melting occurs. The temperature of a mantle plume is around 1550 °C. An increased mantle temperature also causes the geotherm to cross the solidus and generate a melt (B). It can be seen from Fig. 1 that the wet solidus corresponds to lower temperatures than the dry solidus, so that melting may be caused by the addition of volatiles. Melting is also aided by any combination of these three processes: decompression, heating, or the addition of volatiles.

The composition of the melt produced depends on the composition of the material that is melting and the conditions under which it melts. Thus certain compositions of melt are related to specific plate tectonic environments. Figure 2 shows the plate tectonic environments in which magmatism is most likely to occur. Examples of some of these types of magmatism and the processes that generate them are discussed below.

Decompression melting

Decompression melting is melting caused by a reduction in pressure during tectonic extension. As the lithosphere is stretched horizontally it is thinned as the top surface sinks and the bottom surface rises. The mantle flows to fill what would otherwise be a space at the base of the lithosphere, and is decompressed. Where there has been only a little stretching, only a little melting occurs (e.g. in rift valleys). Large amounts of stretching result in the lithosphere becoming so attenuated that it breaks and separates completely. The volcanic rocks that are generated at the breaks form mid-ocean ridges: they solidify to form new crust called oceanic crust. The most common rocks on the Earth are the volcanic rocks formed at mid-ocean ridges. These rocks are basaltic in composition and are called mid-ocean ridge basalts (MORBs). A mid-ocean ridge is a place where two tectonic plates are diverging; as they move apart, decompression melting of the mantle occurs to form basalts that erupt to form new sea floor. Most of this new oceanic crust eventually sinks back into the mantle at a subduction zone and is recycled. The process by which MORB is generated is remarkably constant: it is a melt generated from the mantle by continuous extension of the oceanic crust. The composition and thickness of the oceanic crust is correspondingly constant. Oceanic crust has a thickness of only about 7 km.

Continental rift valleys express much smaller amounts of extension. If there is extreme extension of a rift, it will ultimately become an ocean, as for example, the present-day Red Sea. The volumes of magma generated in rifts are correspondingly smaller than in oceans and their compositions represent much smaller degrees of melting. These smaller volumes of melt are rich in elements such as sodium and potassium (alkali metals), which are the first elements to move into the melt; the resulting rocks are called alkali basalts.

Melting by heating

Melting by heating occurs when a mantle plume rises beneath the lithosphere. The centre of the rising plume is very hot and, where it reaches the top of the mantle, the pressure is low enough for it to melt. The melts thus generated rise through the lithosphere and eventually erupt at the surface. Volcanic activity of this type is often called hot-spot volcanism. Where a plume rises under an oceanic plate the volcanic rocks formed from the magma generated form a sea mount (a mountain on the sea floor). An example of a hot-spot sea mount is the island of Hawaii in the Pacific Ocean. There has been so much melting beneath Hawaii that the sea mount has become taller than the ocean is deep and it emerges as an island. The oceanic plate upon which Hawaii is being built is moving slowly over the plume, producing a trail of islands across the ocean known as the Emperor–Hawaiian chain. The dog-leg in this chain of islands shows that the plate has changed its direction of motion since the hot-spot originated (Fig. 3). Most hot-spot volcanoes of this type erupt ocean island basalts (OIB).

Because the continental crust is much thicker than the oceanic crust, plumes cannot rise to areas of low pressure beneath them without colliding with the base of the lithosphere; continental hot-spot activity is thus rare. However, the hot springs and volcanic activity of Yellowstone Park in the western USA, related to the ‘Yellowstone plume’, are an expression of such activity.

The most common cause of melting by heating is a plume, but during metamorphism (change in the mineral composition of a rock through the application of heat and pressure) part of the rock may be locally melted and escape. This process of partial melting is called anatexis; once an anatectic melt has escaped from the parent rock, it behaves like an igneous melt.

Melting by the addition of volatiles

Under normal conditions volatiles (mainly water and carbon dioxide) are present only in small quantities in the mantle (less than 0.5 per cent). However, in a subduction zone, where an oceanic plate sinks beneath another plate, it takes water down with it into the mantle. The water in the descending plate is trapped in hydrous (i.e. water-bearing) minerals at the top of the oceanic crust. As the crust is heated, the minerals break down and water is released, rising into the wedge of mantle above the plume and causing it to melt. This process generates large volumes of melt by wet melting and forms chains of volcanoes whose composition is andesitic. Examples of chains of andesitic volcanoes occur in the Andes, and in parts of the western Cordillera of North America, both areas of volcanism being related to the subducting eastern margin of the Pacific Ocean. Where one oceanic plate is subducted beneath another, the volcanism can form a chain of islands; Japan is an example. This type of magmatism is very complicated and is still not fully understood.

Mixed melting

All the examples given above involve one melting process. There are also examples of tectonic environments where more than one melting process contributes to the formation of a group of volcanic rocks. Figure 4 shows the possible combinations of melt types, and gives examples of places where they occur.

In Iceland the mid-ocean ridge and a plume are acting together to cause very large amounts of melting (heating and decompression). Without the presence of the plume, the crust would be of normal oceanic thickness, that is about 7 km thick; but the additional heat of the plume means that sufficient melting has occurred for an abnormally thick (30 km) oceanic crust to have formed. This young, thick crust is very buoyant and rises above the level of the ocean surface. It is known that the Iceland plume has been coincident with the Mid-Atlantic Ridge since the ocean began to open. The perennial presence of the plume is reflected in a ridge of thickened crust that joins Iceland to the British mainland. Although when a plume rises under an oceanic plate, melting is restricted to the hottest part of the centre of the plume, it is thought that hot material spreads out in a mushroom-shaped head at the base of the lithosphere, thus raising the temperature of the mantle over a much wider area.

Even though the continental crust is much thicker than the oceanic crust, decompression melting in the presence of a hot plume generates the aptly named continental flood basalts (CFB). These are the largest volcanic effusions on the continents. The most recently active continental flood basalt provinces are those in the USA, which include the Columbia River plateau (active from 17 to 6 million years ago) and the Snake River plain (active from 17 million years ago until the present) provinces. The activity of the Deccan Traps in India during the time of transition from the Cretaceous to the Tertiary is considered by some geologists to have been so catastrophic that it has been proposed as a cause of extinction of the dinosaurs.

In southern Africa, melts that are very small in volume have been generated by wet melting at the base of the lithosphere, enhanced by the presence of a hot plume. These rocks, called kimberlites, came from very deep in the lower lithosphere and were so rich in volatiles that they bubbled up through the crust quickly enough to preserve diamonds picked up from their source region at a depth of about 150 km. Kimberlites are named after one of the most diamond-rich sites in Kimberley, South Africa.

The volcanic rocks of the East African Rift Valley appear to be the product of all three melting processes combined. The African lithosphere is thick (150 km), and there is much debate about the number and location of plumes in the area, but it seems certain that there is at least one plume under the region. The thick African crust displays a low geothermal gradient which inhibits decompression melting. Even though melting would not normally be expected because of the small amounts of extension, the combination of extension, volatiles, and the plume has produced alkali basalts. These basalts, being the result of all three types of melting, have exotic compositions (Fig. 4).

Modification of magma

Melts are less dense than the materials in their source regions; so, as soon as melting occurs, they start to rise. Melts move by percolation and as blobs of melt (diapirs). Percolation is the movement of small amounts of melt in tiny canals along grain boundaries. Work by Mark Spiegelmann in the United States suggests that diapirs can be interpreted in terms of a special kind of wave called a solitary wave. The waves of this type take the form of increased porosity in a sphere of several kilometres radius. However they move, rising magmas eventually come to a region where they no longer have positive buoyancy and they then cease to ascend. In some instances, stationary magmas solidify in situ. As they cool, dense minerals usually crystallize out which sink to the bottom of the magma body. This process is called fractional crystallization. As the density of the remaining melt is lower, it can again rise in the crust. The density of the magma is progressively lowered because it loses magnesium and becomes richer in silica. For example, a gabbroic melt rising through the crust will pond in a magma chamber when it reaches neutral buoyancy. As the magma chamber cools, crystals of olivine form and sink to the bottom of the magma chamber and collect to form a peridotite. The magma, now lighter having lost olivine, continues to rise and can eventually erupt as a basalt enriched in silica.

The composition of magmas is also modified by crustal contamination. Many of the crustal rocks with which a magma comes into contact are silica-rich. Because they have lower melting temperatures than the magma, they are melted on contact with it and can be assimilated, changing its composition.

Judith M. Bunbury

Bibliography

Wilson, M. (1989) Igneous petrogenesis: a global tectonic approach. Unwin Hyman, London.