Magmas and Lavas Definitions

It is clear from our discussion of the layered structure of the earth that although magmas are generated by partial melting in the upper mantle or lower crust, such a process occurs over a range of depths. Accordingly, not all magmas have the same composition. This is evidenced by the variety of igneous rocks that occur at the surface of the earth or at depth. Volcanic eruptions also show that lavas have different viscosities, which in turn are due to their different chemical compositions and/or temperatures. By carefully studying the chemistry of the different types of igneous rocks, and their associations with each other, petrologists were able to classify magmas into four main chemical groups:

In the course of studying igneous rocks, it was found that acidic rocks melt at lower temperature compared to basic and ultrabasic rocks. Because melting is the reverse of crystallization, understanding how a rock melts will help us understand the process of magma formation, as well as the process of formation of igneous rocks from a magma.

The temperature in the earth generally increases regularly with increasing depth, and the variation of temperature with depth in a specific area and at a specific time in the earth's history is known as the "geotherm", the slope of which is known as the "geothermal gradient". Different areas have different geothermal gradients (Fig. 1) which may also change over time (as the tectonic setting of that area changes). As you are aware, the highest geotherms occur beneath mid-oceanic ridges (Fig. 1).

1. 
   the geotherm is perturbed, shifting to higher temperatures.
   
2. 
   H₂O or CO₂ are added to the mantle, thus shifting their solidi to lower T (Fig. 4)
   
3. 
   Adiabatic decompression: This model suggests that packets of mantle material can rise very quickly from the deep parts of the mantle into shallower levels while still maintaining their temperature. This causes these packets to cross the solidus of mantle lherzolites, and thus undergo partial melting. A good example of such "packets" is rendered by mantle plumes. Convection in the mantle is believed to be the driving force behind the adiabatic rise of mantle material. Figure 5 shows the process of adiabatic decompression.
   

Once the mantle undergoes partial melting by about 20% or more, the melt fraction (or primary magma) is squeezed out of it, and begins its journey towards the surface of the earth (Fig. 6). Keep in mind that the mantle layers are not necessarily homogeneous, and that they may contain (in addition to basaltic magma) eclogites (Gt + Cpx), harzburgites (Opx + Ol) and dunites (monomineralic rocks of olivine). The latter two rock types are believed to represent what is left behind after partial melting. Accordingly, if the mantle is found to contain considerable amounts of harzburgite + dunite, it is often termed "depleted mantle" meaning a mantle that has undergone significant partial melting. Figure 7 shows the relationship between the composition of the magma generated by partial melting, and the depth at which such magmas are generated.

By definition, a magma consists of a melt ± suspended crystals ± dissolved or exsolved gases. This shows that the melt (usually a silicate liquid) is the main constituent of a magma. It is therefore important to understand its structure, which will in turn help us understand the physical properties of magmas. Silicate melts consist of polymers of interconnected by distorted Si-O tetrahedra, with other major ions such as Na, K, Al, Ca, Fe and Mg occurring in looser coordination with O (Fig. 8).

1. 
   Network formers: Si, Al, Na, K, Rb, Cs, Ti and P. Al³⁺ and a monovalent ion can substitute for Si⁴⁺, hence the network forming capability of these elements. Ti and P have large charges (4 and 5, respectively) which allow them to act as network formers.
   
2. 
   Network modifiers: Fe, Mg, Ca and Mn, all of which substitute into the melt structure by breaking the Si - O bonds.
   

Note that in alkalic magmas in which Na + K > Al, Na and K become network modifiers instead of network formers. Similarly, in highly aluminous melts, Al will tend to occupy the octahedral (instead of the tetrahedral) sites, and will also become a network modifier. H₂O, Cl, F, and S are all network modifiers. H₂O being the most common volatile in magmas plays a major role in breaking Si-O bonds and lowering the viscosity of the melt (Fig. 9). CO₂ on the other hand appears to be a network former, binding broken Si-O bonds back together, although its role is not exactly known.

Volatiles in magmas are either dissolved (at high pressures) or exsolved (closer to the surface, where the pressure buildup may result in the violent eruptions of some volcanoes). Experimental studies have shown that magmas can dissolve more gases than have actually been observed. However, determination of the exact amount or composition of volatiles contained in a magma is very difficult, as magmas tend to lose volatiles to the atmosphere on eruption, or to the intruded country rocks during their slow cooling. Nevertheless, methods of estimating the nature and amount of volatile constituents of a magma include:

i) analysis of the tops of pillow lavas, and counting their vesicles.

ii) analysis of melt inclusions in crystals

iii) analysis of quenched magmas

iv) phase equilibria experiments.

In addition, volcanic eruptions give us some idea of the amount and composition of volcanic gases. Table 1 lists the composition of volcanic gases. All of these studies show that magmas have ~ 1% by weight volatiles, and that H₂O is the most common volatile constituent of acidic magmas, whereas CO₂ is more common in basaltic ones. Although this may seem like a very small amount, the low molecular weight of volatiles ensures that they will occupy a significantly larger % of the magma by volume. The decrease of the H₂O/CO₂ ratio as the magmas become more basaltic or more alkalic is due to the fact that CO₂ and S gases are more soluble in basaltic magmas. On the other hand, F and Cl are more soluble in acidic magmas.
Because H₂O is the most common volatile constituent of the magma, it is important to understand the form in which it enters (and dissolves in) a magma. Figure 9 shows that H₂O breaks Si-O bonds and is therefore one of the most important network modifiers. Stopler (1982) has shown that at low concentrations of H₂O in the magma, this H₂O is in the form of OH^-; at higher concentrations, H₂O enters the magma in its molecular form (Fig. 10).
Factors affecting the solubility of gases in magmas:

i) pressure: the higher the P, the larger the wt% of H₂O dissolved (Fig. 11a).

ii) temperature: the higher the T, the lower the wt% of H₂O dissolved (Fig. 11b).

iii) chemical composition of the magma: acidic magmas contain a larger amount of volatiles (Fig. 11a). This is because such magmas have already undergone a significant amount of crystallization of anhydrous phases, thus concentrating the volatiles.

iv) composition of the volatiles: An increased amount of CO₂ in the volatiles decreases the solubility of H₂O in the magma (Fig. 11c).

Physical properties of magmas
1) Temperature

Temperatures of magmas can be inferred from:

i) direct measurement of the temperature of lavas using optical pyrometers

ii) direct measurement of lava temperatures using thermocouples

iii) phase equilibria experiments

iv) application of well calibrated geothermometers.

Direct measurement methods yield temperatures in the range: 800 - 1200°C. The lower temperatures represent those determined for acidic rocks or partially crystalline lavas, whereas the higher ones are for basalts. Because most solidi have positive slopes (i.e. the meting temperature increases by ~ 3°C/km, the original temperatures of a magma generated at a depth of ~ 50 km will be 150°C higher than that determined for the corresponding lava at the surface.

2) Thermal conductivity, specific heat and heat of fusion:

The thermal conductivity of both rocks and magmas is very low. It generally increases with increasing temperature. Acidic magmas have a higher thermal conductivity compared to basic ones. Fig. 12 shows the relationship between thermal conductivity and T for several types of magmas.

Viscosity is a measure of the resistance to flow of any fluid substance. Fluids are broadly grouped into two groups according to their viscosity: (i) Newtonian fluids: those which respond directly by flow to any kind of applied shear stress, and (ii) Bingham fluids: those which have a finite yield strength, and will not flow unless this strength is "overcome" by the applied shear stress. Magmas are Bingham fluids.

1. 
   temperature: the higher the T, the lower the viscosity (Fig. 13a & c)
   
2. 
   pressure: has a small effect on viscosity; the higher the pressure, the lower the viscosity (Fig. 13d).
   
3. 
   composition: Acidic magmas are more viscous than basic ones. This is simply a function of the percentage of network formers and network modifiers in each magma; the higher SiO₂ content of acidic magmas ensures that they have the larger % of network formers. This relationship is shown in Fig. 13a.
   
4. 
   amount and nature of volatiles: Again this is a reflection of the ability of volatile components to act as network modifiers or (in case of CO₂) as network formers. Addition of a small amount of H₂O to a magma drastically lowers its viscosity (Fig. 13c), especially if this magma is acidic or intermediate. On the other hand, addition of H₂O to an olivine melt will only have a minor effect on its viscosity. This is because H₂O breaks Si-Al-O bonds, and Olivine has no Al!
   

Viscosity of a magma largely controls the structures formed upon its crystallization. For example, fissure eruptions or shield volcanoes form from basaltic lavas which have low viscosities, whereas cumulo-domes and plugs are formed from high viscosity acidic lavas.

The density of a magma is one of the most important factors controlling its physical and chemical behavior. Density plays a role in controlling the movement of magmas to shallower levels. Moreover, density contrast between a silicate melt and any minerals that may have crystallized from it plays a role in the differentiation of this magma. Magma densities range between 2.2 and 3.1 g/cm³ Mafic minerals such as pyroxenes, olivines and amphiboles are usually denser than the magma, and have a tendency to sink to the bottom of the magma chamber, resulting in a change in the chemical composition of the remaining magma. On the other hand, only calcic plagioclases may be denser than some magmas, but sodic plagioclases and nepheline usually float on the top of all magma. The velocity at which a mineral sinks to the floor of a magma chamber can be estimated by Stokes' law:

v = 2 g r²(r₁-r₂)/9 h

where v is the settling velocity, r is the radius of the settling crystal (assuming it has a spherical shape), (r₁-r₂) is the density contrast between the magma and the crystal, and h is the viscosity of the magma. However, keep in mind that for a crystal to sink, the gravitational force acting on it must be able to exceed the yield strength of the magma.

1. 
   temperature: the higher the T, the lower the density (Fig. 14a)
   
2. 
   pressure: the higher the pressure, the higher the density. Accordingly, the density of a magma will increase with increasing depth, until this magma is actually denser than the solid rocks (this is because fluids are more compressible than solids). This happens at depths of ~ 400 km. Accordingly, magmas generated from depths > 400 km do not rise to the surface!
   
3. 
   composition: The density of a magma depends largely on its Fe content. Accordingly, basic magmas are denser than acidic ones (Fig. 14a & b). This plays a major role in differentiation (Fig. 14b).
   

Now that we understand how a magma is generated, and have some knowledge of its physical and chemical properties, we need to understand how it moves. Movement of magmas is triggered essentially by density contrast between it and the surrounding or overlying rocks, and is controlled to a large extent by its viscosity. As soon as a melt is formed in lower crust or upper mantle (at depths < 400 km), it will have a tendency to rise to shallower depths by virtue of its lower density. This movement is known as "diapiric rise" (Fig. 14). Movement of magma will also take place through connected pores and channelways, essentially following the laws of fluid flow through porous media. At shallow depths, magmas migrate along fissures and fractures. As the magma moves, it essentially creates new channelways for itself by engulfing country rocks, and possibly melting or reacting with them (a process known as assimilation). The whole process describing the movement of magmas at shallow depths by engulfing country rocks is known as "stoping".