Magma is a natural rock fluid beneath the earth’s crust, which may consolidate to form and igneous rock. When magma is erupted to the surface, it is known as lava, the consolidation of which gives rise to volcanic rocks.
Composition and Constitution:
Magmas consist of mixtures of solids, fluids and dissolved gases. Essentially they are very hot silicate melts containing large quantities of water and varying amounts of highly reactive fluids and gases in solution. These reactive fluids include such things as hydrochloric acid and hydrofluoric acid. Magmas do not have a fixed composition. Although the compositions of different magmas undoubtedly vary, many are close to the following composition:
(a) Chemical composition:
In terms of elements:
Oxygen, Silicon, Aluminium, Iron, calcium, Sodium, Potassium, Magnesium, Titanium, etc. They together constitute more than 99% of the fixed constituent of any magma.
Crystallization of magma:
It is well known that magmas become igneous rocks by solidification without crystallization (i.e., formation of glass) or by crystallization with the loss of much of their volatile materials. The crystallization of magma is governed, by several factors like:
(i) Temperature (rate of cooling).
(ii) Viscosity of magma.
(iii) Composition of magma.
(iv) Concentration of volatiles.
(v) Pressure (depth of cooling).
Magmas usually consist of a number of components and’ therefore most igneous rocks are multi-component. Unicomponent rocks are extremely rare. The crystallization can be better studied, in a unicomponent, bi-component and multi-component system.
In the study of unicomponent magma, the temperature region in which the generation of crystals is slow is called the metastable region and the region in which the rate of.” crystallization is rapid is the labile region.
According to this study, it has been observed that slow-cooling, leads to coarse grains whereas rapid-cooling leads to the formation of glass. Besides, multi-component rocks are of finer grains than the one of simpler composition.
The important principle, i.e., “melting point of any of the components in a bi-component system is lowered- down to variable extents due to the presence of variable amounts of the other one” is the guiding principle in the crystallization of bi- component magmas. Bi-component magmas show two types of relationship and method of crystallization as
1. Eutectic crystallization.
2. Solid-solution or Mixed-crystals.
In this case the two components having distinct and different freezing points, shows decrease of their freezing points at various stages of their combination. At a particular point of their combination and at a particular temperature these two components crystallize together. This temperature is called the ‘Eutectic temperature’ and the point of combination showing a specific percentage of both the components in the mixture, (i.e., composition) is called the Eutectic-point.
This type of crystallization gives rise to the intergrowth of the two mineral and results in a peculiar graphic-texture.
Sometimes, it is observed that both the components of a binary magma are isomorphous and miscible in all proportions in the solid-state forming homogeneous crystals. The best examples are plagioclases and pyroxenes.
In a temperature composition diagram (to illustrate this type of crystallization) there are two curves-solidus and liquidus. The melting point curve is called the solidus and the freezing point curve is called the liquidus.
Such crystals do not melt at a definite temperature but melting is spreading over a range of temperature, the lower limit is fixed by the solidus and the upper limit by the liquidus.
If a mixture of the two-components having a particular percentage of their combination start cooling down, the crystallization will only start when a particular temperature is reached.
If a magmatic mixture of composition ‘P’ is cooled to 1400°C, the crystals of composition ‘Q’ will begin to form. As the temperature continues to fall the liquid gets enriched with the albite (Ab) component by the withdrawal of anorthite (An) rich crystals.
In this way, the continuous crystallization of solids, their reaction with the melt and consequent changing over to newer solids of ever changing composition continue till the composition of the *°lid phase is the same as that of the original composition of the
In the case of such solid-solution relationship, with rapid cooling the crystals do not change to a different composition, in consonance with the rate of cooling and therefore zones of different composition appropriate to the temperature (at which it was formed) are formed around the early formed crystals. The examples are best found in plagioclase crystals.
This is also known as continuous reaction-relation.
Besides the above, binary magmas also sometimes show paratactic relationship, in systems like forsteritesilica, etc.
Tri component magma:
These are ternary magmas in which the relationship between all the three components may include both solid-solution and/or eutectic characteristics.
Thus the crystallization of magma has been explained by various geologists through laboratory experiments.