In general, there are three main factors that control the viscosity of a magma: the chemical composition, the degree of crystallinity and the emission temperature. They are variables related to each other to a large extent.
Knowledge of the composition of magma is very important during the development of an eruption. This factor, together with the total gas content, marks whether an eruption is effusive (a mere emission of lava from a fissure on the earth’s surface) or explosive, and how explosive it can be in the latter case.
To classify and give a name to volcanic rocks, and the magmas from which they come, total rock analyzes are used, that is, the sum of the elements that we call the majority, which in general add up to more than 97% of the total the rock, expressed in oxides.
Specifically, the diagram called TAS, English acronym for total alkali content, is used, that is, the sum of sodium and potassium oxides. This is expressed in the ordinate, compared to the silicon oxide expressed in the abscissa.
Silicon oxide is the largest constituent of magmas. It ranges from values that are barely close to 40% of the total in the original melts of the earth’s mantle to those that we call more evolved, which reach 75% by weight of the total.
Silicon is arranged in magmatic liquids, even before crystallizing and forming minerals, into tetrahedra. Silicon is placed in the center of the tetrahedron, surrounded at the vertices by four oxygens. These polymerize being united by the oxygens, a fact that makes the greater amount of silicon the more viscous the magma.
The magmas that we call primitive, those that come directly from the partial fusion of the rocks of the earth’s mantle, as is the case with the eruption of the island of La Palma and most of the Canarian magmas, are called basaltic. They are the ones that contain the least amount of silicon, so they provide the least viscous lava.
As the magmas cools, the minerals (mainly silicates) crystallize and these can separate from the magmatic liquid due to their higher density and be stored in magmatic chambers. Meanwhile, the residual liquid can rise towards more superficial sectors of the earth’s crust and, if necessary, emerge on the surface in the course of an eruption.
A basaltic magma has the order of 0.5 to 1% of water and other gases in solution, but as the first minerals crystallize at high temperature, since they are all anhydrous, the residual liquid becomes enriched in these gases.
Influence of composition and temperature
Thus, during the crystallization process of different anhydrous minerals (olivines, pyroxenes, plagioclase, etc.), magma evolves in its composition, enriching its content in water and gases and in silicon. Upon reaching the surface, the gas decompresses and separates from the magmatic liquid, forming bubbles, as happens when we open a bottle of cava and the CO₂ separates.
If the gas content of the magma is low, and the basalt liquid is not very viscous, the gas present hardly favors the push of the magma towards the surface, while it is easily separated from it. So we have effusive eruptions with predominantly lava emissions.
If the content of silica and gases is high, the presence of gases is much higher, so that the bubbles are larger and can break up the magma into drops that are violently ejected into the atmosphere, producing an explosive eruption.
The magma droplets that rapidly cool on their way through the air until they solidify – and the rocks of the eruptive conduit that can be torn from it and carried away by the violently erupting magma – constitute pyroclasts.
In addition, the high silica content already makes magma more viscous, which can cause it to move more slowly through the eruptive duct, forming a kind of plug. This also prevents the gas from being easily eliminated, which can cause that plug to explode due to the effect of the concentrated gas, causing highly explosive eruptions.
Beyond the composition, the temperature makes the magma more liquid – if it is superheated above its melting temperature – or, as has been seen with the advance of the melts and their gradual cooling, more viscous.
In the case of instant cooling by immersion in the sea, the liquid “freezes” and rapidly lithifies without time to crystallize, forming a glass. On the contrary, in subaerial flows, the gradual cooling also allows a gradual crystallization of the magmatic liquid, and with the increase in crystallinity the lava becomes more viscous.
Evolution of lavas on La Palma
The high viscosity and, therefore, thickness of the flows of the first two weeks allow us to deduce that their chemical composition is not that of a basalt, the most typical basalt rock on La Palma, nor an alkaline basalt, but probably a «distilled magma »In or within a mantle / crust boundary magmatic chamber.
The more fluid lavas of the last days do seem to be more primitive, they come more directly from the earth’s mantle without residence time in the crust. All this is nothing new in the eruptive history of La Palma, for example it already happened in the 1949 eruption.
Continuous sampling and analysis of ash (easily sampled) and lava during the eruption allows dangerous variations in magma composition to be detected early. Therefore, it is an effective instrument in order to make decisions about evacuations and protection of professionals who work in the scientific monitoring of the eruption, and in civil protection.
In short, it allows us to know if, at a given moment, highly “distilled” magma begins to arrive from a more superficial magma chamber in the earth’s crust, and the eruptive scenario could become highly explosive.
The systematic and continuous study of the shapes of the ash particles makes it possible to detect whether the explosive system of the volcano is receiving meteoric water from the aquifers. This is extremely dangerous because it can generate pyroclastic phenomena of waves or pyroclastic flows, much faster and more destructive than the action of the falling strombolian pyroclasts, and that the lava flows themselves.
These analyzes can be done by previously trained scientists, directly in the field with a good binocular loupe. It should be systematically supplemented with samples selected by means of a scanning electron microscope in a university center equipped with this instrumentation.
This article has been published in