The meeting between lava and sea at La Palma is generating more than just great plumes of smoke. Underwater, the interaction between two such huge masses and with such a difference in temperature is altering the nature and shape of the lava stream itself. Some changes will be short-lived, but others will persist for thousands of years. The most immediate and characteristic phenomenon is the glazing of its external part. But it’s not the only one.
Despite the enormous technical and scientific mobilization on land, sea and air, the underwater conditions complicate the study of lava. The Spanish Institute of Oceanography, which operates from the Ramón Margalef vessel, is taking water samples and performing bathymetry in the area. But there are no underwater cameras installed and the arrival of the unmanned submarine it has is awaited. Still, based on the type of eruption, the two types of lava ejected from the volcano, and what has happened in similar eruptions in the past, scientists can anticipate what is happening with the underwater lava flow.
The first thing that happens when you join lava at more than 800 degrees (it lost about 200 degrees on the way since it left the Cumbre Vieja volcano) with sea water at about 24 degrees is that the current breaks up violently and sudden. But it doesn’t explode (outwards, as happens at the mouth of the volcano). What happens is mostly implosions. Thermal contrast causes solidification as it contracts. At the same time, much of the material most exposed to water breaks and comes off, which are hydroclasts, an underwater version of pyroclasts. The process remains active as long as new material continues to arrive from the volcano. And it’s so fast that liquid rock solidifies without crystallizing.
The chemical composition of lava does not change once underwater, but its nature does. The geology professor at the University of Barcelona Domingo Gimeno points out that “the lava stream lithifies [formação de rocha] and the lithification is so fast that it prevents the minerals present to crystallize”. In lava there is a wide variety of elements from the periodic table, many of them metallic: silicon, oxygen, aluminum, iron, magnesium, calcium, sulfur, titanium… Some of them appear in crystalline form, such as iron sulfate or feldspar. Crystals are a form of organization of matter that, simply put, repeats itself in a regular, organized and symmetrical way.
But for the most part molten rock is an amorphous mass rich in volcanic glass: a solid material but without a crystalline structure. The first analyzes carried out on the lava at La Palma, carried out by scientists from the universities of La Laguna and Granada, show that at least 50% of the material is volcanic glass. Despite the confusion that exists in Spain and other Spanish-speaking countries, glass and crystal are two extremes exposed in the solid state of matter. And the collision with the sea causes much of the lava stream to solidify into glass.
“There is a deafening noise, like millions of broken glass,” says Gimeno. “What happens is that, when it arrives, the sea suddenly cools the current, which breaks up. It ceases to be a solidary whole and expands, accumulating gaps in the glass”, he adds. In geology, volcanic breccias are not fissures or holes, they are rocks formed by the agglomeration of smaller materials, the hydroclasts. Another type of stone that is formed are hyaloclastites, also agglomerations, but with more glassy materials, which resemble obsidian, although not so bright.
The volcano at La Palma intrigues scientists because, being of the Strombolian type, with a permanent eruption of relatively fluid lava peppered with explosive spurts, it does not completely fall into this category. One of the elements that doesn’t fit well is the type of lava. During the first few days the lava stream was very dense, rocky and slow advancing. This is what is known as lava type yy, a name taken from Hawaiian, which means rough lava. But, in recent days, the current has accelerated due to the input of more fluid materials. Low viscosity lavas are called pahoehoe (soft, again from Hawaiian). Both types also influence the final shape of the beach being formed. In this part of the island, the new bottom will be slanted, full of edges and tapering shapes.
Professor José Mangas, from the Institute of Oceanography and Global Change (IOCAG), at the University of Las Palmas de Gran Canaria, recalls that an underwater robot would be needed to observe how the lava advances. “It will adapt to the topography of the bottom”, he comments. On top, says Mangas, “the lava type yy it will form a fresh lava field on the new volcanic platform”. Underneath, “as long as there are new contributions, the front and sides will break, coming out like churros”. Part of them will end up in what the Anglo-Saxons call the wash pillows, which are more or less cushioned depending on the composition of the lava. In the case of the eruption of La Palma, they will be pillows, as they are large stones, very rough and sharp. Another part will break up into the hyloclastites mentioned above.
But from the moment the meeting of lava and the sea causes the first to glaze, the reverse process begins. Glass, despite what it may appear to be, is not so stable and, at least in nature, is doomed to crystallize and cease to be amorphous. “Minerals will form in a short time… days, months and years, like clays, zeolites, oxides. And in thousands of years others such as feldspars and low temperature quartz, epidotes…” recalls the geologist from ICOAG.
sign up on here to receive the daily newsletter of EL PAÍS Brasil: reports, analyses, exclusive interviews and the main information of the day in your e-mail, from Monday to Friday. sign up also to receive our weekly newsletter on Saturdays, with highlights of coverage for the week.