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Geology

In the bowels of the Earth

Viscous masses of rock blend together and may escape as lava in volcanic eruptions

FIUR KJARTANSSON/AFPThe E+15 Icelandic volcano: an eruption that is now explained betterFIUR KJARTANSSON/AFP

A mixture of magmas in underground reservoirs is now considered essential to forecast or explain eruptions such as that of the Icelandic volcano in April. According to studies conducted in Brazil and in other countries over the last few years, it now seems that new, viscous, scalding magma from the depths of the Earth reaches older magma that is quietly cooling down in compartments that may be kilometers long. Unable to advance, the new magma becomes stuck in these chambers and starts cooling down, releasing gases and warming up the older material in the process. As a result of the interaction of the two masses of magma, the internal pressure in the chamber rises to the point of forming a hot mass that forces its way through the volcano, overflowing explosively. The gases released from the rocks also escape and stain the sky with smoke that is rich in volcanic rock particles. Thus, it is no longer appropriate to say that a volcano erupts when magma from deep regions of the planet rise up like water in domestic pipes, overflowing and running down as lava along a free path.

“Today, we know that the interaction between magmas and their eventual mixture is the rule,” says Valdecir de Assis Janasi, a professor at the Institute of Geoscience of the University of São Paulo (USP), who has been researching the dimensions, the life span and the internal movements of these compartments called magma chambers, where the magmas meet and transform each other. These studies indicate that new material is constantly re-fed into the chambers and they can provide clues as to what is going on in the deeper, unreachable regions of the Earth. “Until they cool down completely, these magma storing regions can be extremely dynamic,” says Valdecir, who heads a group of researchers from the Institute of Geosciences of USP, the Federal University of Rio Grande do Sul (UFRGS) and the University of Alberta, Canada.

Living chambers
Considered huge pressure cookers that are cooking magma, the chambers of volcanoes such as Yellowstone, in the United States, are drawing more attention and have become the object of constant vigilance: the greater the movement within the chambers, the higher the risk of a catastrophic lava eruption. Surrounded by a national park, Yellowstone is what geologists call a super-volcano. Its eruption might adversely affect the entire planet. In Brazil, there are no more living chambers, to use the words of Valdecir, in which hot and less hot magmas blend together. We only have extinct volcanoes that still release the heat that warms up the waters of spas such as Poços de Caldas, in Minas Gerais state. The nearest living chambers lie under the Andes mountain range, some 20 kilometers deep. “Some have been active for 10 million years, which shows that the lifespan of a magma chamber can be very long,” he says.

EDUARDO CESARMantle rock (peridotite)EDUARDO CESAR

The time it takes for a chamber to close, as a result of the crystallization of the magma, forming rocks, depends on its depth: the closer it is to the surface, the cooler the environment is and therefore the faster the magma cools down. This may not apply, however, when these spaces, even if they are close to the surface, get new, hotter magma, as happened in Iceland in April of this year. One of the approximately 30 volcanoes on this island in the North Atlantic, the Eyjafjallajokull or E+15, as it was nicknamed by US geologists, suddenly acquired world fame when it covered northern Europe with a dense cloud of volcanic ash. Valdecir tells us that this was due to the arrival of basalt magma with a temperature bordering on 1,200 degrees Celsius through cracks in the older rocks. The new magma met a different sort, granite magma, which was cooling down at some 700 or 800 degrees Celsius in the chamber. The new magma cooled and the old one warmed up. After they mixed, an explosive eruption began, further strengthened by the interaction with the ice that covered the volcano. Dense clouds of smoke rose up into the sky, laden with volcanic rock particles that may damage aircraft turbines.

As the Icelandic volcano continued to release black smoke, geologist worldwide debated in blogs how a volcano seen as cold and inert could have become so intemperate. The hypothesis that gained strength was that small earthquakes might have rendered the circulation of new magma easier and allowed it to mix with the resident magma. There were also discussions about where the volcano’s chambers should be and how much of the magma coming up from within the Earth they could hold. The chambers, they estimated, could be some two to five kilometers beneath the surface and possibly connected to those of other volcanoes on the island.

In Brazil, one can see the results of magma mixtures in the gray granite flooring of the older subway stations in São Paulo, for instance. Valdecir and Adriana Laves, a researcher in his group that was recently hired as a professor at the Institute of Geosciences of USP, were intrigued by the rocks that looked like spheres within the granite extracted from quarries in Maua, in Greater São Paulo, and how they had been formed. As these stains were even darker than the granite itself, the researchers thought they might consist of another type of rock, such as basalt. However, they were not. They too were granite, with a similar composition, only darker. “The new magma that invaded the resident magma was even hotter and congealed, turning into large spheres that disintegrated and formed smaller spheres called enclaves,” says Valdecir.

Mantle rarities
In Brazil, there are thousands of old magma deposits. São Paulo state alone is probably home to at least 220 of them of just one type, granite magma, which gives rise to granite rocks, according to surveys conducted over 10 years ago. This number might rise, as the field surveys progress. In the municipality of Itu, in São Paulo state, the USP team found at least four old, large magma chambers and not just one, as had been originally envisaged.

In 2007, the USP team started studying this mass of ancient granite magma that crystallized at a depth of about three kilometers and that today, thanks to erosion, is partially on the surface. The preliminary analyses indicated that some 600 million years ago each one of these chambers was the stage of several magma blending episodes, some of the same type and other resulting from basalt injections, as was the case in Iceland. It is possible that these chambers might be connected to volcanism, as they were fairly close to the surface. In more concrete terms, the current town of Itu may have been an outflow spot for lava from the depths of the Earth millions of years ago.

There were also unexpected findings, such as the first fragments of mantle found in São Paulo. In 2006, Valdecir was on the Vermelha beach near the town of Ubatuba, examining surface rocks with a group of 40 undergraduates when one of them brought him a block of pale green rock. “I can’t remember who it was,” the professor tells us. “I said they were olivines, a type of mineral, but it was rather odd and students aren’t satisfied with just any statement. We examined it with a magnifying glass and found that they were fragments of a rock formed by the combination of two minerals, olivine and pyroxene. For the first time ever, we had gained access to the São Paulo mantle. The weirdest thing is that we had already been there before and had seen nothing. Want to see it? Here it is,” he says, getting a greenish rock, slightly smaller than an apple, standing on the corner of a desk covered with papers, maps and rocks.

Vidyã Vieira de Almeira, who is currently with the Brazilian Geological Service, confirmed in his master’s degree thesis that the 10 fragments brought from the Ubatuba beach were samples from the upper mantle, the layer immediately beneath the crust, the outermost layer of the Earth. These rocks must have been formed at a depth of some 60 kilometers and only reached the surface without melting because they got a ride on basalt magma that rose quickly and was rich in fluids. “These magmas rise through gaps that opened some 80 million years ago, after the Atlantic Ocean started forming. In general, there is no way to get hold of mantle rocks unless they are brought up by magma,” says Valdecir. “And having access to this material is critical, because it is in the mantle that most of the magma formation processes take place, or, at least, start.”

“Nobody can see magma chambers in activity, only the outcome: the exposed rocks,” says Valdecir. Hawaii offers a few exceptions. In 1959, magma from the Kilauea volcano filled a depression forming a lava lake 640 meters wide and 135 meters deep. Geologists waited for the lava to cool down on the surface, walked on the lake and monitored the cooling of the magma for years, by means of successive perforations, thereby understanding better what was going on in the magma chambers.

In 2008, as a result of an unexpected perforation in a region near Kilauea, incandescent magma, which was resting at a depth of 2.5 kilometers, rose to the surface. One of the researchers said that seeing magma in this form was “as exciting as finding a living dinosaur playing on a remote island.”

The project
Contributions of the mantle and different crust reservoirs in neoproterozoic granite magmatism in the Brazilian Southeast (nº 2007/00635-5); Type Regular Line of Aid for Research Project; Coordinator Valdecir de Assis Janasi – IG/USP; Investment R$161,773.20.

Scientific article
ALVES, A. et al. Microgranitic enclaves as products of self-mixing events: a study of open-system processes in the Maua granite, São Paulo, Brazil, based on in situ isotopic and trace elements in plagioclase. Journal of Petrology. v. 50, p. 2.221-47, 2009.

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