To predict what may happen in the future, Brazilian researchers studied the movements of the Earth over the last 15 million years in the southeast of Brazil. Supported by the theory of plate tectonics and continental drift, which today has steers geosciences, they gathered data showing the past and the present, as well as tracing out the geological future of the region.
These results help to ensure the geological stability of construction work – especially highways, tunnels, hydroelectric and nuclear power plants – and to predict disturbances such as landslides, as well as furnishing clues for the discovery of mineral deposits and water.
Completed in June with the aid of financing of R$ 395,000 from FAPESP, the Neotectonic, Morphogenesis, and Modern Sedimentation in the State of São Paulo and Adjacent Regions project covers an area of 400,000 square kilometers including the state of São Paulo and part of the states of Paraná and Rio de Janeiro, plus the south of Minas Gerais. A group from the Department of Petrology and Mineralogy of the Geosciences Exact Sciences Institute of the State University of São Paulo (Unesp) at Rio Claro undertook the project over four years. Professor Yociteru Hasui coordinated the project.
In the quiet streets of Rio Claro, no one imagined the impressive accounts that professor Hasui has recorded in his files, showing the geological movements in the landscape of the Southeast: some parts rise and undergo continuous erosion, while others sink and gather deposits of silt through sedimentation. This produces changes to the relief of the land, the watercourses, the thermal springs, and the soil.
As if they were huge living organisms, the Mar and Mantiqueira mountain ranges rise up, while the western half of the State of São Paulo sinks, changing the relief and altering the course of rivers. A wide band drained by the river Paranapanema, which extends to Rio de Janeiro, has sections that sink and others that slowly rise, something, which was unknown until now, and which will be included in maps and geology textbooks. Rivers adapt to the changes to the relief and follow irregular paths, with canyons, rapids and waterfalls – or they flow calmly meandering through flatlands liable to flooding. They detected many cases where rivers have undergone great detours: in the elbow of the Guararema region, for example, the Paraíba makes an almost 180°-detour.
A dramatic example of a forecast made in the course of the study is that of the Grande river, one of the tributaries of the Paraná: in its headwaters, it flows east, but the slow wearing down of the river Aiuruoca on the Mantiqueira range will eventually capture the headwaters of the Grande, making this tributary of the Paraná flow north and radically altering the area’s environmental conditions. Not to worry, for now, however: this will only happen after a long, still unmeasured geological time – and geological time is long.
Concentrating on the last 15 million years – a blink of the eye in the 4.6 billion years of the planet’s history – the Unesp group employed an innovative set of indirect methods to determine the land’s stability. Hasui’s team also began a more consistent investigation into South American plate tectonics, the rocky raft on which Brazil is journeying. It is moving, like the other plates into which the lithosphere is divided – the Earth’s solid outer layer, including the crust and the upper part of the viscous mantle, which is about 200 kilometers thick. The movement produces earth tremors, volcanic activity, and raises mountains in the Andean region, as well as altering the stability of the continent’s hinterland, including Brazil.
The leader of the Unespo group, with the collaboration of researchers at other Brazilian universities and the University of Freiburg (Germany), Hasui gathers data covering fossilized pollen, records of ancient seismic disturbances (earthquakes), the thickness and type of sediments, the slow growth of mountains and the sinking of regions.
The work is a refinement of the theory of the German geophysicist Alfred Wegener, who explained the origin of the continents based on the movement of tectonic plates (see table). Since it was proposed, this theory has persuaded a growing number of researchers, determined to explain, not just earthquakes, but also volcanism and the formation of mountain ranges as far apart and different as the Himalayas and the Mantiqueira. The theory associates these phenomena to the wear and separation of the edges of these plates.
And, until the end of the 80s, says Hasui, it was thought that the inside of these plates, enormous rocky rafts floating on the mantle, was made up of “static and calm regions”.
The problem is that observation only confirms this theoretical prediction in part: it was not enough. Why, for example, do seismic disturbances occur inside the plates, far from the edges, as is the case of the regions of Pinhal or São Luís do Paraitinga in São Paulo state? This meant that unknown mechanisms were at work producing these tremors and they needed to be satisfactorily explained.
Research into the inside of the plates picked up speed in the 90s. Researchers started from the old clues, the geological fissures. The essential question, still not fully answered, is: what is the structure of the subsoil and how do the rocks behave in each region? Hasui says that, in order to clear up these doubts the inside of the plates have to be studied from a different standpoint from that used to study the edges and also investigation needs to be confined to more recent times.
In search of data to fill in the great gaps in space and time, the team brought together geologists, geomorphologists, geophysicists, soil specialists, and specialists in dating sediment and in information technology. From the combination of studies the working data emerged, to be used in new projects, since the area being studied covered not even 5% of Brazil.
The measurements obtained are almost all indirect and fit into the puzzle that Wegener began assembling almost a century ago. To discover the growth rate of the Mantiqueira and Mar ranges, for example, the profile of the lower areas was examined. This was where sediment brought from above by wind and rain had accumulated. So dating the pollen using the carbon-14 technique enabled to attain the age of the sediment layers. Thence, through relatively straightforward calculations, the material lost through erosion and the ratio between the rising and sinking of the areas under consideration was assessed.
Information garnered by seismographs (earth tremor sensors) enabled the rocky profile of each region being studied to be traced. Sets of samples and observations of the behavior of certain areas show the effects of pressure and rock movements, which, for a meticulous researcher, can unveil facts accumulated over millions of years.
The very topography of a region, such as the mountain ridge near Rio Claro, bears witness to the geological processes begun in remote times and which have still not stopped. Geological faults, responsible for a good part of the topographical profile, result from the forces breaking up the single original continent, Pangaea. Other events came on the scene, however, and the Brazilian group took part in their discovery.
The South American and African plates, for example, separated because of the impact of immense rocky projectiles discharged from inside the Earth. The discharges took place at the limit between the soft mantle and the nucleus, which is the solid structure made up essentially of iron and nickel which are dense and heavy materials. This frontier from where the projectiles came, whose size is up to 200 kilometers wide and 700 kilometers high, is the “D Layer”, a wrinkled sphere whose profile recalled the irregular teeth of a saw disk.
One of these projectiles or fossil plumes discharged around 130 million years ago, without the strength completely to break through the lithosphere, was located under the Paraná basin, in a work recently undertaken by the geophysicist Marcelo Souza de Assumpção (see Pesquisa FAPESP 53), of the Astronomical and Geophysical Institute (IAG) of the University of São Paulo (USP).
Even without erupting fully, this fossil plume was responsible for spillage of lava, which as they cooled and changed over time, shaped the rocks that form the foundation of the Cuesta Basáltica – a mountain ridge between Rio Claro and São Carlos – and break down into the red soil of the region. Thick and fertile, this earth was the basis of many a fortune in the west of São Paulo state and the north of Paraná, where coffee growing prospered for almost a century.
The presence of the Paraná plume enabled an important part of the classical theory to be corrected. Until recently, it was thought that only the crust, the Earth’s external layer about 40 kilometers thick (the earth’s radius at the equator is 6,378 km), slid over the mantle. Assumpção’s work showed that part of the mantle also slid because of the effect of convection – the currents of heat that rise from the hot center of the Earth. The force that feeds the convection currents combines the residual heat from the formation of the Earth with the spontaneous disintegration of radioactive atoms, such as uranium, and even gravitational pressure.
Hasui’s group could not afford to leave any clue unexamined, if it wished to answer satisfactorily to the doubts, especially those of engineers responsible for large-scale works. The geological stability of each region depends on the force to which the rocks are submitted by the westward drift of the plate – at a speed of 6 centimeters a year.
Although they seem solid and unbendable, the rocks forming a tectonic plate such as the South American one, drift because of the seismic activity of sections along the fault lines, that are usually leftover from periods as old as the Precambrian – the geological time from when the Earth was formed about 570 million years ago. These shifts and the earthquakes caused by them put geological stability at risk and may, for example, affect certain buildings. Seismic activity may even take place in the calmness of the interior of the plate that is as powerful as that at the edges, explains the geologist José Augusto Mioto, of Hasui’s team.
In Brazil, the construction of large reservoirs for hydroelectric plants, especially since the 70s, led to deeper knowledge of these conditions, in cases called “induced seismism”: the reservoirs in São Paulo state at Paraibuna and Capivara, for example, with 2.95 billion and 10.5 billion cubic meters of water respectively, produced seismic activity of this sort, tells Mioto. What happens in these cases is not precisely a consequence of the weight of water on the region’s rocks, he explains, but rather the combined effect of the weight and the lubrication of the faults at places subjected to great pressure by the twisting of the plate. With the fault lubricated, the segments of rock slide against one another and release pressure in the form of earthquakes.
The information gathered is broad enough for its use not to be confined to the safety of construction work. Investigating the structure and the composition of the rock can also disclose mineral deposits. “The growing demand for reserves of drinkable water is a good reason for work of this sort”, argues the geologist Norberto Morales, another of Hasui’s associates, quoting the example of the hydrographic basin of the river Piracicaba, already in a critical position in terms of regional supply.
Researchers believe that their work has a great deal to do with contemporary standards of the use of the land. Pioneering use, almost always based on farming and livestock raising, did not require detailed studies and usually reserves of surface water were enough to supply the herds plus some temporary irrigation. The present standard, in the high-valued areas of the Southeast, increasingly depends on heavy construction work and a considerable supply of water, assesses Mioto.
Lack of awareness of the behavior of the ground in works like Itaipu and even in the nuclear power plants of Angra dos Reis (RJ), built in an area that coastal Indians had long identified as unstable (they called the place Itaorna, which means “rotten rock”), does not present any serious risks: the structures of these gigantic works were designed to withstand extreme conditions, according to the researchers. The inconvenient thing is that very large structures imply very high cost, as was the case of Angra’s foundations.
The researchers are optimistic about the work, and not just because of the immediate results: training people is also taken into account.The work has already enabled ten doctoral, three masters’, and two postdoctoral theses to be completed. There are also another five well-advanced doctoral theses.
The purpose of the project – to unravel the evolution of the physical surroundings in recent geological times, to consolidate a new line of investigation, to train people to develop it and set up a dating laboratory for analyzing traces of fission – which has been introduced and is operating – has been achieved. Through it, Geology has gained in knowledge of the recent history of the Earth and Physical Geography moves forward in the understanding of the processes that sculpt the relief of the land and the drainage network.
The challenge of integrating all the fields involved, Hasui and his associates recognize, goes through the availability and use of new technologies, especially three-dimensional computer modeling, which is the area involving interaction with the University of Freiburg. The geologist Hans Dirk Ebert, in charge of this work, is the liaison with Freiburg, known since the end of the 18th century as the capital of European geology. It is through interaction of this sort that the Rio Claro researchers, like Alfred Wegener, seek scientific integration enabling them to progress further in the new stages of unveiling the past and giving a snapshot of the future of the Earth.
Profile:
Yociteru Hasui, 62 years old, graduate in Geology (1961) at the USP’s School of Philosophy, Science and Arts, where he took his doctorate at the Polytechnic School (1967) and was a member of the teaching staff (1989) at the Geosciences Institute. Retired since 1995 as a professor at Unesp-Rio Claro, he is a full member of the Brazilian Academy of Science in São Paulo. He is the editor of Unesp’s magazine Geociências.
Project: Neotectonic, Morphogenesis, and Modern Sedimentation in the State of São Paulo and Adjacent Regions.
Investment: R$ 395,000
