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GEOLOGY

The exposed depths of the Earth

Underwater mapping helps scientists tell the story of the unusual origin of the Saint Peter and Saint Paul Archipelago

Brazilian Navy Aerial image of Saint Peter and Saint Paul: islets formed by rocks that originated more than 10 km deepBrazilian Navy

Thomas Campos knows the Saint Peter and Saint Paul Archipelago like the back of his hand. The Recife-born geologist, a professor at the Federal University of Rio Grande do Norte (UFRN), has already paid 17 visits to these five rocky islets, whose total area is smaller than a soccer field. His trips to the archipelago, which lies in the middle of the equatorial Atlantic Ocean about 1,500 kilometers (km) from the Brazilian coastal city of Natal, began in 1999 as part of the Pro-Archipelago Project supported by the Brazilian Interministerial Commission for Sea Resources (CIRM). Since 1998, the CIRM program has kept these inhospitable rocks inhabited continuously by two to four people—generally geologists, biologists, oceanographers or meteorologists—who conduct research among the birds that are the islands’ only other inhabitants: viuvinhas, or tyrant-birds,  and atobás, or web-footed boobies. This scientific activity serves a strategic objective. Under international law, only if Brazilian citizens continually inhabit these islands will the country be entitled to explore the 200 nautical miles surrounding them—the Exclusive Economic Zone (EEZ)—in this part of the Atlantic richly populated with pods of albacore, tuna and other fish of great commercial value.

During this year’s visit, Campos showed the rocks of the archipelago to his Italian colleague Daniele Brunelli of Italy’s University of Modena and Reggio Emilia, who is a specialist in Atlantic Ocean floor geology. “You kill me!” Brunelli exclaimed as he viewed reefs of peridotite, a rock typical of the upper mantle of the Earth’s interior that lies six kilometers beneath the ocean floor. Mantle rocks rarely emerge onto the surface of the planet. Brunelli had seen this type of rock before in the Atlantic, but from the windows of submarines while exploring fissures at more than four kilometers beneath the ocean’s surface. Of all the oceanic islands in the world, only Saint Peter and Saint Paul are made of rocks that came from the upper mantle to the Earth’s surface—and are still connected to the mantle.

In January and February 2013, Campos and Brunelli took part in the COLMEIA Cruise, an expedition aboard the French naval vessel L’Atalante, as part of a team of European and Brazilian researchers headed by Márcia Maia, a geophysicist associated with France’s National Center for Scientific Research (CNRS). The mission of the voyage, as conceived by Maia and oceanographer Susanna Sichel of the Universidade Federal Fluminense (UFF) in Niterói, was to map hundreds of kilometers of underwater terrain around the archipelago, using high-precision sonar devices and other geophysical apparatus. Based on the data collected on the voyage, Maia and her team think they have shed light on the mystery of how these islets began to emerge from the ocean floor 11 million years ago.

The conclusions of this study, presented in the journal Nature Geo-science in July 2016, solve a nearly 200-year-old geological enigma. In 1832, English naturalist Charles Darwin was one of the first explorers to note that the rocks of this archipelago, unlike those of the Fernando de Noronha Archipelago, were not volcanic in origin. Like most of the Earth’s oceanic islands and the rocky base of the oceans (oceanic crust), Fernando de Noronha is formed of layers of rocks that originated from volcanic activity, such as basalt and gabbro. “During the course of the 20th century, it became clear that the rocks of Saint Peter and Saint Paul were different, and that they came directly from the mantle,” Maia explains. “But nobody understood how they had uplifted nearly 10 km and emerged above the surface of the ocean,” she says.

After graduating from Rio de Janeiro State University (UERJ) in 1983, Maia pursued her scientific career in France, studying islands and underwater areas of the Pacific, Indian and Atlantic oceans. She explains that, while sections of continental lithosphere can date back as far as 4.5 billion years, the oceanic lithosphere is continuously renewed. This enables scientists to determine what has happened in just the last 200 million years.

The Atlantic, for example, began to form 170 million years ago (see Pesquisa FAPESP Issue nº 248). In the Earth’s interior, when the rocks of the upper mantle reach a temperature of 1,300 ºC at about 100 to 200 km beneath the surface, they behave like a viscous fluid that moves slowly over the course of millions of years. Changes in the currents of this fluid about 170 million years ago began to fracture the interior of the supercontinent Pangaea, which then broke apart into the pieces that gave rise to the present-day continents. That rupture opened up enormous valleys in the interior of Pangaea. As the rocks rose towards the surface, they underwent decompression and partially liquefied, creating magma expelled by volcanos in the center of the valleys. As the movement of the tectonic plates elongated the valleys, the valley floors were filled in with the rocks that solidified from the magma. Over time, the valleys increased in size and were eventually inundated, creating an ocean.

This opening of the Atlantic is an ongoing process, with new pieces of oceanic crust still being formed to this day. The volcanic activity is concentrated along the Mid-Atlantic Ridge, a chain of volcanos and tectonic faults that extends from north to south in the ocean and divides it more or less down the middle (see illustration). The oceanic crust forming there is causing the Brazilian coastline to move away from the coast of Africa at an average rate of 3.4 centimeters per year.

The oceanic crust is not being created uniformly, however. From north to south, the Mid-Atlantic Ridge is sliced by what are known as transform faults, which move the north-south axis of the chain alternately eastward and westward. The largest transform faults are located in the Central Atlantic, where the north-south axis of the ridge shows its greatest displacement. The so-called St. Paul system is a group of four faults that have moved the Mid-Atlantic Ridge 630 kilometers to the west. The five islets of Saint Peter and Saint Paul are the peaks of a chain of underwater mountains 3.5 km high, 30 km wide and 200 km long, located along the westernmost fault of the St. Paul system. The mountain range has been named the Atobá Ridge, in honor of the local birds.

The sonar equipment on the COLMEIA Cruise delineated the contours of the Atobá Ridge and the fault on which it lies, completing work begun in 1998 by the first Franco-Brazilian expedition. On that occasion, the researchers used the French submarine Nautile—the same one that identified the wreckage of the Titanic in 1987—to explore the ocean floor in that region. “They had to make 13 dives in order to produce a small profile of the Atobá Ridge and collect rock samples,” Sichel recalls.

Underwater expeditions during the 1980s had already observed an unusually large amount of peridotite on the floor of the Central Atlantic, instead of the volcanic rocks they had expected to find. Ocean floors are generally formed by rocks originating from hotspots in the mantle that liquefy as a result of high temperatures and reduced pressure, as is occurring along the Mid-Atlantic Ridge. The liquefied rocks then undergo transformations and are expelled through underwater fissures and volcanos. The small number of volcanic rocks found in this part of the Atlantic led the researchers to assume that the mantle there was likely colder than in other parts of the planet. These rocks, partially maintained in solid state due to the milder temperatures, were probably exposed directly when tectonic forces opened up faults in the region’s oceanic crust.

Peridotite from the mantle is denser than volcanic rock, and its density decreases when it comes into contact with ocean water and reacts chemically with it. The reduction in density, however, would never create enough force to raise the Atobá Ridge, according to the researchers.

Thomas Campos / UFRN Cracks in peridotite collected in the archipelago are visible to the naked eye: signs of compression forces exerted on the rockThomas Campos / UFRN

Profiles of the deep 
Additional data known as seismic profiles, obtained on the most recent expedition, helped the researchers unravel the mystery. Profiles are measurements that indicate how vibrational waves pass through the rocks in the Earth’s interior and provide an idea of how they are formed. Seismic profiles conducted in the area around the archipelago revealed that the Atobá Ridge is composed of deformed peridotites. There, the mantle rocks are compressed between two blocks of oceanic crust—one to the south and the other to the north—that are colliding frontally at the same time as one block slides along the other. Changes in the Earth’s magnetic field recorded in the area also indicate that the core of the Atobá Ridge consists of mantle rocks that have been changed little by the ocean water—and that core remains connected to deeper rocks.

The seismic profiles, combined with an analysis of fault morphology, have enabled the researchers to reconstruct the history of the Atobá Ridge. Maia and her colleagues maintain that the westernmost fault of the St. Paul system already existed 38 million years ago and that, around 11 million years ago, changes in the tectonic forces expanded it, exposing the peridotite of the mantle. Nearly a million years later, these tectonic forces changed direction and began to compress the exposed peridotite, like butter squeezed between two slices of bread. “The faults we see in the seismic profiles show that the rock is being squeezed upwards,” Maia says. The origin of the ridge is tectonic.”

She maintains that the compression that formed the Atobá Ridge was driven by the influence of a hotspot in the mantle, located 300 kilometers north of the St. Paul system, between the ridge and the eastern coast of Africa. That hotspot is associated with a high-temperature region of the mantle, which likely increased the production of oceanic crust north of the Atobá Ridge and caused continuous compression on the peridotite massif over the past 10 million years. “The irony of the situation is that the upwelling of the mantle that forms the Saint Peter and Saint Paul Archipelago is caused not by the cold temperature of the mantle in that region, but rather by the proximity of a hotspot.”

This explanation is consistent with the fractures and the structure of the grains of rocks that Thomas Campos observes with the naked eye and under a microscope when he examines the peridotites from the archipelago. It also agrees with the dating results that Campos and his colleagues obtained from marine fossils found there. According to the dating results, published in Marine Geology in 2010, the rocks of the archipelago have been upwelling 1.5 millimeters per year over the last several thousand years.

Scientific article
MAIA, M. et al. Extreme mantle uplift and exhumation along a transpressive transform fault. Nature Geoscience. July 11, 2016.

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