Pix: A manned research submersible takes a rock sample from the seafloor near Brazil.
A lost continent off the coast of Brazil may have been found, scientists announced this week.
Granite boulders dredged from the seafloor off the coast of South America two years ago could be remnants of a long-vanished continent, according to Roberto Ventura Santos, the geology director of Brazil's Geology Service.
Santos and his team speculated that the granite—a relatively low-density rock found in continental crust—belonged to a continent that was submerged when Africa and South America drifted apart and formed the Atlantic Ocean about 100 million years ago.
But Michael Wysession, an Earth and planetary scientist at Washington University in St. Louis, noted that granite can find its way onto the seafloor through other means. "There are pieces of granite in the middle of the seafloor that date to about 800 million years ago when we had a snowball Earth scenario and there were large pieces of rock embedded in ice rafts"—mobile glaciers, essentially—"all over the ocean," explained Wysession, who was not part of the discovery. "As those ice rafts were melting, large blocks of rock dropped down all over the seafloor."
Wysession thinks that because the ocean floor has been extensively mapped with satellites, it is unlikely that evidence for any major lost continent will be found. "There's nothing that big that's hidden down there," he said.
The Atlantis-like lost, hidden, or fantastic world is a common theme in fiction. There are J. R. R. Tolkein's Middle Earth and James Hilton's Shangri-La, not to mention Lewis Carroll's Wonderland. The original lost land, Atlantis, was first mentioned by Plato around 360 B.C. According to Plato, Atlantis sank into the earth and drowned beneath the seas. Real continents rarely disappear in such dramatic fashion. "Continents by definition are made of low-density rock and cannot be subducted deep into the earth," explained Staci Loewy, a geologist at the University of Texas at Austin.
Nonetheless, there are real "lost lands" like the Brazilian "Atlantis" that have disappeared from view because of rising seas or the geological upheavals of plate tectonics and erosion. "Parts of continents can be worn down by erosion, and fragments can be broken off and isolated as microcontinents when larger continents break apart," Loewy said.
Here are some actual "lost lands" discovered by science.
Pangaea
A supercontinent believed to have formed around 300 million years ago, Pangaea was an enormous landmass that later broke up to eventually form the continents we know today.
Scientists now think several other supercontinents—such as Kenorland, Columbia, and Rodinia—existed before Pangaea, but the shapes of these ancient land masses are unclear.
Rodinia, for example, was a supercontinent thought to have been formed about one billion years ago; it's believed that it subsequently broke apart to form Pangaea.
"Those pieces are now part of the modern continents, but they have been significantly altered by one billion years of plate tectonics and erosion such that reconstructing the supercontinent of Rodinia is very difficult," explained Loewy.
While they appear stationary, Earth's landmasses shift around over geologic time, carried across the planet's surface by the slow, grinding movement of enormous, shell-like plates.
"The surface of the earth is made up of a rigid layer called the lithosphere; the lithosphere is broken into numerous pieces referred to as tectonic plates," Loewy explained.
"These plates move around the surface of the Earth, colliding into each other, creating mountains such as the Himalaya and Andes; pulling apart from each other, creating volcanic ridges in the middle of oceans like the mid-Atlantic Ridge; and sliding past each other, such as in the San Andreas Fault in California."
Mauritia
Scientists earlier this year announced that they had found evidence of a drowned "microcontinent" off the coast of Africa, near the island of Mauritius.
Sand grains from Mauritius's beaches were found to contain fragments of the mineral zircon that were between 660 and 2 billion years old—far older than the island itself.
One theory is that the sand grains are remnants of Mauritia, a lost microcontinent that once existed off the coast of Africa and which was submerged when India broke apart from Madagascar about 85 million years ago.
Microcontinents are shards of land broken off from continents and supercontinents. The distinctions among the three aren't clear-cut, however, and labeling a landmass a continent or microcontinent can be arbitrary since there are no precise size requirements for each term.
New Zealand, for example, is actually part of a large continental structure that includes the Campbell Plateau. "It's not all that different in size from Australia, but because most of it is underwater, we call Australia a continent and New Zealand an island," Wysession said.
Microcontinents can also merge into larger structures. For example, "the north African edge of the supercontinent Gondwana broke up into slices like the pieces of an apple, and each of those [microcontinents] moved north to form southern Europe," explained Louis Jacobs, a paleontologist at Southern Methodist University in Dallas, Texas.
Beringia
Though Asia and North America are now separated by a thin strait, it is very shallow—about 150 feet (46 meters) deep—and when sea levels are low, such as during ice ages, the two continents are connected by a land bridge known as Beringia.
According to a controversial theory, humans heading east after leaving Asia some 40,000 years ago found their way blocked by glaciers and were forced to settle in Beringia for thousands of years until conditions thawed enough for them to continue to North America.
Less contentious is the theory "that the Clovis people came over from Siberia to North America about 14,000 years ago," Wysession said.
Scotland's Hidden Landscape
In 2011, geologists studying ocean-mapping data stumbled upon a previously unknown landscape now buried beneath more than a mile of marine sediment off the coast of Scotland.
The hidden landscape, which had an estimated area of about 3,861 square miles (10,000 square kilometers), had furrows cut by rivers and peaks that were once part of mountains.
Scientists think it was briefly elevated above the waves by geological processes about 55 million years ago but became submerged again after about 2.5 million years.
No comments:
Post a Comment