Geologist Unearths Secrets of Continent-Forming Process
The process in which molten rock, or magma, migrates to the planet's surface in meandering columns of molten mush is more involved than geologists had thought, said Bruce Marsh, a Johns Hopkins professor of earth and planetary sciences.
His observation exposing this flaw concerns the nature of pea- size crystals that appear in the solidified magma deposits. The crystals settle to the floors of the molten deposits, distilling and enriching the magma and allowing it to form the material from which the continents are eventually produced. The larger the crystals, the more the magma is enriched. Geologists had postulated that these crystals grew and collected, as a sort of precipitate, in the cooling magma. But Marsh's findings have led him to a much different explanation.
"This model ... has been applied for a hundred years, and it's wrong," Marsh said. "It's very much wrong."
The large crystals actually were transported from great depths by the columns of molten slurry, Marsh has concluded. After discovering that detail about the origin of the crystals, he was able to use the golden-green stones as tracers, tracking the chemical and physical path that the magma took as it flowed to the surface.
The geologist recently completed his third expedition to Antarctica within the past four years. He and his graduate students have collected thousands of pounds of rocks over that time, reaching new conclusions about a process that lies at the very heart of planetary evolution.
Some of his findings are detailed in the February issue of Mineralogical Magazine, published by the Mineralogical Society of Great Britain and Ireland.
Understanding the process is not only a matter of ancient history. The same up-welling of magma is going on right now on the ocean floors, but it cannot be studied in great detail because of its remote location.
Over millions of years, molten rock surged up from deep reservoirs, forming and pushing apart the continents, ultimately making it possible for higher, terrestrial animals to evolve. The fulminating magma also might have played a vital role in the beginning of life on the primeval Earth, as the most primitive organisms congregated around hydrothermal vents on ocean floors.
While the key to gaining more insights into this mysterious process is to analyze past magma flows, most of the solidified formations left behind are either covered with plants and other rocky debris, or they have been partly eroded away. The Antarctic formations offer an unusually complete view of what happened millions of years ago, said Marsh, whose research is funded by the National Science Foundation. Each year, he returns to the McMurdo Sound region of Antarctica and then ventures 100 miles inland to the Transantarctic Mountains, a windblown place 800 miles from the South Pole. His goal is to understand the physics and chemistry of magma; specifically, where it comes from and precisely how it is brought to the surface.
The continent is nearly the size of North America, yet 98 percent of it is covered with snow and ice. It's the other 2 percent Marsh is interested in; bare rock is a requirement for geological study.
Frozen in time throughout the spectacular Antarctic cliffs are bands of magma, called sills, because similar material in England once was used to make window and door sills. It was deposited in Antarctica 175 million years ago, around the time a supercontinent that geologists refer to as Gondwana broke up to form South America, Africa, Antarctica and Australia.
After analyzing the way that the largest crystals are distributed in these sills, Marsh has learned that geologists have been profoundly wrong in their assumptions about how magma crystallizes on its way to Earth's surface.
Marsh likens the siphoning of crystals in magma to domestic plumbing.
"If somebody is working on the pipes of your house and you turn on the faucet hard, you get sand, junk," Marsh said. "If you turn on the faucet just a little bit, you don't get any of that junk." Similarly, fast-flowing magma pulls up large, older crystals, grown long before the specific eruptive event that brought them to the surface. Those old crystals have long been misidentified as new crystals, formed after the magma has come to rest. But the newer crystals are actually much smaller, about the diameter of a hair or thin thread. "This has confused us for a hundred years, making us think that magmas chemically change and build continents in ways that they don't," Marsh said.
Marsh has combined field work, theory and laboratory data to design a new model of how magma moves and crystallizes. Geologists had long adhered to the idea that the magma was injected, as if with a syringe, from its deep source to the Earth's surface. They assumed no crystals were present during the trip.
But Marsh has found that the mushy magma is not injected to the surface; it actually is pushed up through a series of chambers, picking up crystals of various sizes along the way and depositing those crystals in pools that solidify as magma sills. One piece of evidence supporting his theory is that some sills contain no unusually large crystals. All of their crystals were born and grew after the magma came to rest, and all of those crystals were too small to settle and enrich the magma enough for it to become continental material.
The reason: like the slow-moving water in the domestic-plumbing analogy, the magma was not flowing with enough force to drag the older crystal debris to the surface.
Marsh spent most of January in Antarctica, just as he has done since 1993, chipping specimens away from bare rock walls. He and graduate students Jon Philipp and David Noe collected 1,250 pounds of rock specimens this time.
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