Stories tagged igneous rocks

Dec
30
2008

Lava fountain: From the collections of the Hawaiian Volcano Observatory
Lava fountain: From the collections of the Hawaiian Volcano ObservatoryCourtesy USGS/Photo by J.D. Griggs
A drilling company in Hawaii tapped into a magma chamber giving scientists a first-hand view of geological processes never seen before.

The fortuitous discovery happened in 2005 when workers from Ormat Technologies, Inc were drilling in the Kilauea basalt fields off the Big Island. It took three years to analyze the data that was presented earlier this month at the annual meeting of the American Geophysical Union in San Francisco, CA.

The geothermal company had been searching for heat sources to create electric power for the Big Island when the drill suddenly broke through hard rock into molten rock. The magma chamber is thought to have been created by Kilauea activity during a 1950s or possibly even earlier in the 1920s.

“If we had hit molten basalt it would not be a big surprise,” said William Teplow, a geologist for the company.

But instead, the drill punched into a magma chamber just 2.5 kilometers beneath the ocean floor. While rising up the drill hole the top layer of red hot magma cooled quickly solidifying into a glassy rock, cuttings of which were gathered for examination. Looking at the shards of fresh rock, Teplow knew something was very unusual. Instead of the normal inky black glass that normally formed out of basalt, the new cuttings were clear and without color.

“It was very striking and we knew immediately that we had some anomalous body of rock,” Teplow said.

The samples were brought to geologist Bruce Marsh at John Hopkins University in Baltimore for study. Marsh determined they formed from dacite, a magma with a chemical composition between that of basalt (an extrusive rock that makes up most of the ocean crust) and granite (an intrusive rock which makes up most of the continental crust). The difference between the rock-types depends on the amounts and kinds of chemicals and minerals present.

Magma is subterranean molten rock that can reach temperatures as high as 1900 degrees Fahrenheit. When molten rock reaches the surface it's called lava.

"This is unprecedented; this is the first time a magma has been found in its natural habitat," Marsh said. "Before, all we had to deal with were lava flows; but they are the end of a magma's life. They're lying there on the surface, they've de-gassed. It's not the natural habitat.”

MAGMATIC DIFFERENTIATION

As magma cools different minerals precipitate out at different temperatures in a process called magmatic differentiation. This means rock that forms early in the cooling process is chemically different from the rock that forms during the middle or end of the process. Differentiation also depends on other factors, such as the melt’s original composition, contamination by wall rock encountered in its ascent, replenishment of the magma chamber, intermixing of different magmas, and the amount of volatiles (e.g. water and carbon dioxide). Watch an animated illustration here. The process is also known as Bowen’s Reaction Series from geologist Norman L. Bowen’s experimental studies in mineral crystallization in the early 20th century.

Essentially what happens is early in a magma’s cooling stage the elements magnesium and iron crystallize into mafic rocks (basalts and gabbros) forming the dense ocean crust. As the magma cools further, felsic rocks (high in feldspar and silica) form into the lighter granitic continental crust. In the theory of plate tectonics when an oceanic plate collides with a continental plate, the denser (mafic) oceanic crust slides beneath the lighter (felsic) continental crust in a process called subduction. The subducting material remelts into the mantle and the process starts again.

The discovery of the material could be the first time the actual process of differentiation of continental rock from older oceanic basalt has been observed in nature.

"As scientists, we've hypothesized about the nature and behavior of magma in literally countless studies, but before now, the real thing has never been found or been physically investigated in its natural habitat within the earth," Marsh said. “It's the difference between looking at dinosaur bones in a museum and seeing a real, living dinosaur roaming out in the field."

LINKS
Earthmagazine.org story
BBC website story
Origin of magmas
Videos of Kilauea volcano
More about magmatic differentiation
Hawaiian Volcano Observatory