January 21, 2013
New Type Of Volcanic Eruption Described By Scientists
Brett Smith for redOrbit.com - Your Universe Online
Neither explosive nor effusive–there´s a new type of volcanic eruption that was recently described in the latest edition of Nature Geoscience.According to the U.K. and New Zealand scientists who authored the description, volcanic pumice produced by the Macauley volcano in the southwest Pacific is the result of a previously unarticulated type of eruption.
“By documenting the shape and density of bubbles in pumices generated by an underwater caldera volcano in the southwest Pacific Ocean — the Macauley volcano — we found large differences in the number and shape of “bubbles” in the same pebble-sized samples, different to anything previously documented,” said co-author Ian Wright, from U.K.´s National Oceanography Centre.
“This range of bubble densities distinct in these pumice samples indicates that the lava erupting from the caldera was neither vigorous enough for an explosive eruption, nor gentle enough for an effusive flow,” Wright said in a statement.
Magma inside a volcano is charged with gas, in the same way that a can of soda contains pressurized gasses that bubble up when it is opened. When magma erupts as lava, the pressure is relieved and the gases come out of solution to form small gas bubbles in volcanic rock or pumice. These bubbles are commonly referred to by scientists as ℠vesicles´. In explosive eruptions, the vesicles created by the gas expand so quickly within the caldera–they fragment the magma. The resulting violent eruption eventually cools and degases. In the case of undersea volcanoes, the result is the formation of airy pumice that can float on water.
The pumice formed from explosive eruptions typically contains a signature vesicle pattern and the scientists first suspected the Macauley volcano could be different when they noted that pumice samples from the volcano had a unique vesicle pattern–marked by evenly spread bubble cavities on the inside, and irregular bubbles near their surface.
After analyzing the Macauley samples, the team determined their pumice characteristics were the result of pressure forces unique to underwater volcanoes-- suggesting that if that volcano erupted on land, it would have blown its top.
According to the scientists, the pumice was the result of expanding magma clouds that created buoyant foam within the caldera. The foam then rose up and out of the seafloor, then buoyantly detached from the volcano.
As the buoyant lava clouds rose, the vesicles would have cooled along their exteriors and eventually the molten interior would have continued to expand as the water pressure reduced. The cooling and depressurizing forces eventually lead to the unique vesicle patterns found in the Macauley pumice.
“These processes explain the unique bubble structure seen in the samples analyzed, which could have only occurred with an intermediate eruption style and in an underwater setting,” said Wright. “We conclude that the presence of widespread deposits of pumice on underwater volcanoes does not necessarily indicate large-scale explosive volcanism.”
The team suggested the newly described type of volcanic eruption be called ℠Tangaroan´, after their research ship and the Maori god of the sea.