A Different Kind Of Volcano

The word "volcano" conjures up dramatic images. There are the fiery fountains from Kiluea and the threatening rivers of lava from Mauna Loa in Hawaii. There's the violent explosion and ashy devastation of Mt. St. Helens in Washington. Then there's the froth and dribble from the mud volcanoes in Southcentral Alaska.

Indeed, lying within sight of dormant Mount Wrangell volcano in the eastern Copper River basin, there are a number of features which, for lack of a better term, are called mud volcanoes. These form low cones generally less than a quarter of a mile across. The top of one of these peculiar objects may be covered by a single shallow crater lake, or it may be dotted with a number of smaller bubbling cauldrons. Turbulent gases sputter through the highly saline water and carry mud to the surface where it spills over, runs off and kills the surrounding vegetation.

The nature and origin of the gases from below continue to puzzle most investigators. To complicate matters, there is a sharp contrast between the composition of the gases from the mud volcanoes to the east of Glennallen (closer to Mounts Drum and Wrangell) and those 25 miles to the west just off the Glenn Highway. Inert carbon dioxide is the primary constituent of the gases from the Drum group of mud volcanoes (as those to the east are known) while flammable methane makes up about half of the gases from the Tolsona group to the west.

Why are the gases so different in the two groups of mud volcanoes? And why are the gases coming to the surface?

Scientists believe that the organic gas, methane, probably derives from buried marsh vegetation and coal deposits. It is not clear exactly how the gases with a high percentage of carbon dioxide are being produced. Although they are being expelled from sites closer to volcanic sources, they are not representative of the volcanoes themselves.

In both cases, overlying pressure can be invoked as the agent responsible for forcing the gases to the surface, although different sources for the pressure can be cited. Some scientists believe that the thick volcanic mass of the Wrangell Mountains created a zone of high pressure, as eruptions flowed onto and weighed down the sedimentary basin that bordered the mountains. Other researchers have suggested that the loading of glacial ice and the large lake that filled the Copper River basin during the last glacial advance is responsible.

While scientists ponder the nature of these geological curiosities, people wishing to visit a mud volcano can do so by walking a couple of hundred yards north from mile 173 of the Glenn Highway. There they'll find mud volcano #1 of the Tolsona group. For a geological oddity, it not much to look at, but at least visitors can say they've been there.