U. BUFFALO (US) — A marriage of magma and water below the surface of the earth is behind a specific type of volcano that erupts only once before dying.
Maar-diatremes, belonging to a family of volcanoes known as monogenetic volcanoes, are not well-known and largely misunderstood, but are actually the most common form of land-based volcano on the planet, researchers say. Though they erupt only once, some eruptions can last for years.
“The hazards that are associated with these volcanoes tend to be localized, but they’re still significant,” says Greg Valentine, professor of geology at the University at Buffalo and lead author of a study in the journal Geology.
“These volcanoes can send ash deposits into populated areas. They could easily produce the same effects that the one in Iceland did when it disrupted air travel, so what we’re trying to do is understand the way they behave.”
Previously, scientists theorized that maar-diatreme eruptions consisted of a series of underground explosions that took place as magma reacted violently with water. With each explosion, the subterranean water table would fall, driving the next explosion even deeper.
Valentine and volcanologist James D.L. White of New Zealand’s University of Otago have cited new geological evidence to propose that maar-diatreme eruptions consist not of ever-deepening explosions, but of explosions occurring simultaneously over a range of depths.
Under the new paradigm, deep explosions break up buried rock thousands of feet below ground and push it upward. Shallow explosions eject some of the debris from the volcano’s depths, but expel far larger quantities of shallow rock.
The new model fits well with recent field studies that have uncovered large deposits of shallow rock ringing maar-diatreme volcanoes, with only small amounts of deeper rock present. This was the case, for example, at two sites that Valentine examined at the San Francisco Volcano Field in the Arizona desert as reported in the Journal of Volcanology and Geothermal Research.
White and Valentine’s description of the eruptive process also corresponds well with White’s investigations into the “plumbing” of maar-diatreme volcanoes, the conduits that carry magma toward the surface. These conduits become visible over time as a landscape erodes away, and the main “pipe”—called a diatreme—often shows evidence of explosions, including zones of broken-up rock, at a range of depths.
Such findings contradict the older model that White and Valentine argue against. According to the old model, Valentine explains, ever-deepening explosions should cause shallow rocks to be ejected from the mouth of the volcano first, followed by deposits of deeper and deeper rock fragments. But this isn’t what scientists are finding when they analyze geological clues at volcanic sites.
The old model doesn’t account for the fact that even when scientists find deep rock fragments at maar-diatreme sites, these bits of rock are mixed mostly with shallow fragments. The old model also doesn’t match with White’s observations indicating that explosions occur at essentially every depth, Valentine says.
The new model uses the strengths of the old model but accounts for new data, giving scientists a better basis for estimating the hazards associated with maar-diatreme volcanoes.
Source: University at Buffalo