An article, by Alexandra Witze, in Nature News says that the ground underneath Yellowstone National Park, located in the western United States (chiefly in Wyoming), contains a large magma reserve, but this is not the greatest geological risk in the region; earthquakes are.
The reservoir of molten rock underneath Yellowstone National Park in the United States is at least two and a half times larger than previously thought. Despite this, the scientists who came up with this latest estimate say that the highest risk in the iconic park is not a volcanic eruption but a huge earthquake.
Yellowstone is famous for having a ‘hot spot’ of molten rock that rises from deep within the planet, fuelling the park’s geysers and hot springs1. Most of the magma resides in a partially molten blob a few kilometres beneath Earth’s surface.New pictures of this plumbing system show that the reservoir is about 80 kilometres long and 20 kilometres wide, says Robert Smith, a geophysicist at the University of Utah in Salt Lake City. “I don’t know of any other magma body that’s been imaged that’s that big,” he says.
Smith reported the finding on 27 October at the annual meeting of the Geological Society of America in Denver, Colorado.Yellowstone lies in the western United States, where the mountain states of Wyoming, Montana and Idaho converge. The heart of the park is a caldera — a giant collapsed pit left behind by the last of three huge volcanic eruptions in the past 2.1 million years.
Yellowstone’s last mammoth volcanic eruption took place 640,000 years ago. Since then, some 50 to 60 smaller eruptions have occurred, with the most recent of these about 70,000 years ago. A much more likely risk than volcanoes, says Smith, is posed by earthquakes of magnitude 7 or greater like those that have struck the region in modern times. “They are the killer events which we’ve already had,” he says. For instance, the magnitude-7.3 Hebgen Lake earthquake that hit near Yellowstone in 1959 killed 28 people.
This area of the western United States is being stretched and thinned by geological forces, causing the crust to fracture in large quakes. The risk of more of these quakes occurring remains high, says Smith, making them a much bigger problem than any chance of a mammoth eruption.This seems surprising when one considers that Yellowstone—almost the whole park of 9,000 square kilometres—is one gigantic super-volcano; and an active one, too. The whole park is a caldera. According to some scientific data, the eruption cycle is every 600,000 years, thus making a large eruption imminent. In a National Geographic article (2012, Sept 20), Richard A. Lovettt says: “Yellowstone's next major eruption will probably be centered in one of three parallel fault zones running north-northwest across the park, a new study predicts.” It might happen, but in ten thousand years, a short period in geological times.
Such sums up the danger posed by having a national park on a historically active volcanic site. Yet, as the article says, of the many geological disruptions that Yellowstone faces, including hydrothermal explosions, rockfalls and earthquakes, the last is the one that that carries the greatest potential of taking place. This does not make the risk of a volcanic eruption any less real, but the risk of an earthquake is now higher and greater than previously considered.
From nature's perspective, it is acting according to the way that nature has always acted, in a seemingly random or capricious way. Or not. Perhaps there is a definite pattern, one that geologists have not yet discovered and ascertained. As for how safe it is to visit Yellowstone, here is what its web-site says:
The science of forecasting a volcanic eruption has significantly advanced over the past 25 years. Most scientists think that the buildup preceding a catastrophic eruption would be detectable for weeks and perhaps months to years. Precursors to volcanic eruptions include strong earthquake swarms and rapid ground deformation and typically take place days to weeks before an actual eruption.****************
You can read the rest of the article at [Nature].