Dawn comes, and the night gives way to a ghostly grey gloom. The rays of the morning sun just barely break through the thick haze of smoke and ash that fills the air. An endless cloud blots out the sky, spreading from the source outwards, engulfing much of the continental United States. Of the source itself, little remains but a vast plain of back rocky debris. Welcome to Yellowstone Park – 640,000 years ago.
These basalt columns, relics of one of Yellowstone’s past eruptions, are similar in form to Ireland’s Giant’s Causeway
Located in northwest Wyoming, Yellowstone is one of America’s most iconic national parks. The mountainous landscape is home to bison, bears, and countless other animal and plant species, roaming free amidst a backdrop of steaming geysers and colorful springs. But far below the ground, geology tells a much more sinister story. Beneath Yellowstone lies a hotspot, a plume of magma that rises from deep within the Earth’s molten mantle.
Hotspots are not uncommon: the Hawaiian islands are a product of thousands of years of undersea eruptions. The main difference between Hawaii and Yellowstone is the thickness of rocky crust that overlies the magma source. As hot buoyant magma rises, it melts the surrounding rock, gradually becoming thicker (think watery dough getting thicker as you add more flour). When magma reaches the surface, its thickness (viscosity) will affect the resulting eruption. The crust under the ocean is much thinner than the continent; Hawaiian eruptions are therefore marked by fast-flowing, non-explosive lava flows. In contrast, the geologic record shows that past eruptions in Yellowstone have been incredibly explosive.
In the last 2.1 million years, the Yellowstone hotspot has fueled 3 massive supereruptions, the most recent 640,000 years ago. Each of these eruptions created a caldera – a large crater formed by a volcano collapsing in on itself. The current caldera measures 45 by 85 km (30 by 50 mi). The last eruption released 1000 cubic km (240 cubic mi) of rock, ash, and debris, reaching as far as Louisiana. Smaller, less explosive eruptions have occurred as recently as 70,000 years ago. Presently, Yellowstone Park is separated from the magma chamber below by 5 to 10 km (3 to 6 mi) of rock. This magma chamber has been approximated as a 4000 cubic km (960 cubic mi) subterranean sponge – the largest of its kind on Earth.
Perhaps the most difficult question to answer is when Yellowstone will erupt again. Since past supereruptions have occurred roughly 600,000 years apart, some have said that we are overdue for another. On the one hand, recent geologic evidence may support this statement. The floor of the caldera has been rising annually on average 8 cm (3 in), and seismic tools have recorded thousands of weak but measurable earthquakes per year. That said, the interval and nature of eruptions is inherently uncertain. There is nothing to say that the next eruption will look anything like the last, or indeed that it will happen within the lifetime of the human race. It is important to remember that ‘soon’ in the language of a geologist is measured not in days but in thousands of years, and the ability to predict events of this nature within a human lifespan just doesn’t exist.
Rest assured, if there’s one thing we learned from last year’s lacklustre ‘end of the world‘, it’s that even the most dangerous volcanoes will never be as active as our runaway imaginations.
Be warned: you may have just sustained a lethal dose of mostly harmless science.
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All images courtesy of Wikimedia Commons.