Editor’s Note: David Phillips is head of the School of Earth Sciences at the University of Melbourne, Australia. He obtained a doctorate in geochemistry from Princeton University and has held prior positions with the De Beers Group and the Australian National University. Current research interests include volcanic systems in Australia and Kenya, diamonds and their host kimberlite volcanoes, and 40Ar/39Ar geochronology of young volcanoes. He has published more than 120 scientific articles on these and similar topics in international science journals. The opinions expressed in this commentary are solely those of the author. View more opinion at CNN.
White Island is the most active volcano in New Zealand. It is also a tourist volcano.
It isn’t hard to see why. Most of it lies under the ocean, with the crater conveniently rising just above sea level — ideal for short visits by boat. Although its dangers include explosive eruptions, deadly emissions of sulphur dioxide and superheated steam, boiling mud pools, earthquakes, mud slides, rock falls and even tsunamis, it erupts relatively rarely.
As an otherworldly, beautifully thrilling place only 50 kilometers (31 miles) off the east coast of New Zealand’s North Island, it has long been popular among adventure-seeking visitors.
But volcanoes like White Island are unpredictable and can erupt with very little warning. It did so tragically on Monday at 2 p.m., killing 16 tourists and guides and critically injuring several others.
The disaster has shocked the world and brought home the unpredictability and inherent risks associated with active volcanoes. So, what led to the eruption and what are the implications for tourism at such sites? To address these questions, we need to understand something about the nature of volcanoes and how they behave.
White Island volcano is a stratovolcano, a classic cone-shaped volcano, like Stromboli in the Mediterranean, Mount St. Helens in Washington and Vesuvius, brooding above Naples, Italy. These volcanoes form along the tectonic boundaries where the Earth’s oceanic plates are forced down below continental plates.
White Island lies along the “ring of fire” that surrounds the Pacific Ocean plate, with stratovolcanoes at the edges of the Americas, the Asia-Pacific region and as far south as New Zealand. The molten rock, or magmas, beneath these volcanoes are typically viscous and gas-charged — and usually erupt explosively.
This contrasts with the shield volcanoes found on Hawaii and Iceland, where basaltic lava pours more quietly out of the earth, although these volcanoes are not immune to explosive activity. In 2010, Eyjafjallajökull in Iceland erupted spectacularly when its magma met melting ice.
The eruption on White Island appears to have been a phreatic, or steam, eruption. The crater zones of stratovolcanoes are often highly fractured, which allows water to penetrate the volcano interior. When this water meets magma or rocks heated by the magma, it turns to superheated steam and usually escapes upward through vents like the fumaroles mentioned earlier.
In photos: Volcano erupts on New Zealand's White Island
If these escape valves become blocked, perhaps due to seismic activity, the steam has nowhere to go. Pressure builds, eventually culminating in an explosive eruption that ejects superheated steam, hot ash and rock debris into the air.
Phreatic eruptions are considered relatively “small” eruptions but still deadly if one gets too close. Being small, such eruptions are difficult to predict and may occur with little or no warning.
Given the dangers, most of the world’s active stratovolcanoes are closely monitored for seismic activity, gas emissions and landscape changes — the warning signs of an impending eruption.
New Zealand’s active volcanoes (not only White Island) are closely monitored by the government’s earthquake monitoring agency GeoNet. In early December, GeoNet raised the volcanic alert level for White Island from one (minor volcanic unrest) to two (moderate to heightened volcanic unrest). However, warnings have oscillated between these levels for many years, with little significant activity observed since a series of eruptions in the 1980s.
Questions will undoubtedly be raised as to whether the monitoring system and warning systems were adequate in this case, but small eruptions are likely to remain unpredictable.
Of course, eruptions on White Island (and other active volcanoes) are relatively rare and the current heightened awareness of the risks will recede with time. As adventure tourism continues to grow in popularity, it is likely that White Island will lure tourists and tour boats again. What may change, however, is what visitors understand about the dangers.
Videos of Monday’s eruption should become essential viewing as part of the risk information provided to adventure tourists to make an informed decision — enter at your own risk.