Once lava from an erupted volcano starts moving, it tends to keep moving that way, regardless of what we want. But Hawaiian researchers have figured out some ways to redirect a lava stream away from you or your property — even if the eruption goes on for several months.

In a new Volcano Watch column, the USGS's Hawaii Volcano Observatory looks at the history of attempts to redirect lava flows away from cities, which have ranged from hastily constructed barriers to what essentially amounts to moat-building.

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While these efforts often end in failure, there have been some success stories, too. In one case in Italy, engineers were able to keep diverting lavaflows for over five months, as the flow's moved ever closer to a small town over the course of an eruption that continued for over a year:

On December 14, 1991, Etna began erupting, sending lava toward the town of Zafferana Etnea, located 9 km (6 mi) downslope of the active vents. On January 1, 1992, workers began constructing a 234 m-long (256 yd-long), 21 m-high (69 ft-high) barrier about 2 km (1.2 mi) above the town. But on January 9, the lava flow front stalled and activity became focused upslope. By early March, another lobe of lava passed the original stalled front, reached the barrier on March 14, and overtopped it by April 10.

The barrier had successfully delayed the lava for a month, but flows continued to threaten Zafferana, and the population prepared for evacuation. Three more short barriers were built to slow the lava flow's advance, but they, too, were overtopped.

Meanwhile, plans for a different kind of lava-control project were enacted farther upslope. Per this plan, explosives were used to open up the feeder lava tube in an attempt to slow the flow's advance. After four unsuccessful attempts, the lava was successfully redirected into an artificial channel in late May. Robbed of its supply, the flow advancing toward Zafferana stalled.

By June 1992, the eruption rate had decreased by half and lava flows were only active upslope. Lava was no longer threatening Zafferana and efforts to slow or divert the lava were no longer required. The eruption ended in March 1993, after 16 months of volcanic activity and about 5 months of work to control the flow.

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You can read the full story, with some more case studies in lava diversionary tactics, right here.

Image: A more recent Mount Etna eruption from 2013 / ESA