I needed to know because this was part of the premise of my novel (book #3 Hearts and Thrones series, untitled). My hero Rayn is a fire mage living on a volcanic island, and part of his job is to redirect lava flows when the volcano erupts, directing them into unpopulated areas where they won’t do as much harm. When it’s a fantasy world, one can always fall back on, “But it’s magic! Anything is possible!” Still, I wanted my premise not to be too far outside the realm of possibility.
I learned that there have been three volcanic eruptions in history near populated areas in which an attempt was made to stop or redirect the lava flow.
The first two were eruptions of Hawaii’s Mauna Loa, one in 1935 and another in 1942. In both cases, a lava flow advanced on Hilo, threatening to overrun the city. Guess how we tried to stop both lava flows? U.S. Army Air Corps bombed the lava flows from the air. I find that kind of funny–what’s the use in bombing lava? Scientists are uncertain whether it was effective, because both lava flows did stop short of Hilo, but it’s believed that they would have done so on its own. In a 1984 eruption, the decision was made not to intervene, and that lava flow stopped before reaching the city as well.
The third incident was in Iceland. An eruption occurred 200 meters east of the town of Vestmannaeyjar. The resulting lava flow devoured houses, but the most serious threat was to the harbor, which was vital to Iceland’s economy. They considered bombing the lava flow, but ultimately dismissed that idea. Then they tried spraying the lava with water from fire trucks, which was ineffective because the lava flow dwarfed the tiny amount of water they could spray, but it did succeed in cooling the lava at the front, creating a bit of a wall. They realized they’d probably hit on the solution, but they needed more water.
So they brought in a dredging ship and used its pumps to pour seawater on the lava at a rate of 20,000 liters per minute. More pumps were added, and the lava began to slow. Eventually, they managed to solidify enough lava to create a basalt barricade protecting the harbor. The barricade held, and the harbor was saved. So it is possible for human intervention to stop a lava flow.
That battle against the lava took months, however. My fire mage will have to work faster!