A couple of Sitka residents who were fishing for halibut last month noticed an unusual feature on their depth sounder as they set gear a few miles north of town. Two-hundred feet below the surface of the water, the sounder slowly painted a picture of what appeared to be a volcano: A perfectly-formed cone about 100 feet tall, with a plum of gas trailing from the top.
Experts who’ve seen the image say it is no cause for alarm. It’s most likely a “mud volcano,” and it’s also not the only one near Sitka.
Locals don’t use rods and reels much to fish for halibut in Sitka. Rather, most set a subsistence skate – a long line baited with a dozen or more hooks that is laid right along the bottom from a slowly-moving boat.
Jacyn Schmidt was doing just this in November near the entrance to Nakwasina Sound, 5 miles north of Sitka. Schmidt was on deck, paying out the line.
“And then my friend who I was out fishing with was inside watching the depths on the sonar, and he noticed just a weird shape, and called me in to look at it with him,” said Schmidt.
KCAW – “And what was it?”
“Well, I didn’t know what it was when I first saw it,” Schmidt continued, “but it certainly looked like some things I had seen before. I think we both immediately saw that cone shape and the kind of trail coming out of it, thinking that it could be an underwater volcano of a sort.”
Had it been anyone else, this might have been an invitation to head for the hills. But Schmidt is the regional geoscience specialist for the Tlingit and Haida Central Council. She’d been in Sitka for what has been a very interesting year – geologically – for the region. Her first impulse was to try and find the feature again, and then get on the phone.
“I’m aware of the geologic context of Sitka and the surrounding islands,” said Schmidt, “and spatially where the Queen Charlotte Fault is and where the Mt. Edgecumbe Volcanic Field is, and this location was just not adding up with the story that I know about our geology here. And so I just called the people who I knew had spent a lot of time thinking about the seafloor here.”
“It’s a classic example of what we have seen out on the (Queen Charlotte) Fault, down around Dixon entrance,” said Dr. Gary
Greene, one of the people Schmidt called.
Greene is a marine geologist, emeritus professor at the Moss Landing Marine Labs, and a leading expert on the undersea geology of the Queen Charlotte Fault.
“These types of plumes that come out of this volcanic-like cone are associated with what what we call mud volcanoes,” said Greene. “They’re not really active volcanoes, where magma comes flowing out, but it’s mainly fluids that come up from depth and carry with it sulfide-rich gasses and what have you.”
Greene has done sonar studies of the Queen Charlotte Fault that have produced images that are surprisingly similar to what Schmidt and her fishing partner saw on their depth sounder. The term “mud volcano” tends to conjure up thoughts of destruction, but – with a few exceptions – their only similarity to terrestrial volcanoes, like the nearby Mt. Edgecumbe, is their shape.
“If there’s a better term, I’d love to use it,” said Cheryl Cameron, “And right now, I don’t know about one.”
Cameron is a state geologist at the Alaska Volcano Observatory, and another expert Jacyn Schmidt consulted. Cameron is from Sitka, and is familiar with the depth sounders people use here while fishing. She hesitates to draw any conclusions from the image Schmidt captured, but admits that there are plenty of possibilities.
“A fishfinder is looking for reflections within the water column,” she explained. “And so anything that has a density different from that of the rest of the water will cause a reflection. And so that reflector could be indicating a change in temperature or composition, it could be any kind of gas leaking out of the ground that would generate a plume with a different impedance, and thus a reflection. So it could be CO2 (carbon dioxide), it could be methane, it could be fluid with particulates in it – there are a lot of possible causes that are not related to volcanism.”
Nakwasina, where Schmidt was fishing, is not within the Mt. Edgecumbe Volcanic Field – but it’s right next door. Edgecumbe generated headlines last spring when an earthquake swarm signaled that magma was stirring beneath the long-dormant crater. Subsequent radar studies showed that the flanks of the mountain had inflated by several inches over the last couple of years. Although the mud volcano is probably not volcanic, Gary Greene says there’s not enough information to rule out that it might be related to what’s happening beneath Mt. Edgecumbe.
“It could very well be associated with that magma that’s moving up into the cone,” said Greene. “It could be that the heat from that magma has started a convection of warm or hot waters that are now venting at that locality.”
The mud volcano is near a place called the Beehive. Everyone who’s been to Sitka on the ferry passes just a stone’s throw away. Jacyn Schmidt didn’t catch any halibut on the cone, but that unhappy fact hasn’t kept her away. Now, she’s fishing for information.
“Well, we keep going back again,” she said. “My friend, whose boat I was on, has gone back to the same place to check again and again, and it’s still a bump under the water. I still have so many questions, and would like to go back with better instrumentation where you could really see but I’ll leave that to Gary and to Cheryl to direct.”
Although Schmidt is a professional scientist, accidentally discovering an underwater mud volcano is a form of citizen science that is very useful to researchers. Cheryl Cameron welcomes any and all observations of geologic phenomena at the Alaska Volcano Observatory. Gary Greene is going a step further, and looking for research funding to go down and take a look at the mud volcano, or at least take its temperature.
