You may have seen pictures of the Mendenhall Glacier ice cave flooding social media over the last few weeks. National news websites have even recirculated pictures of Juneau residents standing in awe inside the blue-tinted ice walls or looking up into daylight through a giant vertical shaft in the back of the cave.
But is it a safe place?
Laurie Craig, a naturalist at the Mendenhall Glacier Visitors Center, has this advice for people embarking on the mile-and-a-half hike on the frozen Mendenhall Lake out to the cave:
Just for people to be really cautious, be safe, (and) be prepared.”
The ice cave is located near the western terminus of Mendenhall Glacier. While the Forest Service is not encouraging people to visit the cave, it’s not prohibiting access either. In case of an emergency, it will be Capital City Fire & Rescue – not the Forest Service – that will arrive on scene and try finding a victim in distress in the vast expanse of the glacial area.
It’s kind of thing that people need to be aware of, particularly those folks taking a lots of children out. Keep the children with you, be prepared for rescuing yourself because it’s very difficult for anybody else to get out there.”
Signs are posted warning visitors of the dangers of the glacier and lake ice, part of which she calls a dynamic environment.
Jason Amundson is associate professor of geophysics at the University of Alaska Southeast and has researched tidewater glaciers and glacier ocean interactions. He said the cave was likely formed by a stream coming down off of Mount McGinnis.
The water is carrying heat with it and that heats goes to melting the ice. As it’s gotten bigger in the summer, there’s water running into that ice cave, but there’s also warm air that can make its way into the ice beneath the glacier.”
Another stream on the glacier surface found a fissure or crack and, over time, created the giant circular shaft or moulin that allows daylight into the far, accessible end of the cave. Amundson said it’s a fairly common feature on glaciers.
You get water running over the surface of the glacier and it drops into a crevasse. Water is more dense than ice and it wants to move downward, and so it basically drills a hole through the glacier. That’s probably just something that formed at the surface. It could’ve been there for a long time and maybe – as the glacier has moved down-valley – it’s now just right in the same spot as where that ice cave is.”
Amundson said different layers of the glacier may flow at different rates. It’s the same for the center versus the edges where friction with the ground and surrounding hills can slow the ice movement.
UAS environmental science associate professor Eran Hood believes the ice in that area may be over 200 years old.
You can see in the walls there’s a lot of subglacial sediment that’s been entrained. Some areas of the ice looks actually quite dark because there’s a lot of the sediment in there which the glacier has just picked up.”
Visitors to the glacier can be deceived by the apparent stillness and sublime beauty of the area.
The Mendenhall calves all the time, even in the winter. Craig said the five-story high, snow-covered blocks of blue ice near the western edge broke off about a week ago. Calving events can cause lake ice to undulate or even shatter over large areas.
Someone who was skiing on the lake when that happened on Thursday afternoon, he said he could feel the ripple as it rolled across the lake.”
Landslides off Mount Bullard near the eastern edge of the glacier and the slow, constant movement of the glacier could mean perpetually thin and unstable lake ice at the terminus.
Underwater currents can also erode the underside of ice-locked icebergs, causing them to unexpectedly flip or roll with a change in the center of gravity.
Laurie Craig said she will never cross the lake ice.
There’s no way to predict what’s going to happen, but you can always hear creaking and groaning because it’s continuously moving down the slope, even in the winter time. If you get a look at the terminus, you’ll see that’s just a huge jumble of great big icebergs and they’ll stay there until the lake ice thaws. But it’s indication of how much ice is falling off all the time. So, any place that people congregate, it should hopefully be on land.”
While the interior of the ice cave is located on land and several dozen feet under the glacier, the overhead ice at the entrance may only be a few feet or even several inches thick.
Eran Hood said he escorted a National Geographic photographer to the back of the cave two years ago, but he won’t take his child there.
The one dangerous place, in my view anyway, is near the entrance to the cave where you have some overhanging ice that’s pretty thin and you can actually have blocks breaking off there. I just didn’t feel like taking my five year old daughter back through there and feeling like something could fall down on us.”
Hood said the ice that’s deep inside the cave may be relatively solid, but there is always a risk of collapse.
Below is a video from Firefight Films which appears to include drone shots of the Mendenhall Glacier ice cave that were filmed during the summer. Near the end of the video is a shot that seems to show the apparent thickness, or lack thereof, of the roof ice at the cave entrance.