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Here’s another view of the enormous ice crystals and hoarfrost on the ceiling of a glacier cave in the Castner Glacier. This cave is an example of a Röthlisberger channel or R-channel. These are channels formed at the base of the glacier by flowing subglacial water, melting a semi-circular tube through the ice. Near the exit of the channel warm air in the summer melts the channel further, sometimes leading to large cave entrances like this.
Further, into the cave, the tunnel is much narrower and will be completely full of water in the summer. Since the meltwater in the channel is almost always at or very near freezing, the primary method of melting the ice is viscous heating from the water due to flow rate and water pressure. Without that water flow and pressure, ice deformation from overburden pressure will slowly close the channel.
The hoarfrost and giant ice crystals on the ceiling grow from water vapor present in the cave. Their formation requires a unique environment like that present in the ice cave. There is a large temperature gradient between the floor of the cave and the ice that drives convection that allows for the deposition of ice from supercooled water vapor on the cold surface of the glacier ice. The photo above shows an area full of skeletal growth patterns with much branching. The picture below shows these crystals up close.
In another area of the cave, there was a section of chandelier-like long c-axis ice spikes that had limited skeletal growth on them. It’s not fully understood what mechanism drives the growth of these spikes, but it is thought that they form much faster than the surrounding skeletal crystals.