Ice Stars: Fantastic flow patterns in snow ice crated by water flowing from under the ice into snow covering an ice sheet.
Vicki Gluek sent this picture from Canyon Ferry Reservoir in Montana.
This formation is an ice star (1). They have other names including ice crab, ice optupus and a few others.
They are upflow holes associated with water flowing up through a hole or crack as the ice sheet is being submerged by the weight of a new snow cover. Given the size and roundness of this hole it may have started as a fishing hole but they often start as small drain holes that form in a thaw just before the snow arrives. The dark arms around the hole are infiltration conduits that fully melted the slush. The streamline like pattern seen in this picture and in the ones below are partial melting of the slush as it infiltrates the snow. They appear to be smaller conduits that occur near the surface so they have grey snow ice underneath them.
One plausible sequence is:
- A new black ice sheet forms and thickens.
- Just before a snow storm the sheet thaws enough to develop sporadic drain holes. They may be tripple junctions between ice grains and they may have been enlarged by surface melt/rain water flowing down through them. Any passage that allows water to wick into the snow will do.
- Alternatively, the the insulation effect of the snow and moderate temperatures should allow the ice to become isothermal at 32 degrees. Triple junctions between ice crystals have a very slightly lower melting point. A few of them may melt enough to form the first very narrow passages that then erode into larger ones as water flows up through them.
- It snows which submerge the ice sheet with a combination of the weight of the snow and the weight of water wicked into the snow as water comes through the ice holes.
- Water infiltrated from the inflow hole melts conduits in the slush layer that are clear and appear as dark, water ice 'octopus arms'.
- The streamline pattern fans out from the conduits to the edge of the area fed by each hole. The streamlines appear to only be in the top 1/4" of the slush layer (this layer often 1/2 to a couple inches thick).
- The slush freezes into snow ice and clear ice in subsequent cold weather.
- The ice surface gets Zambonied in a thaw to remove the layer of white ice enough to make the patterns visible.
- We come along and enjoy the show.
Click here for an annotated gallery of the probable formation process.
First published in January 2011. The text has been significantly modified since, most recently on 11/14/12.
1) Ice: The Nature, The History, And The Uses Of An Astonishing Substance Mariana Gosnell, Published by Alfred A. Knopf. Chapter 1. Comment: This is an excellent and broad look at ice.
2) Charles A Knight wrote a paper on Slush on Lakes in 1987 that explains the formation process. See CRREL record ID 51000661
3) Jan 15, 2013: Dave Dermott just sent a reference to a technical paper on these formations. It gives a general description and provides a mathmatical model for their behavior.