Tension Cracks in the Brunt Ice Shelf
Yesterday the BBC news website published images showing a large section of the Brunt ice shelf in Antartica, which has separated from the main body of ice and formed a large iceberg. The associated article notes the giant berg to be 1,290 square km on plan.
The reason this interested me was because, despite the macro scale of cracking on display, it has the appearance of something that is familiar to me at a much smaller scale.
It follows that my explanation of the BBC’s photos is based on extrapolation from small to big. I don’t really know anything about ice shelves or icebergs so admittedly it’s going to be a bit of speculation on my part; though I hope not too much of a stretch.
The defining characteristics of the crack are; clean edges edges; and what seems like debris in the gap. Almost all of the debris is on the left side of the crack and has accumulated adjacent to the iceberg rather than the shelf. To me that is interesting.
Clean edges, or more importantly the absence of tearing, indicates that the crack is a tension crack i.e. the iceberg has pulled directly away from the ice shelf and has not slid past it.
If there has been a clean break the question arises as to why there is debris within the crack and why it is clustered on one side. I think the most important clue is to be found in the profile of the debris’ edge. It matches the profile of the ice shelf much closer than the profile of the iceberg. This indicates that the debris must have been in contact with the ice shelf.
From this we can deduce that the original crack must be old and that it must have been dormant for a reasonable period of time. We can also deduce that it must have remobilised more recently. The inferred mechanism would be as follows:
At some point in the past a crack opened in the ice shelf. The images do not convey why, but for some reason the crack stopped becoming larger and was for a time stable. This would have allowed new snow to fall within the crack and over time to pack down and form new layers of ice. The crack was starting to fill.
For some reason, again the images do not convey why, the crack remobilised and the iceberg started to drift further away from the ice shelf. The forces carrying the berg also carried the debris moving it further away from the shelf. I suppose I shouldn’t really call it debris, as that would imply that it fell from the sides of the ice pack, whereas I think it was new snowfall. If it had fallen from the edge I don’t think the crack edges would be as clean as they are.
I like this explanation, because I think it fits the visual evidence in the photo, however it also suggests that the same principles apply at a human scale as they appear to at a continental scale.
For example, old and dormant cracks in a wall can often be identified by evidence of dust and debris accumulating in the gap. If the debris has become dislodged it could indicate that the crack has started to remobilise. Similarly, cracks with clean edges will generally indicate the presence of tension. This in turn indicates movement perpendicular to the direction of the crack. Together these observations can often be used to infer the underlying cause.
In the case of the Brent ice shelf we can infer that the berg has moved perpendicular to the crack, perhaps due to ocean currents at the water’s edge, or perhaps under the influence of gravity if the underlying shore slopes toward the sea. I don’t know the reason, as it is not evident from the images.
Nevertheless, the visual appearance of the crack in the ice shelf does imply the remobilisation, for some reason, of an older crack that was stable for a period of time. Something has changed.
Or, since I don’t know anything about ice shelves or icebergs, perhaps none of my conjectures are true. I shall allow the reader to decide.
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