Tuesday, 29 November 2016
So what is an Ice Shelf?
The Brunt Ice Shelf
If you search for Halley VI research base on Google maps (Go on on try it!) you’ll get a place marker sitting in the sea about 30km from the coast of Antarctica. Strange as it appears this is actually perfectly accurate.
Sitting here in what feels like a very solid and stable structure is is easy to forget that we are quite a long way from solid ground. Halley VI is built on the Brunt Ice Shelf, a 200m thick block of ice that floats on the Weddel Sea. But the Brunt is more than just a block of ice, it is a constantly changing, dynamic environment and it is precisely these changes that are responsible for the design of Halley VI and the reason that this summer and winter seasons will be such a challenge here.
So what is an Ice shelf?
1) Snow falls on the Antarctic mainland, it is so cold here that is never melts, it just continues to pile up and up and up, eventually the weight of the snow on top compacts the snow underneath into hard ice and the this starts to slide down hill in much the same as a normal river only much more slowly. This is called a glacier and it happens in many mountainous areas of the world (Sadly not in the UK – it’s too warm in the summer for the ice to stay!) In Antarctica the amount if ice is so deep and covers such a huge area that it is called an ice sheet
2) The ice continues to accumulate and slide slowly down hill until it reaches the sea.
3) Ice is less dense than water so when the glacier reaches the sea it floats forming an ice shelf. Over hundreds and thousands of years the snow from Antarctica falls and adds to the the ice sheet and eventually the ice shelf. Most ice shelves slide gracefully from the land into the sea but the Brunt is different. At the point where the land meets the water there is a drop so the ice falls off in great big chunks (Icebergs!), these chunks can’t float away because of the surrounding ice shelf and eventually get glued back together by snowfall and freezing sea – this leaves a very irregular area close to the coast – large peaks of upturned ice and deep crevasses.
4) Year on year the ice sheet continues to extend out to sea. It moves (and by extension everything on it – including us) at roughly 400m per year, a little over a metre per day.
All of the previous bases at Halley have succumbed to this steady march. Buried by snow they have long since fallen off of the edge of the ice. (Halley V was built on stilts and every year was raised up to keep it from being buried. In 2011/2012 is was dismantled and Halley VI constructed. The remains of the base were shipped away but the remains of the stilts are still buried deep in the ice and continue to make their way north!) Halley VI was designed with this movement in mind and is built on skis so that it can be moved back towards the coast – increasing the life of the station. As more snow builds up on the ice sheet it becomes smoother and safer – 40km from the coast it is almost perfectly flat out here.
Halley IV emerging from the ice decades after it was abandoned.
5) Eventually stresses and strains in the ice form fractures and large blocks come away in a process called calving – leading to ice bergs. Some can be only a few meters in diameter, some can be many kilometres. The rate of snow accumulation changes over years as well as the point at which the cracks and fissures form in the ice so the size of the ice shelf can vary considerably.
This is obviously a very simplified version of what goes on – the ice is influenced by a host of factors including the weather, sea currents and the geography of the seabed.
One thing I hadn’t realised is that the ice shelf is tidal (well it responds to the tide in the Weddel Sea underneath it) we move up and down by approximately 1 metre twice a day, though you can’t feel it – we only know because of the GPS sensors on the base.
It is one of the cracks in the Ice that is causing us so much work this year!
Chasm 1 formed around 30 years ago but has been dormant until 2012 when it started to expand again – extending north at around 1.5km per year. While it doesn’t directly threaten the station, as it progresses it will eventually separate the piece of ice that the station is on from the rest of the ice shelf and we will float out to sea!!
To prevent this from happening this year we are moving the base 25km to the west – out of the way of the crack.
The picture above shows out current location and the new site. Doing the move now means that we can drag the base in a roughly straight line – leave it any longer and we would have to take a dog leg north to avoid the crack. The tight parallel ripples you can see in the ice close to the continent are the cracks and crevices thrown up by the ice as it comes off of the land. These slowly soften and flatten out the further from the mainland you get. The new site is in a safe area (it has been thoroughly examined by ground penetrating radar to make sure there are no cracks hidden underneath it.
The N9 resupply site is an area of the shelf where it is safest for the ship to dock – the ice here is a good height and thickness for the ship to more up alongside and safely unload cargo. The site of Halley V is also marked.
What we will be doing this summer – moving the modules around the crack in the ice to a safer part of the shelf!
The penguins on the left are on sea ice – this is different to the ice on the shelf in that it is frozen sea – much thinner and in the summer breaks up much more easily. It can be moved around by wind and currents. The sea ice forms in the winter and can extend hundreds of kilometres out to sea. At the nearest site to Halley (the appropriately named Windy Bay) this year the sea ice has broken up and been blown out of the bay.
The MacDonald Ice Rumples are caused by a small island just under the surface. The ice shelf hits the land and is forced up, turning and rolling over. It puts a lot of resistance on the flow of the ice shelf and creates stresses and strains that cause cracks in the ice.
MacDonald Ice rumples
These photos show the edge of the ice shelf at Windy Bay (we were there on Saturday digging out an automated weather station) – you can see the sea ice has broken up and moved way from the shelf.
Right - that's it for now. I'm going to be off station from tomorrow if the weather holds. We are setting up a camp at the new site for the base and will be away from the internet connection for a while - my first taste of proper Antarctic living!!
I'm going outside - I won't be long!!!
Till next time - N
(PS - huge thanks to our resident Glaciologist/Ice Shelf-ologist, Jan de Rydt for his help with this!)