Thursday 21 April 2005
A new iceberg
All these stories in the news about the huge iceberg breaking off part of an ice stream near McMurdo station have reminded me of a similar (but smaller scale!) event that we saw at the end of the summer. The ship had just returned to Halley to take all the summerers away and pick up the last of the cargo. Unfortunately its usual mooring site was now unsuitable, as the sea ice had broken right back to the cliff edge so there was no space to turn vehicles around. The nearest suitable relief site we could find was 60km up the coast which meant making several long land journeys by sno-cat to ferry all the people and supplies from base.
We had just arrived at the relief site in the second convoy of 6 sno-cats with most of the remaining passengers and were hanging around waiting for the okay from the ship to venture down onto the sea ice. When we first arrived someone had pointed out the overhanging cliff edge at the entrance to the bay and said that it looked like it was going to fall off soon. They were right but they had no idea how soon!
Around 5 minutes later I glanced back at the edge to find that instead of the overhanging edge I remembered there was a huge tower of ice 10 stories high in its place. At first I couldn't quite believe my eyes - how could something that big have just silently appeared so quicky? To start with I just assumed it was an iceberg that had drifted along in front of the cliffs. The crew of the ship had a better view of it from the bridge so they told us what happened. A large tabular berg had been pushed along the coast by the currents and clipped the edge of the shelf. The force of several million tons of ice hitting the shelf sheared off the corner of the bay. Once the corner was free from the shelf it then slowly and gracefully flipped over, creating a huge triangular berg right before our eyes.
The really strange thing about all this was that the whole thing happened without us noticing - we were only a few hundred metres away but didn't hear a sound. The collision occured on the far side of the bay so the sound must have been absorbed by the intervening ice shelf. The flipping over was a very slow and gradual so didn't make a lot of noise either. There were a few tense moments when it looked like the new iceberg might have grounded on the sea bed, trapping the ship in the bay! Fortunately it too got picked up by the currents and drifted off as quietly as it had arrived.
The iceberg just after it broken away from the shelf. The blue ice at the base was submerged below the water line before it broke free. The block of ice to the right is the tabular berg that hit it. To give an idea of the scale, it's around 30 metres between the top of the tabular berg and the water line.
The berg continued to tilt as it drifted away, revealing more of it's underside. The flat white surface on the left hand side was the surface of the ice shelf just minutes before.
A view of the whole bay when the tabular berg was passing by the entrance. A rough calculation based on its size shows the tabular berg weighed more than 5 million tons. A large amount of brash ice (small chunks in the foreground) was also generated by the collision.
Posted by simon at 3:38 PM | Feedback (3)