Suppose you’re sailing your Hobie 16, and you saw off one of the pontoons. The boat will flip over. (Unless you put a counterweight way off to the other side – and it would be very unstable.)
Let’s try the toy bird again, only this time let’s build something. You set up a couple of stands with an axle between them. The axle passes through an eight-foot two-by-four, three feet from one end. So on one side of the pivot there is three feet of board and there are five feet on the other side. When the axle is on the stands, the board will hang upright because the five-foot end is heavier than the three-foot end. That’s your iceberg.
Now let’s say you cause your simulated iceberg to ‘melt’ by cutting wood off of the long end an inch at a time. For a long time nothing will happen. But eventually you’ll reach a point where the ends are equal in length. If you push the board (simulating wave action, for example) it will no longer remain upright. But if you continue the ‘melting’ by cuttin off more wood, the bottom will be lighter than the top and the board will flip over.
Hombre, you’re right not to be convinced by the explanations you’ve been given. Everyone is giving you bad analogies and examples to explain why iceburgs tip.
The heart of the reason is not that they are just getting smaller – in which case you are right, they would merely sink lower in the water. The key here is that they are changing shape as they are getting smaller, due to uneven melting. Once it assumes a new shape its center of gravity can shift to a different place, and the iceburg will flip suddenly to bring its new “top” up, and its new “bottom” down. Throughout the entire, of course, the iceburg will still only show 15 (or whatever the number is) percent of its total volume above the waterline.
But the ice just doesn’t melt off of just one side, nor all at once. If it did I could understand tipping.
This not a good analogy. As ice melts away from the iceberg the pivot point (the aforementioned CG) will change when the iceberg sinks a little. This is what I meant by saying the iceberg isn’t balanced on the surface. If the iceberg would ride at the same waterline while the underwater ice was melting then, sure, at some point it would flip over. But unlike your 2X4 and axle which has a fixed pivot point, it seems the iceburg would have an ever changing center of gravity.
They also change shape and CG for reasons other than melting. In the process of simply moving, an iceberg must force its way through a hell of a lot of water, against resistance; fragile bits might break off.
Why would it be against resistance if it’s moving with the current? Well, it’s moving with a current. It might be moving in two currents at once: a surface current moving NW and a deepwater current moving E (and the deepwater current may affect more of the berg’s total surface area, and it might not). If the eastern current wins, the berg’s SE side will be taking the brunt of the current and may melt faster.
Furthermore, those suckers are huge and cold and they create their own local currents and downdrafts and turbulence, which can carve the ice into some pretty weird shapes, creating nooks and hollows and channels.
There could be an air pocket in a 'berg from a previous rollover that had been acting as buoyancy and suddenly melted through and filled with water.
Last, the top of a tabular iceberg (the big blocky squarish ones) is probably snow, not ice. Snow can be blown off by the wind, or melted off by a light rainfall, changing the weight distribution, and… well, you get the idea.
Apparently a tabular glacier (which has snow, generally not packed) has 7/8 below the surface. Once it loses its snow cover it becomes comparatively denser and the 90% figure is said to apply. I believe that was on one of Johnny L.A.'s cites, or maybe it was one I found. Hafta look.
Interesting. The picture linked to above, on the Greenpeace site, looks like the previously-submerged portion of the iceberg is the same white color as the top. I wonder if the white color is just from snow or freshly frozen rain or something that the bottom part wouldn’t have covering it, so immediately after it turned it could be a different color until snow or rain or whateer turns it white again.
Do you know of any videos on the Internet that show icebergs turning over? That would be pretty cool regardless of color.
You might be right. I’ve only paid attention to the ones being shown actually rolling, so they have the blue shade. No idea how they look after floating for a while.