Minerals in the water make vertical veins in the ice, these veins melt much faster than the rest of the ice causing vertical channels and the ice to weaken along those channels.
Minerals in the water make vertical veins in the ice melt much faster than the rest of the ice. This ice is causing vertical channels. This is to weaken along those channels. This ice is causing the ice. This is to weaken along those channels. The ice is these veins.