More With the Ice

More about ice spikes and the like....

This from a reader: a 7.5 cm protrusion from a rock pothole:



The caption gives a clear explanation:

Water expands when it freezes and as it typically freezes along the edges first in a confined space the tendency is for the volume of water in the center to be slightly raised as it freezes. Ice cubes made in a freezer usually have a somewhat convex surface. When factors such as water purity, the rate of freezing and the preferred growth of ice in crystalline planes are considered, expansion of freezing water can result in a variety of interesting structures, such as linear spikes with triangular cross-sections.

And in a variation of this physics, here's a underwater icicle formed in salt water, from the BBC (skip ahead to 1:45):