[Jessarah]

While contraction in the vertical direction reduced the flow thickness (without fracturing), horizontal contraction could only be accommodated by cracking throughout the flow.

So why did it contract in a vertical direction instead of a horizontal contraction?

[Andyman]

Sometimes the formation happens vertically, sometime more horizontally. In the hill above the more famous formation most commonly pictured at Giant;s Causeway, there is this formation jutting out of the hill:

These ones are called Organ Pipes. Why they are not geometrically shaped at the top, I don't know.

This shot shows the horizontal and vertical fracturing:

[wafflehouse]

it contracted in both directions.. however vertical contractions along with gravity just resulted in settling of the lava, where as horizontal contractions caused it to pull apart and crack. thats what i got from wikipedia anyway

[wafflehouse]

here is a good page to check out.. Devils Postpile National Park