lmao! You have to remember that this formed within a lava flow, this isn't pluton related.

Imagine a flood basalt volcanic eruption. Millions of cubic feet of lava get released all at once from fissures creating a layer of basalt that is several meters thick and is erupted onto some sort of wet ground (or maybe a topographical low, like a valley, where water tends to congregate). After the eruption, while the lava is cooling, perhaps rain occurs and quickly cools the top of the lava flow, so a simple cross section would look like this:

• [thin layer of H2O]
• [Flood Basalt]
• [Flood Basalt]
• [Flood Basalt]
• [Flood Basalt]
• [Flood Basalt]
• [thin layer of H2O]

The water on the top and bottom of the basalt cool quickly, but the interior of the lava flow stay hot. As science dictates, matter contracts as it cools, and the top and bottom of the lava flow contract in the horizontal plane. On the top of the lava flow, the basalt begins to contract from itself, creating the hexagonal shape. This happens on the bottom of the flow as well. Effectively, a fracture in the rock forms, which is what I meant by jointing. ( see: http://en.wikipedia.org/wiki/Joint_(geology) )

So right now, our cross section looks like this:

• [thin layer of H2O]
  
• [Columnar Basalt]
  
• [Flood Basalt]
  
• [Flood Basalt]
  
• [Flood Basalt]
  
• [Columnar Basalt]
  
• [thin layer of H2O]
  

The water will continue to percolate downward through the joints that the basalt is forming and will cool the entirety of the lava flow in the same fashion until the lava flow has completely cooled. Like discussed downward in the thread, this happens at a relatively quickly, not slowly.

After continued cooling:

• [thin layer of H2O]
• [Columnar Basalt]
• [Columnar Basalt]
• [Columnar Basalt]
• [Columnar Basalt]
• [Columnar Basalt]
• [thin layer of H2O]

I hopes that answers your question!

Edit: Formatting