Columnar basalt forms when flowing lava spreads several feet think over a large area. The lava cools from the top as it loses heat to the atmosphere and from the bottom up as it loses heat to the ground below. Like almost any solid, it contracts as it cools. The entire body does not contract, because the margins of the flow would have to be dragged inward. Instead, the contract is localized and cracks form, resulting in polygonal columns of basalt that are at most a few feet wide. The people of Nan Madol used these columns in the same fashion that many mainland cultures have used logs for buildings. The basalt columns were just much heavier (the downside) but remain centuries later as an indicator of the industry and sophistication of the people who built these islands.