Nan Madol was a Micronesian urban complex built between 500 and 1500 C.E. on artificial islands. The islands consisted of seawalls made of columns of basalt stacked like logs, with coral rubble fill behind the seawalls. Buildings of basalt columns may be present too.|
Columnar basalt forms when flowing lava spreads several feet think over a laarge area. The lava cools from the top as it loses heat to the atmosphere and from the bottom up as is 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.
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