A gymnasium of basalt in Arco, Idaho
Building Image
 
      One of the least likely structures to be built of stone is a gymnasium, but the building above is the Recreation Hall (and sometime city offices) of Arco, Idaho. The Rec Hall was built as a WPA project in 1937 and 1938, and renovations including a new roof in the 1990s allow basketball and other activities to continue in the Rec Hall today.

      Another unusual feature of this building is that the stone is vesicular basalt. Basalt, or "lava-rock", is a dark volcanic rock formed when mafic magmas reach the earth surface and flow across the landscape. Magma, or molten mineral material, contains gases that usually escape to the atmosphere. However, sometimes the gases haven't time to escape and form bubbles or vesicles in the rock, making the porous or vesicular texture seen below.

      Basalt is a very hard rock, and it presented many challenges for this building. One challenge during the construction process was that the WPA's masons couldn't hammer the basalt into regular blocks for a tightly fitting wall. Instead, they mortared the irregular stones together and then traced small "seams" in their mortar to give the appearance of tightly fitting stones. A challenge that continues today is that the porous basalt leaks a little water when it rains, so that moisture appears inside the walls. That could be a huge problem, but Arco gets only an average of about 10 inches (25 cm) of precipitation per year, some of it as snow, so the leakiness of this stone isn't the issue that it might be elsewhere.

 
Stone Image
 
Stone Image
 
The gymnasium or red hall shown above is not unique in Arco - the church shown below is also built of basalt:

 
Stone Image
 
      The choice of this less-than-ideal stone for such an unusual building as the Rec Hall has everything to do with Arco's location. Arco sits at the edge of the Snake River Plain, which is covered with basalt flows that flooded the landscape (in fact, they're called "flood basalts"). The image just below shows Arco in the foreground and black expanses of flood basalt in the distance to the west (in the direction of Craters of the Moon National Monument, twenty miles to the west). Our Rec Hall is the nearest of three red-roofed buildings in a line on Arco's main street; the church's red roof is the most distant of the three.

 
Stone Image
 
      The final image below shows one of the basalt flows south of Arco. Arco sits at the foot of the mountain in the right background of the image. The outcrop shows the vesicular nature and irregular breakage of the basalt that made it such a challenge for the building above.

 
Stone Image

 


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