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Roman concrete consisted of a mixture of a white powder known as slaked lime, small particles and rock fragments called tephra ejected by volcanic eruptions, and water.

The Connecticut-based artist, whose “What Makes the Earth Shake” is at Tephra Institute of Contemporary Art, often depicts people reading, pondering or gazing out a window.

The two artists employ such solid materials as stucco and plaster, but softer, more nebulous aspects also characterize the pieces in O’Brien’s “Cross Sections,” at Tephra ICA at Signature, and Sohn’s “Unspoken Volumes,” at the University of Maryland’s Stamp Gallery.

That’s one reason the Boston- and New York-based artist’s show at Tephra Institute of Contemporary Art is called “Mother Line.”

Before January, Tephra ICA was the Greater Reston Arts Center in Virginia, which sponsored several installations in the park across the street from its Reston Town Center home.