Today when I stepped out of my garage, I flushed a male eastern tiger swallowtail from my gravel driveway. Later when I returned to the house, I found the handsome male tiger swallowtail had returned and appeared to be actively feeding on what might be described as rock flowers.
The answer to this question is obviously no. Although we often see butterflies seemingly feeding on moist sand, soil, pavement, and rocks, they are definitely unable to extract calorie-rich food from these lifeless surfaces.
The butterfly illustrated in the accompanying photo was actually using his long, coiled proboscis to sip much-needed sodium and other minerals found on the gravel. Often these minerals originate from the materials themselves. In other cases, they originate from the unseen residues left behind by dead plants or animals. Family pets or wildlife that urinate or defecate on the hard surfaces can also deposit the minerals.