This lovely cinder cone is Pu'u ka Pele, sitting on the flank of Mauna Kea volcano, Hawaii. Cinder cones are piles of volcanic ash so fluffy and loose that lava doesn't build up inside to flow out of the top. Instead, lava flows break out the side.
Cinder cones are useful sources of landscaping material, like the lightweight frothy gravel used on running tracks or pathways. The industry calls it "lava rock" but geologists call it scoria.
See another cinder cone in the California desert photo tour, and see the insides of one in the Cascades Range/Columbia Plateau photo tour.