Image courtesy U.S. Geological Survey (fair use policy)
Hornito is a Spanish term meaning "little oven," and small lava cones like this one from Hawaii resemble old-fashioned outdoor wood-fired ovens. Hornitos form along the tops of lava tubes, wherever an opening in the solid upper crust appears. As the lava beneath pulses and flows along its channel, small gouts of it spatter up and outward, building a little pile around the opening.
Hornitos are also called driblet cones. Larger ones are called spatter cones and have a smoother, more gently sloping form.