Hornito

Hornitos are formed in lava flows and are small openings that release small amounts of lava. Similar to spatter cones, Hornitos are rootless, meaning they form on the surface of lava flows like spatter cones. They are usually created by the slow upwelling of lava through the roof of a lava tube. High pressure causes lava to ooze and spatter out. The lava builds up on the surface and solidifies creating the initial structure. Hornitos can grow and exceed 10 meters in height.

...
Wikipedia