A Lava Boulder (火山弾) (also known as Volcanic Debris in other games than Mario Kart: Super Circuit) is a flaming chunk of rock that spews from volcanoes, first seen in the original Mario Party. Racers in Mario Kart have occasionally had to deal with these fiery boulders falling from the sky, hoping not to get seriously burned by them. Literally.
These half-melted rocks are sent shooting through the sky after a volcanic eruption, and they often times end up smack in the middle of the racetrack. Just like running into any other obstacle, a Lava Boulder will spin players out pretty badly. Luckily there's no permanent damage.
They first appear in the deadly sequel to Riverside Park under the name Lava Boulder, on the 2nd lap, the skies turn dark as volcanoes erupt violently in the background. Lava Boulders rain down on the track. These rocks almost seem alive, as they move slightly from side to side. Any racer who even glances off of them will suffer a fiery fate of being spun around.
Grumble Volcano, a track named for a fire-spewing mountain, is, of course, rampant with Lava Boulders. Near the middle of the course (after the tunnel with split paths), players enter the outside air, only to be greeted with blistering stones. These versions of Lava Boulders hit the ground and explode into a fiery puddle. A direct hit is rare but devastating. Driving through the flaming remnants of the debris is equally bad, as it will spin out any racers.