Cinder cone volcanoes explode abruptly, building very steep, but not very tall, slopes.
Far underground, the magma combines with water and gases to form a bubbly mixture. As it approaches the surface, the reduced pressure causes the gas bubbles to explode, throwing debris high into the air.
The larger fragments tend to fall on the slopes, building them up further. The smaller pieces are carried on the winds.
Additional lava may make its way out near the base of the cone after the initial explosion.