Medium-sized tephra are called lapili. Tephra are rock fragments ejected from a volcano. The smallest tephra are called ash, while the largest are called blocks if they are solid and bombs if they are liquid.
Tephra can cause many problems in addition to the accompanying volcanic eruption. Tephra clouds can spread hundreds of kilometers and cause periods of darkness affecting plant and animal life. These clouds reduce visibility and cause respiratory issues. They interfere with electric transformers and power lines and cause problems with communication devices. They may also start forest fires and are known to generate electricity for lightning. Larger tephra can collapse structures.
Tephra consist of pumice, glass, mineral crystals and different types of shattered rock. The size of particles falling from a volcano is largest near the volcano's source, with particles like lapilli and ash decreasing in size the farther they are from the volcano. Particles more than 1 millimeter in diameter fall to the ground within 30 minutes.