Most of the volcanic rock and tephra is of basalt or basaltic andesite. Beerenberg is composed primarily of basaltic lava flows with minor amounts of tephra. Explosive eruptions are characterized by gas-driven explosions that propels magma and tephra. Amorphous clays are young, and commonly found in recent volcanic ash deposits such as tephra. An estimated of magma erupted, producing around of tephra. Large pieces of tephra are called volcanic bombs. The calcium sulfate, anhydrite, was present as phenocrysts in the erupted tephra. The volcano also erupted lapilli (a form of tephra) and pumice bombs. These deposits are composed of layers of tephra and sediment. Much of the inland is covered by salt flats, tephra and lava flows. No tephra or lava volumes were recorded at this time. They have wide craters and are formed of highly altered, thickly bedded tephra. A small portion of the erupted tephra was deposited locally. Most of the tephra was deposited west-southwest of Samalas. The eroded area consisted mostly of loose tephra, easily washed away.