English: Rhyodacite pumice from the late Holocene of Indonesia. (CMNH 12513, Cleveland Museum of Natural History, Cleveland, Ohio, USA)

Krakatoa (also spelled Krakatau) is a subduction zone volcano in Indonesia’s Sunda Strait between the large islands of Sumatra and Java. It had a catastrophic eruption in late August 1883, culminating in an enormous steam explosion as its magma chamber collapsed. The steam explosion generated a large tsunami, which wiped out many villages in surrounding areas. Tens of thousands were killed. The steam explosion was heard several thousand miles away, and the shock wave went around the entire globe. For some time after, Krakatoa ash high in the atmosphere resulted in gorgeous orangish-red sunsets around the world. An artist in Scandinavia was inspired by the Krakatoa sunsets to make the famous surrealistic painting “The Scream”.

Krakatoa’s eruption resulted in the generation of enormous volumes of ash and pumice. Sailors noticed floating Krakatoa pumice for months afterward, sometimes mixed with the bodies and bones of victims killed by the tsunami. The rock shown above is a large pumice piece from Krakatoa’s August 1883 eruption - it was recovered floating in the ocean. Most of Krakatoa’s erupted ash and pumice is rhyodacite. A small percentage of the erupted material is dacite and andesite.

The lower (incomplete) vintage label refers to how many thousands of people were killed from the eruption. The upper vintage label reads: “From the burning mountain of Krackatao. Ship H.W. Dudley sailed three days through dead bodies and pumice stone [from] this island.”