KRAKATOA
Krakatoa (also spelled Krakatau) is a subduction zone
volcano in Indonesia's Sunda Strait between the large islands of Sumatra and
Java (see
image of volcano from before 1883; see
pic of it today). 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 rocks shown below are large pumice
pieces from Krakatoa’s August 1883 eruption. Most of Krakatoa’s erupted
ash and pumice is rhyodacite. A small percentage of the erupted
material is dacite and andesite.
Rhyodacite pumice (above & below) from the late August 1883
eruption of Krakatoa Volcano, Indonesia. This sample floated in the
Indian Ocean Basin for almost a year before washing ashore in eastern
Africa. As is all pumice, this rock consists of highly porous, finely
vesiculated volcanic glass. The abundance of gas bubbles (frothy texture)
makes the rock lightweight. Many of the vesicles (gas bubbles) are
noticeably stretched. Sample generously donated by Frank Brown.
Collection locality: Takwa Beach, southeastern shores of Manda Island,
Lamu Archipelago, coastal Kenya, eastern Africa.
Above:
8.6 cm across at its widest.
Below:
field of view ~2.7 cm across.
Rhyodacite pumice from the late August 1883 eruption of Krakatoa
Volcano, Indonesia. This large pumice sample was recovered floating in
the ocean.
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."
(CMNH 12513, Cleveland Museum of Natural History,
Cleveland, Ohio, USA)