On the Indonesian islands just north of Krakatau, some local strata contain suspiciously well-sorted, well-rounded pumice but are lacking the dense components commonly found in this type of material.
On the Indonesian islands just north of Krakatau, some local strata
contain suspiciously well-sorted, well-rounded pumice but are lacking the
dense components commonly found in this type of material.
[At right: Anak Krakatau erupting in 1999. ]
Steven Carey and researchers from the University of Rhode Island’s School
of Oceanography and from the Volcano-
logical Survey of Indonesia have suggested that tsunamis, generated
by the eruption of Krakatau in 1883, deposited rafts of pumice on the shores
of these islands, which lie in the Sunda Strait between Sumatra and Java.
Their article in the April issue of Geology outlines the important role
tsunamis can play in depositing volcanogenic clasts, as well as in assessing
volcanic hazards to coastal areas.
The inundation of coastal areas by tsunamis during the eruption of
Krakatau deposited several relatively thin exotic sediment layers above
the high-tide mark, says Carey. Understanding where these layers came from
and how they were deposited on the islands is critical to helping predict
how future eruptions may affect coastal areas.
Shallow submarine earthquakes, volcanic flank failure or the discharge
of pyroclastic flow into the sea typically generate tsunamis. These giant
waves wash volcanic fallout or pyroclastic flow material, which was floating
on the sea surface, inland where it can later be traced back to the original
eruption. On the islands north of Krakatau, a typical 1883 volcanic deposit
is massive, poorly sorted sand with some pumice component. Carey and his
co-workers found two anomalous layers, one on islands about 20 kilometers
north of the volcano and another 35 kilometers north on the Sumatra coast.
These layers suggest a different depositional method than the surrounding
strata.
This first layer of pumice fragments looks very similar to deposits
from a Plinian eruption — an explosive eruption that releases a steady,
turbulent stream of large volumes of fragmented magma and gas. But the
pumice on these islands is better rounded than Plinian-fall pumice and
is also missing the heavier lithic and crystal components typical of Plinian
fragments. In order to identify how the pumice in this layer differs from
other pumices of known origin, Carey used fractal analysis to compare the
particle shapes with those in Plinian-fall deposits.
His team found that the particles were quantitatively more rounded
than the clasts taken from fall or surge deposits, as if some post-eruption
process had abraded them. Carey suspected that the pumice was not dropped
on the island directly, but were dropped on the ocean and then rafted to
the island. One option was that the pumice fragments abraded each other
as the tsunami carried them up the coast of an island and deposited them
there as it receded. The missing lithic and crystal components would have
sunk into the water before deposition.
Carey and his co-workers also attribute the second unusual layer to
tsunami deposition. This layer also contains rounded pumice, but is poorly
sorted. The lithics and crystals missing from pumice in the other layers
are present and mixed with broken coral pieces and carbonate sand. This
deposit would also have been washed up in a tsunami, but here the water
entrained local beach material, including fresh pyroclastic material from
the eruption. In this case, the pumice did not drop its crystal and lithic
cargo at sea.
Characterizing these deposits can help researchers locate and assess
the origin of tsunami deposits. And by locating and linking these peculiar
strata to tsunamis, researchers can begin to determine how far tsunamis
of volcanic origin can travel and thus inundate far away shores.
Audrey Slesinger
Geotimes contributing writer
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