Crater delta, tephra sand

Flush sediment into a body of water and a delta will form – regardless of scale or environment, it’s just one of those ubiquitous natural processes.

Taking advantage of a recent visit to the city of Bandung in central West Java, my companion, familiar with the region, suggested the scenic route back to Jakarta – and some scenery it proved to be. Being prominently positioned on one branch of the Pacific “ring of fire,” Java owes its topography to its volcanoes, and it’s these towering and long-lived eruptive centres that form the backbone of the island and the backdrop to Bandung. One of them, Tangkuban Perahu (or Parahu in the Sundanese dialect) reaches an altitude of nearly 2100 meters and is something of a tourist destination, since an easy drive brings you to the rim of the crater and a spectacular landscape. Climbing above the tea plantations, it’s not long before you recognise that you’re in a volcanic neighbourhood: the entire atmosphere becomes sulphurous, the whiffs of hydrogen sulphide developing into an all-pervasive stink. And then, there you are on the lip of a crater in what is described technically as a dormant volcano. “Dormant” is fine when you’re there, but tomorrow? Tangkuban Perhau last erupted in 1983 and in 2005 an alert lasted for some time as rumbling seismic activity suggested the threat of an impending eruption. The palm-roofed “shelters” along the road, were, I presumed, in case of rain not volcanic pyrotechnics.

The crater is but the current focus of a long period of eruptive history and is but a minor feature of a much larger caldera, the roughly circular chasm formed when the entire edifice blew itself up around 190,000 years ago. For a minor feature, however, it’s still awe-inspiring and huge. Steam vents from a distant fumarole, but its roaring sound can be distinctly heard from the rim. In the floor of the crater is a toxic-looking lake – and a complex of deltas, water from rain and the fumaroles, sediment from the crumbling crater walls (recent landslides are disturbingly visible around the cliffs) and the ongoing minor eruptions. Well, mainly minor: close inspection reveals that the surface of the delta is littered with what must be boulder-sized debris – grain sizes that would seem to be inconsistent with those of the rest of the delta sediments….

The majority of the recent eruptions have been phreatic – explosive events resulting from molten magma encountering near-surface groundwater and causing instantaneous detonation of vast volumes of steam and debris, hurled into the air and cascaded back to the surface. These deposits, tephra, have no respect for grain size organisation – they contain everything from clay to boulders:

And, of course, tephra deposits contain large amounts of newborn sand. Small streams transport this down the mountain, and provide the opportunity for a sampling of this geological nursery. Angular grains of volcanic glass, individual crystals, rock fragments, and the occasional perfect globule:

The name Tangkuban Perahu refers to an upturned boat – the volcano of course comes with a legend, one of eternal youth, unrequited Oedipal problems, demons, a dam, and a faked sunset. But the reality of the landscape outdoes any legend.

[A good geological report on the eruptive history and construction of Tangkuban Perahu can be found here – note the designation of “sandy ash” layers.]