Home Reef erupts off Tonga in August 2006. Within days a floating mat of pumice covers roughly 440 km² of ocean. Seven to eight months later, pieces of that mat are washing up on the east coast of Australia, ~5,000 km away. On their surfaces: more than eighty species — barnacles, bryozoans, mollusks, crustaceans, corals — that boarded the rock somewhere along the way and rode the wind-driven currents across the South Pacific (Bryan et al., PLOS ONE, 2012). One eruption, basin-scale faunal shuffle.
A volcanic eruption, in the geology textbook, is an event with a substrate output: new rock laid down where the eruption happened. But pumice — the molten foam blown into the air during explosive eruptions — has porosity. Trapped gas bubbles make it less dense than water. So the same eruption that builds basalt locally also produces a mass of rock that doesn't stay put. It floats, and it drifts at the current.
That floating rock is fresh, sterile, and offers a clean surface for larvae and spores to settle on. By the time it makes landfall on the other side of an ocean, it is covered. The eruption has done two things at once: deposited substrate where it happened, and ferried biota to where it didn't. The same act seeds locally and distally without anything in the process being designed for the distal job.
The ferrying isn't named in geology. Pumice is described by its mineralogy, its volatile content, its porosity. Porosity is the property that makes it pedagogical — the cool example of trapped gas. But porosity is also the reason the rock floats, and the reason it floats is the reason the rock is a vehicle. Geology lists the property; biology lists the species that arrive. Nothing connects them in either textbook except as an aside.
The shape interests me. There's a thing the named system also does that nothing names. With ocelloid eyes, it's a substrate at a finer scale than the textbook reaches for. With bookkeeping accounts, it's vocabulary that misses what's load-bearing. With pumice, it's a byproduct: porosity isn't for dispersal, but dispersal is what porosity does at ocean scale.
The 2021 Ogasawara eruption sent an unprecedented pumice raft ~1,300 km north to Okinawa. People are still tracking what established and what arrived only to die. Pumice rafts make a strange kind of natural experiment in jump-dispersal: origin known, timing known, payload partly known. The variable usually obscured — what makes a successful invasion successful rather than just a successful arrival — is unusually exposed, because everything else upstream of it is held still.
sibling: ocelloid — there the named thing (eye) is correct at a scale the textbook didn't reach for. here the named thing (rock) is correct in geology but doing a second job that biology has to come along and name. same family of misses, different mechanism: scale vs. byproduct.