To many people Geology may seem like its about rocks, and only rocks. Geologists look at rocks that tell them something about other rocks, for the purpose of understanding things about rocks. Geologists know otherwise, of course. Not only is a rock a springboard for immense spatial and temporal leaps of imagination, but the lithosphere (rocks) is tied to every other aspect of earth in intimate and complex ways. In so many words, rocks aren’t just interesting because they are the marks left behind from environments long gone, but they actively interact with the biosphere, atmosphere, and oceans as well. The subject of this week’s blog is on a particularly fascinating connection between rocks and the biosphere.

Sessile Benthic organisms are oceanic organisms that affix themselves to a surface and live a life stationary life. Mussels, corals and barnacles are some examples of organisms that adopt this lifestyle. How does an animal adapt to become a stationary organism like a plant? That itself could be the subject of countless blog posts as there is an amazing diversity in strategies, but it all has to do with growing up. The organism modifies its life cycle to include a free floating (benthic) stage and a stationary (sessile) stage. Organisms in the phylum Cnidaria, such as corals, go through a larvae stage. The organism is called a planula in this stage and lives a free floating life largely subject to the currents of the ocean. Ideally, the ocean carries the planula to a prime location to settle down.

This stage only lasts several weeks or months though before the larvae must settle down and begin its polyp stage. This places a limit on how far away sessile benthic organisms can colonize, especially when there is a large expanse of uninhabitable habitat to cross for the organism.

Here’s where rocks come to the rescue. It turns out pumice created from explosive eruptions near or under the ocean can serve as perfect vehicles of transport for sessile benthic organisms. A quick aside for anyone not familiar with pumice, it is a rock created in explosive, usually dacitic (felsic) eruptions that cools and hardens in mid air. It contains such a high percentage of open spaces created by gases contained in the lava that it floats.

A 20 dollar bill with a pumice hat. Courtesy of

In a recent study called “Rapid, Long-Distance Dispersal by Pumice Rafting” a pumice raft created from a volcanic vent to the northeast of New Zealand was tracked. The pumice raft created from the eruption was on the order of >440 square kilometers. This mass of pumice was found to travel >5000 km away reaching the eastern coast of Australia up to 3 ½ years later.

Computer model of the trajectory of the pumice raft. 

For several weeks after the eruption the pumice remained sterile, with no colonization by sessile benthic organisms.  As time went on however, coral planula and other sessile benthic larvae looking to settle down attached to the pumice and began to colonize it.

16 months after the eruption, more the 3/4ths of most pumice surfaces were covered in organisms. For Goose Barnacles alone, the study finds a conservative estimate of the organisms transported to be 10 billion individuals. These pumice colonizers weren’t just passive passengers either. The colonization of the pumice by organisms reinforced the buoyancy of the pumice in several ways, of which two I will mention. First organisms that grew on the surface of the pumice created a shell that locked out water from seeping into the air pockets in the pumice. Second, algal and cyanobacterial respiration released gas into the poor spaces of the rock, increasing buoyancy.

        This isn’t a rare occurrence either. In the last 200 years, pumice rafts have occurred in all the major oceans. In the south west of the Pacific a pumice raft occurs about every 10 years! This all suggests that pumice rafts play a very significant role in the spread of sessile benthic/corals.

What more eloquent connection between rocks and the biosphere could there be? -TB




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