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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Condit Dam

        The history of earth is a story full of the massive, brief events. Most of the time, we only catch a glimpse of the mark they left behind such as a turbidite from an underwater avalanche, or a ignimbrite deposit from an immense volcanic eruption. The geologist is left to imagine the power and sheer awesomeness of these processes that only once in a great while manifest themselves. However, with the rise of the internet and dissemination of technology more and more moments of amazing natural phenomena are being captured and shared. In this case of this blog post, its’ the removal of the largest dam ever in the United States.

       The Condit Dam is located on the White Salmon River in Washington state. It was dismantled on October 26th, 2011. Fortunately, a man by the name of Andy Maser captured this process with time lapse photography.

       While the impressive torrent of water released downstream is cool to watch, the more interesting thing to me is what happens upstream of the dam. If you watch carefully, in the last 30 seconds or so there are three periods where the entire surface of the river bed appears to move. If you missed them, I made three gifs showcasing the movements. 

         What is triggering the mass wasting? What interrelated factors are changing such that the river bed is oscillating from a stationary state to a state of motion? I am unsure of the real answer to this as the makeup of the stream bank is unknown to me, and even if it was the mechanics are complex. However, if I were to make an educated guess I would say that a high angle of incision may have caused the mass wasting of the river bed.

figure 1. the dam before the removal

figure 2. Steep incisions, or drops in river beds are transmitted upstream through river erosion. Before the introduced hole into the dam, this process was stuck in limbo due to the structural integrity of the dam. Once the dam was bypassed, the steep incision was free to migrate upstream. This figure simplifies the reality of the situation by assuming the vertical drop created by the dam is accommodated by one large incision when in reality several probably occurred, each migrating upstream independently. This could explain the multiple mass wasting events.

figure 3. At some point the incision migrating upstream reached a point where the hanging wall of the incision was structurally weak, or an influx of water from upstream pulsed. Either way, a large block of sediment detached from the ground and slid downstream. It is interesting to note that when the mass wasting occurs the block of loose sediment acts essentially as a solid and the internal geometry is mostly preserved. That is, the whole thing moves, but it still looks the same afterwards.

        Again the process going on here is probably much more complicated than I am presenting it as. For example, the first gif/movement appears to fit my model and the third gif/movement appears to be a rotational variation of it. However, the second gif/movement may have something to do with a wave from upstream dislodging the sediment (notice how the sediment rises before it moves).  Watch the videos and gifs for yourself and see what you think!

Until next time -TB

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Student Blog: Logan B

Check out Logan B’s blog as she spends the spring 2011 semester in Turks and Caicos!

We all wish we were somewhere tropical too.

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Student Blog: Sam

Check out Sam’s blog as he spends the spring 2011 semester in Tanzania, Africa!

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Student Blog: Aurora

Check out Aurora’s blog as she spends semesters abroad in Costa Rica (fall 2010) and New Zealand (spring 2011)!

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Welcome to the Skidmore Geoscience blog!

This blog has been created so our students, faculty, and alumni can share their adventures in geology with all of us.  We have several current students heading abroad for the 2010-2011 school year, as well as several faculty members conducting exciting new research and field work!

Please check back often to hear about their ventures!

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