Geology Home Page physical geology historical geology planetary gems
Roger Weller, geology instructor
by Daniel Quarto
Pumice Rafts, and What They Mean to You
Let’s Learn Some Basics
is the common result of explosive volcanic eruptions in which gasses are
expanded from cooling viscous high silica content lava flows. What remains from
cooling is a lightweight and abrasive rock that has a high percentage of void
spaces within, known as porosity, from the hundreds and thousands of tiny gas
bubble holes, called vesicles and microvesicles. The composition of pumice is
usually silicic, felsic, and even rhyolitic (intermediate), but in rare
instances basaltic variations are found. Pumice is considered a glass due to
having no crystal structure.
Including white, pumice colors range from pale cream
to grey, even green-brown and black
Pumice, in addition to being very common, has also been applied to many practical uses. For most people, the following picture is pumice in its natural form:
This rectangular piece of pumice is sold in any hardware store worth your time.
It’s cheap, accessible, and is pretty effective for removing stubborn stains
when a scouring brush just wouldn’t cut it.
Pumices’ natural lightweight has been applied to make a normally dense and heavy material lighter and easier to work with; like Cement. Pumice has been used as a lightweight aggregate in cement for over two millennia, but its sophisticated modern equivalent is called Pumice-Crete, and they’ve built entire houses out of it. Because of its comparatively lighter weight to other concretes, this material can be shaped and fitted to almost any style of home (although all samples appear to be Adobe). Pumice, or rather it’s fine grained form of ash, Pozzolan, can also be mixed with lime to form lightweight plaster, so it’s probable that the outer coating as well as the innards of a home could be pumice. Naturally one shouldn’t get carried away with this idea that pumice can replace most construction materials lest they start considering the idea of pumice floor tiles. It’d make people understand the definition of aa (ah-ah) lava flows pretty quickly.
Do not adjust your web browser.
What you’re seeing is real.
This is a house made of Pumice.
Pictured above, cosmetic exfoliates and Lava soap. Both are derivatives of this plucky volcanic glass.
Porosity at Work
This is the key component that gives pumice is most identifiable features. The
above rendering of porosity can make it clear that while the shape of the rock
isn’t necessarily altered, its total volume is dropped considerably. The gaps
left behind will continually lower the weight and volume of the material until
it attains its notable lightweight, fibrous look, and numerous, however tiny,
sharp edges. Without these gases, the rock would cool into a solid, smooth,
black volcanic glass called obsidian. Obsidian, while being an aesthetically
pleasing smooth black rock has done little for humankind other than being an
effective cutting tool.
Pumice does float in water, but only under the condition that the vesicles leave little solid space within the rock. If the pumice is extremely porous on only some parts of the material, then the other, less porous sections will be too dense and the rock will sink. The rock will only float if the density is generally the same over the entirety of the rock.
Pumice rafts are aptly named rafts of pumice created by underwater volcanic
activity. They sprout up on occasion and notably in the South Pacific areas.
Study of the pumice rafts only began several decades ago, but people have been
observing them for an undocumented amount of years.
The most well known and documented occurrence of a pumice raft was when an
eruption on August 10th 2006 in the Tonga Islands in the South
Pacific east of New Zealand not only created a 19 mile long pumice raft, but
also in the aftermath of the eruption, a new volcanic island appeared to the
southwest of the raft and neighboring Late Island. The following picture was
taken from NASA’s Aqua satellite.
According to news articles, the “clouds” above the emerging island are not clouds; they are volcanic plumes, citing how the plumes appear drab grey or beige in contrast to the clouds. Also, it is said that the light greenish blue area around the raft itself is probably just fine grained sediments from the pumice, which upon reflection makes the water appear brighter.
The new volcanic island, known as “Home Reef” is nearby and
related to the Metis Shoal, another underwater volcano that has erupted 9 times
since 1851. Home Reef has remained prominently above the surface until November
of the year, when observers had noted that it had retracted slightly. This
volcanic vent has been steadily been building up and retracting for years, 2
steps forward 1 step back, if you will. But why is it pushing up and back like a
pulsating zit of the sea? As the magma is pushed up to the surface it erupts
from a vent, a crack in the surface, and begins pouring lava outward wherever
gravity will take it. As the rock cools, the process quickened by the seawater,
it solidifies and becomes the new layer of the volcano. It is in this way it
builds up, but how does it break down? It’s simple erosion. Water, salty ocean
water in particular, has the uncanny ability to, over time destroy, ruin, and
permeate anything it touches. The current activity around the underwater volcano
will drag sands, salts, any matter of sea minerals colliding into this pile of
solidified lava flows and break it down over the course of several decades, even
Just so you can get an idea of where this happened.
One would think that this raft, nearly larger than both of the islands it neighbors, would be known for its sheer size rather than what it’s really known for. A news article printed the story of a yacht called Maiken and her crew that encountered this particular raft. The group had been on the nearby Vava’u Islands, visiting the touristy cities, watching humpback whales play in the deep waters surrounding the archipelago. Upon sailing to the floating “pumice islands” however, they had some engine trouble that would force the group to cut the trip short. The danger of these rafts and the underwater volcanoes they come from isn’t from pumice rocks scratching holes into ship hulls, it’s from LAZE (LAva haZE), the result of contact between hot lava and sea water. It creates a strong mix of hydrochloric acid and steam which concern boat tours that operate in such areas. Also, the gas bubbles (which, remember, have to be released for pumice to be made) that permeate in shallower waters can reduce the density of said water to so low a point that a ship would no longer float in it, which is exactly what once happened to a Japanese research vessel. The Metis Shoal and Home Reef are past this particular stage of danger, as these problems are really only harmful from volcanoes that haven’t crested sea level.
Story of the Yacht Maiken along with many nifty pictures of the raft
Describes the volcanic activity of this particular area in the South Pacific. Dated, but fascinating.
Information and pictures of Pumice concrete. I promise you that I’m not getting paid any royalties for mentioning them, and I’m sure they don’t mind a little free advertisement.
Main source of information on the “Home Reef” eruption that caused the 19 mile long pumice raft.
Three Wikipedia links. This is where I got some pictures and the initial ideas of how I was going to branch outward with the paper. The links here lead me to the others.