Pumice Rafts: The Floating Rocks of the Sea

Do rocks float? The answer to that is “Depends on which rocks you’re talking about.”

We just looked at what happens in the atmosphere when a volcano like Copahue erupts. We also looked at the impact the 1912 eruption of Novarupta still has today. And, before VIIRS was launched into space, there was Eyjafjallajökull – the Icelandic volcano that nobody could pronounce. (Think “Eye-a-Fiat-la-yo-could” [click here to hear audio of some guy saying it properly].) These are examples of what geologists would refer to as an “explosive eruption”. Not all volcanoes blow ash into the atmosphere. Think of Kilauea in Hawaii – this is an example of an “effusive eruption” where lava oozes or bubbles up out of the ground in a rather non-violent manner. These are the most common volcanic eruptions on land that everyone should already be familiar with.

But, what happens when the volcano is underwater? You get what a group of New Zealand geologists are calling “Tangaroan” (named after the Maori god of the sea, Tangaroa). This article explains it in more detail, but the short version is this: at the bottom of the ocean, there is immense pressure from the weight of the water above the volcano that prevents an eruption from being truly “explosive”, yet the eruptions are often more violent than an effusive eruption. The magma, filled with gas, erupts into the ocean where the outer edges are instantly cooled and solidified. (The water is cold at the bottom of the ocean.) This traps all the gas inside and you get a rock that’s filled with millions of tiny air bubbles, which is called pumice. This new rock can be so light, it floats to the surface.

What does this have to do with VIIRS or a blog about imagery from weather satellites? Large underwater volcanic eruptions can create large quantities of pumice that float to the surface of the ocean and create what are called pumice rafts. VIIRS has seen these pumice rafts.

Here is a “natural color” or “pseudo-true color” RGB composite of VIIRS channels I-01 (0.64 µm, blue), I-02 (0.865 µm, green) and I-03 (1.61 µm, red), taken at 01:40 UTC 27 August 2012. Notice anything unusual in the water?

False color RGB composite of VIIRS channels I-01, I-02 and I-03, taken 01:40 UTC 27 August 2012

False color RGB composite of VIIRS channels I-01, I-02 and I-03, taken 01:40 UTC 27 August 2012

As always, click on the image, then on the “2798×2840″ link below the banner to see the full resolution image. All those pale blue-gray swirls in the ocean surrounding Raoul Island and Macauley Island are the pumice rafts. They almost look like someone sprayed “Silly String” in the ocean.

To get a sense of the scale of these rafts, the latitude lines plotted on the image are ~111 km apart. Some of these rafts are 1-2 km wide in places. In this image you can see pumice rafts stretching from about 27.5 °S to 31.5 °S latitude and from about 175 °W to 178 °E longitude. That is a lot of floating rocks!

Here is a zoomed version of the previous image:

False color RGB composite of VIIRS channels I-01, I-02 and I-03, taken 01:40 UTC 27 August 2012

False color RGB composite of VIIRS channels I-01, I-02 and I-03, taken 01:40 UTC 27 August 2012

The main concentration of floating pumice is in the box the covers the area from 29 °S to 30 °S latitude and from about 176 °W to 178 °E longitude, although there is plenty of pumice south of that box – it’s just a little harder to see.

As an aside, Raoul and Macauley islands are part of the Kermadec Islands of New Zealand. If you’re interested, the New Zealand government is always looking for volunteers to spend six months on Raoul Island pulling weeds and keeping invasive species off the island. (There, that saves me from doing a Remote Island post to cover this.)

These pumice rafts have been traced back to the eruption of the Havre Seamount (an underwater volcano) on 18 July 2012. This new eruption is part of the “Ring of Fire” in the southwestern part of the Pacific Ocean, roughly 1,000 kilometers northeast of New Zealand. If you believe the Wikipedia article linked to first in this paragraph, the eruption was unknown until an aircraft passenger took pictures of the pumice raft from her plane on 31 July 2012. I have been able to track this pumice back to 26 July 2012. Before that, it is too cloudy, making it difficult to see anything. (Apparently, MODIS saw it on 19 July 2012.)

False color RGB composite of VIIRS channels I-01, I-02 and I-03, taken 01:39 UTC 26 July 2012

False color RGB composite of VIIRS channels I-01, I-02 and I-03, taken 01:39 UTC 26 July 2012

The red arrow points to the pumice raft. There’s a nice looking cyclone southwest of the pumice, but I’m not sure if it was given a name. If you zoom in, you can see Cheeseman Island and Curtis Island off to the east of the raft. These islands were obscured by clouds on the 27 August 2012 overpass. Cheeseman Island is only 7.6 ha (19 acres) and Curtis Island is 40 ha (99 acres), yet VIIRS has the resolution to see them!

In an effort to highlight these pumice rafts, a PCI analysis was performed on the five VIIRS high-resolution imagery (I-band) channels. PCI analysis uses principal components to identify the major modes of variability within the data. Analysis of the 5 VIIRS I-bands resulted in 5 PCIs or component images. Of those components, PCI-2, 3, and 5 appeared to show the pumice rafts. A particular RGB combination of those three components (red = PCI-5, green = PCI-2 and blue = PCI-3) resulted in the pumice appearing red on a green-blue ocean. Clouds are white, then cyan and then red for colder cloud-top temperatures. (Certain pepper-like black pixels are out of range in the PCI analysis.) The three principal components that highlight the pumice rafts are shown in the figure below, along with the resulting RGB composite. Unfortunately, these images were made using McIDAS-X, which has a habit of plotting VIIRS data upside-down. Therefore, north in each image is at the bottom.

PCI Analysis of the 5 VIIRS I-band channels from 01:40 UTC 27 August 2012

PCI Analysis of the 5 VIIRS I-band channels from 01:40 UTC 27 August 2012. Panels A, B, and C are the second, third and fifth principal component images from this analysis (PCI-2, PCI-3 and PCI-5). Panel D is an RGB composite of these three images with PCI-5 as red, PCI-2 as green and PCI-3 as blue. Images courtesy Don Hillger.

This in an image you’ll want to zoom in on to see the details as you consider the information in the previous paragraph. There are two main results of this PCI analysis: it can be used to highlight pumice rafts (although they have the same color as cold cloud tops) and the temperature information from channel I-5 (11.5 µm), which shows up in PCI-5, indicates that the pumice has a tendency to collect along gradients in sea surface temperature.

Being able to track the pumice rafts is important for geology, biology and oceanography. They can act as a tracer for following ocean currents. Some of them crack and fill with water, causing them to sink to the bottom, depositing the newly formed rock in other parts of the sea floor. The nature of the pumice gives clues about what happens in underwater volcanoes, a process that is not well known at this point. And, as these floating pieces of pumice are carried around, organisms like algae, coral, and barnacles will attach to them and grow, eventually settling in far away places. Studying these rafts may shed new light on how life can spread across the oceans.

So, yes – rocks can float. And they can be seen by a weather satellite with 375 m resolution.

Copahue, the Stinky Volcano

On the border between Chile and Argentina sits the volcano Copahue. (If you say it out loud, it is pronounced “CO-pa-hway”.) In the local Mapuche language, copahue means “sulfur water”.  This name was given to the volcano as the most active crater contains a highly acidic lake full of sulfur.  An eruption in 1992 filled the area with “a strong sulfur smell.” Later eruptions have involved “pyroclastic sulfur” (molten hot sulfur ash) and highly acidic mudflows. That doesn’t sound very pleasant.

Right before Christmas, Copahue was at it again. It erupted on 22 December 2012, sending a cloud of sulfur ash into the atmosphere, and MODIS got there first. VIIRS got there 4 hours later and took this image:

VIIRS "true color" RGB composite of channels M-03, M-04 and M-05, taken 18:38 UTC 22 December 2012

VIIRS "true color" RGB composite of channels M-03, M-04 and M-05, taken 18:38 UTC 22 December 2012

This is a “true color” image just like the MODIS one in the link. Make sure you click on the image, then on the “3200×2304″ link below the banner to see it in full resolution. Then see if you can spot the volcanic ash cloud from Copahue. I’ll give you a hint: it’s the only cloud that appears brownish-gray.

If you still can’t see it, here’s a zoomed-in image with a yellow arrow to help you out:

VIIRS "true color" RGB composite of the Copahue volcano, taken 18:38 UTC 22 December 2012

VIIRS "true color" RGB composite of the Copahue volcano, taken 18:38 UTC 22 December 2012

Now compare the ash cloud in the VIIRS image with the ash cloud in the MODIS image from 4 hours earlier. (This is easier to do if you can locate in the VIIRS image the lakes marked as “Embalse los Barreales” in the MODIS image.) There’s a lot less ash in the VIIRS image, right?

Not so fast. As the ash dispersed, the plume thinned out, making it harder to see against the brown background surface. But, that doesn’t mean that it’s not there. Here’s the “split window difference” image from VIIRS at the same time:

VIIRS "split window difference" image (M-15 - M-16) taken 18:38 UTC 22 December 2012

VIIRS "split window difference" image (M-15 - M-16) taken 18:38 UTC 22 December 2012

That whole black plume is volcanic ash detected by the split window difference. The yellow arrow points to Copahue and the ash plume that is visible in the true color image. The red arrow points to the ash plume that is not visible in the true color image, yet is detected by this simple channel difference (M-15 minus M-16). A victory for the split window technique!

It was also a victory for the EUMETSAT Dust RGB, which didn’t work for the 100-year-old ash cloud over Alaska. Here’s what that RGB composite looks like when applied to VIIRS:

EUMETSAT's Dust RGB composite applied to VIIRS from 18:38 UTC 22 December 2012

EUMETSAT's Dust RGB composite applied to VIIRS from 18:38 UTC 22 December 2012

It is interesting that the ash plume right over Copahue is tough to detect in this RGB composite because it is red, just like a lot of the other clouds. As the plume thins out away from the volcano, its color changes to a variety of pastels of pink and blue, and even appears to extend out over the Atlantic Ocean. Where clouds and ash coexist near the coast of Argentina, pixels show up orange and yellow and green (click to the high-resolution image to see that).

Why does the plume appear to extend into the Atlantic Ocean in the EUMETSAT Dust RGB, and not in the split window difference? It is due to the fact that the Dust RGB uses channel M-14 (8.55 µm), which is sensitive to absorption by sulfur dioxide (SO2) gas. The split window difference is better at detecting sulfuric ash particles, which may have mostly settled out of the atmosphere before reaching the Atlantic coast. There are likely still some ash particles in the plume, though – just not enough to show up easily in the split window difference. Detection of SO2 gas plumes has been used to infer the presence of volcanic ash.

Being able to see the location of the volcanic ash very important to pilots. Aircraft engines don’t work that well when they are sucking in particles of liquified sulfur and other abrasive and corrosive materials spit out by stinky volcanoes like Copahue.