Woods Hole Oceanographic Institution

Author Archive for Jocelyn McPhie

What are Submarine Lava Domes?

So this expedition was supposed to be about Havre volcano exploding in 2012 and how the huge pumice rafts we all saw were formed. We will get to that eventually but so far, what we have found on the seafloor at the Havre is mostly new lavas. In fact, most of the world’s ocean floor is made of lava called basalt (plus a thin layer of sediment above that volcanologists like to ignore). Basalt lavas on the seafloor are commonly found in thin layers or in tube-like features known as pillow lava.

Although finding lava on the seafloor is no big deal, the Havre lavas are very different from the black basalt lavas and not at all common. They appear to be rhyolite. Rhyolite lavas have a chemical composition that makes them very sticky and stiff (viscous) when they emerge from the vent. Rather than flowing away, the pasty rhyolite lava piles up over the vent forming a mound (dome) that may grow to tens or even a couple of hundred meters high. If enough rhyolite comes out, then the dome may spread forming a short tongue of lava.

We have been using the autonomous underwater vehicle Sentry (a vehicle that can move underwater independently of the ship, collecting data) to make a very detailed map of the seafloor right at Havre volcano. The maps show new lava domes on the seafloor. We know for sure the Sentry features are domes because we have been down there with Jason, observed and sampled them.

So what do the rhyolite domes look like? They have an overall lumpy dome shape and close up, they look like big piles of jumbled up rock fragments, twisted blobs and jagged spires–in the words of one of the crew, “a big dump of sticky concrete.” Some of the spires are as high as 10 meters but only a few m across. All the rocky rubble comes from the sticky lava breaking up when it quenches and cracks up in the cold seawater. The broken pieces collect in talus around the edges of the domes and on the top. Most of the inside of the dome is solid rhyolite or broken just a little.

Only a few other rhyolite domes have been found and sampled on the floor of modern oceans because the rhyolite composition is not common. Rhyolite domes are reasonably common in volcanic areas on land. We know from the ones on land that sometimes growth of the domes is interrupted by explosions.

What is Pumice?

We’ll be spending a lot of time examining and talking about pumice on this trip, so we thought it would be a good idea to take a closer look at pumice now, before we get too busy.

Pumice is usually formed when volcanoes explode. Volcanoes explode when there are lots of gas bubbles trapped in the magma. As the gas pressure builds, it eventually causes an explosion. When that happens, the magma is torn apart into many small pieces that still have gas bubbles in them–we call these bubbly shards of cooled magma pumice. When it is still hot, the pumice and gas escape quickly from the vent, producing lots of explosions as they do.

If you live somewhere near an active volcano, you have probably seen pumice on the ground or in cliffs. If you don’t live near an active volcano, you still might have seen pumice because it often floats, thanks to of all the holes, and it can be found washed up on beaches far from the volcano that made it.

We have seen many explosive eruptions at volcanoes on land but only two very small ones at volcanoes under the ocean. This expedition is going to Havre volcano, which is entirely submerged. When Havre erupted in 2012, it produced a large amount of floating pumice so we know the eruption was explosive. Ocean currents swept the floating pumice along, some of it as far as the eastern coast of Australia. Pumice doesn’t float forever—water seeps slowly into the holes in the pumice, making it heavier and heavier until it sinks to the seafloor.

Floating pumice is relatively easy to find and study but freshly erupted pumice doesn’t always float, even though it is very light and full of holes. When the pumice coming out of a vent on the seafloor is very hot and just the right size, it can quickly soak up water and fall back down to the seafloor, so chances are that some of the pumice produced by Havre in 2012 is on the seafloor close to the volcano. In a couple of days, we will know for sure.