This week it’s all about volcanoes. And for once, Kilauea, active and interesting though it unquestionably is, isn’t the headline news. That distinction goes to an entirely different type of volcano with an entirely different style of eruption.
Volcano: Fuego Erupts
We’ve been talking a lot recently about Kilauea and the awesome power of nature which it displays — but Kilauea is a pussycat of a volcano in comparison with others, and the eruption of Fuego, in Guatemala, on 4 June, is a stark reminder of exactly how dangerous a volcanic eruption can be.
Latest reports indicate that the devastation and damage far exceeds that of Kilauea, with the BBC reporting that “192 people are missing and 75 are dead…More than 1.7 million people have been affected by Sunday’s eruption, with more than 3,000 evacuated.”
The devastation stemmed from two specific types of volcanic hazard — pyroclastic flows (or pyroclastic density currents) and lahars.
Pyroclastic flows are the inevitable consequence of an explosive eruption: when a vast amount of material is thrown into the air, the laws of gravity dictate that it must fall to Earth. So large volcanic plumes collapse and, being invested with so much energy, create flows which can cover significant areas.
If you’ve seen some of the images in the news on the eruption of Fuego, you’ll understand the advice offered by the USGS and other volcanologists: “If you witness a pyroclastic flow, run in the opposite direction as quickly as possible”. They consist of a mixture of material, mainly volcanic rock, which is hot (typically between 200-700C) and usually move faster than 50 mph. Though the flows tend to follow valleys, they are not constrained by the valley. The flows can travel significant distances and deposit enormous layers of debris.
Lahars often follow volcanic eruptions, though they can occur without them. The USGS has the best definition: “a hot or cold mixture of water and rock fragments that flows down the slopes of a volcano and typically enters a river valley”.
The subduction of the Cocos and Nazca plates beneath central and South America has given rise to a chain of hundreds of active volcanoes, similar in style and capable of devastating eruptions. Fuego isn’t unique — and that’s a sobering thought.
And so to Kilauea, with its fire fountains and its lava flows. This week it’s been anything but quiet and, after over a month, there are as yet no signs of the activity lessening. The attached infographic from the USGS summarises the activity up until 4 June, 2018. Since then, the flow of lava has reached the sea at Kapoho Bay and completely filled it.
What we are seeing in action at Kilauea is the creation of new land. The lava comes from deep within the Earth and the flow of lava that is currently covering the south-eastern tip of the Big Island is just another in the series that have built the Hawaiian Islands, one after another, as the Pacific plate moves slowly over an upwelling plume of magma.
Kilauea has destroyed property but it has, as yet, cost no lives and its (relatively) slow progress as an eruption means that, with common sense and correct response to scientific advice, it shouldn’t cost any lives in the future. Interestingly, the hazards it poses are very different from those at Fuego. The low explosivity and gentle slopes mean that pyroclastic flows and lahars aren’t a risk, but that the significant hazard comes instead from gas and from volcanic fog and haze.
Compare and Contrast
So why are these two volcanoes so different? The answer lies in their tectonic setting. I noted above that Kilauea overlies a magma plume and that Fuego is on a subduction zone. This is significant. Because of this, Kilauea is one type of volcano (a shield volcano) and Fuego another (a composite, or stratovolcano). The differences are illustrated in the graphic.
The magma that feeds Kilauea and other Hawaiian volcanoes comes from deep inside the Earth and is primarily basalt. Its mineral composition is geochemically very different from more evolved magmas, such as Fuego. Without going into too much mineralogical detail, the key difference is that Hawaiian lavas are relatively low in the mineral silica. This means that they are thinner and less viscous. They don’t hold gases as easily (hence the volcanic gas emissions) and their eruptions are typically longer and slower.
In Fuego and other subduction zone volcanoes, the process is different. Rather than deriving directly from the Earth’s mantle, the magma is caused by the melting of subducted crust, and evolves into a different mix of minerals. Silica content is higher, which means that the molten rocks is more viscous (hence the steeper slopes of the characteristic subduction volcano); the rocks hold gases and the pressure builds up — resulting in short, extremely violent eruptions with associated hazards of pyroclastic flows and lahars.
Tempting though it is to treat both of these volcanoes as related, and as interacting parts of a single volcanic system, that is not the case. They are both volcanoes — but they’re otherwise unconnected.