7 November 2013 11:20

Small eruptions of runny red basalt lava happen almost daily on Earth, at famous volcanoes such as Etna in Sicily and Kilauea on Hawai’i. We are all familiar with images of rivers of molten lava flowing down volcano flanks, both from news reports and the movies. Scientists have a good understanding of basalt lava flows as there have been ample opportunities to observe how they advance.

However, basalt is just part of a spectrum of types of lava erupted on Earth, and rhyolite lava, the least runny of all, is far richer in silica, seldom observed and little understood. Rhyolite lavas are also known as obsidian flows as they are largely composed of obsidian, a black volcanic glass prized throughout much of human history for tool manufacture. Although obsidian flows are found in many places, from Armenia, Turkey and Iceland to Mexico, New Zealand and the US, the last one to be erupted, in 1960, was not closely observed, leaving volcanologists guessing about how they advance.

I’ve been carrying out research on obsidian flows since the late 1990s but as they erupt so infrequently I never imagined I’d see one advancing during my career. However, in June 2011 there was a major eruption of rhyolite from a remote volcano called Cordón Caulle in southern Chile, which also produced the previous obsidian flow in 1960. I couldn’t visit immediately as the eruption coincided with the birth of our second child, but was able to join an expedition to the volcano in early 2012, with Victoria University of Wellington’s Ian Schipper, and led by Jon Castro from the University of Mainz. As captured in a film shown on BBC’s Volcano Live, we climbed for hours through the eerie silence of an ash-blanketed rainforest before hearing the deep booming of explosions that sent ash and huge chunks of lava high into the sky.

An unforgettable moment on the trip came when we reached the top of a ridge near the crater and suddenly saw an enormous, steaming, ash-covered obsidian lava flow before us. Its movement was imperceptibly slow, but frequent collapse of table-sized blocks from the lava edge reminded us that this lava was on the move. The lava really was the antithesis of the stereotypical red river of runny lava, being a mass of rock as tall as 10 double decker buses, with a thick crust of shattering black obsidian hiding lava that crept slowly within at temperatures as high as 900°C.

Our task was to measure how the lava was advancing, and so I used the latest imaging techniques being developed by Dr Mike James in LEC: I took a series of hundreds of photographs as I walked around the edge of the lava, and then repeated this six days later. Back in Lancaster Mike used my photographs to create two high-resolution 3D models of the lava. The difference between them allowed us to precisely quantify how the lava had advanced.

As reported by the BBC today, the results were startling – the lava advanced at only 1 to 3 metres per day, a rate more commonly associated with glaciers than lava flows. The advance mostly occurred at its edges, where hot lava appeared to be bursting through the thick obsidian crust, which was both insulating the interior of the lava and holding it back. This style of advance has been observed in basalt lavas but was not thought to occur in obsidian flows, so we were surprised to discover that, despite their drastic differences, basalt and obsidian lava flows actually behave in very similar ways. A unifying model explaining how the full spectrum of lavas on Earth advance is therefore within reach!

We returned to Cordon Caulle in January 2013, eight months after the eruption had ended, to find signs of renewed life in the ashy rainforests. Our aim was to closely study the lava, which we expected to have now stopped, and to attempt to climb inside the crater to collect ash samples. As we neared the crater during a difficult climb in thick, acrid mist we began to hear loud banging sounds reminiscent of the explosions of a year earlier. However, as the mist cleared we were amazed to see part of the obsidian lava in a completely unexpected place, and realised that the rumbles were made by huge blocks cascading from lava that was still on the move.

The still-active lava showed us that obsidian flows can continue to advance long after eruptions stop, as their insulating crust, which may be over ten metres thick, slows heat loss from the lava interior, keeping it hot enough to gradually flow and spread. An advancing lava flow can be hazardous, especially on steep slopes, as collapse of blocks of hot lava can trigger devastating pyroclastic flows – clouds of gas and ash that travel down volcano flanks at enormous speeds and destroy much in their path. Volcanoes that erupt obsidian flows may therefore threaten nearby communities over a protracted period, well after the end of eruptions.

We will return to Cordon Caulle in January 2014 to collect more samples and data. I hope to see many more flowers and insects in the recovering rainforest and perhaps hear, once again, the extraordinary sound of obsidian lava still on the move.

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