 |
|
Eruption
plume above Eyjafjallajokull volcano, Iceland, April 17, 2010.
In the foreground is the meltwater floodpath from eruption-induced ice melting. Photo credit: Eyjoful Magnusson, University of Iceland. Source |
In 1973, a new
volcano erupted on the island of Heimaey, part of the Westman Islands off
the southern coast of Iceland, and threatened the very existence of the
inhabitants and their way of life as they knew it. The main town of the island,
Vestmannaeyjar, became partially inundated with lava but the inhabitants knew
they would be able to rebuild their town. However, the lava was also advancing
toward the only harbor on the island, threatening to seal off the harbor and
destroy this fishing community’s livelihood as well as one of Iceland’s main
fishing port. The Icelanders came up with a novel plan to desperately attempt to
save the town. The idea? Throw enough cold water on the lava in hopes of
creating a solidified lava wall to divert the rest of the lava away from the
harbor. It took two months of water pumping and the entire arsenal of pumping
ships from the area (high-capacity pumps were even brought in from the US) but
the harbor was eventually saved. This event is one rare case of
a “successful” fight against an advancing lava flow; however, half of the town
was still destroyed and a similar event could occur at any time.
Lava flows are not the only
volcanic threat to life and infrastructure. Lava flowing over ice or snow can
induce rapid melting, leading to an unexpected debris filled flood event. Volcanic
eruptions are dangerous and need to be quantitatively measured in order to
better understand the hazards associated with them. Specifically, understanding
the interactions between lava and ice is critical in developing hazard
assessments. We care about the interactions between lava and ice because of the
rapid melting involved and the increased risk for flash flooding and debris
flow events, such as lahars
and jokulhlaups. However,
because volcanoes are often in difficult to reach places and an eruption
involving lava and ice is inherently dangerous, it is difficult to observe the
lava/ice interactions closely. Moreover, a lava flow traveling over ice may
melt sufficiently through it and begin flowing underneath and/or through the
ice, where it is no longer observable from the surface.
In an attempt to address
these observational and measurement difficulties, a group of scientists out of
Dickinson College, Pennsylvania, in collaboration with Syracuse University, New
York, have created a way to observe and measure the interactions of lava with
ice and snow (figure 1). In short, their experiment is to pour basaltic
lava onto layers of ice or snow in a controlled setting to observe and measure
the interaction. Multiple experiments were performed using blocks of ice,
shaved ice, and sand (to simulate an ash covered ice surface).
 |
| Images of the experimental lava flows advancing over ice and snow. The top image is lava flowing over an ice bed 12 cm thick (the width of the flow is 15 cm). The second image is lava flowing over snow 12 cm thick (the width of the flow is 20 cm). Images are from the original article. |
Experiments reproduced two
end-member melting behaviors consistent with field observations: (1) flows that
melted quickly through ice and (2) flows with slow initial melting. In
experiments without a sand layer, lava advance was inhibited due to it quickly melting
through and sinking into the ice. Eventually the lava melted through the ice to
the container bottom and again flowed downslope along the container-ice
boundary. These experiments show the ability of lava to exploit these crevasses
to flow downward and through ice. By controlling the starting geometries for
ice-lava boundaries we can test different hypotheses for ice confinement of
lava flows.
One of the more interesting
observations is that during some experiments the lava would appear to skate across
the ice; this is thought to occur from the trapping of a vapor phase between
the ice and lava (the ice transitions to steam, is trapped, and acts as a lubricant
between the ice and lava). The existence of this trapped gas may also act as a
buffer for heat flow between the lava and ice, which would melt the ice slower
and allow the lava to advance further before sinking into the ice.
 |
| Formation of bubbly Limu o Pele surface in experimental lava flow. Liquid water and stream from the melting of the ice blocks below the lava is incorporated into the lava. The steam expands within the lava and forms large bubbles that then solidify, known as Limu o Pele. Image is from the main article. |
Rapid and extensive
formation of large bubbles (Limu o Pele, see figure 2) occurred within the lava
during the no-sand experiments. This may be the first published demonstration
of large-scale Limu production with water originating underneath lava. The
abundance of bubbles that formed as lava flowed across the ice indicates that
there may be a strong relationship between external water incorporation and the
formation of Limu.
Additional reading:
No comments:
Post a Comment