Iceland is the land of fire and ice. Whenever Earth’s climate warms, however, there is less ice and more fire in Iceland. More specifically, geologists have learned in the last ten years or so that whenever large glaciers in Iceland melt, volcanism increases on the island. This is because the pressure released through deglaciation causes more of the mantle to melt, which can lead to some unusually large volcanic eruptions.
About 10,000 years ago at the end of the last glacial period, the ice began to melt in Iceland. Ice melted and melted and melted. An island-sized, 2 km thick ice sheet was melted over about 1000 years. This melting triggered a large pulse of volcanic activity that lasted for a few hundred years then subsided.
But how do geologists know that certain lavas were produced by greater degrees of melting? I mean, don’t all those Icelandic lavas look somewhat similar? Most generally, geologists can tell by estimating volumes of various volcanic flows in the field and making use of tephrachronology for dating. There is much more volcanic material from about 10,000 years ago than there is from older and more recent volcanism. A simple volume estimate doesn’t tell geologists much about the process that led to the eruption of these lavas, however. Maybe there was just natural variation with the Iceland hotspot at that time. How do we really know this increased volcanism is due to decompression melting of the mantle associated with the deglaciation?
Well, one way to study mantle melting is to look at the chemistry of the rocks on the surface. As a geochemist, I can infer changes in the degree of mantle melt that produced certain lavas by looking at the chemistry of trace elements in a single lava rock. The incompatible trace element chemistry of the post-glacial lavas suggests that they were produced by larger degrees of melt than the “normal” Iceland lavas.
Essentially, an incompatible element is an element, such as a rare earth element (those elements found at the bottom of the periodic table, stuck down below the rest), that is incompatible in the minerals found in Earth’s mantle. When a melting event occurs, incompatible elements are preferentially incorporated into the melt. These elements are incompatible because their ions don’t quite “fit” into the structure of mantle minerals. That is, their ions are too big, don’t have the right charge, or both.
Incompatible elements are always enriched in melts. However, these elements will be more enriched in smaller degrees of melt. Why? These elements tend to leave right away, from the very first stages of melt. So, a fairly small volume melt will have essentially the same amounts of incompatible elements as larger volume melts. However, the concentrations of these incompatible elements in larger melts will be lower because they are diluted by the large volume of material. I oversimplify a little, but that’s the basic idea. The post-glacial Iceland lavas in general have lower concentrations of incompatible elements than the “normal” lavas, so this seems to indicate that they were produced by larger degrees of melt of the mantle (Mervine, Sims, and Jull, to be published).
Geologists estimate that about 3100 cubic kilometers of lava were produced as a result of deglaciation-triggered melting (Jull and McKenzie, 1996). Since that large pulse of volcanism, only about 200 cubic kilometers of lava have been produced (Jull and McKenzie, 1996). That’s a big difference in the volume of lava. Personally, I find it somewhat incredible that climate change has such a big affect on volcanism. Logical, yet still incredible.
So, do we need to worry about increased volcanic eruptions as a result of global warming? Maybe. Fortunately, there are not too many volcanoes located underneath giant ice sheets. Iceland volcanoes are good examples. Another is Mt. Erebus in Antarctica. Also, we are not in a glacial period now, so the glaciers which are melting now are much smaller than those which were melting in Iceland 10,000 years ago. I wouldn’t worry too much. Rising sea level is more likely to be a problem in the near future, but it is something for volcanologists to think about.
My advisor and I are in the process of finishing up a paper on deglaciation and volcanism in northern Iceland. We’re not the first ones to come up with this idea, but we do have some valuable data to add to the story. Essentially, I’m finishing up a project that was abandoned by a geology post-doc a couple of years ago when he gave up geology for architecture school. I was handed a beautiful data set: major and trace element data on a set of samples from the wall and rim of a pit (collapse) crater in Iceland. The sample set is beautiful because the samples represent a vertical sequence, so the samples give relative time information and allow the study of how the volcanism changed through time.
I’m in the process of gathering isotopic data for these samples to learn more about the lava’s source. Isotope ratios are not affected by changes in concentration, so it doesn’t matter how much material melts or how much the major element chemistry changes. The isotopes stay the same, more or less. They’re sort of like fingerprints of different parts of the Earth. Looking at isotopic signatures allows geochemists such as myself to determine if the source of the lava was deep or shallow, what minerals might be present in the source, if different sources were mixing over time, et cetera. Isotopes are a very powerful geochemical tool, so I’m looking forward to having this new data in a few week’s time.
For those who learn more about deglaciation and Iceland, here’s a good starting reference.