{"id":525,"date":"2012-01-20T16:03:01","date_gmt":"2012-01-20T16:03:01","guid":{"rendered":"http:\/\/all-geo.org\/volcan01010\/?p=525"},"modified":"2012-06-10T09:12:19","modified_gmt":"2012-06-10T08:12:19","slug":"icelands-melting-glaciers-and-other-stories","status":"publish","type":"post","link":"https:\/\/all-geo.org\/volcan01010\/2012\/01\/icelands-melting-glaciers-and-other-stories\/","title":{"rendered":"Iceland’s melting glaciers and other stories from the Nordic Geological Winter Meeting"},"content":{"rendered":"

This post describes my highlights from the 30th<\/sup> Nordic Geological Winter Meeting<\/a> that took place in Iceland last week. The most interesting results include that Iceland’s glaciers may be gone in 200 years, that the May 2011 Gr\u00edmsv\u00f6tn eruption was supplied by a fresh input of magma, and that melt flows upwards into the columns that form when lavas are cooling.<\/p>\n

\"\"<\/a>

The meeting took place in Harpa, Reykjav\u00edk's newly-opened opera house and conference centre. It was attended by over 250 participants, mainly from Nordic countries. Sessions covered topics such as tephrochronology, glaciology, and igneous, metamorphic and resource geology.<\/p><\/div>\n

Iceland’s shrinking glaciers<\/h3>\n

The alarming conclusion of Helgi Bj\u00f6rnsson’s talk<\/a> was that Iceland’s glaciers may have less than two centuries left. This seems incredible, given that they are so huge, but Bj\u00f6rnsson demonstrated how their rapid demise results from a positive feedback loop or vicious circle: the smaller these ice caps get, the faster they will shrink. Understanding this depends on a concept known as the Equilibrium Line Altitude (ELA).<\/p>\n

Above the ELA, more snow and ice accumulate on the glacier in the winter than are lost in the summer. The opposite is true beneath the ELA, which is known as the ablation zone. As long as the area above the ELA is large enough to collect enough snow to replace the ice lost beneath it, the glacier will be stable. With rising global temperatures, the ELA is rises, the glaciers lose more ice than can be replaced by annual snowfall, and they shrink. But this alone doesn’t explain the rapid shrinking.<\/p>\n

Bj\u00f6rnsson and colleagues have mapped the bedrock surface beneath Iceland’s glaciers using ground-penetrating radar. This is how we know, for example, that Katla’s glaciers are ~500 m thick. But this means that the surfaces of the glaciers are only above the ELA because of the ice itself<\/em>. For example, 65% of the area of Vatnaj\u00f6kull is above the ELA, but only 20% of the bedrock is. As glacier shrinks, the surface altitude of the ice decreases, which reduces the area above the ELA, which makes the glacier shrink faster and so it continues.<\/p>\n

There are equations that show how snow is compacted into ice. There are equations that show how glaciers flow under their own weight. There are equations that show how ice melts at lower, warmer, elevations. Combining these with the surface shapes of the glaciers, the radar maps of the bedrock and estimates of future climate (2\u00b0C increase by 2100), Bj\u00f6rnsson and colleagues modelled the glaciers to see how they would change shape in the future. They concluded that they would be mostly gone in 150-200 years.<\/p>\n

The disappearing ice was a theme in other glaciology talks and even in other sessions. Another glaciologist, Thorsteinn Thorsteinsson<\/a> showed the results of 25 years of monitoring the H\u00f6fsj\u00f6kull glacier, during which period it has lost 6.5% of its mass. The summers of 1991 and 2010 were particularly bad for the glacier, when a coating of fresh volcanic ash caused extra melting. Over in the geodynamics session, Th\u00f3ra \u00c1rnad\u00f3ttir<\/a> presented results from GPS stations that are looking for tiny movements of the land surface caused by plate tectonics or volcanic activity. Every station that they put on the mountains poking through the glaciers (nunataks) shows a strong upward motion. This is caused by the land rising as the weight of the ice disappears, and is something to bear in mind when looking for signs of inflation at ice-covered volcanoes.<\/p>\n

\"Reykjanes\"<\/a>

The Reykjanes peninsula basking in the midday January sun. This is where the mid-Atlantic ridge comes ashore. If you swam due south from here, the next land that you would hit would be Antarctica.<\/p><\/div>\n

Fresh magma drove the Gr\u00edmsv\u00f6tn 2011 eruption<\/h3>\n

Olgeir Sigmarsson’s talk<\/a> looked at the geochemistry of the magma from the Gr\u00edmsv\u00f6tn eruption in May, and concluded that the reason that the eruption was so powerful is that it was driven by a fresh batch of hot, gas-rich magma from deep beneath the volcano (see my post from August to see the deposits close up<\/a>).<\/p>\n

Geochemistry is a very important tool in understanding volcanoes. It relies on two facts, which will be familiar to any students of a Volcanology 101 class:<\/p>\n

    \n
  1. Magma is a \tcocktail<\/em><\/strong> of many different chemical elements and their oxides \te.g. SiO2<\/sub>, Al2<\/sub>O3<\/sub>, MgO, CaO, F, Cl.<\/li>\n
  2. Magma doesn’t \tall melt or freeze at once<\/strong> <\/em>or at a single temperature.<\/li>\n<\/ol>\n

    As magma solidifies, crystals grow, and the concentration of the ingredients of those crystals is reduced in the remaining liquid. For example, MgO is removed from the liquid when crystals of the mineral olivine<\/a> ([Mg,Fe]2<\/sub>SiO4<\/sub>) form. This process is called fractional crystallisation<\/a>.<\/p>\n

    A key piece of evidence in Sigmarsson’s talk was the concentration of the element thorium (Th) in samples of ash and pumice from each Gr\u00edmsv\u00f6tn eruption of the 20th<\/sup> century.\u00a0 These show an increase in Th concentration over time.\u00a0 This suggests that they all came from the same source beneath the volcano (the magma chamber), where growth of crystals of low-Th minerals results in an increase in concentration in the remaining melt. This melt was erupted in each of the events that produced the samples e.g. in 1998 or 2004, and was probably left over from the huge Laki eruption in 1783. The Gr\u00edmsv\u00f6tn 2011 samples contain much less Th than previous eruptions. This implies that it is a different magma: hotter, richer in gas, and closer to the composition of the original melt that formed in the mantle.
    \n    <\/p>\n

    Melt migration into lava columns<\/h3>\n

    <\/a>
    \n    Hannes Mattsson<\/a> gave a presentation suggesting a previously-undescribed process within cooling lavas that explains the patterns in polished lava tiles (see image) as part of a study into the formation of columnar joints in basalt.\u00a0 This work has already been published<\/a> in the journal Nature Communications.\u00a0 Joints<\/em> are a system of cracks in a rock, and columnar joints break the rock into columns. Famous examples of columnar jointed lavas are the Giant’s Causeway<\/a> in the UK, the Devil’s Postpile<\/a> in the USA and Svartifoss<\/a> in Iceland.\u00a0 Mattsson and colleagues found that melt flows into the columns as they form.<\/p>\n

    \"Floor<\/a>

    Basalt tiles on the conference centre floor. These were quarried from columnar-jointed lava, and each tile is a slice through a column like a plank of wood. The pattern, which resembles the grain in wood, results from melt movement within the columns as they cooled.<\/p><\/div>\n

    Columnar joints form because lava contracts as it cools. Usually, the top of the lava, exposed to the air, cools most quickly and the cracks grow downwards into the flow. Experiments show that basalt lava shrinks by 10-15% when it solidifies, but the volume of the gaps created between the columns is much less than this.\u00a0 Mattsson’s group thought that this is because more material flows into the columns as they form.\u00a0 Sometimes you can see where finger-like parcels of lava have done this, but they are not very common, so another mechanism is required.<\/p>\n

    This mechanism is also based on the fact that magma doesn’t freeze or melt all at once<\/em>.\u00a0 As the lava cools, the first crystals to form are tiny, white, matchbox-shaped grains of plagioclase feldspar.\u00a0 The dark bands in the tiles are rich in a black-coloured mineral called titanomagnetite, which is the last mineral to start crystallising.\u00a0 If lots of plagioclase crystallises, then the crystals can lock into each other and form a network with some strength.\u00a0 By this point, the lava is effectively rigid, but the remaining melt is still free to move through the gaps between the crystals.\u00a0 It is equivalent to sucking coffee through a sugar cube.<\/p>\n

    Titanomagnetite, as the name would suggest, is magnetic. To test their hypothesis, Mattsson and colleagues measured the magnetic field across the column. It showed that the titanomagnetite grains were all lined up parallel to the column’s edge, instead of begin randomly orientated, or lined up with surface of the ground.\u00a0 This demonstrates that melt flows into the columns from the molten interior of the flow, and seems to confirm their idea.\u00a0 The different colours of the individual bands could be explained by different pulses of melt moving in.<\/p>\n

    If you visit Iceland, you will surely see these patterns, as the same tiles are used in the floor of Keflav\u00edk airport. Now you know what they are.<\/p>\n

    Other highlights<\/h3>\n
    \"Football<\/a>

    The conference was opened by the president of Iceland. He knew how to play to a room full of geoscientists. In his address, he said that seven-day creation myths are clearly false because Iceland was still being created. I imagine that he is equally unimpressed by stories about walking on water; in Reykjav\u00edk, people play football on the city pond.<\/p><\/div>\n