Katla Volcano
Photo: Halldór Ólafsson (hallo@hi.is)
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Katla is one of the most active volcanoes in Iceland,
with two eruptions each century on average. The volcanic centre is located
on the east margin of the eastern volcanic zone in Iceland, above a junction
of linear rift tectonics and a steep increase in crustal thickness, compared
to the rift zone north of the volcano. The volcano massif rises to an elevation
of 1512 m and is partly covered by the 200-700 m thick Mýrdalsjökull
glacier which fills up a caldera of about 70 km3, and covers
its eruptive vents. The volcanic centre of 300 km3 is mainly
formed in the time interval from 220 thousands years ago to Holocene. The
present composite caldera is formed stepwise during the last glaciation.
The beginning of Holocene activity of the volcano is marked by the major
explosive eruption named Sólheimar eruption, dated 12.000 BP by
the Skógar (Vedde) tephra layer. The volume of eruptives from this
eruption is estimated 6-7 km3, contributing significantly to
the caldera subsidence.
Results of a 2-D, seismic undershooting experiment on the Katla volcano indicate a shallow magma reservoir with bottom at depth of 3 km below the surface of the ice cap(Gudmundsson et al., 1994). Thickness of the magma reservoir was estimated 1 km, and roughly assumed volume could be 10-12 km3 . According to these measurements the magma reservoir can only be defined as a volume where the relative melt content is high enough to create a significant S-wave shadow. The volcanic activity in historical times is characterized by high eruption frequency, and production of Fe-Ti basalts and minor amount of silicic tephra. In spite of the dominating basaltic composition of magma, all eruptions are highly explosive due to the subglacial mode of extrusion. The most recent major eruption of Katla occurred in 1918, but possibly a small eruption that did not break the ice cover occurred in 1955. Similar smaller events may also have occurred in historical times. |
Table 1. Eruptions in the Katla volcanic centre in historical times (G.Larsen, 1993).
Eruption site Year/century Date of eruption Duration (days) Repose time (years) Katla 1918 October 12. 24 58 - 1860 May 8. 20 37 - 1823 June 26. 28 68 - 1755 October 17. ~120 34 - 1721 May 11. ~100 61 - 1660 November 3. ~60 35 - 1625 September 2. 13 13 - 1612 October 12. 32 - 1580 August 11. ~80 - ~1500 - 15. century - 1416 - ~1357 - 1262 - 1245 - ~1179 - 12. century Eldgjá-Katla ~934 Katla ~920 - Late 9. cent. or early 10. cent.
Chemical composition of the Katla products is characterized by Fe-Ti basalts. (Table 2.) Remarkable is the uniform composition of the basaltic products through time. Newertheless, some production of silicic magma from the Katla volcanic centre has taken place through its activity. During the last glacial period (Weichsel) rhyolite domes were erupted around the caldera rim, and during postglacial time at least 11 dacitic tephra layers originated within the caldera are known, the youngest one dated 1700 C14-years ago (G.Larsen, 1994). The volume of these tephra layers (except the Vedde tephra) are relatively small compared to the basaltic production. As mentioned before, minor amounts of silicic tephra has been erupted together with the basaltic tephra in historical eruptions, commonly encountered as separate pumice fragments. As a whole, rhyolitic, sub-alcalic rocks produced from the Katla volcanic center is estimated 10 % of its total production, while 90 % consist of homogenous, aphyric Fe-Ti basalts (Hildebrand, 1999).
Table 2. Chemical composition of products from the Katla volcanic centre. (%)
(Compiled by Níels Óskarsson)
LABELS K1000* K1485** K1625 K1660 K1860 K1918a K1918 Kx-Xen. Eyjafj. 1821-23 SiO2 47.19 46.61 46.87 47.17 48.22 47.66 47.55 63.83 60.29 TiO2 4.39 5.28 4.77 4.97 4.71 4.66 4.52 1.49 1.36 Al2O3 13.45 13.22 12.84 13.11 13.08 12.84 12.98 14.05 15.02 FeO 15.54 15.49 15.55 15.21 15.08 15.18 15.01 7.30 9.44 MnO 0.27 0.22 0.25 0.25 0.24 0.24 0.23 0.21 0.23 CaO 9.93 10.06 9.51 9.59 9.30 9.63 9.73 4.11 5.36 MgO 5.14 4.78 5.06 4.92 4.61 4.84 5.15 1.48 1.81 Na2O 2.80 2.92 3.46 3.11 3.14 3.31 3.27 4.85 4.30 K2O 0.61 0.61 0.71 0.71 0.76 0.71 0.67 2.14 1.50 P2O5 0.51 0.65 0.82 0.80 0.71 0.78 0.72 0.36 0.54 Ba 0.0173 0.0176 0.0218 0.0202 0.0209 0.0196 0.0269 0.0539 0.0396 Co 0.0089 0.0082 0.0075 0.0083 0.0073 0.0086 0.0069 0.0020 0.0019 Cr 0.0027 0.0010 0.0013 0.0009 0.0011 0.0011 0.0029 0.0007 0.0006 Cu 0.0125 0.0072 0.0060 0.0058 0.0055 0.0060 0.0064 0.0010 0.0014 Ni 0.0040 0.0022 0.0025 0.0017 0.0015 0.0017 0.0022 0.0006 0.0005 Sc 0.0035 0.0033 0.0032 0.0032 0.0031 0.0030 0.0031 0.0012 0.0019 Sr 0.0330 0.0357 0.0398 0.0386 0.0375 0.0396 0.0375 0.0341 0.0307 V 0.0435 0.0457 0.0380 0.0385 0.0377 0.0358 0.0363 0.0050 0.0033 Y 0.0038 0.0040 0.0043 0.0044 0.0042 0.0043 0.0039 0.0073 0.0069 Zn 0.0102 0.0099 0.0102 0.0104 0.0104 0.0105 0.0116 0.0131 0.0127 Zr 0.0209 0.0215 0.0250 0.0246 0.0244 0.0225 0.0228 0.0594 0.0439 Rb 0.0008 0.0010 0.0013 0.0012 0.0012 0.0012 0.0012 0.0044 0.0027 * =Katla ~934 **=Katla~1500 (G.Larsen 1993)