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Overview of Mapping of Tephra Layers in Iceland

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Overview of Mapping of Tephra Layers in Iceland Overview of mapping of tephra layers in Iceland Gudrún Larsen Institute of Earth Sciences University of Iceland MeMoVolc Workshop Askja May 4, 2016 Photo M.T. Gudmundsson 2010 Overview • Volcanic systems • Tephra layers in Icelandic soils • Maps – some examples • Preservation of tephra - volume • Basaltic tephra deposits - long fissures Photo S.Thorarinsson Photo G. Larsen Photo G. Larsen Photo G. Larsen Volcanic system consist of: • About 30 volcanic systems active during Holocene Central volcano (high eruption frequency, silicic and basaltic magma) • Explosive eruptions on most systems Fissure swarm (lower eruption frequency, basaltic during Late-glacial - Holocene transition magma only) • Explosive eruptions on about half of them during the Holocene • Nine systems with most numerous explosive eruptions shown in red • 11 volcanic systems partly covered by ice at present Adapted from Jóhannesson and Sæmundsson 1998 T The volcanic products from many - but not all - the volcanic systems have chemical characteristics that can be used to “fingerprint” the tephra layers. This also allows correlation to source volcano. Different colours indicate different chemical characteristics. Tephra layers in Icelandic soils Because of partial ice cover and high ground water frequency of explosive eruptions is particularly high on EVZ Number of tephra layers in soil and sediment is highest within the EVZ Between 600 and 700 tephra layers are known in soil, sediment and ice. >500 basaltic, ~100 silicic identified (Óladóttir et al. 2008, 2011, various other sources) Estimated number, postglacial explosive eruptions (Thordarson and Höskuldsson 2008) Total ~1930 Basaltic ~1770 Silicic ~160 (Larsen and Gíslason 2013) A very substantial part of the Icelandic eruption record during the Holocene is represented by tephra layers. K. Magnússon Three out of every four eruptions in the last 11 centuries left a tephra layer. In 2/3 of all the eruptions tephra was the only product. This detailed record covers only about 1/10 of the Holocene. The record for the other 9/10 is less detailed - research is ongoing. The ratio may be slightly K. Gudmundsson different for the Holocene in general. S. Hjaltason In the last 11 centuries 4 out of every 5 tephra layers are of basaltic composition Hekla – possibly 5 out of every 6 tephra layers in the last 8000- 9000 years. Katla Silicic Katla tephra Grímsvötn Surtsey Silicic Hekla tephra Silicic Katla tephra Photo B.A. Óladóttir Of 18 tephra layers 15 are basaltic, 3 silicic. Grímsvötn Maps of Icelandic tephra (dispersal maps not included) Tephra from at least 94 eruptions mapped (for several eruptions units mapped separately) 47 silicic, 47 basaltic eruptions, isopach and isomass maps Volume range <0.001 to 10 km3 (excluding the 26 m3 borhole tephra layer, Krafla fires in 1977) Maps of tephra from 64 eruptions published 32 silicic (17 Hekla) eruptions, 32 basaltic eruptions (15 Katla), some very small) Volcanic Explosivity Index 50 silicic, 30 basaltic tephra deposits VEI n km3 VEI 2 2 <0.01 VEI 3 16 0.01-0.1 VEI 4 48 0.1-1 VEI 5 10 1-10 VEI 6 4 >10 Volumes as freshly fallen Photo G. Larsen Maps of Icelandic tephra (dispersal maps not included) Tephra from at least 94 eruptions mapped (for several eruptions units mapped separately) 47 silicic, 47 basaltic eruptions, isopach and isomass maps Volume range <0.001 to 10 km3 (excluding the 26 m3 borhole tephra layer, Krafla fires in 1977) Maps of tephra from 64 eruptions published 32 silicic (17 Hekla) eruptions, 32 basaltic eruptions (15 Katla), some very small) Volcanic systems Askja Krafla Bárdarbunga Ljósufjöll Eyjafjallajökull Reykjanes Grímsnes Snæfellsjökull Grímsvötn Torfajökull Hekla Vestmannaeyjar Hengill Öræfajökull Katla Photo G. Larsen Calculations of volume/mass of tephra layers from isopach/isomass maps - from planimeters to computer programs Most of the isopach/isomass maps are hand-drawn from point data in an appropriate scale. Volumes/mass have been calculated from the maps by various methods, from simple area/thickness plots to programs such as Surfer© Golden software. Currently done by integrating the isopach/isomass maps using computer programs. Maps adapted from Larsen and Thorarinsson 1977; Thorarinsson 1968; Grönvold et al. 1983. Isopach maps of Hekla tephra layers representing four orders of magnitude, ~10 – 0.06 km3. 1 and 10 cm isopachs bolded. All but the H-1980 map are currently being revised. Volumes of tephra are published as: •Bulk volume or compacted volume •Freshly fallen or uncompacted volume S. Hjaltason 1980 •Dense Rock Equivalent (DRE) New tephra layers - mapped as freshly fallen, or uncompacted, at least in the distal and medial areas. Thick, near-vent deposits may begin to compact as they accumulate. Old tephra layers are mapped in the compacted, eroded state. In publications they are sometimes both presented as compacted/bulk and K. Gudmundsson1918 as freshly fallen/uncompacted volume. K. Magnússon1963 The area within certain isopachs may be off-shore. Hence two volumes may be listed, on land and on land-and-sea. Why giving uncompacted volume: It is closer to the actual input into the environment – the volume we have to deal with in the aftermath of an eruption (removing, cleaning, etc) For comparison with recent tephra deposits and explosive eruptions observed in modern time. For hazard evaluation when predicting/discussing certain categories of eruptions (such as the large Hekla eruptions). Th. Högnadóttir 1998 Some recent eruptions Volumes km3 (isopach maps) Grímsvötn 2011T 0.7±0.1 Eyjafjallajökull 2010T 0.27±0.07 Grímsvötn 2004 tephra on Vatnajökull Grímsvötn 2004T 0.044±0.009 MODIS satellite image 07.11.2004. Hekla 2000T 0.01* Hekla 1991 0.02* Hekla 1980 0.06* Hekla 1970 0.07* Vestmannaeyjar 1973T 0.02 Hekla 1947 0.18* T: Tmax measured *Opening phase •Photographs, videos, television •Observations from ground and air •Numerous descriptions •Measurements by state-of-the-art equipment •Sampling of relatively undisturbed material •Analytical equipment Even with all this – the true volume or mass of a tephra deposit/layer will never be obtained The closest so far: The volume/mass of the Grímsvötn 2004 tephra, deposited on snow and preserved in snow Grímsvötn 2004 tephra layer, isomass maps (Oddsson 2007, Oddsson et al. 2012) Total mass: 5.2±1.0 x 1010, total volume 4.4±0.9 x 107, density (dry) 1020-1290 kg/m3, mean 1190±40 kg/m3 Grímsvötn 2004 tephra mapped and sampled on Vatnajökull in summer 2005 Sometimes in fairly good weather, note the smiling geologist Oddsson coring for the tephra Sometimes under difficult conditions, note the line securing the geologist Photo K. Audunardóttir Hannesdóttir Photo B. Oddsson New tephra deposits have to be measured almost as they fall - IF you want to get the “right” thickness Photos Sigurdur Hjaltason Opening stages of the Hekla 1980 eruption, seen from 50 km distance, first 7 minutes. Photo Ævar Johannesson Photo Sigurdur Thorarinsson Prof. Sigurdur Thorarinsson and Niels Oskarsson measuring thickness of freshly fallen Hekla 1980 tephra on day 2. Note the lava flow and its rough surface behind Oskarsson Collecting data on tephra, even a newly deposited one, is not as easy as it may seem. Tephra on the ground some hours after deposition, fairly evenly distributed over the landscape .. Höskuldsson 2011 Höskuldsson 2011 … and the next day after a storm. Bedding indicates partly primary deposit. Óladóttir 2011 Óladóttir 2011 Eyjafjallajökull 2010 A modest size eruption 0.27 ± 0.07 km3 bulk volume 0.14 ± 0.02 km3 on land 0.13 ± 0.05 km3 at sea Medial deposit mostly ash size grains Measured during tephra fall or within days Eroded during and after deposition Proximal deposits (glacier) ash-lapilli Measured within weeks - months Adequate preservation Unprecedent wealth of information (satellites, etc) Maps from Gudmundsson et al. 2012 Tephra mapped one day after deposition Eyjafjallajökull eruption MERIS 17042010_1209 on April 17, 2010 Höskuldsson 18042010 Preliminary map from April 18, 2010 (a race against time) Gudmundsson 17042010 But we have the proximal deposits Thickness of near-vent deposits, including Tmax, can be measured in recent eruptions A great problem when constructing isopach maps for older eruptions Photo A. Höskuldsson What about preservation of the old tephra layers? I- 60 m -I Photo G. Larsen •An “exposure” in basaltic tephra deposit from the phreatomagmatic Vatnaöldur ~870 CE eruption. •About 12 m thick tephra deposited over a small hill - and eroded into horizontal surface exposure •Bedding shows as concentric circles. Width of exposure 60 m. Footprints for scale. (From Larsen et al. 2013) What about preservation of the old tephra layers? Volume uncertainities – mapping of old tephra deposits • Preservation of deposit • Exposures available for measuring • Knowledge of deposits characteristics • - and in case of eruptions within the glaciers – lack of proximal data These are the actual limiting factors The clustering of measured sites, the uncertainties in thickness measurements, etc, have less effect The characteristic explosive volcanism in Iceland: Hydromagmatic basaltic eruptions The eruption cloud at the beginning of the Grímsvötn 2011 eruption within Vatnajökull glacier. Photo Björn Oddsson Explosive basaltic eruptions within glaciers are the most common eruptions Less common but much larger: Explosive phreatomagmatic basaltic eruptions on fissures outside glaciers, tens of km long The really large explosive basaltic eruptions, >20 km long source, bulk volumes 3-7 km3
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