Overview of mapping of tephra layers in Iceland

Gudrún LarsenInstitute of Earth Sciences

University of Iceland

MeMoVolc Workshop

Askja May 4, 2016

Photo M.T. Gudmundsson 2010

• Volcanic systems• Tephra layers in Icelandic soils• Maps – some examples• Preservation of tephra - volume• Basaltic tephra deposits - long fissures

Overview

Photo G. Larsen Photo G. Larsen Photo G. LarsenPhoto S.Thorarinsson

The volcanic products from many - but not all - the volcanic systems havechemical characteristics that can be used to “fingerprint” the tephra layers. Thisalso allows correlation to source volcano. Different colours indicate differentchemical characteristics.

T

Volcanic system consist of:

Central volcano (high eruption frequency, silicic andbasaltic magma)Fissure swarm (lower eruption frequency, basalticmagma only)

• About 30 volcanic systems active duringHolocene

• Explosive eruptions on most systemsduring Late-glacial - Holocene transition

• Explosive eruptions on about half of themduring the Holocene

• Nine systems with most numerousexplosive eruptions shown in red

• 11 volcanic systems partly covered by iceat present

Adapted from Jóhannesson and Sæmundsson 1998

Because of partial ice cover and highground water frequency of explosiveeruptions is particularly high on EVZ

Number of tephra layers in soil andsediment is highest within the EVZ

Tephra layers in Icelandic soils

Between 600 and 700 tephra layers areknown in soil, sediment and ice.

>500 basaltic, ~100 silicic identified(Óladóttir et al. 2008, 2011, various other sources)

Estimated number, postglacial explosiveeruptions(Thordarson and Höskuldsson 2008)

Total ~1930Basaltic ~1770Silicic ~160

(Larsen and Gíslason 2013)

K. Magnússon

K. Gudmundsson

Three out of every four eruptions in the last 11centuries left a tephra layer.

In 2/3 of all the eruptions tephra was the onlyproduct.

This detailed record covers only about 1/10 of theHolocene.

The record for the other 9/10 is less detailed -research is ongoing. The ratio may be slightlydifferent for the Holocene in general.

A very substantial part of the Icelandic

eruption record during the Holocene is

represented by tephra layers.

S. Hjaltason

In the last 11 centuries 4 out of every 5 tephra layers areof basaltic composition

– possibly 5 out of every 6 tephra layers in the last 8000-9000 years.

Silicic Katla tephra

Silicic Katla tephra

Silicic Hekla tephra

Katla

Grímsvötn

Surtsey

Grímsvötn

Hekla

Of 18 tephra layers 15 are basaltic, 3 silicic.

Photo B.A. Óladóttir

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 published32 silicic (17 Hekla) eruptions, 32 basaltic eruptions (15 Katla), some very small)

VEI n km3

VEI 2 2 <0.01VEI 3 16 0.01-0.1VEI 4 48 0.1-1VEI 5 10 1-10VEI 6 4 >10

Volcanic Explosivity Index50 silicic, 30 basaltic tephra deposits

Volumes as freshly fallenPhoto 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 published32 silicic (17 Hekla) eruptions, 32 basaltic eruptions (15 Katla), some very small)

Volcanic systems

KraflaLjósufjöllReykjanesSnæfellsjökullTorfajökullVestmannaeyjarÖræfajökull

AskjaBárdarbungaEyjafjallajökullGrímsnesGrímsvötnHeklaHengillKatla

Photo G. Larsen

Calculations of volume/mass of tephra layers fromisopach/isomass maps - from planimeters to computerprograms

Most of the isopach/isomass maps are hand-drawn frompoint data in an appropriate scale.

Volumes/mass have been calculated from the maps byvarious methods, from simple area/thickness plots toprograms such as Surfer© Golden software.

Currently done by integrating the isopach/isomass mapsusing computer programs.

Isopach maps of Hekla tephralayers representing four orders ofmagnitude, ~10 – 0.06 km3.1 and 10 cm isopachs bolded.

All but the H-1980 map arecurrently being revised.

Maps adapted from Larsen and Thorarinsson 1977; Thorarinsson 1968; Grönvold et al. 1983.

S. Hjaltason 1980

Th. Högnadóttir 1998

K. Magnússon1963

K. Gudmundsson1918

Volumes of tephra are published as:•Bulk volume or compacted volume•Freshly fallen or uncompacted volume•Dense Rock Equivalent (DRE)

New tephra layers - mapped as freshly fallen, or uncompacted, at leastin the distal and medial areas. Thick, near-vent deposits may begin tocompact as they accumulate.

Old tephra layers are mapped in the compacted, eroded state. Inpublications they are sometimes both presented as compacted/bulk andas freshly fallen/uncompacted volume.

The area within certain isopachs may be off-shore. Hence two volumesmay be listed, on land and on land-and-sea.

Why giving uncompacted volume: It is closer to the actual input into theenvironment – the volume we have to deal with in the aftermath of aneruption (removing, cleaning, etc)

For comparison with recent tephra deposits and explosive eruptionsobserved in modern time.

For hazard evaluation when predicting/discussing certain categories oferuptions (such as the large Hekla eruptions).

Some recent eruptions

Grímsvötn 2011T 0.7±0.1

Eyjafjallajökull 2010T 0.27±0.07

Grímsvötn 2004T 0.044±0.009

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

Volumes km3 (isopach maps)

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 on VatnajökullMODIS satellite image 07.11.2004.

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 tephramapped and sampled onVatnajökull in summer 2005

Sometimes under difficult conditions,note the line securing the geologistHannesdóttir

Sometimes in fairly good weather,note the smiling geologist Oddssoncoring for the tephra

Photo B. Oddsson

Photo K. Audunardóttir

Opening stages of the Hekla 1980 eruption, seen from 50 km distance, first 7 minutes.

Photos Sigurdur Hjaltason

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

Photo Sigurdur ThorarinssonPhoto Ævar Johannesson

New tephra deposits have to be measured almost as they fall- IF you want to get the “right” thickness

Tephra on the ground some hours after deposition, fairly evenly distributed over the landscape ..

… and the next day after a storm. Bedding indicates partly primary deposit.

Collecting data on tephra, even a newly deposited one, is not as easy as it may seem.

Höskuldsson 2011

Höskuldsson 2011

Óladóttir 2011Óladóttir 2011

Eyjafjallajökull 2010

A modest size eruption0.27 ± 0.07 km3 bulk volume0.14 ± 0.02 km3 on land0.13 ± 0.05 km3 at sea

Medial deposit mostly ash size grains Measured during tephra fall or within daysEroded during and after deposition

Proximal deposits (glacier) ash-lapilliMeasured within weeks - monthsAdequate preservation

Unprecedent wealth of information (satellites, etc)

Maps from Gudmundsson et al. 2012

Tephra mapped one day after deposition

Eyjafjallajökull eruption on April 17, 2010

Preliminary map from April 18, 2010(a race against time)

MERIS 17042010_1209

Gudmundsson 17042010

Höskuldsson 18042010

Thickness of near-vent deposits, including Tmax, can be measured in recent eruptions

A great problem when constructing isopach maps for older eruptions

But we have the proximal deposits

Photo A. Höskuldsson

I- -I60 m

•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.

What about preservation of the old tephra layers?

Photo G. Larsen

(From Larsen et al. 2013)

• Preservation of deposit• Exposures available for measuring• Knowledge of deposits characteristics• - and in case of eruptions within the glaciers – lack of proximal data

Volume uncertainities – mapping of old tephra deposits

What about preservation of the old tephra layers?

These are the actual limiting factorsThe clustering of measured sites, the uncertainties in thickness measurements, etc, have less effect

The eruption cloud at the beginning of the Grímsvötn 2011 eruptionwithin Vatnajökull glacier. Photo Björn Oddsson

The characteristic explosive volcanism in Iceland: Hydromagmatic basaltic 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 on land.Proximal deposits accessible, realistic volumes can be calculated/estimated,

Other eruption source parameters more problematic

Explosive basaltic eruptions within glaciers are the most common eruptions

Source: 26 km long part of the Veidivötn fissure. Fall area on land: 53 000 km2. Bulk volume on land: 6.7 km3. Total of ~450 sites measured, 2 – 150 sites per 50 x 50 km area. Map modified from Larsen et al. (2013)

Originally mapped in a hazard-related project financed by the National Power Company,some of the questions of interest were: How long did such voluminous explosive eruption last and how high was the eruption plume? Duration of the explosive phase was estimated by taking into consideration mass eruption rate calculated for Laki fissure, 8-12 June 1783, in kg/sec per m of fissure (after Thordarson and Self 1993), resulting in duration of 12-18 hours.

Photo G. Larsen

Grímvötn 2004 tephra, bombs scattered.litics unbroken. Scale is 100 cm

Photo G. Larsen

Lithics, Grímvötn 2004 eruptionScale is 200 cm

Column height of 14-16 and 18-19 km for one venton the Veidivötn fissure was obtained from twoisopleths of lithics (Carey and Sparks 1986)

The use of maximum pumice size was limited bybreakage of pumice clasts (note that the photosare from Grímsvötn tephra)

Vei!ivötn tephra ~14774 km from source,5.5 m thick

Veidivötn tephra

~200 km from source.Primary structures partlypreserved (red box).

~35 km from sourcescale is 20 cm long~2 km from source

Spade is 105 cm long

Photo G. LarsenPhoto G. Larsen

Photo G. Larsen

Tephra layers, mostly from

Grímsvötn, in Vatnajökull ice

Photo O. Sigurdsson

• Information on volume and dispersal, ~100 eruptions• Volumes range from <0.001 to ~10 km3

• 50% silicic and 50% basaltic• Information is in many cases on land only• Explosive basaltic eruptions are under-represented• Preservation of deposits deserves more attention

Some concluding remarks:

Thank you for the attention