© G

. Ber

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Autonomous Underwater Vehicle (Atlas Sea Cat, Atlas Elektronik GmbH)

By the Sea Cat detected gas exhalations at the lake bottom

Why does the Size of the Laacher See Magma Chamber and its Caldera Size not go together? – New FindingsUlrich Schreiber & Gabriele Berberich

University of Duisburg-Essen, Essen, Germany

EGU General Assembly 2013 | Poster EGU2013-5908 | B444 | GMPV36/TS3.4 Corresponding author: ulrich.schreiber@uni-due.deReferencesSchmincke U (2009): Vulkane der Eifel. Spektrum Verlag Heidelberg.Holohan E, van Wyk de Vries B & Troll V. (2008): Analogue models of caldera collapse in strike-slip tectonic regimes. Bull. Volcanol., 70:773-796Acocella V, Funiciello R, Marotta E, Orsi G, and de Vita S. (2004): The role of extensional structures on experimental calderas and resurgence, J. Volcanol. Geotherm. Res., 129(1-3), 199-217.Roche O, Druitt TH, Merle O (2000): Experimental study of caldera formation. Journal of Geophysical Research, 105: 395Schmincke U (2000): Vulkanismus. Wissenschaftliche Buchgesellschaft, Darmstadt, pp. 264.

Loos J, Juch D & Ehrhardt W (1999): Äquidistanzen von Blattverschiebungen - neue Erkenntnisse zur Lagerstättenbearbeitung im Ruhrkarbon.– Zeit. f. angew. Geol., 45: 26 – 36.Ochmann N (1988): Tomografische Analyse der Krustenstruktur unter dem Laacher See Vulkan mit Hilfe von teleseismischen Laufzeitstudien. Mitt. Ing.- u. Hydrogeol. 30, pp. 108Viereck LG & v.d. Bogaard P (1986): Magma- und Wärmeinhalt der Magmakammer des Laacher Sees und des Riedener Vulkans. Forschungsbericht T86-174, Bundesministerium für Forschung und Technologie (BMFT), Karlsruhe, pp 1-98.Bahrig B (1985): Sedimentation und Diagense im Laacher Seebecken (Osteifel). Bochumer geol. geotechn. Arb. 19, pp 231v.d. Bogaard P & Schmincke HU (1984): The Eruptive Center of the Late Quarternary Laacher See Tephra. – Geol. Rundschau, 73: 933-980.v.d. Bogaard P (1983): Die Eruption des Laacher See Vulkans. Dissertation, Ruhr-Universität Bochum, 348 S.

Blo

ck b

ound

ing

faul

t

Thrust

Norm

al

Zone bounding fault

ConjugateStrike slip

Rotation

Marker

Voids

Source: Dibblee, 1977

a)

b)

c)

d)

R’-Shear plane

Development of scales & duplex structures

en-échelon R-Shear plane

Connection withP-Shear planes

Faults 2nd order

Position and conncention of Riedel shear planes along of a dextral strike-slip fault

(after Swanson 2006, J. Struct. Geol. 28, 456-473)

Our findings:

Erupted magma volume incl. bedrock: 0.5 km³

Bed rock

Bed

ro

ck

Bed

ro

ck

Dra

wn

by:

G. B

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Pre-eruptive Szenario 1a (Schmincke 2009):

Magma chamberVolume: 18 km³ Width: 1kmDepth of top: 3 km

3 km

6.0 km³ eruptedMagma

Post-eruptive Szenario 1b (Schmincke 2009):

Erupted bed rock volume: 0.5 km³ Erupted magma volume: 6.0 km³ Total erupted volume: 6.5 km³

Magma chamberVolume: 18 km³ Width: 1kmDepth of caldera bottom: ~7 km

1 km

10.6 km

2.5 kmToday‘s caldera width

Bed

ro

ck

Bed

ro

ck

Pre-eruptive Szenario 2a(Schmincke 2009):

Magma chamberVolume: 18 km³ Width: 1.6 kmDepth of top: 3 km

Surface

1 km

18 k

m³

Mag

ma

cham

ber

vo

lum

e

12 k

m³

Mag

ma

cham

ber

vo

lum

e

Bed

ro

ck

Bed

ro

ck

3 kmBed rock

1.6 km

6.0 km³ eruptedMagma

Surface

18 k

m³

Mag

ma

cham

ber

vo

lum

e

Bed rock

Bed

ro

ck

Bed

ro

ck

0.5 km³erup. Magma

3 km

Bed rock

Surface

Bed

ro

ck

Bed

ro

ck

2.5 kmToday‘s caldera width

Debris of bed rock

Debris of bed rock

Bed

ro

ck

Surface

6 km

1.6 km

12 k

m³

Mag

ma

cham

ber

vo

lum

e

Post-eruptive Szenario 2b(Schmincke 2009):

Erupted bed rock volume: 0.5 km³ Erupted magma volume: 6.0 km³ Total erupted volume: 6.5 km³

Magma chamberVolume: 18 km³ Width: 1.6 kmDepth of caldera bottom: ~3 km

2.5 kmToday‘s caldera width

Debris of bed rock

>1 km³ ?

Mag

ma

cham

ber

volu

me

3.5 km

6.8 km

2.7 km

Munich

Hamburg

Amsterdam

Brussels

ViennaBudapest

Marseille

Rome

Bonn

Stockholm

Kopenhagen

WarsawBerlin

Prague

Genf

Ice border11 000 BP

ocooStooc

Turin

Stockholm

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2 mm

5 mm

1 cm

5 cm1cm

5cm

10cm10cm

10cm

5 cm

1 cm

5 m

m

Gotland

Turin03

Total thickness of Laacher See ashes and tephra fans in Central Europe (after: van den Bogaard, 1983)

New EQ-Center of the last 40 years

Laacher See strike-slip fault (LSSSF)

Processed mining fieldExamples A and BMining exposureTectonic projectionProjection due to equidistances

of the large-scale strike-slip faults}

Analogue Models of Caldera Collapse in Strike-Slip Tectonic Regimes (from: Holohan et al. 2008)

Ruhr carboniferous: Equidistances of strike-slip faults (from: Loos et al. 1999)

10 m 8 m 4 m 2 m 1 m1,1 %/ 1,1 %

0,3 %/1,0%

0,13 Mio. ppb

0,10 Mio. ppb/0,21 Mio. ppb

0,16 Mio. ppb/0,22 Mio. ppb

0,7 Mio. ppb

0,8 Mio. ppb

0,23 Mio. ppb

0,16 Mio. ppb/ 0,21 Mio. ppb

0,16 Mio. ppb

0,30 Mio. ppb

0,17 Mio. ppb/0,11 Mio. ppb

0,38 Mio. ppb

0,16 Mio. ppb0,18 Mio. ppb/0,46 Mio. ppb

Strike-slip faults and crustal block rotation with voids (after: Dibblee, 1977; Swanson 2006)

a) Pre-caldera strike-slip deformation

Outline of magmachamber at depth

Y-shearChamber-localised

graben and fault

Regional faults (Riedel shears)

b) Early downsag phase

Diffuse zone of peripheral

extension

Elliptical downsag zone inside

chamber outline

c) Initiation of reverse faulting

Reverse fault scarp forms a short axis end of chamber

Focus of asymmetry is initially to the NW

Component of horizontal movement during early

central zone subsidence

d) Final caldera structure Reverse fault propagates toward chamber long axis

Regional fault reactivated

Regional fault reactivatedto accomodate peripheral extension and/or central

subsidence

Second reverse fault forms.Central zone subsidence

becomes less asymmetric and purely vertical

e) Interpretation

Subsidence controlling

reverse fault

Zone of diffusive peripheral extension

Peripheral extension

localised on reactivated pre-existing

fault

Subsidence controlling

reverse faultReactivated Riedel shear

Pre-collapse regional faults(Riedel shears)

Pre-collapse faults(unreactivated)

f) Excavation of remnant chamber

Subsided, almost flat, upper surface of remnant chamber

Ridge bends sharply into

Y-shear trend. Displacement transfer from reverse fault to Y-Shear?

Cut into chamber made by regional R- and Y-shears. Associated with arrest of reverse fault propagation

Ridge with parallel outward dipping furrow

Vertical ridge. Inward dipping at SW end

Scale 10 cm

Scale 10 cm

Scale 10 cm

Scale 10 cm

Scale 10 cm

Scale 10 cmN

N

N

N

N

N

Under pressure experiments of Roche et al. (2000)

Platewithhole

Sandpack

Silicone

Tube

a) Apparatus, section view

Smalldeep chamber

Breccias

Roof

Chamber

Chamber

RoofLargeshallow chamber

Breccias Breccias

c) Final model summarizing the deformation pattern for funnel (small deep chamber) and piston (large shallow chambers) calderas (modified after Roche et al. 2000)

San

d-p

ack

Silicone

San

d-p

ack

SiliconeS

and

-pac

k

b) Section view of 3 experiments characterized by very different aspect ratios of the chamber roof

Silicone

Schematic models of Laacher See magma chamber sizes and erupted volumes

Sea Cat tracks of 2010 and 2011 campaigns

Profile of Laacher See (reflection seismic; Bahrig 1985)

Break-up zone below Laacher See (Ochmann 1988)

2010

2011

Sonar image of rising gas bubbles from Laacher See bottom (300 kHz side scan sonar)

New model concepts on the tectonic evolution of the young East Eifel lead to a contradiction in terms of size of the postulated magma chamber and the Laacher See Caldera

Due to the slow movement rates of active tectonic faults (mm per year), an estimated 18 km³ magma chamber beneath the Laacher See (v. d. Bogaard & Schmincke 1984) cannot be

confirmed. Discrepancies are given by

• the volume of the Laacher See caldera of approx. 0.5 km³ with regard to the pre-eruptive surface (Viereck & v.d. Bogaard 1986) and the erupted volume of 6.3 km³ dry rock equiva-

lent of lava and bed rock (v. d. Bogaard & Schmincke 1984) resp. 6.5 km³ magma (Schmincke 2009),

• a comparison of modeling of caldera evolution with the Laacher See Caldera formation (Holohan, de Wries & Troll 2008; Acocella, Funiciello, Marotta, Orsi & de Vita 2004),

• no geophysical prove of such a large magma chamber,

• a volume compensation of approx. 6 km³ by ascending magma from the mantle which could have prevented a further subsidence of the magma chamber

(over a period of several days of the estimated duration of eruption) appears unrealistic,

• performed sonar recordings of the post-eruptive Laacher See sediment layers (Bahrig 1985) that do not show any displacements that might indicate a doming caused by magma.

Our findings

• No statistical significant data set with regard to spatial distribution of the erupted tephra volume, e.g. only one sample point for North Italy (v.d. Bogaard 1983).

• Overestimation of the tephra thickness in published isopach maps of the Westerwald and other regions.

• More critical evaluation of interpretations of tephra samplings from old maps and literature is required.

• Inclusion of atmospheric effects (e.g. atmospheric turbulences, dune formation, dust storms long after the eruption, congestion of air masses at the alpine orogene) is required.

• An order of magnitude smaller magma chamber stretched over a longer vertical crustal section can help to better match the given tectonic movement rates and

the size of the caldera.

• All sampling locations would also be explained by an erupted volume of only 10% of the estimated one by Schmincke (2009).

© J

. Kal

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AT

LAS

Ele

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© J

. Kal

wa,

AT

LAS

Ele

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© J

. Kal

wa,

AT

LAS

Ele

ktro

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Gm

bH

© J

. Kal

wa,

AT

LAS

Ele

ktro

nik

Gm

bH

Pre-existing fault reactivated to accomodate

peripheral extension

Reverse fault truncates

against regional fault