Journal of Volcanology and Geothermal Research 202 (2011) 73–82

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Journal of Volcanology and Geothermal Research

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Toothpaste lava from the Barren Island volcano (Andaman Sea)

Hetu C. Sheth a,⁎, Jyotiranjan S. Ray b, Alok Kumar b, Rajneesh Bhutani c, Neeraj Awasthi b

a Department of Earth Sciences, Indian Institute of Technology Bombay (IITB), Powai, Mumbai 400076, Indiab Geosciences Division, Physical Research Laboratory (PRL), Navrangpura, Ahmedabad 380009, Indiac Department of Earth Sciences, Pondicherry University, Puducherry 605014, India

⁎ Corresponding author. Tel.: +91 22 25767264; fax:E-mail address: hcsheth@iitb.ac.in (H.C. Sheth).

0377-0273/$ – see front matter © 2011 Elsevier B.V. Aldoi:10.1016/j.jvolgeores.2011.01.006

a b s t r a c t

a r t i c l e i n f o

Article history:Received 13 August 2010Accepted 26 January 2011Available online 26 February 2011

Keywords:volcanismlava flowbasalttoothpaste lavaBarren IslandAndaman Sea

Toothpaste lava is a basaltic lava flow type transitional between pahoehoe and aa and has been described fromParicutin, Kilauea and Etna volcanoes. Here we describe a spectacular example of toothpaste lava, formingpart of a recent (possibly 1994–95) aa flow on the active volcano of Barren Island (Andaman Sea). This flow ofsubalkalic basalt shows abundant squeeze-ups of viscous toothpasate lava near its entry into the sea. Thesqueeze-ups are sheets and slabs, up to several meters across and tens of centimeters thick, extruded fromboccas. They are often prominently curved, have striated upper surfaces with close-spaced, en echelon linearridges and grooves, broad wave-like undulations perpendicular to the striations, and sometimes, clefts.Textural, geochemical, and Sr–Nd isotopic data on the squeeze-ups and the exposed aa flow core indicate verycrystal-rich, viscous, and isotopically very homogeneous lava.We envisage that a greatly reduced speed of thisviscous flow at the coastline, possibly aided by a shallowing of the basal slope, led to lateral spreading of theflow, which caused tension in its upper parts. This, with continued (albeit dwindling) lava supply at the back,led to widespread tearing of the flow surface and extrusion of the squeeze-ups. The larger slabs, whileextruding in a plastic condition, curved under their own weight, whereas their surfaces experienced brittledeformation, forming the en echelon grooves. The extruded, detached, and rotated sheets and slabs werecarried forward for some distance atop the very slowly advancing aa core, before the flow solidified.

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1. Introduction: Pahoehoe, aa, and toothpaste aa

The surface morphologies of basaltic lava flows are a reflection oftheir rheological behaviour and emplacement dynamics. Pahoehoeand aa are the two fundamental types of basaltic lava flows (e.g.,Macdonald, 1953, 1972; Peterson and Tilling, 1980; Rowland andWalker, 1990). Toothpaste aa (Bullard, 1947) is a third structural typeof basaltic lava flows that is transitional between pahoehoe and aa interms of viscosity and flow velocity (Rowland and Walker, 1987).Toothpaste lava can form primary lava lobes, or can extrude fromrootless openings (boccas) on other flows, usually late in an eruption.It has a surface characterized by longitudinal grooves and ridgesoriented parallel to the lava movement, caused by irregularities in theshape of the feeding bocca. Its surface is rough and spinose on acentimeter scale (Rowland and Walker, 1987).

Toothpaste lava is common on basaltic oceanic volcanoes such asthose of Hawaii. Here, we report an excellent example of toothpastelava from the active mafic stratovolcano of Barren Island in theAndaman Sea, northeastern Indian Ocean. We describe the morpho-logy of this toothpaste lava flow in detail, compare it to toothpastelava flows and some broadly similar lava types described fromHawaii,

Paricutin (Mexico), Etna, and continental U.S.A., and discuss theformation and emplacement dynamics of the Barren Island toothpastelava flow.

2. The Barren Island volcano and its aa flows

Barren Island (India) (Figs. 1 and 2) is an effusive as well asexplosive mafic stratovolcano in the Andaman Sea, northeasternIndian Ocean. It is the northernmost active volcano of the greatIndonesian arc. The volcano is 3 km in diameter, has restricted publicaccess, and no regular monitoring. The volcano is known to have beenactive from 1787 to 1832 (its “historic” eruptions), when it producedbasalt and basaltic andesite tephra and lava flows from a cinder conelocated in a 2-km-diameter caldera. The lavas flowed into the seathrough a breach in the caldera wall on the western side (Luhr andHaldar, 2006). Activity older than this (hitherto undated, “prehistoric”eruptions) is represented on the caldera wall (Alam et al., 2004;Chandrasekharam et al., 2009). It includes lava flows (of basalt andbasaltic andesite, and rare andesite) and volcaniclastic deposits,including lahar deposits, and pyroclastic fall and surge deposits thatform a hydrovolcanic tephra ring (Sheth et al., 2009). There isevidence in ash layers sampled in a marine sediment core near thevolcano that the prehistoric activity goes back to at least 72 ka, and thecalderamay have formed at≤10 ka (Awasthi et al., 2010). Four recenteruptions (1991, 1994–95, 2005–06, and 2008–ongoing) from the

Fig. 1. Map showing the major geological and tectonic features of the northeastern Indian Ocean and southeastern Asia, along with the locations of the Andaman Islands, BarrenIsland, and Narcondam, an extinct (possibly dormant) Andaman volcano. White triangles are Holocene volcanoes. Based on Luhr and Haldar (2006) and references therein.

74 H.C. Sheth et al. / Journal of Volcanology and Geothermal Research 202 (2011) 73–82

same polygenetic cinder cone have produced aa and blocky aa lavaflows of basalt and basaltic andesite. Lavas of the former threeepisodes covered the historic flows and flowed into the sea on thewestern side (Fig. 2a,b). The 1991 lavas buried a 12-meter-highlighthouse on the western coast that was the only man-madestructure on the island (Shanker et al., 2001). The ongoing eruption(2008-) (Fig. 2c) has produced an aa flow entering the sea over thecliffs on the northern side (Sheth et al., 2009, 2010).

As described by Sheth et al. (2009, 2010), all Barren Island lavaflows, prehistoric through currently forming ones, are aa flows, andpahoehoe is absent. The flows are distinctly channelized, as is typicalof aa flows, and the whole caldera-filling aa flow field is made up ofridges of the flows sloping towards the sea and with an overall surfaceamplitude of as much as 25 metres. From a distance the aa lavastreams can be distinguished by subtle colour differences (shades ofgrey through black, with darker shades for younger lavas), and theyounger lava streams can be seen to have left some “islands” of theolder lavas between them (Fig. 2b). All these aa flows have jagged,very sharp and highly vesicular clinker at the top, and usually havelevees made up of coarse aa rubble. They also show massive cores atseveral places, typical of aa flows.

3. The toothpaste lava of Barren Island

The toothpaste lava described in this paper is part of the recent aaflow exposed at the Landing Site 1 (N 23° 17.08’, E 93° 50.92’) on thewestern coast of Barren Island (Fig. 2c). This flow issued from thecinder cone 1.5 km to the east and flowed to the coast and into the sea.Its exact eruption age is not known, since no regular monitoring ofthis active volcano has been in place, but Sheth et al. (2009) inferred

from various evidence that it probably formed during the 1994–95eruptions.

This flow at the coastline is notable in the entire recent aa flow fieldfor its striking morphology. Its surface crust has been extensivelybroken into sheets and slabs, up to several meters across, some ofwhich are strongly curved (Fig. 3a). The sheets and slabs, which wedistinguish in this paper with an arbitrary boundary of 10 cmthickness, are often razor-sharp and stand on end, with meters-deepchasms in between (Fig. 3b,c). They were carried atop along a mobileaa core, well exposed at the Landing Site 1 (Fig. 3d). The curved sheetsand slabs, including the largest ones illustrated in Figs. 3a and 4a, areelongate in the N-S direction, i.e., perpendicular to the flow direction(due west). They are convex upstream. We measured the two largestcurved slabs (S1 and S2 in Fig. 4a); S1 has a length of 14.5 m, width of2.1 m, and a maximum thickness of 2 m, whereas S2 has a length of8.8 m, width of 2.4 m, and thickness of 1.5 m.

The abundant sheets and slabs have the morphology of toothpastelava. The upper surfaces of S1, S2, and most other sheets and slabsdisplay close-spaced striations (Figs. 3a, 4, and 5), whereas the lowersurfaces notably do not. The striations are linear grooves and ridges,parallel to the long dimensions of the slabs. Note that the strong visualimpression of continuous grooves and ridges imparted from a distance(Fig. 3a, 4a–c, and 5a) is illusory; when traced these terminate andsplay laterally, and are actually arranged en echelon (Fig. 5b–e).

The sheets and slabs extruded from fissure-like boccas, identifiablein some cases (e.g., for S1 in Fig. 5a). The upper surfaces of sheets andslabs, most notably the curved ones including S1 and S2, also havefold-like or wave-like undulations, which are transverse to their longdimensions and to the surface striations (Figs. 3a, 4a,5a,d,e, and 6a,b).As observed in toothpaste lavas described from Paricutin (Bullard,

Fig. 2. Panoramic views (in April 2008 andMarch 2009) of Barren Island taken from aboard Indian Coast Guard vessels, showing the general morphological and geological features ofthis active volcano. The cinder cone is 450–500 m high above the sea and the southern caldera wall 334 m. The view in (a) is roughly due NE, and views in (b) and (c) are roughly dueE. Yellowish white patches on the cinder cone in (a) and (b) are fumarolic deposits. The beach at Landing Site 1 in (c) is ~20 m wide, and the lava flow described in this paper ismarked.

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1947) and Hawaii (Rowland andWalker, 1987), these undulations areparallel to the lava movement direction, reflect the shape of thefeeding bocca, correspond to the longitudinal grooves and ridges ofHawaiian toothpaste lava, and are preserved in the extruded lava,already out of the bocca, because relaxation due to flowage did notflatten them, attesting to a high lava viscosity. Finally, thick curvedslabs such as S1 have local striation-parallel and transverse clefts(Figs. 4a, and 5a,b), which expose the dense, unjointed slab interior.

Sheth et al. (2009), who noted the similarities of these features tothe Hawaiian toothpaste lava (Rowland andWalker, 1987), describedthe curved slabs like that shown in Fig. 5a as “tumuli-like uplifts” dueto their domical shapes and clefts. They noted that tumuli, common inpahoehoe flows (e.g., Walker, 1991; Rossi and Gudmundsson, 1996;Anderson et al., 1999), are also known from aa flows (e.g., Duncanet al., 2004). However, tumuli form by localized lava inflation upliftinga surface crust (Walker, 1991), and a closer look at these features inthe present study shows that they are not tumuli. They are thick,curved slabs of toothpaste lava, usually detached from the flow(Fig. 4a). Their morphology, comparisons to flows or flow typesdescribed elsewhere, and formation mechanism are discussed in thispaper in much more detail than Sheth et al. (2009) could attempt.

4. Samples, mineral assemblages and textures

We sampled some of the slabs' upper crusts (samples BI-08-02 andBI-09-05, Table 1), the upper crust and interior part of the thickcurved slab S1 (samples BI-09-03 and BI-09-04, respectively), the aaflow core exposed at Landing Site 1 (sample BI-08-14), and one of theslabs (sample BI-08-15) atop this core. The aimwas to determine theirmineralogical and textural features and geochemical compositions,

which should provide valuable information on the likely lava temper-ature and cooling history, crystal content, and viscosity of this unusuallava flow on Barren Island.

Petrographic features of the various samples are illustrated in Fig. 7(see Luhr and Haldar, 2006 for a general description of petrography ofBarren Island rocks). All our samples are crystal-rich and have a fewvesicles. Several photomicrographs were taken of each slide with aLeica DM2500P microscope attached with a camera, and transportedto Adobe Illustrator 10, in which a 20 x 20 grid was superimposed onevery photomicrograph. The number of cells occupied by thephenocrysts and microphenocrysts and the groundmass werecounted and converted to volume percentages, and these percentageswere averaged for every sample. The rocks contain up to 46.7%crystals by volume (sample BI-08-02, Table 1). Plagioclase isextremely abundant as a phenocryst, commonly showing zoning ortwinning, sometimes both (Fig. 7a–h). Many plagioclase phenocrystsare very well-developed euhedral, tabular crystals and contain largenumbers of glass inclusions (Fig. 7b–d,g,h). Olivine (Fig. 7b) andclinopyroxene (Fig. 7d) phenocrysts also occur, but in small amounts,the latter showing twinning as well as zoning. The zoning common inthe plagioclase crystals suggests their incomplete equilibration withsurrounding liquid, and Luhr and Haldar (2006) have reported thatsome of the plagioclase and olivine megacrysts in Barren Island lavasare not phenocrysts but xenocrysts, representing disaggregatedtroctolitic cumulates from a shallow magma chamber.

The most common groundmass phase is also plagioclase, withsmaller amounts of clinopyroxene and very fine Fe–Ti oxides. Thegroundmass in the samples of slab crusts (BI-08-02, and BI-09-03 fromslab S1) shows substantial amounts of basaltic glass (dark brown overthe polarizer), which is much less or nearly absent in the slab S1 core

Fig. 3. (a) Sheets and slabs of toothpaste lava in the Barren Island flow. (b) Upturned sheets, with meter-deep chasms in between. Flow direction is directly away from the viewer.(c) Other upturned sheets. Flow direction directly away from the viewer. (d) The flow in cross-section at the Landing Site 1 (beach is just to the left and front of the person). Note thewell developed, thick, jointed flow core, and upturned slabs on it.

76 H.C. Sheth et al. / Journal of Volcanology and Geothermal Research 202 (2011) 73–82

Fig. 4. Close-up views of the curved slabs. Slabs S1 and S2 are marked. Note that whereas S1 is still rooted in its source fissure (bocca), S2 is detached. Note also that slabs adjacent toS1 are rotated and overturned because their lower surfaces, without striations, are exposed. Views in (b) and (c) are from opposite ends of the curved slab S1. Note pervasivestriations on the upper surfaces of S1 and the other slabs, and the wave-like undulations on S1 and S2. The active cinder cone is 1.5 km towards the east (left) in (a). Whitishappearance of the slab surfaces is due to a millimeter-thick deposit of light grey ash, erupted by the cinder cone between our April 2008 and March 2009 visits.

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sample BI-09-04 (Fig. 7e,f). This confirms the expectation that the crustsof the slabs cooled at a rapid rate and the cores cooled more slowly.

5. Major oxide geochemistry and Sr–Nd isotopic ratios

The samples were analyzed for the major elements by X-rayfluorescence (XRF) spectometry, at the PLANEX Facility of the IndianSpace Research Organization (ISRO), located at the Physical ResearchLaboratory, Ahmedabad. The analyses were made on pressed powderpellets using an Axios (from Panalytical Limited) XRF following themethods of Norrish and Chapell (1977). The data are presented inTable 1. The major oxide totals are almost 100% in all cases. Loss inweight on ignition (LOI) was also determined on sample powders bydrying them in an oven at 110 °C overnight, and then weighing themand igniting them in a preheated muffle furnace at 1070 °C for threehours. The LOI values are useful in providing an idea of the secondaryalteration suffered by volcanic rocks. Here, the values are all low andnegative, indicating that the sample powders gained rather than lost alittle mass due to oxidation. All samples are subalkalic basalt by thetotal alkali-silica diagram of Le Bas et al. (1986, not shown). The rocksare moderately evolved, with Mg numbers of ~48–56.

We also carried out bulk rock strontium and neodymium isotopicanalyses of the lava samples at the Physical Research Laboratory,

following the methods of Awasthi et al. (2010). Sample powders weredissolved using a standard HF–HNO3–HCl dissolution procedure forsilicate rocks. Sr separation was done by conventional cationexchange chemistry and Nd was separated from other REE (rareearth elements) using Ln specific resin from Eichrom with dilute HClas the elutant. The isotope ratio measurements were carried out instatic multi-collection mode on an ISOPROBE-T mass spectrometerand are reported in Table 1. The six samples from the several slabs andthe aa core are identical within analytical error in the Nd isotopicratio, and almost identical in the Sr isotopic ratio, indicating that themagma being supplied during the eruption and sheet and slabformation was very homogeneous.

6. Viscosities

The viscosity of a magma or lava is strongly dependent on itstemperature and composition (silica content), among other factors(e.g., Shaw, 1972). Hawaiian basalts have eruption temperaturesbetween 1100 and 1200 °C (e.g.,Wright and Okamura, 1977; Philpottsand Ague, 2009, p. 20). Basaltic liquids (45–52 SiO2) at 1200 °C rangein viscosity between 10 and 100 Pa s, and andesites are 3500 Pa s(Dingwell, 1995; Philpotts and Ague, 2009, p. 23). At Barren Island,clinopyroxene crystallization is known to follow that of plagioclase,

Fig. 5. (a) Pervasive longitudinal striations on the upper surface of the thick curved slab S1. Note also its undulating upper surface, and local striation-parallel and transverse clefts.The slab extruded out of the bocca underneath and the lava movement direction inferred is indicated by arrows. (b) to (e) Close-ups of the curved slabs’ upper surfaces, showing theen echelon ridges and grooves that produce the striated appearance. Photographs (c) through (e) were taken in March 2009. In the close-ups (d) and (e), the en echelon ridge-and-groove surface topography, as well as the transverse saddles and humps, are especially distinct because of fine ash (erupted between April 2008 and March 2009) deposited on theupper surfaces, and swept by wind into the grooves relative to the ridges and the saddles relative to the humps.

78 H.C. Sheth et al. / Journal of Volcanology and Geothermal Research 202 (2011) 73–82

which follows that of olivine±spinel (Luhr and Haldar, 2006). With afew available temperature estimates based on olivine melt-inclusiongeothermometry (Luhr and Haldar 2006), and the extensive crystal-lization (including that of clinopyroxene) observed in our samples, weconsidered a temperature of 1100 °C suitable for input in the viscositycalculation we carried out using the major oxide data following themethod of Shaw (1972). For this purpose we used a convenientspreadsheet provided by Barbara Nash (http://serc.carleton.edu/NAGTWorkshops/petrology/teaching_examples/24507.html).

Here, noting the recent age, negligible negative LOI contents andminimal alteration of the samples, H2O was taken to be zero for thecalculation, andwe took a Fe2O3:FeO ratio of 0.20 as recommended forbasalts by Middlemost (1989) and Verma et al. (2002).

Crystals and vesicles both increase the viscosity of a magma (e.g.,Philpotts and Ague, 2009). Bulk effective viscosities were thereforecalculated using the method of Shaw (1972) and the Einstein-Roscoeequation (Roscoe, 1952), ηB=ηL (1−φ /σ)−2.5 where ηB is bulkeffective viscosity, ηL is liquid viscosity, φ is crystal fraction, and σ is a

constant (0.6 according to Marsh, 1981; see also Sato, 2005).Calculated bulk effective viscosities are high: N3800 Pa s for five ofthe six samples and as high as 12,500 Pa s (sample BI-09-04) (Table 1).The values are to be treated as no more than rough estimates becauseof the assumptions involved (such as the temperature, and noting thatthe crystals are not spherical). Nevertheless, these crystal-rich lavaswould have been very viscous, and comparable viscosity estimates of12,000 Pa s for Hawaiian toothpaste lava were derived by Rowlandand Walker (1988) based on observations of olivine settling.Krauskopf (1948) also estimated a viscosity of 104 to 105 Pa s for the1944 aa flow from Paricutin that we describe in some detail below, forits closely similar features to the Barren Island flow.

7. Some broadly similar lava flow morphologies

A number of flow morphologies have been described in theliterature that have broad similarities to, as well as differences with,the Barren Island toothpaste lava, and it is useful to compare them

Fig. 6. Striation-perpendicular, wave-like undulations (alternating saddles and humps)on the upper surfaces of slabs and curved slabs.

79H.C. Sheth et al. / Journal of Volcanology and Geothermal Research 202 (2011) 73–82

before we compare toothpaste lava flows described from variouslocalities to the Barren Island flow.

“Slab-crusted” flows have been described from the 1983 eruptionof Mount Etna (Guest and Stofan, 2005), and Sheth et al. (2009)alluded to these as well. These flows have continuous, flat slabs ofpahoehoe crust normally 4–10 cm thick, 1–2 m wide and tens ofmetres long, and these continuous slabs ride passively on a lava plugwithout significant disruption or deformation. Guest and Stofan(2005) consider that the coalescence of vesicles below successivecrusts within these flows causes separation between core and crust,periodically giving a new cooling surface within the flow, on whichropy (or more rarely aa) surfaces may develop, depending on therheology of the crust (Kilburn, 1990). The slab-crusted Etna flowstypically erupt from late-stage, ephemeral boccas. These have severaltypes of surface ornamentation, one of them the longitudinal surfacestriations (see Figs. 2, 6, 9–11, and 13 of Guest and Stofan, 2005). Theslab-crusted flows may contain lava squeeze-outs, and may producetumuli, and their cleft walls show horizontally ridged surfacesconsistent with incremental crack opening.

Nichols (1938) described “grooved lava” from the McCartys flow,New Mexico. This contains blocks of surface crust that are turned onend, deeply grooved and scratched. The grooves vary from merescratches to marks a few centimeters deep, and are straight andparallel over many tens of centimeters (rarely, slightly curved), unlikethe striations observed on the Barren Island toothpaste lava. Nichols(1938) interpreted the grooves to have formed when the bottoms ofthese blocks, still soft and plastic, were scratched by the irregular solidedges of other, collding blocks. He also illustrated grooved wedgesand squeeze-ups of viscous lava extruded out of fissures and archedover by gravity. Nichols and Stearns (1940) described similar grooved

lava from Big Craters, Craters of the Moon National Monument, Idaho,and interpreted their striated, grooved and pock-marked surfaces ashaving resulted from the falling and rolling of blocks, bombs andlapilli, thrown out by the associated cinder cone, on the molten lavasurfaces. Despite an associated active cinder cone at Barren Island, thismechanism can be completely discounted there.

Anderson and Fink (1992) have described “crease structures” fromMount St. Helens and elsewhere, which are surface features found inlava flows and domes of all compositions and crystallinities. Theyconsist of a fracture with curved walls extending outward from alinear valley. The curved fracture walls have striations as a primaryfeature, and these extend roughly parallel to the axis of the centralvalley. Crease structures form due to stresses induced by lava coolingand, much more important, lateral spreading and extension. Accord-ing to Anderson and Fink (1992), cyclic fracturing due to tensilefailure of the cooled crust of the spreading lava, in the mannerdescribed by Ryan and Sammis (1978) for columnar joints, producesthe striations. The Barren Island flow's sheets and slabs are similar tocrease structures in their striated upper surfaces, but it is not clearwhat might cause the separation of the slabs from whole creasestructures. Besides, the field evidence is clear that they are extrusionsof the nature of squeeze-ups, through boccas on the surface of the aaflow (Figs. 4a and 5a).

8. The toothpaste lava flows of Paricutin, Kilauea, and Etna

The 10-km-long 1944 aa flow that issued from the cinder cone ofParicutin (Mexico) and buried the village of San Juan was described atlength by Bullard (1947) and Krauskopf (1948). It is Bullard (1947)who coined the term “toothpaste lava”, and the example in his Fig. 1and our own examples from Barren Island are exactly identical in theirwave-like undulations, close-spaced striations perpendicular to theseundulations, and the fissure-like boccas through which this lava typeemerges.

Krauskopf (1948) noted abundant rough curved slabs forming thesurface of this aa flow over its entire lower half, very different from theblocky surfaces of all other Paricutin flows. He described curved slabsof viscous lava, several centimeters thick and with roughly corrugatedsurfaces, slowly issuing from narrow cracks. He also describedelongate domes 2 to 3 m wide and 1 to 2 m high, split down themiddle and divided transversely into curved slabs convex upstream.These are precisely the features we observe in the Barren Islandtoothpaste flow. He described the formation of elongate domes alongfissures thus: “Lava is pushed up slowly from beneath, splitting intoslabs as it cools and contracts; then each slab grows as it is pushedoutward, plastic at its base but cold and hard in its upper part… Thecurved slabs apparently grow upward out of the massive interior andbend forward as they are pushed up.” At other times, he observed thelava coming up through a crack as a single thin sheet, curving to oneside or both sides as it emerged. Krauskopf (1948) further wrote: “Inmany places the slabs are somewhat thinner and curved into fantasticshapes. Often the slabs stand on end, with chasms several meters deepbetween, so that the flow is almost impossible to cross on foot.” Thisdescription perfectly fits the Barren Island toothpaste flow asillustrated in Fig. 3b,c of this paper.

Rowland and Walker (1987) described toothpaste lava from the1960 Kapoho eruption on the East Rift Zone of Kilauea volcano,Hawaii. They observed that boccas usually occur in pahoehoe flowstens to thousands of meters away from the vent, and lava issuing fromthem may have cooled and degassed, and thereby become moreviscous than pahoehoe. They found that toothpaste lava andproximal-type aa have comparable, moderate to high viscosities, buttoothpaste lava emerges more slowly (1 m in a few minutes to a fewtens of minutes) from its bocca than proximal-type aa. As a result, itssurface does not tear to form aa clinker. Toothpaste lava forms asurface skin within a short distance of the point of emergence. They

Table 1Major oxide and Sr–Nd isotopic data for lava slabs, crusts and cores, with crystal contents and calculated viscosities.

Description Striated crust ofa curved slab

Striated crust ofcurved slab S1

Interior of curved slab S1(30 cm below BI-09-03)

Striated crust ofan upturned slab

Core of flow atLanding Site 1

Slab atop flow coreat Landing Site 1

Reference Measured

Sample no. BI-08-02 BI-09-03 BI-09-04 BI-09-05 BI-08-14 BI-08-15 BHVO-2±2 s BHVO-2±2 s

SiO2 49.00 49.45 50.72 50.43 51.13 50.58 49.95±1.00 49.12±0.08TiO2 0.81 0.86 0.86 0.87 0.80 0.73 2.73±0.16 2.72±0.04Al2O3 23.45 22.63 22.32 22.03 22.24 21.74 13.38±0.26 13.78±0.02Fe2O3T 7.90 8.62 8.04 8.33 7.57 7.23 12.23±0.48 12.32±0.86MnO 0.14 0.15 0.14 0.14 0.14 0.14 0.17±0.02 0.16±0.01MgO 3.79 3.76 3.22 3.41 3.51 3.91 7.23±0.28 7.28±0.02CaO 11.39 11.26 11.09 11.06 10.92 10.99 11.45±0.46 11.53±0.03Na2O 2.69 2.92 3.12 3.09 3.53 3.23 2.14±0.18 2.38±0.02K2O 0.36 0.39 0.40 0.40 0.43 0.41 0.51±0.04 0.52±0.004P2O5 0.10 0.08 0.12 0.12 0.10 0.12 0.22±0.02 0.24±0.005LOI -0.08 -0.18 -0.34 -0.32 -0.11 0.07Total 99.55 99.93 99.69 99.56 100.25 99.12 100.01 100.05Mg no. 52.9 50.5 48.3 48.9 52.0 55.8Vol.% crystals 46.7 28.2 44.2 35.8 36.0 41.7Temperature 1100 °C 1100 °C 1100 °C 1100 °C 1100 °C 1100 °CBulk effectiveviscosity (Pa s)

11970 1411 12566 3891 4457 8224

87Sr/

86Sr 0.70400 0.70405 0.70415 0.70409 0.70396 0.70398

143Nd/144Nd 0.512879 0.512872 0.512862 0.512868 0.512877 0.512881εNd +4.7 +4.6 +4.4 +4.5 +4.7 +4.7

Notes: Fe2O3T is total ironmeasured as Fe2O3. The high accuracy of the XRFmajor oxide determinations is indicated by the recommended andmeasured values on the USGS referencematerial BHVO-2 (analytical errors shown are 2σ, and the reference values are of Wilson, 1997). Mg Number=[atomic Mg/(Mg+Fe2+)]×100. Sr and Nd isotope ratios werecorrected for fractionation using

86Sr/

88Sr=0.1194 and

146Nd/

144Nd=0.7219, respectively. NBS987 strontium gave a value of 0.71023±0.00001 (2σ error on the mean), and JNdi-1

neodymium gave 0.512104±0.000004 (2σ error on the mean), with εNd of −10.4±0.1. εNd is computed using a present-day chondritic average143Nd/

144Nd ratio of 0.512638.

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reported that a 5 cm thick crust had formedwithin 5 m of the bocca inthe 1960 Kapoho flow on Kilauea volcano, and this crust took 30 minto form. At Barren Island, we find comparable thicknesses of thesurface crust at similar distances from the boccas (Fig. 5a,b), thoughunfortunately there are no observations of these actively forming.Rowland andWalker (1987) also described imbricately stacked platesof toothpaste lava, typically 5–10 cm thick, and often standing nearlyvertical (their Fig. 8). They thought that these plates may haveseparated from the underlying lava because of coalescence of gasbubbles beneath them, and were stacked when the flow encounteredan obstacle. The extensive development of toothpaste lava instead ofaa in the 1960 Kapoho flow field was ascribed by them to the veryshallow ground slopes (mostly b1o).

Applegarth et al. (2010) have recently described curved slabs oflava in the 2001 lower flow field on Etna. The slabs, up to several tensof centimeters thick andwith surface striations, are exact analogues ofthe curved slabs in the Paricutin and Barren Island flows. Theymention the widespread occurrence of late-stage squeeze-ups of lavain a range of rheological states. They classify their squeeze-ups intoflows (relatively fluid), tumuli (not the same as the classical inflation-formed tumuli), and spines (highly viscous). They believe that highyield-strength lava during waning activity (meaning much cooling,degassing and crystallization) would not have moved on its own on agentle slope, but pressurization formed new boccas due to eithercontinued or resumedmagma supply to the front of the flow, and suchpressurization may have occurred because of late-stage flowsuperposition. The squeeze-ups were fragmented due to high yieldstrength and/or strain rate, their surfaces were torn transverse to thelocal flow direction, and on fragmentation produced blades and sheetsof lava, rather than relatively equant aa clinker. The last lava to beextruded had a very high yield strength, as evidenced by sheets andspines that are often almost vertical.

9. Formation of the Barren Island toothpaste lava

Our field observations and textural and geochemical results showthat the Barren Island flow with the toothpaste lava squeeze-ups wasconsiderably cooled and crystallized, and thus highly viscous, by thetime it arrived at the coast. High viscosities have been inferred also for

toothpaste lavas of Paricutin (Krauskopf, 1948), Kilauea (Rowland andWalker, 1987), and Etna (Applegarth et al., 2010). We believe that thehigh viscosity of the Barren Island aa flow had greatly reduced itsspeed of advance, and the speedmight have been further retarded dueto its entry into the sea, or a local shallowing of basal slope near or atthe coastline. Under these conditions the flow began to spreadlaterally and its upper parts began to experience tension, much ashappens in the formation of crease structures (Anderson and Fink,1992). Lateral spreading and preferentially E–W-oriented extensionin the flow (as indicated by boccas aligned N-S), along with continuedlava supply at the back, led to the extrusion of squeeze-ups of viscouslava, as sheets and slabs. The larger sheets and slabs, while they wereextruding and still growing in their lower parts, were still pliable, andthemselves spread and bent over under gravity, thus acquiring curvedand convex shapes. While whole sheets and slabs were undergoingductile bending under their own weight, their upper crusts experi-enced brittle extensional deformation, again oriented E-W, whichproduced the en echelon ridges and grooves, oriented N-S.

Applegarth et al. (2010) have illustrated small-scale platy cleavingof the surface crusts of the 2001 Etna flows. They suggest that these“peeling textures” indicate brittle deformation of the outer layer,while the curvature of the whole sheets and blades with these surfacepeeling textures shows that they were ductile enough to be folded.Precisely the same combined ductile and brittle deformation featuresare exhibited by individual sheets and slabs of Barren Islandtoothpaste lava; a particularly striking example is in Fig. 6b (compareFig. 13 of Applegarth et al., 2010). We believe that the grooves thatproduce these striated toothpaste crusts are tensional gash fractures(Davis and Reynolds, 1996, p. 364), i.e., the grooves and ridges areformed by pervasive centimeter-scale normal faulting of highlyviscous lava. Davis and Reynolds (1996, their Fig. 6.146A) haveillustrated gash fractures in a lava flow from the Pinacate volcanic fieldin northern Mexico that have produced identical surface ornamenta-tion to the Barren Island toothpaste lava crusts, and their photographalso shows the same broad, striation-perpendicular saddles andhumps. As noted, the ridges and grooves on the Barren Islandtoothpaste lava crusts are only superficially continuous; in detail theyterminate and splay laterally, and are arranged en echelon in detail(Fig. 5b-e). Large-scale, including regional-scale, normal faulting, such

Fig. 7. Textural features of the toothpaste lava slabs and core of the Barren Island aa flow. Abbreviations used to identify some of the crystal grains are: pl, plagioclase; ol, olivine, cpx,clinopyroxene. Black spaces marked “v” are vesicles. All photomicrographs have been taken between crossed polars, except (e) and (f) taken over the polarizer.

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as in the Basin and Range province, is also typically en echelon (e.g.,Davis and Reynolds, 1996).

As the very slowly progressing aa flow arrived at the Barren Islandcoast, boccas began to open in its upper part, and the squeeze-upsextruded. The extruded sheets and slabs, when they became cool andrigid, were detached, rotated and overturned, when the underlyinglava changed direction or velocity, or as new squeeze-ups continuedto extrude from the same or newly opening boccas (see Fig. 4a,b,c). Allthese were carried forward atop a very slowly advancing aa core,which solidified as the lava supply waned and finally ended. Squeeze-ups such as S1 (Fig. 5a) were frozen in the process of emerging fromthe boccas.

10. Summary

This paper describes the morphology and formation mechanismsof squeeze-ups of viscous toothpaste lava in a recent aa flow of theBarren Island active volcano (Andaman Sea). The squeeze-ups (sheetsand slabs, many prominently curved) have pervasive striations (close-spaced, en echelon linear ridges and grooves) on their upper surfaces,broad, wave-like undulations perpendicular to the striations, andsometimes, clefts. The flow has remarkable similarities to toothpastelavas previously described from Hawaii, Paricutin, and Etna, all ofwhich must involve similar rheological behaviour. As inferred forthese flows, the Barren Island flow was significantly cooled andcrystallized and highly viscous. The squeeze-ups of viscous toothpastelava extruded from boccas in its upper surface as the flow, movingvery slowly at its entry into the sea, began to spread laterally andexperience extension, and received continued, though waning, lavasupply at its back.

Acknowledgements

This work is funded by the Department of Science and Technology(DST), Govt. of India (Project no. ESS/16/262/2006/P-1) and by thePhysical Research Laboratory, Ahmedabad. We highly appreciate theinvaluable support and logistical help of the Indian Coast Guardduring three field trips to Barren Island in 2007, 2008 and 2009. Wethank Pooja Kshirsagar for preparing thin sections, and S. V. S. Murtyand Anil Shukla for access to and assistance with the ISRO-PLANEXXRF facility at PRL Ahmedabad. This manuscript was significantlychanged and greatly improved from its initial version by the detailed,critical reviews of Steven Anderson and Scott Rowland, and theeditorial comments of Lionel Wilson.

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