Bull Volcanol (1987) 49:631-641 Volcanology© Springer-Verlag 1987

Toothpaste lava: Characteristics and origin of a lava structuraltype transitional between pahoehoe and aa

Scott K Rowland and George PL Walker

Hawaii Institute of Geophysics, 2525 Correa Road, Honolulu, HI 96822, USA

Abstract. Toothpaste lava, an important basaltstructural type which illustrates the transitionfrom pahoehoe to aa, is particularly well dis­played on the 1960 Kapoho lava of Kilauea Vol­cano. Its transitional features stem from a viscos­ity higher than that of pahoehoe and a rate offlow slower than that of aa. Viscosity can bequantified by the limited settling of olivine phe­nocrysts and rate of flow by field observations re­lated to the low-angle slope on which the lavaflowed. Much can be learned about the viscosity,rheologic condition, and flow velocity of lavaslong after solidification by analyses of their struc­tural characteristics, and it is possible to make atleast a semiquantitative assessment of the numeri­cal values of these parameters.

Introduction

Three types of surfaces are generally recognizedon basaltic lavas, i.e., pahoehoe, aa, and blocklava (e.g., Macdonald 1972, p. 68). In this paperwe consider a fourth type, i.e., "toothpaste lava"(Bullard 1947; Einarsson 1949; Macdonald 1967).Such a type has also been called "drawn-surfacepahoehoe" (Foster and Mason 1955), "spiny pa­hoehoe" (Peterson and Tilling 1980), "semi-hoe"(Malin 1980), and "fine aa" (Jones 1943). Thename toothpaste lava directs attention to the out­standing feature of this type of flow, i.e., the sur­face grooves and drawn-out spines imparted atthe orifice where the lava encounters the air. Wefind that toothpaste lava is common on many lavaflow fields on Hawaii and many basaltic volca­noes elsewhere (e.g., Mount Etna).

Offprint requests to: SK Rowland

Toothpaste lava forms either primary lavalobes or is extruded from rootless openings orboccas (see for example, Pinkerton and Sparks1976) on pahoehoe flows usually late in an erup­tion. About 30% of the lava flow of Kapoho (Figs.1, 2), the main study area of this paper, consists ofprimary toothpaste lava and innumerable second­ary toothpaste-lava tongues which occur between,around, and on the primary lobes. Much of therest of the flow consists of "slab pahoehoe" (Pe­terson and Tilling 1980) derived from mobiliza­tion and breakup of toothpaste-lava tongues.

The Kapoho eruption took place low on theeast rift zone of Kilauea Volcano (Fig. 1), threeweeks after the end of a month-long summit erup­tion. Macdonald (1962) and Richter et al. (1970)give excellent eye-witness accounts.

Differences from and relationships to pahoehoe

Toothpaste lava differs from pahoehoe in sevenimportant respects:

1. The surface is characterized by longitudinalgrooves and ridges (Fig. 3), lacking on pahoehoe,oriented parallel to the direction of lava move­ment and caused by scraping against irregularitiesin the orifice roof. These grooves and ridges char­acteristically maintain the same separation fromeach other along the entire length of a toothpastelava stream, good evidence for a lack of spreadingor deformation of the crust.2. The surface is spinose on a centimeter scale(Fig. 3), whereas pahoehoe is smooth. The spinesare typically 1-5 cm long and up to 1 cm wide;they were drawn out of the fluid lava as it issuedfrom the orifice and generally point in the up­flow direction. Vesicles in the upper part of the

632 Rowland and Walker: Toothpaste lava

L' lighthouse

..... roads and trills

unclassified 1960 lavas

D pre-1960 surface

toothpaste lava, primarylobes outlined within

~ thick a'a flow,LL::.J pressure ridges dashed

Fig. 1. Location of 1960 Kapoho lava flow, and map of part of flow showing distribution of structural types. Primary toothpastelava lobe (see text) in Figs. 2 and 9 indicated by arrow pointing in flow direction

Rowland and Walker: Toothpaste lava

lava also show structures caused by drag of thesurface (Fig. 3).3. The surface has transverse undulations (Fig. 3)on a much broader scale (tens of cm) than that ofthe wrinkles (ropy structure) of pahoehoe. The

633

transverse undulations on toothpaste lava consistof convex-upward waves interpreted to form bynonuniform, pulsing (surging) flow out of theorifice ("discontinuous extrusion" of Macdonald1972, p. 96) that buckled the surface. When these

d

Fig. 3a-d. Characteristic features of toothpaste lava. Direction of flow is left to right. Arrows approximately I, 0.5, and 0.01 mlong in a, b, and c, respectively. a Toothpaste tongue issuing from curved bocca. S I-S2, lateral shear zone evidenced by imbricateshearing and clinker. Note how longitudinal lineations maintain same spacing along length of tongue. b Cut-away view showingpulse buckles (Pb) and pulse flaps (PjJ, features of discontinuous extrusion. Note how vesicles deformed because crust was re­tarded relative to underlying lava. c Close up showing spines pointing back toward bocca. d Photograph of small toothpaste-lavatongue. Flow direction is toward the left

634

"pulse buckles" became detached and wereforced to override the underlying lava, "pulseflaps" were generated (Fig. 3B).4. Flow units are significantly thicker than thoseof pahoehoe. The minimum thickness we ob­served in 20 or more measured toothpaste-lavaflow units is about 60 cm, and the median thick­ness is about 150 cm. In contrast, the minimumthickness observed in 3000 Hawaiian pahoehoeflow units is about 3 cm, and the median is40cm.5. The glassy surface rind is dull due to an abun­dance of microlites, unlike the shiny surface layerof sideromelane glass, commonly 1-2 cm thick(depending on vesicularity), which is invariablypresent on Hawaiian pahoehoe. The glassy rindof toothpaste lava is generally less than 1 cm

Rowland and Walker: Toothpaste lava

thick, and crystallization of the lava was well un­der way when it formed.6. Lava tubes are scarce. Many pahoehoe flowson Hawaii contain abundant lava tubes from lessthan 20 cm to 20 m high or wide, where lavaflowed beneath a surface crust and then drainedout. We found only one in the toothpaste lavaflows we examined on Hawaii. A lack of impetusto drain out (as when lava flows over nearly hori­zontal ground) will inhibit the formation of tubeseven though lava may be flowing under a crust.The ground slope at Kapoho was mostly less thanone degree. Gas blisters, formed of coalesced gasbubbles which have uplifted the overlying lavacrust, superficially resemble lava tubes and arecommon on the Kapoho lava though less so thanon most pahoehoe flows.

o.

ao

Pahoehoe, Kilauea

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h 0

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aa,

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9 1'\=6.8x103

0

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2

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~0

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Toothpaste lava,Kilauea

1'\=1.2x104

02 cca

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Fig. 4a-c. Graphs of number ofolivine crystal (c, crystals perI cm2 of exposure) vs height (h inmeters) for (a) pahoehoe exposedin wall of Kapoho graben, (b)toothpaste lava in 1960 now atKapoho, and (c) proximal-type aaexposed in gulch eroded inKo'olau volcano, 4 km SE of Hal­eiwa. Points are field data. Linesare distributions generated by nu-

2 C merical modeling from which vis-cosities (" in Pa s) are calculated

Rowland and Walker: Toothpaste lava

7. Mafic phenocrysts show a slight but measura­ble downward concentration, whereas pahoehoeunits show a very pronounced concentration inthe lower halves of their flow units (Fig. 4).

These features characterize toothpaste lava asa type distinct from typical pahoehoe. In the timeavailable before solidification, relaxation by flo­wage did not flatten surface perturbations, i.e.,longitudinal grooves and spines, in toothpastelava which would have been flattened in pahoe­hoe, nor did olivine crystals settle to any appreci­able extent. The toothpaste lava had a higher vis­cosity than pahoehoe and had crystallized to agreater extent at the time when it erupted.

The olivine crystal distribution is important inquantifying the difference between pahoehoe andtoothpaste lava. The Kapoho toothpaste lava car­ries olivine crystals up to 12 mm across. We foundthat the content of crystals 4 mm or more across isrelatively uniform in the upper 10 to 30 cm of allflow units, regardless of distance from vents. Ap­parently very little settling took place while thelava was flowing and the surface crust forming.The olivine content is 2-4 times greater in thelower interior part than in the upper interior partof units a meter or more thick; we attribute this tosettling of olivine in situ in the static lava.

Numerical modeling allows us to generate ol­ivine concentration profiles across lava flows ofgiven thicknesses using different crystal settlingrates (Fig. 4). We matched our field profiles withthe modeled profiles and obtained a best fit for asettling rate of 1 cm per 5.5 h. For crystals 4 mmin size, the corresponding viscosity from Stokes'law is 104 Pa s. For the porphyritic pahoehoeflows that we examined elsewhere in Hawaii, thesettling rates are about 1 cm per 40 min for olivinecrystals of the same size, indicating a viscosity ofabout 103 Pa s. The content of crystals in the crustof these flows is moreover not uniform, indicatingthat some crystal settling took place while theflows were moving. A fuller discussion of theserelationships is in preparation.

The fact that olivine crystals settled to a smallyet measurable degree through the toothpastelava indicates a low yield strength for the lava. Ayield strength of about 4 N/m 2 is required to pre­vent a spherical olivine 4 mm in diameter fromsinking (Sparks et al. 1977).

Differences between and relationships toproximal-type aa

Aa is a lava type whose surface consists of jum­bled and loose rubble overlying coherent lava. We

635

distinguish here between proximal-type aa anddistal-type aa (Fig. 5). Proximal-type aa advanceswith a rolling caterpillar-track motion commonlyat a rate of greater than 1 mlmin on low-angleslopes and tends to occur relatively near its vent.The surface rubble consists of scoriaceous frag­ments (clinkers) with spinose surfaces.

Distal-type aa forms lava lobes commonly10m or more thick such as occur in the distalparts of many Mauna Loa flows. Distal-type aadoes not advance with a caterpillar-track motionand generally moves slower than 1 milO min onlow-angle slopes. Many rubble fragments aremassive nonscoriaceous lava derived by uprise ofthe massive flow interior on inclined shear zones(ramp structures). The crumbling of rubble at thesurface generates a high content of submillimeter­sized particles.

In the following discussion toothpaste lava isrelated specifically to proximal-type aa, a com­mon associate in the field. A flow unit of tooth­paste lava typically has shear planes along eitherside (Fig. 3), and rubble litters the surface alongor near these planes. The rubble fragments arespinose, and many have a spiral form like the"lava coils" described by Temperly (1966), Peck(1966), and Peterson and Tilling (1980). Theshapes and positions of the coils show that theyformed by the rotation and tearing of spongy sur­face lava where it was subjected to a shear torquedue to the lateral velocity gradient in the underly­ing lava.

Such shear zones are a key to how aa lavasform. We envisage that emerging lava rapidly de­velops a surface skin which, being cooler, is moreviscous than the underlying lava (Fig. 6). Skinabove a lateral velocity gradient is subjected to atorque because of variable drag exerted on its un­dersurface. The torque rotates the skin counter­clockwise left of the lava median line and clock­wise right of the line. Such rotation is possibleonly if the skin can tear away from adjacent skin.An aa lava is one in which the skin has been torninto fragments due to this torque. The spines of aaclinker are stretched and torn fragments ofspongy lava that is unable to coil. An additionalfactor in the formation of aa is that the underlyinglava must be too viscous to rise between the torn­apart fragments and so heal the tears.

The distribution of olivine crystals in proxi­mal-type aa flows at Kapoho and elsewhere inHawaii is similar to that in toothpaste lava. Weinterpret that the crystal settling rate and the lavaviscosities were about the same.

We also interpret that toothpaste lava emerges

636 Rowland and Walker: Toothpaste lava

a

b

Fig. 5 a-b. Photographs of ec­tions of proximal-type aa at Ka­poho (a) and distal-type aa oncoastal plain below Hilina pali,Kilauea (b). Note thinness ofclinker cover and lack of internalshear structure within proximal­type aa. In distal-type aa, surfaceand flow front are covered withlarge blocks. In b, arrow indicateshammer

more slowly from its bocca than does proximal­type aa. A surface skin forms within a short dis­tance of the point of emergence and a small rota­tion occurs, twisting and coiling some strands oflava. However, the torque is insufficient to tearapart the skin. The rigid crust that characterizestoothpaste lava can thus develop.

Our field observations on toothpaste lavatongues with a crust broken into slabs show that acrust about 5 cm thick had commonly formed

within about 5 m of the bocca. Our measurementson active lava flows of Kilauea show that a crustof this thickness forms in about 30 min. The flowvelocity of the toothpaste lava was thus 1 m per6 min. One of us (G.P.L.W.) measured a flow rateof 1 m per 30 min on toothpaste lava in Heimaey(Iceland) in 1973. One of us (S.K.R.) measuredflow rates of 1 m per 25-30 min on a toothpastelava flow in August 1986 in Kilauea.

We consider toothpaste lava and proximal-

Rowland and Walker: Toothpaste lava 637

a

c

Fig. 6a-d. Schematic block dia­grams (not to same scale) of lobesof the four basaltic structuraltype in Hawaii, showing velocityprofiles (arrows proportional inlength to now velocity). a pahoe­hoe, differential now accomo­dated by wrinkling or stretchingand disruption and rapid healingof skin; b toothpaste lava, differ­ential now of rigid crust accomo­dated by shearing (S) at margins;c proximal-type aa, differentialnow causes tearing and rotationof portions of skin and also grad­ual outward movement, even­tually to become attached to thelevees (L); d distal-type aa, plugnow of lava having a yieldstrength, with loose rubble con­veyed on surface, and marginalzones of shearing (S), and rotatedrubble

type aa as alternative types that develop fromlavas with similar viscosities, since at Kapoho 01­ivines have sunk to similar degrees in both types.The surface of lava emerging rapidly from abocca tears and generates aa; the surface of lavaemerging slowly resists tearing and forms tooth­paste lava (Fig. 7).

I 1:100 I ~IToothpaste --III

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102 103 104 105

lava viscosity (Fa s)

The relation between lava structure and slopeangle is easily observed at Kapoho where an in­crease in slope occurs (A in Fig. 1). Upslope, theflow field is 90% toothpaste lava and brokencrusts of primary lobes. From the top of the 30­m-long steep section, through a second break inslope back to sub horizontal, and all the way to the

Fig. 7. Semiquantitative diagram showingrelation between lava viscosity, now vel­ocity, and now type. The reciprocal ofnow velocity plotted on vertical axis; vel­ocity therefore decreases upward

638

sea, 50% of the lava surface consists of clinker.We interpret this increased amount of aa as dueto more rapid shearing as lava flowed down thesteep slope. The lava did not revert to toothpastelava where the slope flattened, indicative of thenonreversible structural transitions that take placein lava flows (Peterson and Tilling 1980; Kilburn1981).

Toothpaste lava in the Kapoho flow field

The above discussion relates to toothpaste lava ingeneral; here we describe two main types of oc­currence of toothpaste lava in the 1960 flow fieldat Kapoho.

Most toothpaste-lava units emerge from boc­cas on top of an earlier flow surface, but they mayalso form at the front of a lava flow; they are thusable to both thicken and widen the lava flow. Thetypical secondary toothpaste-lava tongue from abocca is 1-3 m wide, 1-2 m thick, and morethan 10m long. The distribution and orientationof these tongues on the main flow tends to be ran­dom.

The surface of some toothpaste-lava tongues isbroken into imbricately stacked plates (Fig. 8).These plates apparently separated from the un­derlying lava because of the coalescence of gasbubbles beneath them and became stacked whenthe tongues encountered an obstacle. The stacksoften stand nearly vertical and consist of spiny

Rowland and Walker: Toothpaste lava

lava plates typically 5-10 cm thick (see alsoMacdonald 1972, p. 79).

Other tongues were evidently cooler and moreviscous. Gas bubbles could not coalesce to allowbreaking of surface crust, and the tongues weretherefore more able to plow their way through ob­stacles while maintaining surface integrity.

Clinker on most of the lateral margins oftoothpaste lava tongues indicates that shearingtook place. The clinker formed as pieces of pastylava rotated and tore off above the shear zone.The formation of shear zones was apparently re­lated to the velocity at which the tongue was ad­vancing. Those tongues which were somewhatmore fluid became attached more easily to the ad­jacent rock, and relatively wide shear zones (up to25% of the tongue width) were formed. Thosetongues which moved more slowly due to higherviscosity or slower effusion rate were better ableto maintain their integrity as a whole, and shear­ing was confined to narrower zones. The faster ormore fluid lava was more prone to form clinkerthan the slower or more viscous lava. Since theseshear zones are narrower on the more viscoustongues, so too is there less clinker.

Boccas usually occur tens to thousands of me­ters away from the vent, and lava issuing fromthem may have cooled slightly and lost gas. Thiscooling and gas loss causes a rise in viscosity sothat lava from boccas on pahoehoe lava fields issometimes toothpaste lava. The distal ends of pa­hoehoe flows are also sometimes toothpaste lava.

Fig. 8. Imbricate stacking of sur­face plates on toothpaste-lavaflow tongue that moved fromright to left. Plates are about10 cm thick. Width of view atbOllom edge of photograph isabout 5 m

Rowland and Walker: Toothpaste lava

10m

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3

Primary toothpaste lava, solid lines are cracks,dashes indicate spine orientations

Subsequent spread ing, lines and arrows showspreading directions

Crust fragments less than. 5 m2

Broken toothpaste tongues and boccas

"streamers" of clinker

639

Fig. 9. Map of large primary toothpaste-lava lobe at Kapoho (arrow in Fig. 1). Flow direction is bottom to top. Breakup andspreading of surface crustal plates occurred at many places, as did significant rotation of some plates. Insets show spreading ingreater detail. Numbers on insets and cross sections show relative ages of lava surfaces with one being the oldest

640

This is generally manifested by small flow unitsthat bud from the main body of the flow.

In places at Kapoho the primary flow lobes(as opposed to lava from boccas) consist of tooth­paste lava. These lobes are similar in form to thesecondary lobes, but are much larger. Figure 9 il­lustrates characteristics of a primary lobe. A solidcrust 10-20 cm thick formed within 10 m of thebocca, preserving on its surface longitudinalgrooves and upflow-pointing spines. This crustwas relatively rigid and broke into separate plateswhere movement in the underlying fluid lava div­erged, converged, or changed velocity. The platesthen diverged, rotated, or became thrust on oneanother.

New crust formed in the spreading zones be­tween diverging plates. In places this new crustwas broken by further spreading, forming third oreven fourth generation crust (Fig. 9). By matchingoutlines of surface plates and noting orientationsof surface spines on the new crust, the detailedspreading history of the toothpaste lava can be re­constructed. These histories can be quite complex,and the analogy to plate tectonics is close (Duf­field 1972). Neither did the surface plates sinkonce they fractured, nor did lava ever well up toheal the fractures completely.

Summary and conclusions

Pahoehoe characterizes the most fluid, and aa (oroccasionally, block lava) characterizes the mostviscous of Hawaiian basaltic lavas. It is temptingtherefore simply to correlate flow character withlava viscosity and identify the transition from pa­hoehoe to aa as the passage of the cooling flowacross a viscosity threshold. Toothpaste lava is arecognizable transitional lava type; all gradationsare found between pahoehoe and toothpaste lavaas well as between toothpaste lava and proximal­type aa. The incorrectness of ignoring flow move­ment during the viscosity transition was demon­strated by Peterson and Tilling (1980). The recog­nition of toothpaste lava and study of its charac­teristics provide further evidence that the lavaflow rate was an additional important factor.Slow flowage of moderate to high viscosity lava atone meter per minute to several tens of minutescaused the formation of toothpaste lava which infaster-flowing conditions would have formed aa.We attribute the extensive development of tooth­paste lava instead of aa in the 1960 Kapoho flowfield to the very low-angle ground slopes (mostly

Rowland and Walker: Toothpaste lava

less than one degree) onto which this flowerupted (Macdonald 1962).

The rheologic condition and slow rate ofmovement of toothpaste lava can be deducedfrom a number of features distinctive of this lavatype; these features reflect the slow rate of relaxa­tion of the lava when it was deformed. The viscos­ity correspondence between toothpaste lava andproximal aa is demonstrated by the comparabledegree of crystal settling that they exhibit.

The passage from pahoehoe to proximal-typeaa might be correlated with the crossing of a rheo­logic threshold and the acquisition of a yieldstrength. However, in situ crystal settling occurredin both pahoehoe and proximal-type aa at Ka­poho so that the yield strength of both types musthave been less than 4 N/m 2

•

Our studies demonstrate the feasibility oflearning much about the viscosity, rheologic con­dition, and flow velocity of basaltic lava flowsafter solidification by an analysis of their struc­tural characteristics and of making at least a semi­quantitative estimate of the numerical values ofthose parameters.

Acknowledgements. The authors thank Donald Swanson forvery helpful reviews of this manu cript, and Michael Sheridanand Michael Malin for reviews of an earlier draft. Researchfunded by NASA grant no. NAGW-541. Visit by helicopter tothe August 1986 eruptive vent of Kilauea made possible byNSF grant no. EAR-8601183. This is Hawaii Institute ofGeo­physic contribution number 1872.

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Rowland and Walker: Toothpaste lava

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Received June 9, 1986/Accepted December 4, 1986