Bull Volcanol (1991) 53:546-558

Volcanology«.'l Springer-Verlag 1991

Structure, and origin by injection of lava under surface crust, oftumuli, "lava rises", "lava-rise pits", and "lava-inflation clefts" inHawaiiGeorge PL Walker

Hawaii Institute of Geophysics. Honolulu, HI 96822, USA

Received June 10, 1989/Acceptcd February 20,1991

Abstract. Tumuli are positive topographic features thatare common on Hawaiian pahoehoe lava flow fields,particularly on shallow slopes, and 75 measured exam­ples are presented here to document the size range. Tu­muli form by up-tilting of crustal plates, without anycrustal shortening, and are thus distinguished frompressure ridges which are up-buckled by laterally di­recled pressure. The axial or star-like systems of deepclefts that characterize tumuli are defined here as "lava­inflation clefts"; their tips advanced into red-hot lavaand they widened as uplift proceeded and while thelava crust was thickening. Flat-surfaced uplifts, formedlike tumuli by injection of lava under a surface crust,were previously called pressure plateaus, but "lavarise" is proposed instead. The pits that abound amonglava rises, previously attributed to collapse or subsi­dence, are generally formed because the lava aroundthem rose, and the name "lava-rise pit" is proposed.Unique examples of tumuli and lava rises, from whichlava drained out under a surface crust 1.5 to 2.5 mthick, are described from Kilauea caldera. These exam­ples show that in tumuli and lava rises the crust floatson considerable bodies of fluid lava, and is able to doso because of its higher vesicle content: the fluid lavaloses many of its gas bubbles during residence beneaththe crust. The bulk densities of samples from tumulishow a general downward increase. The form of thedensity profile is consistent with the relationship thatfor any given crustal thickness the density of fluid lavaclosely matched the average density of that crust, sug­gesting that the lava was stably density-stratified. It isinferred that stable stratification was regulated by out­flows of the more vesicular lava fractions, loss of bub­bles through the lava-inflation clefts, and entry of in­jected lava at its level of neutral buoyancy. Below theuppermost meter the downward decrease in vesicularityclosely conforms with that expected by compression ofa uniform mass of gas per unit mass of lava.

Introduction

Tumuli are positIve topographic features found onmost, if not all, pahoehoe lava flow-fields. They typi­cally form mounds or whaleback ridges I to 10 m highand are deeply gashed by systems of axial to more orless radial gaping clefts. TIley are common in Hawaii,particularly on the gently sloping coastal terraces.Many have already formed on the currently active pa­hoehoe flow-field that began to develop in mid-1986from Kupaianaha vent on Kilauea's east rift zone.

The name "tumulus" was proposed by Daly (1914)(from the Latin "a swelling-up", following its applica­tion to certain prehistoric burial mounds). 'Schollen­dom' (Friedlander [914) is a synonym. Daly recognizedthat tumuli are common in Hawaii and explained themas having been raised by "the local hydrostatic pressureof still-fluid lava beneath the already chilled crust", aview with which the author concurs. As pointed out byCotton (1952), tumuli are not hollow and are, thus, notgas blisters as was assumed by Dana (1887), Sapper(1919), and Tyrrell (1931). Anderson (1910), in his ac­count of the formation of the 1910 pahoehoe lava onSavaii, observed that: "the surface frequently floatedup and was raised by the intrusion of fresh lava under­neath, so that what had previously been the course ofthe valley now became the highest part of the field".

Skeats and James (1937) called the tumuli in Victoria"basaltic barriers", and explained them variously as be­ing ramparts formed by temporary stoppage of thefront of a lava sheet, gas blisters, and structures result­ing from the irregular collapse of lava crust consequenton drainage of underlying fluid lava. Oilier (1969, 1988)equated tumuli with squeeze-ups and also apparentlywith gas blisters, and described them as being un­breached (that is, lacking clefts). He thus significantlydeparted from the original meaning of tumulus, andlike Skeats and James called tumuli "pressure ridges"or "barriers".

Guba and Mustafa (1988) described "volcanicridges", evidently tumuli, on the surface of Jordanianlava flows and interpreted them as primary vent struc-

lure, the ridge length and cleft being parallel withtheir eruptive fi ure. They interpreted the wide scatterand diversity of orientations to reflect a like diversity ofpo itions and orientations of the eruptive fissures.

Most worker e.g. William and McBimey 1979)ha e followed Daly' usage, but a general confusionha de eloped between tumuli and pressure ridge .Wentworth and Macdonald (1953) commented that:"pre sure ridge generally resemble tumuli except intheir greater elongation". Macdonald 1972) attributedpre ure ridges to upheaving of lava crust into an elon­gate anticlinical fold where the flow crust was pushedagain tome ob tacle being "aided by the hydro taticpre ure of the liquid beneath it '. Furthermore he com­mented that "tumuli are formed in much the same waya pre ure ridge, and in fact, there i a completegradation from one to the other".

This view has preva.iled (see, for example, Green a.ndShort 1971; Gary et al. J972; Bullard J976); but Mac­donald (1967) acknowledged that mo t ridges res,ultingfrom lateral pre ure are "irregular jumble of blocks ofnow crust", and Wae che (1940) ga a graphic eye­witne s account of ucb jumble forming in MaunaLoa' caldera in 1940. Champion and Greeley (1977)called all structure "pressure ridges", following Ru ­sell (1902); those aligned with axes parallel to flow di­rection they called "now ridges '.

Although tumuli are therefore well-known volcanicfeatures confusion exists regarding their origin and noy tematic quantitati e tud of them ha yet been

made. This paper i intended to sati fy a perceivedneed to document tumuli more fully, e tablish distin­gui hing criteria inve tigate pre ure plateaus andJava-collapse pit that are closely a sociated with tumu­Ii, and propose nomenclatural change necessitated bylhi tudy. The paper al 0 de crib a group of tumuliand related tructure on the floor of Kilauea caldera

Table 1. List of localitie , on the island of Hawaii, of measuredtumuli and related tructures

A. 1986-1989 now from Kupaianaha vent on Kilauea's east riftzone

B. Prchi toric lava from Kane ui 0 Hana pahoehoe shield: ex·po ed on coa tal Oat, outheast of Kilauea

. 1859 lava from Mauna Loa, on coastal l1ats al KiloloD. Prehistoric lava at Kaholi Point, Puna di lrict of Kilauea

In lava of 1919 from Halemaumau, below Uwekahuna Bluffand also near the middle of Kilauea Idera

F. At Pu'uhonua 0 Honaunau ational Hi tori cal Park westernfooL of auna Loa

G. Prehistoric lava from Hualalai, on coastal nats near KalokoPi hpond

H. 180 J lava from Hualalai, on coastal Oals 9 miles north of Kai­lua

I. 1972-1974 lava on outh side of Mauna Ulu, KilaueaJ. Prehistoric lava around Mauna lki on Kilauea's southwe t rift

zoneK. Prehi lorie lava expo cd in roadcul, 1.6 mile SW of FOOI­

prints trail·head, Kilauea' oUlh\ esl rift zoneL. I 43 lava from Mauna Loa, <0.5 km we I of Humuula Sad­

dle

547

which appear to be unique for Hawaii in that they havedrained oul' thi occurrence provides valuable in ightinto detail of the tumulus-forming mechanism.

Tbe new d ta et

The present paper is based on a morphometric tudy of75 randomly elected tumuH and related structure on12 pahoehoe flow fields (listed in Table 1) on the islandof Hawaii. A profile was mea ured across each truc­ture, generally cro ing at or n ar the ummil, u ing ameasuring calc, clinometer, and magnetic compa .From thi profile the height and width and the widthof eru t acro th structure were determined. Thelength of each tructure was al a mea ured. Figures 1and 2 show typical tumuli and ig. 3 portrays profilesacross a number of them to illu trate variations in theirize and morphology. Figure 4 plot the dimension of

the measured examples.Bulk-rock deo ity was determined for ample col­

lected from om tumuli a a mean of assessing theirvesicularity. Rectilinear block were sawn from the eamples, and the density wa calculated from their

mass aod linear dimen ions.

Characteri tic features of tumuli

Figure 2a illu trate a typical moderate- ized tumulufrom the coastal flat of Kilauea. It is polygonal andsomewhat elongate in plan and lopes at mostly 20 0 to40° to a height of 3.0 m. It is distinctly asymmetric be­ing higher and teeper on one ide. A main axial cleftup to 2.2 m wide follow the long xis and e erallesser cleft branch off from it. The clefts narrow down­ward and are al lea t 1.4 m deep but are partly infill dwith debris and may be considerably deeper than this.The lesser cleft have about the same depth as the axialcleft.

Lava squeeze-up occur in the axial and les er cleft.They have an upwardly conve top urface and are oflinear bulbou type described by ichols (1939). leftdevelopment continued after olidification of thequeeze-up and eacb squeeze-up now occurs attached

to a wall of the cleft. Two succe ively formed squeeze­ups occur at different depths on the ame wall and eachmerges down into the underlying lava within 1 m belowthe bulbous top.

The shallower queeze-up is the wider and more vo­luminou and lava from it flowed out from one end.The deeper squeeze-up bas a negligible volume anddid not flow from the cleft. The exposed side urfaceof the squeeze-ups have a lineation of millimeter-highwrinkle, inferred to be parallel with the lava-flow di­rection. The lineation plunge teeply in the middlepart of the tumuJu shallowing toward either end of themedial cleft where the lava flowed mainly laterally. Thequeeze-ups have a low vesicle content apart from a

moderate concentration near their top.

548

ig. 1:1-. ypi altumuli. :I Tumulu Ct, 5.3 m high, on 1859 lavaon coa tal flat at north we t foot of Mauna Loa. b Tumulus 820,2.8 m high, near H lei sea ar h on outh side of Kilauea. c Tumu-

lu E9. 2.4 m high, on Kilauea caldera floor; the ropy structure onthe cru"t surfa e indicate the lava-flow direction (now up anda 1'0 s the cru t before tilting

o

++ + /

IDg+

o

oa a

2b 30TUMUlUS wroTH tm)

10 WIDTH

,.....; C +C-CRUST WIDTli

~, TUMULUS

WIDTli

549

Fig. 4. Linear dimension of the Hawaiian tumuli and lava ri cmeasured in the present study. Diagonal lines on a and b denotea peet ratio

20 -

10

30

CRUSTWIDTH

(m)

IUfTllll on moderate slopes (>4'1

o Iumuh }on shallow slooest<4'llava "saG

HE'G~OT"'----'---'--'-----'VC;V 06/~(m)t ~\

E2

~~ dip of cruslol surface (d grO"o)I£: OuiliOw ., SQueez ,,"p

." .. -"',,83

". :,' .?

LAVA RISES

TUMULI

Fig. 2. Typi . Itumuli in plan iew and cro -profile. B3 occurs onthc coa tal nat of Kilauca near Holci sea arch; E2 occurs on Ki­lauea caldera noor below Uwekahuna Blufr

\

" ,

114

~ LAV:RISE PITS .~~

...\,.-I ~::~ii" Ii ii' ,"I,"""'" ,I " ',Ii Ii Ii illllljLL·.~.~

~B2~'i:~i 7C4~ (:, CS ~~~ -"-~:;,\;m::;;,-f',, "

? 1 1 ' 1 ? I 1 1 1 'POll squ zeup "",orl Inal crusl

Fig. . Profile, on the arne cale, acro typical lUmuli, lavarises, and lava-rise pits. A tumulus-like feature is also known, thatoriginated by irregular ubsidence of lav crust over an underly­ing ob tacle (Ia a draped around the stump of a tree

Figure 2b illustrate a larger and more symmetricaltumulu from J<jlauea caldera floor. It has a single ax­ial cleft up to 2.5 m wide by 3 m deep with bends at twoplaces where there ar triple cleft junctions. Mo t of thecrust urface evidently consist of one pahoehoe flowunit, but locally this i overlain by thin outflow whichemanated from the clefts at an early tage of tumuluuplift.

The foregoing descriptions relate to two sp cHic tu­muli but with small modifications are applicable toHawaiian tumuli generally. The linear dimensionrange over nearly two order of magnitude (Fig. 4). Noclaim i made as to the maximum size attained by tu­muli in Hawaii and examples larger than tho e tudiedoccur for example in Victoria (Skeat and Jame 1937)and on Etna Gue t et al. 1984 . A minimum ize is dif­licult to define becau e many incipient tumuli occur in

550

which the system of crack i well developed but thela a eru t ha been uplifted by les than] m.

In plan view tumuli vary from equant uplifts toridges in which the length exceeds four times the width.The more nearly equant examples a.re cut by a, sy ternof approximately equal clefts having a more or lesslar-shaped arrangement and the more elongate the tu­

mulus the more dominant the axial cleft. Cleft depth iscomparable to tumulus height and exceeds 8 m in thehighe t tumuli. One or more squeeze-ups almost invari­ably occur in the clefts, and as many a five may befound at different level in the arne cleft. In general,the uppermost i the largest, most ve icular, and mostcommonly the ource of outnows that drained out fromthe cleft.

M ny tumuli are raised above their uITounding bya more or less horizontal cleft extending around part oftheir periphery. These horizontal clefts appear to bemore or Ie identical in character to but are more dif­ficult to inve tigate than the vertical clefts.

The distinction between tumuli and pressure ridges

The main uncertainty in the Ii erature concern the dis­tinction between tumuli and pressure ridges. The crite­rion hitherto u ed is the relative length. A simple crite­rion is now proposed that permits discrimination be­tween structure in which the buckling was due to lat­eral pressure transmitted through a rigid cru t (a in thepre sure ridges on sea ice) and structures in which thebuckling wa caused by a imple up- welling. Thi newcriterion is that in the former the total crustal width

~rr"''''- mea ured in eros -profile exceed the width of theI,) structure (shortening by lateral compression has oc­

curred), wherea in the latter it is equal to or less thanthe width of the structure. 'vp"""'j

In practically all the tumuli mea ured the total wid hof cru t (within the limit of measurement error) i equalto OJ' Ie s than the width of the structure (Fig. 4c). Theyformed without any lateral shortening or evidence forlateral compression. In the few examples where crustwidth exceed tumulu width, it is u peeted that lavarose to infill early formed clefts and hence created newcru t during tumulus growth.

Tn several measured example the cru t width i ig­nificantly less than the ttlmulus width and sugge t thattumuli tend to form in lava undergoing a modestamount of lateral extension. One of the largest tumuli,GI, ha a crustal width 4.5 m less than the tumuluswidth and part of the cru t in this instance probablysub ided into an axial graben where it is concealed bysqueeze-ups and outflows.

Although tumuli are regarded as formed by upljft, iti acknowledged that in many example the uplift is re­lative. Undoubted example of tumulu -like featuresformed by the irregular ubsidenee of crustal platesover underlying obstacle are common in Hawaii. Inexamples in the Mauna Ulu lava of 1973 we t of PuuHuluhulu, th ob tacle were lava trees (lava coatingthe tump of a tree).

Tumulus growth: geometrical con traints

The tilted cru tal plate of tumuli are up ardly con exand thei I' dip increa e typically by about 10° towardthe tumulu periphery. Assuming that the plates weretoo rigid to bend while being uplifted this implies thaItumuli form where the pahoehoe crust has an initiallyup-arched form. Broad upwardly convex swell sueh 3

mi.ght have become the sites for tumuli are indeed verycommon on pahoehoe flow-field . The prior existenceof such a well facilitates uplift of crustal plates, andmay indeed be a nece ary pre-condition for tumuluformation, for the following rea on.

Consider I1rst two horizontal crustal plate of rec­tangular cro - eelion hinged at their top outer endFig. 5a). The e plate are geometrically incapable of

rising by rotation about their hinges because their diag­onal length D exceeds their width H. The greater theaspect ratio V/H of the plates the greater the differ­ence between 0 and H and the greater the re i tanceopposing plate-rise.

If the two horizontal plates are bounded at each endby cooling (contraction) joint, as umed to be ofwedge- haped fonn decreasing to zero width at the hotlower plate urface c Fig. 5b) it i then g ometocallpo sible for the plates to rise initially by rotation abouthinges at thei r lower ends (h', Fig. 5b) until the outerjoints are closed (Fig. 5c). The angle through whichthey can rise before cia ure would be about 5° for a 1%linear thermal contraction of the plate. Further riseabout hinge at the upper ends of the plates is then pos­sible provided that the plate a pect ratio V/H is sum­ciently maJI 0.05 or Ie s) 0 that point A i not lowerthan the plane containing the two hinge Fig. 5d).

Con ider now two plates ituated on an upwardlyconvex lava crust and hinged at their top outer end(Fig. 5e). Then provided that th apex of the well, A, inot lower than the plane containing the two hinges the

"I" "Ingec_cooing Join! (wldtl' exagg r led)

Fig. 5. Geometrical con traints on tumulus formation, as ex­plained in the text

551

plate are geometrically free to rise without any restric­tion .

Position of tumuli in pahoehoe now-fields

Three kind of tumulu occurrence can be distin­guished in Hawaii, namely: (1) shallow-slope tumulioften large and forming cluster can isting of manymembers; (2) moderate-slope tumuli found on lopeof about 40 or more mo lIy mall, and commonlymarking rootle vent feeding m ny lava outflows; and(3) flow-lobe tumuli, occurring at the leading edge of apahoehoe flow-lield and occupying the entire lobewidth.

Both hallow and moderate- lope tumuli tend to beelongate down lope, and commonly occur in inuourains aligned downslope. It is tempting to explain tu­

mulus-trains a overlying major lava tube as reportedby Gue t et at. 1984) on Etna. Whene er a search wamade along a train it failed, however, to reveal a majortube, and whenever the line of a known major tubemarked by skylights) was followed tumuli did nothow any obviou association with it.

Tumulus trains may well overlie les er lava tubesthat form in the middle of the larger p hoehoe lobe.

ven though such tube may never develop into mastertubes, they can persist for some time and carry a suffi­

ient volume of lava to generat tumuli and la a ri e .Moderate- lope tumuli tend to be mall and evi­

dently formed when the crust was thin (ca. 10-30 em).Most of them were ources of many mall outflowwhich tend to conceal the tumulu to give rise to whatMacdonald (l972) called a "tumulus covered with en­trail pahoehoe". "Lava-coated tumulu "i now pro­po ed. The re ulting structure resemble a hornito (dri­blet cone; driblet spine; spatter cone), but hornitos, bydefinition con ist of spatter re ulting from weakly ex­plosive activity.

Flow-lobe tumuli are less common in Hawaii than inmany other ba altic area, and orne of the be t exam­ples occur in toothpa te lava that wa distinctly morevi cou than pahoehoe (Rowland and Walker 1987).The example illu trated (Fig. 8) occur at the edge ofthe 1960 now-field at Kapoho at the ea tern lip of Ki­lauea volcano, and cia ely approached, but failed to

lava top urface only in their upper several ten of cen­timeters, and the walls tend to depart increasingly fromnormality with increa ing depth Fig. 6e). The uppercleft wall could ha e formed imply by the uplift oflabs of already formed crust, but the lower cleft wall

could not have formed in this way.Careful mea urement show that the ariation of

cleft width with depth approximates to the distance be­tween two circle centered on the crustal hinge-point(Fig. 7). The be t lit is obtained if increments are addedto the arcs to account for squeeze-up added duringcleft growth. The interpretation, hown a a sequence in

ig. 6 i that formation of a tumulus typically beganwith the uplift of plates of lava crust that initially wereevera[ tens of centimeters thick; the plates thickenedubsequently by olidilication of lava on their under­urface ynchronou Iy with uplift.

1 mWIDTH

b

t-

~wo

2

5.Om---

,t.. /" ll\ -'/t \ ..-.-' /

I \ I .\\ J /~ \( /\. 1_ l\ "

i--7.3m

Fig. 7. Origin of cleft cutting tumuli. a Profile aero s cleft in tu­mulu E2. Ar of circle having the same radii as the width of theuptilted crustal slabs are inserted. b Width variation with depth,compared with distance apart of the two arcs of circles shown in(a). A better lit is obwincd by adding the width of queeze-ups(short arcs

Fi . 6. Stage in the growth of a tllmulu , Crustal thickening byaccretion of material to the lower urface of the plate accompa­nies tumulus uplift

\ /\i

i?:i:~: Ii!: """.':'" 'i" :I.'·::·t·' ','" ':'1 !.',".'"'' ", ".~r""

Clef hape and its geometrical ignificance

The geometry of the cleft that cut tumuli ha an impor­tant bearing on how tumuli develop. The two oppo itewall of a cleft are orientated normal to the original

a

~-" ,\

552

01

10m.., ............' ......1 ......1 .....1

Fig. 8. Flow-lobe tUll1ulus in plan view and cross-profile, al edgeof 1960 lava beside Kapoho lighthouse, Kilauea

Fig. 9. Flow-lobe Lumulus (c1efllava lobe) on edge of dacite lavanow 31 WNW foot of San Pedro volcano in Northern Chile. Thetop three diagram how inferred stage in tumulus growth. Do/­led lines 1-5 delineate uccessively injected lava increments

engulf the lighthou e. It issued from the side of aa lavaand nowed for 25 m, splitting into two lobes thatmoved in opposite directions for a further 15 to 20 m.The lobes are deeply cleft, and the original lava cru t isnearly vertical in places, while the upper cleft wallhave been rotated until tbey are nearly horizontal. Inplaces lava wedges orne of which have agged undertheir own weight project from tbe cleft and appear tobe squeeze-up.

It is appropriate to call the e lobes "cleft lobe";their identity a tumuli, and their origin by the samemechanism that forms tumuJi, are however, clear. Cleftlobe are particularly common on andesitic and otherhigh-vi co ity lava . The example illustrated in Fig. 9 ione of everal that occur around the edge of the greatdacite lava at the we t-northwest foot of San Pedro vol­cano in northern hi Ie (Francis et al. 1974). The cleft

walls curve over from the cleft and fonn 010 t of theupper surface of the lava. The wall surfaces have chiselmarks (Jame 1920),otherwi e known as cyclic fracturestriations (Ryan and Sammis J978), resulting from theintermittent advance of the crack-tip into hot lava. Lavalobes on the growing lava dome of Mount St. Helenare similarly cleft (called .. mooth preading centeratop lobe" by wanson et al. 1987' called "crea truc­tures" by Anderson and Fink 1988).

Characteristics and origin of lava-inflation clefts

The wall of clefts in tumuli can ist of two mor or Iewell-defined portions having different surface texture.The upper 0.1 to 1.0 rn are clean and crudely pri maticjoint surface uch as form by contraction of olid lavacrust. The lower portions have a thin surface veneer,generally I to 5 mm thick, of red vesicular lava that isbanded parallel with the initial Java top surface. Thebands are typically S to 10 cm wide and consi t of v n­eer having different texture. In ome band bubbleproject into the cleft or upward along the cleft.

The urficial banding wa regarded by Nichols(1939) to be formed where the cleft tip propagateddown into till-nuid lava and he interpreted the bandedveneer to be due to nuxing of the cleft urface by es­caping ga . The writer prefer to interpret the ven er toconsi t of e icular la a that oozed up from the clefttip.

Locally the walloI' cleft are cut by sharks-toothprojections these being lipped cracks plucked when amass of lava was dragged down over and pulled awayfrom a hot and still-plastic lava surface. Locally thewall show lava groove ("1 va lickenside" Bartrum1928) formed when solid lava craped aero hot andstill-plasti lava. ichols 1939) interpreted both r a­ture to re ult from differential movement of one cleftwall again t the other while one or both were till hotand in a pia lie condition.

The lower portions of the gaping clefts tbat charac­terize tumuLi belong to a very di til1ctl~e cIa of crackformed where the tip propagated into hot and still-nuidor plasti lava as cooling proceeded while at the arnetime the cleft progressively widened in response to aninflation or welling of the hOl lava. The name "Iava­inflation cleft" is proposed.

Two other ituations in which lava-inflation cleftsdevelop i in breadcrust block and pseudo-pillow lava.The former re blocks of hot but highly viscou la a,thrown out in many vulcanian-style explosi e eruptionand found aloin many pyrocla tic-flow depo its,which then become inflated by vesiculation (Walker1969). Breadcrust structure locally develop al 0 on thesurface of many rhyolitic and andesitic nows. Pseudo­pillow lava (Watanabe and Kat ui 1976) is Java cut bydeep crack evidently formed by a general expansionof the now which pennit acce of water and ag instwhich clo e- paced cooling joint occur.

553

c lava-lnHa hon cletls

Lava-ri e pits ("subsidence pi ')

marked by tilted cru tal plate . The lava between someclefts sagged under is own weight. Squeeze-ups projectinto, and small outnow i ued from, some clefts. Figure10 illu lrate the mechani m by which lava rise form.

Fig. 10. rage in the formation of tumulu. lava ri e. and lava­ri e pit. olid black denotes nuid lav inje led below the urfac'rul

lumulus

Pit commonly occur among Java rises and have almo tinvariably been interpreted to form by sub idence andtherefore called sub idenc pits or collapse depres-ions. This designation would be appropriate for tho

pits formed by subsidence, but most pits formed in­stead at places where the lava urface failed to be ele­vated; thus they are pit becau the lava around themro e, a hown by Fig. 10. It i now proposed to renamethe e tructures "Iava-ri e pit ".

Thi origin of lava-ri e pit a witnessed on thecoa tal flats of Kilauea near Kalapana in the 1986-19 0current eruption. It can al 0 be deduced from struc­

tural detail of the pits them elves. in particular, thewall of each pit are identical in character to the edgeof lava rises.

Nichols (1946) regarded the depre sions on Mc ar­ty's flow a having formed by the collapse of the roofof lava tunnels, but Champion and Greeley (1977) ob-erved that of the collap e tudied by them in the

Wapi lava field "none were found with any connectionto a lava tube ystem at either the upnow or downflowand of the depre sion". The arne is true of exampleeen in ection in coastal cliff cutting the Mauna Loa

flow of 1859.Lava-rise pits are equally numerous as tumuli and

have imilar dimension (Fig. 3). They generally widendownward and the surface cru t forms an overhangaround the top. Alternatively crustal plates are tilted 0

that one end rests on the pit floor. Pits having a wideoverhang can be dangerou to animals and people'many een in the 1859 flow-field on the coa tal flat of

auna Loa, for example contain the bones of goathat evidently jumped into the pit but were unable to

climb out again.

Characteri tics and origin of lava ri e ( pressureplateau )

[nitial la a-cru t thickness in tumuli

The term "pressure plateau" wa applied by Wentworthand Macdonald (1953) to a more or less flat-toppedportion of a flow-field that was bodily uplifted by theinjection of lava beneath the surface crust. Profiles nowmea ured across several examples demonstrate that, asfor tumuli, the crustal width i equal to, or less than, thewidth of the structure, conlirming an origin by simpleuplifl. "

"Pre ure plateau" is a very expre ive name, but iti confu ing in that the pres ure involved (hydrostaticpre ure in fluid lava) is different from the pressure in­volved in generating pressure ridges (lateral compres­sion tran'mitted across rigid cru tal plates).

The alternative term "lava rise" i now proposed toremove this confusion; "rise" implies both the elevatedmorphology and the origin of the feature by a physicalelevation of the lava surface.

Lav ri e on the coa tal flat of Hawaii are very ex­ten ive and may be hundreds of meter wide, althoughtheir margins are not everywhere well delined. Themea ured examples Figs. 3 and 4 are relatively smallbut well-delined structures. Each i circumscribed by asteep or overhanging escarpment one to several metershigh, cut by sub-horizontal lava-inflation clefts or

How thick wa the lava cru t when tumulu growth be­gan? On indicator is the depth at which cleft walls be­gin to depart from normality to the cru t surface. Thisdepth varies from about 0.1 to about 1.0 m.

Another indicator is the depth to which a clean ten­sion fracture extends, below which the cleft walls areveneered by glass. This depth varie generally fromabout 0.1 to about 0.5 m but if the glass veneer is dueto ga fluxing or upward oozing of fluid lava the depthis likely to be less than the initial cru t thickness. Smalltumuli observed forming on Kilauea recently had acrust thickness of less than 0.2 m mea ured down to thered-hol cleft tip. The drained-out tumulus EI (describedbelow) had fully formed by the time its crust was 1.5 mthick.

Some tumuli uplifted more than one lava-flow unit,implying that the upper unit or units plus at least thetop cru 1 of the underlying unit had olidilied beforeignilicant uplift occurred. In orne of these examples

the upper unit or unit were erupted from the cleftsafler the tumulus began to form. One of the measuredtumuli (818 ha an axial c1efl at least 2.5 m deep thattransects compound lava consisting of many flow unitsapparently not erupted from the cleft and implies thatat lea t 2.5 m of units had accumulated before signifi­cant uplift began. The general conclusion is that tumu­Ius growth can begin as early a when the crust is only0.1 m thick, or as late as when the crust exceeds 2.5 mthick, and can continue until the crust is at least 8 mthick (the maximum measured cleft depth).

e:1eeil

S

r

554

(b)

Fig. 12. Sketch map, not on the allle calc, of collap ed lavari ·cs E5 (a) and 3 (b), and profile across the structures. Stippledareas repre en! the outwardly dipping and non-collapsed outerparts of the ri e . Arrows give dip direction and amount in de­grees

spacing of about 3 em; tbis and the absence of "tidemark " indicate that the draining out of liquid lavafrom th cavern was accompJi bed in a ingle uninter­rupted act.

The roof of EJ i 1.5 01 tbick both where it consi Is oforiginal cru t and where the broad axial cleft is infilledwith queeze-up. From the known cooling rate (Peck1978; Hardee 19 0) thi cru t would ha e taken about12 day to form. The cru t on the outheastern side wapu hed up farther than that on the northwestern ideand an abrupt tep about 1 m high in the cavern roofrenect the under ide of thi a ymmetric tructure.

E I was thu a fully developed tumulus, pushed upto 501 above it urrounding by the time that a crust1.5 m thick had formed, and the cru t overlaid a pool offluid Ia.va 4.5 m or more deep. Outflows are thin, haw­ing that the lava vi cosity was low. Normally tumulisol idify to solid tructu res, but E I occurs in an intracal­dera setting where lis ures readily form, and it is pre­sumed that draining out of the remaining nuid lavatook place into uch a fi ure.

E3 and 5 are collapsed lava rises. E3 is crudely rec­tangular in plan Fig. 12b) and mea ures 110m by

~======:::::::::~

10,

thenEOU

Es I~

'",

<E7Ea· E9

El rise L...!QQ!!!.....

oI

Cf-"'-'" l?,-I. i,. ..,\

I .. I\ : ~

,I \( ~~·tm, I ... ~.I \ '. '.\1 ....\I; '.," .. ','

l'i .: "'" .\, ".~. """ •...

, (I ~ ,I' /. ">. ... ...

,:! .:f/" r.........'\\!! .: ~."1 Ii"'j" it '" }SKYLIGHt I ~i 1 i \ / _AR~OF ::: II ~OOF :: ! l I OUAPS I':. I ;I. I • '

'. \. \, "t ••# ... ",'

~\ '., ': 'v' ... :' :.'.1'';......... I?···.~:··'i ....::;g./':';/ ~:'~ \

..~~~:::.:>~.o~ DRAINAGE

TUSE

Drained tumuli alld ri es ill Kilauea crater

Fig. 11. a Plan view of the cavern in drained-out tumulus E I; thecontour of cavern r 0 height arc ba ed on 320 measurements. bProlilc acros tUlI1ulus I on the same scale. c Sketch map of partof Kilauctl caldera 11 or howing the locations of EI and othermeasured tumuli nd I'va ri es in relation to the well-knownUwckuhunu "laccolith" on Uwckahuna Bluff

A field of cattered tumuli and lava rises occur on thenoor of Kilauea caldera below Uwekahuna Bluff andincludes ome apparently unique drained and partiallycollap ed example. A detailed tudy was made ofome becau e their internal features are highly relevant

(0 unde tanding tumuIi and lava rises. They occur onthe pahoehoe 1 va now of 1919 which forms a low lavashield loping outward at I to 30 from Halemaumau pitcrater. The tumulu field occurs on the Jaw-anglenorthern foot of the hield cia e to the caldera walls(Fig. II).

Tumulu I i 24m wide by 5.3 m high (Fig. 11) andhas a slightly a ymmetric axial cleft that is largely filledwith, and partly concealed by, squeeze-ups and out­flows. Part of the oulheast side has collapsed givingaccess to a flat-floored cavern 43 01 long by up to 28 01wide and 4.5 01 high. The roof height was measured at320 points on a rectilinear I m grid and contoured, giv­ing a cavern volume of about about 900 01 3

. The noor isnat, locally with a ropy tructure, and slope gentlyouthwe t to a lava tube that extend from tbe lowest

point on the cavern noor down into a smalJ side cham­ber. Around 010 t of it circumference the floor curveteeply up to join the roof unlike ga bli ters wbere tbe

roof and noor generally converge asymptotically).A notable feature of (hi ca em i the great number

of worm talactite imilar to examples illustrated bFriedllinder I 14, plate L IV) Jaggar (1931) and Per­ret 1950). They are hollow and quite fragile about6 mOl wide, nd up to about 30 em long. Small stalag­mit occur on the noor. The talactites are uniformlydi tributed a er the entire cavern roof at an average

555

2.5 '--+-'---'--+"-----+I---II·-J20 30 40 50

POROSITY %

Fig. 13. Density and inferred porosity (based on non-vesicularlava density of 3.0 mg/m J

) of samples from Five tumuli or lavarises as they vary with depth below the surface. Solid CUnie is oneof a family of computed curves for which, at any given depth ofsolidified-lava density D, the average density of overlying crust is0.95 D (the approximate equivalent density of nuid lava at thatdepth). TIle reasonably good Fit is interpreted to indicate that thecrust was just noating. Dashed /inc is one of a family of computedgas-compression curves relating the porosity to depth, assuming auniform mass of gas per unit volume of lava and the density pro­rile of the solid curve

o B4

~ Elx E3+ El:2V L1

BULK-ROCK DENSITY Mg/m3

o_r-2_5-+-+-+-+,_2.+'O-t-t-,--f-,--f-----!1,5---1H'3

• ~ lI. I 0_ I .;r'r ~

+ ,A+ //?

x JI 0+ ,

v ++1 •oJ x

+KI,

l\10

+ /I K

I' •I tI<I

E) x t+IIIIIIIIII vj x

"':IIII

+ II

1.5 -

1.0 -

2.0

0.5--

across the lava crust of E1 and E3 and through thethickness of several tumuli. The resulting profiles (Fig.13) show that the crust density is lowest at its top andincreases downward as the porosity decreases (whilevesicle sizes increase). On a gross scale the porosity de­creases consistently downward; on a finer scale (5­10 cm) small fluctuations in porosity occur about thistrend.

Another line of evidence is provided by outflowsand squeeze-ups from tumuli clefts. In general, the ear­lier outflows are the largest and have the lowest densi­ty; the small later squeeze-ups have a higher density,and the latest and deepest squeeze-ups have the highestdensity. This shows that as tumulus development pro­ceeded, the lava that rose in the clefts became progres­sively less vesicular because its source, the underlyinglava, became less vesicular.

The form of the density profiles in Fig. 13 is consis­tent with the view that on a gross scale the lava wasstably density-stratified, the requirement for which be­ing that at any given time the density of fluid lava was

DEPTHm

The formation of tumuli and lava rises by uplift of lavacrust over fluid lava, and evidence from El, E3, and E5for the presence of a considerable pool of fluid lava be­low developing tumuli and rises, implies that the crustmust have floated on a denser liquid. Since crystallinerock is in general about 5% denser than its melt, this inturn implies that the crust was lighter because it wasmore vesicular. Several lines of evidence confirm thisinference.

One line of evidence comes from a lava-densitystudy made on sample suites collected from profiles

Densities of crust and melt: stably density-stratifiedlava?

70 m. Proceeding inward from its periphery, it has asmooth crust that slopes up for an average of 7 m at5-25°, and culminates 3-3.5 m higher than its sur­roundings. A series of fractures occurs at or near theculmination delineating a sunken area of 4200 m 2

, inplaces rimmed by a scarp 1-2 m high, within which thelava crust dips inward at up to 25 0 into a horizontal­floored central area 2-3 m lower than the culmination.Remnants of a low-roofed cavern occur in places underand beyond the culmination, roofed by crust 2-2.5 mthick.

E3 was evidently a flat-surfaced lava rise, consistingof a crust that would have taken about three weeks toform, overlying a pool 2-3 m deep of fluid lava with agently down-bowed noor. About 12000 m3 of lava thendrained out, and the roof settled onto the noor withminimal break-up. The fractures bounding the collapsearea include some (having a banded glass-veneered sur­face) related to uplift of the rise and others (clean sur­faced) initiated at the time of collapse. Locally lavaflowed up the circumferential fractures and veneeredthe collapsed crust.

E5 (Fig. 12a) is broadly similar to E3 but is smallerand is not flat-floored; the collapsed lava crust disinte­grated to a mosaic of blocks which evidently rest on anirregular floor having two low points 9-10 m lower thanthe rim. The western of these contained a pool of lavawhen the crust collapsed and some of the lava splashedover the subsided crust. The pool in turn crusted overand later drained out. Part of a flat-floored cavern withroof-height up to 2 m extends west of the collapsedarea.

E5 was evidently, thus, a lava rise consisting of acrust 2-2.5 thick overlying a pool in the low points ofwhich fluid lava was 7-8 m deep. About 10000 m 3 oflava then drained out. The collapsed crust broke intoblocks that settled to the floor sequentially.

These drained tumuli and lava rises are significantbecause they demonstrate that during their activegrowth their crust overlaid large pools of very fluidmolten lava from 4 to 8 m deep. The geometry of out­flows and squeeze-ups show that this lava was veryfluid. It is presumed that similar pools of lava underlieactive flows across which it is possible to walk withoutdiscomfort with fluid lava visible only in lava-inflationclefts and small flow-outs or squeeze-ups.

of~d:kutis

dev-a:r-

e,)f

a

556

2 3

~.0lil"'" ..

Fig. 14. Schematic view of proposed mechanisms by which a sta­ble den ity-stratification may be hieved in tumuli and lava ri e'.I, lhin lava erupted; 2, lava injected below surface; 3, accumula­tion of bubbles leads to den ity in tability: 4. 5, bubble-rich lavafraction form outflow: 6. 7, bubble' esc pe through cleft

equal to or greater than the average density of overlyingcrust.

The computed solid curve of Fig. 13 is one of a fam­ily for which, at any given depth where the ve icularcru t has a density D and the corresponding vesicularlava ha a density 0.95 D (Mura e and McBirney 1973)the average density of overlying crust is 0.95 D. Thecurve thu defines the situation in which the crust haneu ral buoyancy relative to the immediately underly­ing lava. In reality the top of the crust is less dense thanspecified by the curve so that the crust is just floating.Below a depth of about 0.5 m the downwardly increas­ing density can be explained by gas compressibility(dashed line, Fig. 13).

Note that the density profiles are very similar totho e given by lava ponded in lava lakes (Peck et al.1966: Wright and Okamura 1977; Peck ]978).

echani ms leading to stable stratirication

Mea urements made on chilled margins at the top andboltom of pahoehoe lava demonstrate that the flowinglaya at the time of its arrival at medial and distal sitestypically possesses about 40 or 50 volume % of smallve icles interpreted to be remnants of an initialcomplement that originated at the vent (Walker 1989).On moderate slopes mo t flow unjts soon becometatic and olidify to form pongy pahoehoe in which

little or no bubble ascent or e cape occur. On shallowlope the top and bottom cru t olidify rapidly and

generally have a ve iculariry imilar to that of pongypahoehoe. Ln tumuli and lava ri es the flow interiorconsists of basalt that is injected between these twocru t and this basalt is significantly less vesicular thanthe crusts, implying that a con iderable loss of bubblefrom it occurred.

Density profiles acro tumuli and lava rises are con-i tent with the establishment of a table density stratif­

ication on a gro scale, and thi requires a significant10 of bubbles from the flow in erior. Loss of bubblei indeed nece sary for the development of tumuli andlay ri es ince the e tructure require that a more ve·icular crust floats on a den er and less vesicular li­

quid.

It is proposed that there are three regulator bywhich a table stratification i achieved: (1 buoyanlri e of the more highly e icular lava through the Cn! Ito form outflow unit : (2 e cape of an exce of bub·ble from the flow; (3) injection of newly arrived lavafractions at levels appropriate to their den ity.

The formation of outnows was clearly an importantregulator, particularly in the earlier stages of tumulusdevelopment (early ouenow are uptilted together withthe tumulu crust), and outflows also emanated laterfrom many of the Jav -innalion clefts. Some tumulihowever lack outflows, ugge ting that 10 of bubbl(a di tinct from ve icular melt also occurred.

It is proposed that the deep cleft of tumuli and lavari e projecting into the nuid lava, and the upwardlyloping crustal plates of tumuli which would direcl

bubbles into the clefts are loci from which bubble e ­cape is concentrated. The vesicular glass that veneerspart of the cleft wall j one manifestation of thi es·cape. One can only pecul te on the effectivene of thethird mechanism. The clo e fit of the 95% curve im­plie , however not only that olid crust floated on li­quid basalt, but al 0 thal the den ity of liquid w reg­ulated 0 that it did not ri e to ignificantly greater thanthe cru t density.

Tumuli and lava ri e on hallow- lope pahoehoelield characteristically form a complex and intercon­nected network overlying an equally complex ana, ta­mosing network of ubcrustal lava conduits. It is envi .aged that increments of fluid lava coming into positionbeneath a given tumulu may have travelled variabledi tances through thi network from the ma ter tubethat feeds fresh lava into th y tern, and may vary a, ­cordingly in their ve icle content (having 10 t variableamount of vesicles according to their residence time inlhe y tem); when the e increment enter a density- tra­tj(jed lava pool under th cumulus they are injected attheir level of neutral buoyancy in the pool.

Conc.lusions

Tumuli (previou Iy often referred to as pressure ridge)nd lava rises (previou Iy called pre sure plateau)

have long been known to be ubiquitous features of pa­ho hoe now on hallow lopes, and it may be con id­ered urpri ing that a quantitative study of them habeen so long delayed. The pre ent tudy reveals unex­pected relationships and introduce a new concept:that of achievement of a table density-stratification inlava, to explain some of these relationships. The crustof tumuli and lava rises noats on fluid that i denserbecause it is less vesicular.

This study has important implication to under-tanding the development of the pahoehoe flows that

occur on shallow lope. Ob ervations of nowing lavaindicate that pahoehoe flow units initially seldom e ­ceed I m thick, but they commonly thicken ub e­quendy to several meter by injection of lava under aurface crust. Thjckening form exten ive areas of lava

ri e , diversified by tumuli (where the rise was greater

Jynt

th-la

ItIS

:h~r

~s

'-

's

than average), and lava-ri e pit (at points of failure tori e). There i evidence Ihat these tructures form byimple uplift of I a cru t without any horizontal com­

pres ion; a few meter f hydro tatic head would beadequate to generate th ob erved amount of uplift.

Aubele et al. (19 9 tudied the esicle distributionpattern in vertical prolile acro s lava flows, and pro­posed a imple model whi h relate this pattern to theascent of ve i Ie through ta6c lava. One consequenceof their model hould be the common formation of azone having ignificanl concentration of vesicles inthe upper flow int rior. Thi concentration should de­velop becau e ri ing ve icle are trapped under a down­ward-moving olidificalion front. Such a zone is not,however, clearly hown by their density profiles, whichuggest that their model is incomplete and that orne

other mechani much a bubble loss also occurs.Studies briefly commented on by Rowland and

Walker (1987) show that a progressive elimination ofbubble i an important a pect of the flowage of aaflows, and lead to the formation of the distinctive di ­tal facie of aa which i barren of ve icles. The presenttudy now how that ve ide e cape, albeit by different

mechani m al 0 occur from orne pahoehoe flows.Tumuli and lava ri e are cut by deep lava-inflationclefts and it i propo ed that there clefts are the locifrom which the bubble escape was concentrated.

On hallow- lope Hawaiian pahoehoe flow fields,tllmuli and la a ri cover a ubstantial proportionprobably exceeding 50% of the total area. These struc­ture are not therefore morphological curio ities butare manife tation of important mechanisms by whichlava flow develop; the pre ent study is highly relevantto under tanding emplacement mechani ms and inter­preting internal tructure of flood basalts.

Acknowledgements. Thi re earch was funded by the Jaggar Be­que t -und of the Univer ity of Hawaii. SOEST Contribution no.2531. My thank to T. L. Wright for directing my attention at ve­siculation tudics of lava ponded in pit craters; to J. P. Lockwoodand F. Trusdcll for directing mc to the 1843 Mauna Loa lavawhcrcin occurs thc largest lumulus of my data set; to J. E. Guestand an anonymous reviewer for their very thoughtful commentson thc manu cript; and to the late Kimo Kabatbat (National ParkRanger) and Ken Hon, who shared with me their observations onthe growth of lumuli and the aClive uplift of lava rises on the cur­rently active now-field of Kilauea.

Referenc

Anderson Wink JH 19 ) rea e structures as la a emplace­ment rate indicator ab tr). OS 69: 1486

Anderson T 19(0) The olcano of Matavanu in Savaii. Geol SocLond Quart J 66:621-639

Aubele J , rumpler LS EI ton W (1988) esicle zonalion andtrtical ·tructure of ba It nows. J Volcanol Geolherm Res

35:349-374Bartrum J 192) Lav licken ide at Auckland. NZ J Sci Tech-

nol 10:23-25Bullard FM (I 76) Volcanoes of the arth. University of Texa

Press, Austin, 579 PPham pion D -, Greeley R (1977) Geology of the Wapi lava field,

Snake River PI:lin, Idaho. In: Greely R, King JS (eds) Volcan­i In f the eastern nuke Rivcr Plain, ldaho. NASA: 133-152

557

otton A 1952) Volcanoes a land cape forms. Hafner Pub 0,ew York, 41 pp

Daly R 1914) Igncou rock and their origin. McGraw Hill,ew York

Dana J D I 7) haracteri ties of olcanoes. Dodd, Mead and0, ew ork, 99 PP

ranci P ,Roobol MJ, Walker GPL, obbold PRo Coward M(1974 The an Pedro and an Pablo volcanoe of northern

hile and their hot avalanche depo it . Geol Rdsch 63:357­3

Friedl nder I (1914) Ober die Kh:informen der vulkanischen Pro­dukte. 111 Laven. Zeit Vulkan 1:219-224

Gary M, Mc fcc R, Wolf L cds) (1972) Glossary of Geology.Am Geolln t

Green J, hort M 1971 Volcanic landforms and surface fea­lure. pringer, Berlin Heidelberg New York, PI' 519

Gub I, Mustafa 1-1 (I ) tructural control of young basaltic Ii -sure eruption in thc plateau basalt area of the Arabian plate,northea tern Jordan. J Volcan Geotherm Res 35: 319-334

Guest J E, Wood ,Greeley R (1984) Lava tubes, terrace and me­gatumuli on the 1614-24 pahoehoe lava now field, Mount

tna, Sicily. Bull Volcan 49: 635-648Hardec H (1980) Solidification in Kilauea Iki lava lake. J Vol­

can Gcothcrm Re 7:211-223Jaggar A 1931) Lava stalactite, talagmites, toes and "squeeze­

up n. 11le Volcano Letter, Hawaiian Volcano Ob , no 345: 1­2

Jame AVG 1920 producing columnar jointing in lavas.J Geol 2 :45 9

Macdonald A 1967) orm and truetures of extru i e basalticrock. In: He 1-1 H, Poldervaart (eds Basalts. The Polder-aan lreati e on rocks of baltic composilion. lnter cience

Publ, e\ ork 01 I: 1-61Macdonald G 1972) olcanoc Prentice-Hall, Englewood

liff. J. 510 pplurase , cBirne R 1973) Propenies of ome common ig-

neou rock and their melt at high temperalUres. Geol ocAm Bull 4:356 -3592

ichol RL 1939) urlicial banding and shark' -tooth projectionsin the crack ofb alii lava. m J ci 237:188-194

ichol RL (194 ) Mc arty basalt no Valencia Country, ewMexico. Geol 0 Am Bull 57: 1049-1086

Oilier 19) Volcanoes. Austral Natl Univ Pres Canberra, 177pp

Oilier 1988) Volcanoes. Blackwell, Oxford, 228 ppPeck DL 1978) ooling and vesiculation of Alae lava lake, Ha­

waii. US Geol UI'V Prof Pap 935-B, 59 ppPeck DL, Wright TL, Mo I'e JG (1966) rystal\ization of tholeiitic

basalt in Alae 1 va lake, Hawaii. Bull Volcan 29 :629-655Pcrret FA (1950) Volcanological observations. Carnegie 1nstn,

Wa hinglon Pub 549. 162 PRowland K, Walker GPL (1987) Toothpaste lava: characteri lies

and origin of a lava truetural type transitional bctween pa­hoehoe and aa. Bull Volcan 49:577-587

Ru ell 1 19 2 Geology and WaleI' resource of the Snake RiverPlain of Idaho. U Geol Surv Bull 199: 192 pp

Ryan M P, ammi (1978 yclic fracture mechani ms in cool-ing ba alt. eol oc m Bull 89: 1295-1 08

apper K 1919)" bel' hornitos und ven andl gebilde. Zeit Vulcan5: 1-39

keat , Jame A G (1937) Basaltic barriers of western icto-ria. Proc R oc ietoria 49 n ):245-27

w n on DA (1973 Pahoehoe nows from the 1969-197\ aunalu eruption, Kilauea volcano Hawaii. Geol Soc m Bull

4:615-626Swan on OA, Ozuri in 0, Holcomb RT, lwatsubo EY, Chadwick

WW. as dcvall TJ, Ewart JW, Heliker C (1987) Growth ofthe lava dome at Mount St Helens, Washington, USA), 19 1­19 3. Geol 0 Am pec Pap 212: 1-16

Tyrrell GW (1931) Volcanoe . Henry Holt and Co, New York, 252pp

'"Wacsche HH (1940) Mauna Loa summit crater eruption. 1940.

The Volcano LeUcr, no 468: 1-9Walker GPL (1969) TIle breaking of nl:lgma. Gcol Mag 106: 166­

mWalker GPL (1989) Spongy pahoehoe in Hawaii: a slUdy of vesi­

cle-distribution patterns in basalt and their significance. BullVolcanol 51: 199-209

Watanabe: K. K:lIsui Y (1976) Pseudo·pillow lavas in the: Aso cal·dc:ra, Kyushu, l:apan. J Jap Assoc Mineral ~trol Eoon Geol71:44-49

Wcnlwonh CK, Macdonald GA (1953) Structures and forms orbasaltic rocks in Hawaii. US Gcol Sun' Bull 994, 98 pp

Williams H, Mcllirney AR (1919) Volcanology. Freeman Cooperand Co. San Fransisco. 397 pp

Wright TL. Okamura RT (1977) Cooling and cryslalliulion oftholeiitic basalt. 1965 Makaopuhi lava lake. Hawaii. US GtOISurv Prof Pap 1004. 78 P

Editorial responsibility; R. S. J. Sparks