16
Volcanoes as landscape forms
Ordinary, non-volcanic landforms are the results arid environments such as the Atacama Desert or of erosion by wind, water, and ice. Erosion is an the Moon's surface, erosion rates are indeed irreversible process which ultimately reduces immeasurably slow, but in others, such as the even the loftiest mountain range to a flat plain. humid tropics, they can be startlingly fast, even Volcanic landforms by contrast, are the results of by human standards. Catastrophic processes opposing constructive and destructive forces. such as avalanching accomplish in a few Constructive processes operate only while volca moments what it might take millennia to achieve noes arc active. This may be an extremely short otherwise. Whatever their rate, one thing is period-a matter of days or weeks-or rather certain: erosion starts work on a volcano as soon long, with activity continuing intermittently over as it starts growing, even before its lavas cool. tcns ofthousands of years. Paricutin, a common Erosion never ceases. A large volcano may or-garden basaltic scoria cone was born in a experience several phases of rapid construction Mexican cornfield on 20 February 1943. After a in its lifetime, during which the rate of construc year of activity it was 325 m high; when it finally tion exceeds the rate oferosion, but once eruptive simmered into silence in 1952 it w3s410 m high. activity wanes, erosion instantly gains the upper About 2 cubic kilometres of lava and tephra were hand. On a large volcanic massif, erosion may be erupted during its nine years of activity. By proceeding in one part, while new lava is being contrast, Stromboli in the Mediterranean has added to another. bccn erupting throughout history, but is still only All these variables yield volcanic landscapes 981 metres above sea-level. Many rapidly con that are as richly diverse to analyse as they are structed volcanic landforms are not 'volcanoes' pleasing to behold. Strangely, though, volcanic at all-the Valley of Ten Thousand Smokcs was landscapes have been little studied. There is a buricd undcr 15 cubic kilometres of ignimbrite in book on the subject, which remains an authorita less than 60 hours. tive account although it was published almost 50 When contemplating the impassive grandeur years ago; onc which is still capable of supplying ofa mountain range, one intuitively thinks ofthe volcanologists with insights into why their volca destructive processes of erosion as acting infini noes look the way they do. 'Volcanoes as tely slowly. Erosion is often conceived as the Landscape Forms' was the title which C. A. epitome of the slowness of geological processes. Cotton selected for his book; his title is used for But this is a considerable oversimplification. In this chapter to acknowledge his contribution.! Volcanoes as landscape forms 341 - 16.1 Monogenetic volcanoes
16.1.! Scoria calles sions of 910 scoria eoncs. and found that their A monogenetic volcano is the product of a single mean basal diameter was 0.9 km. In a sample of eruptive episode. This may last a few hours, or a 83 fresll scoria cones, he found some regular few years, but the essential point is that once geomctrical relationships: the heigll/ of Ihe cOile eruption has ce;.sed, the plumbing connecting proved to beO.18 times the basal width, while the the vcnt to its magmatle source freezes over, so crater diameter was 0.40 limes the basal width. the volcano never erupts again. Basaltic scoria This emphasizes a characteristic feature ofscoria cones are good examples of monogenetic volca cones: lheir craters lire large in relation to I he si7.e noes. They arc found in thousands all around the of the edifice as a whole. Naturally, lheir crisp world, in many lectonic environments, either as profiles soften with age, but Wood showed that components of scoria cone fields, like P.lricutin the ratio of crater width to basal width changes in Mexico, or as p•• rasitic vents on the flanks of remarkably lillie. Thus. scoria cones remain larger volcanoes-Etna has dozens, All over the easily recognizable, even after millennia of world. they have the same distinctive morpho weathering. logy. They arc rarely more than lwO or three hundred metres high, and arc often asymmetri 16. J.2 Mnors cal: either elongated along a fissure. orelse higher Scoria concs arc the results of minor basaltic on Ihe side Ihat was downwind al the time of eruptions taking place in dry conditions. When eruption. A breach on one side often marks the basaltic mllgmas interact with water, the nature sile from which bva has flowed. A distinctive of the eruption is explosively different, prod ucing feat ure is their sim pic geomet rical profile, def! ned sllrlseya/l pyroclastic deposits (Section 6.4.1). It by the angle of rest for loose scoria (Fig. 16.1). All is not necessllry for the eruption to lake place young scoria cones have side slopes elose to 3r. under water to produce explosive consequences In a statistical study of scoria cones, Chuck -a water-bearing stratum (aquifer) in sedimen Wood showed that 50 per cent were formed tary rocks is all that is nceded. In the simplest during eruptions thaI lasted less than 30 days; case, shallow phreatic explosions caused by 95 per cent of them during eruptions that lasted magma-ground-water interactions blast less than one year. 2 Wood looked at the dimen- upwards through to the surface, forming large
Fig. 16.1 La Poruiia. north Chile,:1 300m-high scoria cone. La Poruiia appears youthful, but may be many thousand years old. since it is located in a hyper-arid pari of Ihe Alacama Desert. In Ihe shadow al the foot of the cone. 3 lrain on lhe Antofagasta-La 1)3Z railro;.d provides scale. (Compare Ihe air phOlO in Fig. 7.8.) 342 Volcanoes: a planetary perspective
holes in the ground. In the Eifel ;lrea ofGermany, metamorphic basement underlying the sand eruptions of this kind formed 30 craters about a stone. Explosive activity thoroughly commi kilometre across, now occupied by lakes. which nuted the relatively weak sandstone. while the gave their name to the landform: maars. J\faur granites and gneiss were more resistant. At the cr:Hcrs 3fC simple. circular depressions sur bottom ofthe Malha maar is a small lake. fed by a rounded by low rims of ejected debris. Their series of small springs seeping through the walls arc steep-sided initially. but arc quickly ubian sandstone. Because they arc the only eroded away to gentle slopes. Since they 3fC by source of fresh water for thousands of squaTC definition holes in the ground rather than kilometres, thescsprings are vital to thc people of structures buill up above it. maars typically fill the arc;:\. Their me.."lgre flow illustratcs how little with water and arc thus manifested as lakes. water needs to be colllained in an aquifer for In his statistical study, Wood showed that explosive magmatic interactions to t"ke place. muars arc Iypically small [eatures, most having At Malha and other mallrs, there is often only a diameters of about one kilometre. His preferred small proportion of magmatic material in the examples arc from the Pinacate region of north ejecta. This can lead the unwary into grievous west Mexico, where eight young maars are errors of interpretalion, si nee basi n-shaped maar beautifully exposed in the Sonoran Desert. They craters have been mistaken for meteorite impact arc circular to oval, with diameters between 750 craters when there is little obvious volcanic and 1750 metres and range in depth between 36 malerial present, as at Jayu Khot:l in Bolivia and 245 metres. (F;g" 16"3)" In the desiccated heart ofthe Sahara Desert, an improbable place to look for magma-water 16.1.3 Tuff rillgs interactions, there is an instructive //Iaar at A convenient. but not iron-clad distinction. Malha, in the Darfur province of the Sudan. between moars and tuff rings is that maars arc About one kilometre in diameter and a hundred excavated inlo the substrate, whereas luff rings metres deep, the maar was blasted through a arc buill up above it (Fig. 16.4). And whereas layer of ubian sandstone, depositing an apron "wars arc the results of shallow explosions, often of ejl.'Ctcd debris around the crater, now well involving scanty amounts of juvenile material, exposed in the rim (Fig. 16.2). Conspicuous in tulT rings contain an abundance of highly frag the debris arc rounded boulders of gneiss and mented basaltic scoria: they arc essentially ac gr..Illite up to a metre across derived from the cumulations of surtseyan tephm. TulT rings arc
. " Fig. 16.2 Camels and goats drinking al the Malha /lllIlIr. Darfur province, weslern Sudan. White Nubian sandstone is exposed in the wall of the erater. while darker overlying material is ejccta. Animals in the foreground are clustered around small springs which all rise at the same
strJtigraphic le\'el, probably ." the contact between the • sandstone and the underlying ,.".:"-"= crystalline basement. •
343
Fig. 16.3 Jayu Khot:l U1(/{lr on the altiplano of Boli\ia. iniliall) I1101l£h11O Ix: an impact cr:.uer. Tufa dcpo~ilS from the high sl;l1ld of glacial Ltke Tauca are prcscnt around the cr:llcr. which lhcn.:fon: mll~l be more than 10000 years old. Erosion has subdut:d the ejecta rim.
Fig. 16.4 A superbly s}lllmclrical tun'ring, ncar the En,I'AIe \'olc:lno. Ethiopia. (photo: 1·1. Ta1.idf.)
formed when magma comes ncar to the surfucc tuff ring formed in prehistoric times (Fig. 16.5). before being ex plosivcly fmgmcnlcd. Cerro Xico, For years it domin.llcd the cxotic p Fi~. 16.5 Diamond Head luff ring. Honolulu. H:lwaii. Trade y,inds COluscd l11:tXll1ll1nl :Ish 3ccumul;lIion do\\nwind. forllllng lhe high pomt on the rim. furthest :I\\ay from c:.llllCrJ just as thc)' do through much orthe yem IOday. form a tuff ring. while anotller only a short Ejccta inevitably accumulated much more distance aw:.y forms a cone. bUI it probably has thickly on the downwind side. to do wilh the relative ;tmounts of water :Illd magma. and the duration ofthe eruption. Tephra 16.1.4 Tuff COI/es has 10 be widely dispersed to fOfm a lulT ring. Tulr cones arc smaller. stccper versions of tuff calling for a violent hydromagmatic eruption. rings. composed of similar surtseyan tephra. with relatively high column and mass eruption Morphologically. they resemble scoria cones. rate. Tuffcones may forlll from less violent. more Only a few kilometres from Cerro Xico, on the prolonged eruptions. sallle lake bed near Mexico City. is a prominent The tephra erupled arc hal and water satu tulT cone called EI Caldera (Fil:l. 16.6). It is not rated. so wet. muddy deposits are formed. Their immediately obvious why one eruption should stickiness results in turT cones which develop Fig. 16.6 El C'lldcra.;1 250-m-high 1un' COliC constructed above the bed of former Lake Te~coco. near Mexico City. Morphologic:llly. it rescmbks a scoria cone. but is composed of surtscyan tephra. • • Volcanoes as landsca~ forms 345 startlingly steep slopes on erosion, sometimes the differences between tulT rings and cones. steeper thall the 33 0 angle of rest of loose. dry Wohlctz and Sheridan suggested that the mas scoria; sometimes so steep that it is barely sive bedding found in tulT cones is due to possible to walk up the dip slopes of the tephra emplacement in cool. wet conditions, less than layers. Koko crater, only a few kilometres from 100 c. l In contrast. the thinner-bedded deposits Diamond Head on O:lhu. is an excellent example of tulT rings arc emplaced hOI and relatively dry. of a sleep tun" COIlC (Fig. 16.7). Koko crater is As increasing amounts of wa ler mix with magma smaller than Di Fig. 16.7 Prolil..: view of Koko crater. Oahu. I-hlwaii. Trade winds blowing from righl 10 left arc responsible for Ih..: pronounced as)'mmelry of Ihis 366-rn-high l\lIT COlle, which shows good p;lrasol ribbing. 346 Volcanoes: a planclary perspective Mechanical energy Scoria Cone lillie or ...... water Turf Ring • ~m '''''' Pillow Lavas Fi~. 16.M Effect of incrcasint; amounl.s of water on morphology of sm;lll basallic volcanic constructs. Internal structures arc shown only schematically: slumps from S\L'Cp crater w"lIs arc observed in some lulT rings, while inward and outward dips ;11'C characteristic of IlifT COIlt:S. After Wohlcl~.. K. 1-1. :Ind Sheridan. M. F. (1983). Hydrovolcanic explosions 11. Evolution of basaltic tulT rings and lufT cones. Am. J. Sci. 283, 385-413. of about 1600 square kilometres. They are all widlh oflhe carrol-shaped pipe is only 30 metres, between 15 and 20 million years old, and so their while at the present surface levcl it is 300 mel res. surface expression is 1l1uted. Some arc identifi Il may have been linked with :.l maar :.lbout 500 able as vague depressions, but mosl can only be mel res across at the original surface. Soulh picked up through geophysical surveys. Africa is 110t now a region of active volcanism: At Kim bcrley in $OlHh Africa, a deep mine was from iSOlopic evidence, thc diamonds themselves cxc.l\,.llcd to probe thc limits ofa large diatreme. 'lppcar to be more than one thous:lnd million This expensive undertaking was nol C • Volcanoes as landscape (arms 347 Fig. 16.9 Hyato Ridge. Wells Gray Pro\'iocial Park. British Columbia. ~l 1()(X) J11 high IUra formed about IO())(} ye:us ago by subglacial eruption of albli olivine basah, now thickly forested. Photo: courtesy of Catherine l'liekson, Canadian Gcoto1;ical Sur\'ey. 16,1.6 Ltmdforms reslf/li/lg from sub-glacial crupfiolls ;\t the present day. permanent glacicrs arc confined to the polar re~ions, a few sub-arctic ice caps such as those ill Iceland and Patagonia, and shrinking valley gl:Jciers on high mountains around the world. It is easy to forgel th,lI huge areasoflhe Earth were covered by continental icc sheets which receded only II 000 years ago. When the ice receded, the steep sides and flat. lava-capped topS of subglacial VOlc;:lllQCS were revealed (Section 6.4.3). In British Columbia, Fi~, 16.10 Detail ofa 'pillow' of basalt in easily recognizable 'table mountains' developed hyaloclastitc nl3trix. showing quenched margins and radial jointing. formed by subgl3cial baS31t eruption. by subglacial centr - 16.2 Polygenetic volcanoes Polygenclic volcanoes arc thosc that havc cxperi cones which carried on erupling. They ha\'e a eneed more than one eruptive episode in their single summit vent and radial symmetry. These history. Most ofthe world's volcanoes fit into this arc the volcanoes whose graceful profiles adorn rather loose category. but several different sub so many calendars and postcards. There arc groups can be identified, based on the number innumerable examples ofsplendidly symmetrical and location of the venlS from which eruptions simple cones around 'the world. MI. Mayoll lOok place. (2400 III high) in the Philippines is often ciled as the world's most beauliful vokano (Fig. 12.14). 16.2.1 Simple cOlles Licancabur (5916 m high) has a striking geo Simple cones are overgrown scoria cones: scoria metrical purity of line. dominating the oasis of 348 Volcanoes: a planetary perspective over-impressed by the apparent simplicity of such volcanoes. however. Subtle structural com plexities oflen \Urn up on closer inspection. so that the volcano should fonnally be regarded as composi/e-magnillccllt Mount Fuji is a case in POlilt. Simple cones arc characterized by rather small summit craters; often tiny for Ihe size of the edifice as a whole. and not much bigger than that of a scoria cone-Mt. M,lyon's for example, is less than 200 m in diameter. Nestled within Lieancabur's sunllnit crater is a miniscule fresh water lake, 90 melres by 70 melres. At an elevation of almost 6000 m. it is probabl)' the world's highest lake. but none the less a plank tOllic fauna of considerable inlerest to biologists manages to exisl. Weak volcanic thermal emis, sions probably help to prevent the lake from Fi~. 16.11 Licancabur. north Chile. a 5916-m-high simple cone. The dale of its I,ISI eruption is not freezing into a solid mass-stalwart divers have kllOI...I, but appears to h:l\'c I:k.--en rca:1II. since lavas explored its depths during the world's highest on its l1ank~ are fl\.'Sh. There arc In\.-;\ ruins and a altitude dive. small lake in its summit crater. In their summit regions_ simple cones arc often armoured by lava nows (Fig. 16.13). These San Pedro de Alae.lIna in Chile (Fig. 16.11). In provide mechanical strength. so vertiginously Peru. daily life in the city of Arequipa is carried steep slopes, over 40. arc possible. as for out beneath tile sublime curves of EI Misti (Fig. example on the 1157.m-high volcano St Eusta· 16.12). while in Guatemala. Agua and Fuego tius in the Dutch Antilles. On Ihe mid-l1anks. volcanoes form an appropriate backdrop for the interstralillcd scoria. l:tva. and talus eroded from strombolian eruptions of I>acaya. It is casy 10 be the lavas predominate, so slopes case otT. Fig. 16.12 EI Misli (5822 Ill) ri~s direel1y abo~e the suburbs of Arcquip'l. Peru's second city. preselHing an ob~ious hazard to Ille CilY (rorcground). There have bccn many eruptions since lhe Spanish conquest. A lava dome was active in the summit crater as recently as Ihe 1950s. Photo courtesy of F. Bullard. University of Texas. - Volcarloes as landsc "-ig. 16.13 Acam:lrachi. north Chile.;I 6046·m.high simple cone with S[lOCP slopes rc~ulling from extrusion of la\'11 nows from (hI:: summit cr~tlcr. Edifice heigh! is only 1200 m. Although lav:I !lows may snake down to low lc\'cls. Ihe lowest slopes aTC moslly construclCd of talus c:.rried downwards by mass wasting. Thus. where f'= volume. r= radius. and II = height. the elegant concave profiles of simple cones In reality. the shape ofa 'conical' volcano such rcnCCI the interplay between erosion and erup as MI. Mayon is actually expressed by a more tion. complex exponenti:.1 relationship of the forlll: Gi"cn a single \cnt. the shape and height of a simple \olcano arc immutably controlled by geometry: evcry additional increment of heighl requires a huge additional increase in w/l/me. When a volcano reaches a height of more than where IJ and M arc constants. lis volullle can be 2(X)() m. c:lch additional metre of height requires found by integr:lling this expression. Geologists the eruption of lens of millions ofcubic metres of with a mathematical bent established these rock. And as height increases. so erosion relationships Illore than a century ago. but since becomes more effective, though not in a simple Ihe pioneering work of Milne6 and Becker. 7 the geometrical manner. So. for :lny given combin subject has been largely ignored. so we still do ation oferuplion and erosion rales.the height of no! understand many of the su btleties underlying a simple cone is self-limiting. As volcanoes the geometrical form of volcanoes. become larger. approaching 3000 m. the volume A factor which further complicates consider inCrelllelll required for each additional height ation of the height of a volcano is that once a increment is so huge that the required eruption volcano grows large. it begins to deform under its rates begin to exceed the geologically plausible. own weight. When volcanoes arc built on thin This explains why voleanoes on Earth rarely oceanic lithosphere, the mass of the volcano also exceed 3000 m in edifice height. (Note that the causes the lithosphere to sag downwards into the SIll/lilli' heights of many voleanoes are much asthenosphere. In the case of the large Hawaiian higher. but these edifices are constructed on volcJ.nocs. this subsidence has been very con clev.lIed basements.) siderable. According to J. G. Moore. as much as These geometrical relationships can easily be a half or two-thirds of the upbuilding of the appreciated by ex(X:rimenting with the formula volcanoes may be offset by lithospheric subsi for the volume of a simple right cone: dence. Thus. if the lithosphere were not so 350 Vo1c;:moes: a planetary perspective llcxiblc. the Hawaiian volcanoes would be kilo high. It has bro:ld radial symmclry. although mel res highcr.~ there ;Ire several summit vents and innumerable parasitic monogenetic vents. On its northern cOlles 16.2.2 Composite flanks, vents arc aligned along a rift-likc exten Composite cones have had morc than one sion. while on the eastern flanks ,I great amphi c... olutionary stage in their existence. but still theatre (the Valle deillove) takes a great bitc out retain an overall radial symmetry. Throughout of the edifice. From the ground. these features their complex eruption history. the locus of make the volcano look distinctly complex, but aClh·ity has been csscllIially confined to a single from the perspective of space. the volcano takes sile. Vesuvius is all example of a volcano where on a symmetrical shape, and its identity as a an earlier edifice was wrecked by an eruption single conical edifice is obvious. All its many (AI) 79) and a younger one buill up in its place. vents ha,c tapped a single mantic sourcc. such thai from sOl11e vantage points the ruins of Etna also exhibits conn~nicntlyanother prop the older edifice (MoniC Somma) are not erty of many apparently simple major volcanoes: obvious. and the VOIc;lIlO appears \0 be simple the edifice we see now is only the youngest of a and sYlllmetrical. II is surprising how extensively $Cries constructed on more or less thc samc site. a \olcano may be modified. yet still retain an Present-day Etna (also called Mongibello). overall symmetrical sh;IJ>C. AI Parinacota in appears to havc been constructcd ovcr the last Chilc. a hugc dcbris avalanchc cviscerated thc 34000 years. but $Cyeral edifices cxisted prc wc:.tcrn flank of thc volc,mo 13000 ycars ago "jously. stretching back more than 100000 years. (Fig. 16.14). So much reconstruction has takcn For example. between 80000 years and 60000 place since then that no trace of the amphithcatre years ago. an older volc Fig. 16.14 Large 11l0Ul1d~ in the middle distance Oflhis photography arc lorel!{/ blocks. cmplacc Fig. 16.15 Vcnical air photograph of Ceboruco volcano. Mexico. an excellent example of a cOl11posil~ volcano with a ("llnpl~x history bUI an imJividual idcntit)·. M;lI\~ dilfercnt cpiwdes of activit} are identifiable. including nested calderas and 1:Iva-l1ow fields of different age:. Ccboruco is 21~ III high. ;lIld its outermost C;Lldcr;t ;tbout -l km in diameter. It last erupted in 1872. gled with domes and craters. Remarkably. the case, as in many others, there is e\'idence for detailed anatomy of this. the world's highest activity migrating through time along the ridge. volcano. has never been studied. so it is not certain exactly how many componenlS make up 16.2.4 Vo/cmlo complexes the massif. or how their magmatic plumbing Compound. or multiple. volcanoes have an systems arc interconnected. Allhough the sum individual identity. On a map. one could I>ointto mit of the massif is very high (6887 m) the the volcano and say there. Volcano complexes individual cones arc not especially large: b Fi~. 16.16 Auc:tnquilcha. north Chile. Vicw of north p:lrl oft!'c lO-km-long caSI west-trending compound \·olcano. which docs not possess ob\ iOlls \ okano morphology. Actin: fum:ITolcs ha\'c deposited sulphur which is CXIr:tctcd from the "orld's highcsl mine al the summit ;1\ o\"cr 6000 m. PJ:1Il1 in the fOfCGround IS :11 :tn clc\:Jlion of about 5CXXl m and is p;trl ol "orld's highl"l>l pcrm:lIlcntl) inhabited scttlement. almost buried under a confusion of lavas. It is ico. represent another type ofdistributed volca dinicult to tell which lavas came from which nism, In an area near the centre of the Mexican cone. Volcanic Belt. directly south of Mexico City. one Volcano complexes like Cordon Punta Negra hundred and forty-six Quaternary scoria cones represent a form of -dislribul(,.'(r volcanism. If were counted within an area of about 1000 erupted frolll a single vent. all the magma found square kilometres, :1 cone density of 0.15 per ill a complex would make a decent large COile, but square kilometre. Basal diameters varied from rather Ihan erupling through a single conduit, O. t to 2 km.OJ For years, many of the buildings on !iCveral closcly spaced conduits were active more Ihe campus of the University of Mexico have or less contemporaneously. It remains 10 deter been renowned for their e:wberanlly colourful mine the length of time required to create a murals. It is less well known that the campus is complex like Punta Negra, Ihe range of composi canst rueted on Ihe la va l1eld from one of the mosl tions present, :llld how these V:l ried through lime. recenlly aClive cones in the field. Cerro Xitlc Scoria cone fields. like those of Central Mex- erupted only 2400 years ago. - 16.3 Shield volcanoes Whereas cones arc either slraighl sided (scoria shaping growing COlles, the gcomclry or shield cones) or eOIlC:lVC (most large cones), shic1d volcanoes is dictated only by the rheology ol'the volcanoes arc conpe,\" upwards. And while cones lavas or which they arc m:ldc. Thus. a young may be ralher steep. somelimes I'e:lching above shield volcano is a ralher subtly shaped con 40" in lheir summil regions. shields arc genlly struct. with a gelltly swelling prolile, made sloping. often less thall 10'. Simple and compo entirely or basalllavas. Whereas ,trtists respond site cOlles include lavas. pyroclastics. and talus, readily 10 the sweeping. uplifting curves ofa Fuji but shields arc constructed almost entirc1y of like cone. the bland profile of a shield volcano lavas. Finally. rocks with compositions ranging alTers less inspir;l\ion. from b:IS:lllic to rhyolitic turn up in cones. but shields arc almOSI exclusively basaltic. Whereas 16.3.1 Hawaiiall shields erosion and mass wasting pl;ly importanl parts in Mauna Loa and Mauna Kea arc shield volca- Volcanoes as landscape forms '53 noes in a class of their own, rising nearly nine spread rapidly over the surface. at a rate ofaboul kilometres from the noor of the Pacific. Mauna 50 cubic metres per second. burning their way Loa reaches 4169 Illetres above sea-level. II has a through oliia rain forest and marijuana plan tOlal volume of about 40000 cubic kilometres. a tations indifferently. Once the eruption is well hundred limes greater than a lyrical composite established. a large fraction of the lava (pcrh:lps conical volcano like Mt. Fuji. Despite its huge as much as 80 percent) flows through lava tubes. proportions, Mauna Loa is an unpretentious This dramatically extends the distances lhe nows volcano. ils smoothly arching whale-back profile can reach, enabling lavas initialed high on more reminiscent of the gelltle contours of the Kilauea's cast rift to debouch into the sea. Dorsel downs than a mountain ofalpine altitude extending the coastline. Apan from creating (Fig. 2.26). Only when the snowline defines the opportunities to photograph sizzling red lava summit region wilh sharp whiteness unexpected and white steam clouds against an azure ocean. in a tropical island is the magnitude ofils edifice these tube-fed nows account for the gently apparent. sloping profiles of shields: most of tile erupted While Maunas Loa and Kea are the largest volume ends up a long way rrom its point of and youngest shields in the Hawaiian Islands. origin, adding to the nanks of the \'okano. rathcr there arc many others of difTerent ages and than the summit region. Eruptions of more degrees of erosion. Each of the eight major viscous magmas, which do not flow in tubes. lead islands represents one or more dissected shields. to steeper, conical volcanoes. gelling progressively older westwards. A young Maunas Loa and Kea arc similar in altilude. shield has gentle slopes of only 2 3 at its base. But whereas Mauna Loa is activc (it erupted in steepens slightly to about 10 in its middle slopes. 1984). Mauna Kea has not eruptcd in historic and then llallens off again in the summit region. times. One reason why astronomers have been Each shield is composed of myriads ofindividual willing to risk building huge :md hugely expen 11ows, many of them compound /la/we/we 110ws. sive lelescopeson Mauna Kea is thaI Mauna Kea averaging only a few melres Ihiek. While each appears to be 1110re mature. This conclusion is major shicid probably had a summit caldcra in based on the presence on its upper nanks of which some acti\'ity was focused. a characteristic abundant small scoria cones and lavas of alkalic feature of the Hawaiian shield volcanoes is thai composition: markedly different from the tho they arc elongated along rift zones. from which leiites making up IllOSt of the volume of the most lav,ls were erupted. Dykes propagaling vO!c:lno (Section 3.2.1). Morphologically. these laterally from the cenlralmagma chamber carry alkalic lavas somewhat resemble andesites. in basalt magma laterally until it emergcs from a that the nows arc thicker and chunkier than parasitic \'en! on the nanks orthe vokano. These ordinary bas•."tlts. In the summit region of Mauna dyke-fed rift zones arc prominent topographic Kea where a small ice-cap existed during the laSI fcaturcs, extcnding for tcns of kilomct res, and arc Ice Age. glaciers scoured the surface of the flows. marked by many small spaller eOlles. pit eralers. creating topography more reminiscent of lhe and l1ssures. Kilauea. the smallest but most high Andes than a tropical OCC:ln island volcano. active shield at present. has experienced almost Mauna Ke.. has reached what is termed the continuous lav3 effusion since 1983 from the "alkali cap' stage in its e\'olution: a Slage which Pu'u 0'0 vent. 17 kilometres frol11 the summit the older shields on the Hawaiian Islands have caldera. During the period 1969-74, activity was also reached (Fig. 16.17). Unfortunately for lhe centred all the Mauna Ulu ('growing mountain') astronomcrs. reaching thc alkali cap stage docs vent. 10 kill along lhe rirt. In 1955.1a\'as spewed not nccessarily mean the vokano is extinct-o", out from:l \'ent25 km :lIang the rift. while in 1960 the neighbouring island of MauL Haleakala a major outburst engulfed lhe village of K:lpoho. volcano. also a mature shield, erupted as recently almost at sea-le\el, and nearly 30 km distant as 1790. Should an eruption take place on along the rift. Mauna Kea. the damage done to astronomical When a lypk:,i1 rifl eruption begins, lava Ilows rescarch programmes would be incalculable. 354 Volcanoes: OJ planetary perspective 1 Deep submarine stage Shillld ,'ole,1II0 Fig. 16.17 Scoria (,.'Qncs cluster (background) 31 2 Shallow submarine stage 4000 m at the ~ummil of Mauna Kca. J·bwaii. During Ihe Icc Age. a sm'lll icc-cap 5 Postcalder.l stage 9 Atoll stage ..,.",'''''-,",' •. "« •<, ,,' ...~ ,'",.;' ... i" .' ' t,"," 't Cinder (ones'5;:::~"~':t!;~',"12'" , --""'-L '~-- "'.le\'el Fig. 16.18 Stages in the morphological evolulion of 6 Erosional stOlge Hawaiian shield volcanoes. (After Macdonald, G. A.. Abbon. A. T" and Peterson. F. L. (1970). Volcanoes ill/he Seu. Univ. Hawaii Press. Honolulu. 517pp.) Each major Hawaiian volcano is an enormous shield. flut individual rirt eruptions may them selves construct small, gently sloping lava shields. Mauna Ulu. in the Volcanoes National Park was born in 1969, Innumerable pllllOeiloe lavas welled up and over the rim of the vent. spreading out to form an apron ofanastomosing 7 Stage of reef growth flows. In less than a year. a shield almost 100 metres high and a kilometre in diameter had grown. Subsequently. its summit crater was _... ---- ...... occupied by a small lava pond which, persisted for ...... - ...... several years. Pahoehoe lava erupted from ...... /'0...... __ ..... ~twcen • ~ 5e~ Mauna Ulu 1969 and 1974 ultimately ~<.~ ~ ~.,..:,~~vd reached the coast, covering almost 45 square kilometres. Fringing cora) r..'Cfs (n.....h b..'Comc wide ifupw.,rJ growth 16.3.2 Galapagos shields .1CC1Jln 1'" nk'S ~il1 king of islJ nd) Clustered on the Equ;ltor 1100 km west of Ecuador, the Galapagos Islands arc belter known for their contribution to Darwin's ideas on the origins of species than for their volcanoes. None the less, the volcanoes occupy :l hot-spol selling similar to, but more complex than, the Hawaiian volcanoes, Each island is either a shield VOIC:lllO or a coalescence ofscveral shields, each 45-80 km across, which risc about 1500 m above sca-Ievel. In detail. the Galapagos shields differ from those of Hawaii in three ways: First, they lack the gentle, wh:lle-back profiles of Mauna Loa, but instead have profiles tradition ally likened to upturned soup-plates. with a 356 Volcanoes: a planetary perspective Fi~. 16.19 Volcano Cumbrcs. Isla Fernandina. G:J.I:lpayos. shoy,ing the im'Crlcd ~up plate profile characlcri~ljc or Galapagos ~hiclds. Cumbrcs is 1250 metres high. Photo courtesy I'cler Mouginis-Mark. marked change of slope from gentle to steep (> 10 ) on the mid-flanks. and flatfish tops (Fig. 16.19). 16.3.3 lee/aI/die shields Second. whereas the active Hawaiian shields Iceland's shield volcanoes arc modest in size. but Mauna Loa and Kilauea have summit calderas elegantly symmetrical. Twenty have been con scveral kilollletres across. these are shallow, less structed in post-glacial times. They arc topo lhan two hundred llletres deep. On the Galapa graphically subdued and arc usually only a few gos Islands. by contrast. summit calderas are hundred metres high. Some have slopes of as spectacularly deep. That on Fernandina, for little as 1°, renecting the low viscosities of the example. is Illorc than SSO In deep. basaltic lavas involved. They rescmble the small Third. Ihe Galapagos volcanocs arc more Hawaiian lava shields such as Mauna Ulu. ncarly radially symmclric~l than the Hawaiian Skjalbreidur. the classic Icelandic shield, has shields. While there is some evidence for dyke uniform slopes of 7_8°, is 600 III high, and has a intrusion, linear rift zones like lhose of Kilauca diameter of about 10 kill. 1ts total volume is only and Mauna Loa ;Ire subdued. Surrounding the about 15 cubic kilometres, a mcrc pimple com summit calderas arc prominelll sets of circumfer pared with Mauna Loa. Georgc Walker has enlial fissures. These circumferential fissures arc suggested that the Icelandic shields were buill up almost uniquc lO thc Galapagos. On Earth, a few quickly, by almost continuous cruplion of thin terrestrial volcanoes exhibit comparable fissures, pahoehoe basaltic lavas irom the central vent. for example Deception Island (Antarctica) and Some of these fluid flows wcre :lble to lravcllong Niuafoou AlOli ('Tin Can Island': Tonga), but distances over gentle slopes. A flow from Trolla the best analogues arc on Mars. dyngja may have tr'lvellcd more th'lIl 100 km Is is not clear what factors arc responsible for over a 10 slope. the marked differences in topography between Small, flat lava shields of Icelandic type arc the Hawaiian and Galap'lgos shields. One widely characteristic of many fluid bas'lll provinces, hcld hypothesis is that their internal architecture such as those of the Snakc River 1'laill. 1O They is dilTerent, with ring dykes in the Galapagos and provide important clues to the morphologies to rectilinear dykes in the Hawaiian shields. In be expected in the source regions of extraterres some ways, the Galapagos shields resemble trial basaltic volcanism, for example on the overgrown seamount volcanoes. smooth plains of Mars (Section 18.5.3). Volcanoes as landscape forms 351 - 16.4 Volcanic landforms resulting from erosion It is unusual for parasol ribbing to remain 16.4,7 Singes il/ Ole erosio/l of Wiles intact for long. For one reason or another. The topography that il volcanic cone displays as usually reflecting subtle variations in original il succumbs to the depredations of erosion topography. 'master' gullies begin to prevail. depends on the climate. and what it is made of. capturing the heildwaters of lesser gullies. and Scoria cones arc esscntially heaps of loosely cutting rapidly downwards into the hear! of the packed, porous pyroc1asls, and therefore they volcano. Youthful volcanic materials arc often absorb water like sponges. Even under condi poorly consolidated, and so th is process can t;1 ke tions of heavy tropical rainfall. water soaks place amazingly rapidly. especially in tropical immediately into a scoria cone rather than conditions. After the great K rakatau eruption of running olT. gi\ ing erosion little chance of taking 1883.thc Dutch geologist Verbeck reported that hold. Scoria cones thus remain rccognizitble for only two months after the eruption, gullies 40 millennia. In cones which contain a high propor me/res deep had been cut into the pyroclastic tiOll of welded spatter, and in t ulT cones where the deposits. Even in the more temperate conditions tephra arc fine grained or muddy, porosity is of the US". deep gullies had been cut in the M t. llluch less, and runolf is enhancL-d. Once runolT St Helens debris avalanche deposit within a few gcts under way, it radically resh:lpcs the volcano. weeks of the eruption of May 1980, Within a few On a symmctrical cone, the first stage in this years, gullies tens of metres deep in places process is the developmcnt of parasol ribbilly: presented formidable obswcles to movement. evenly spaced V-shapcd radial gullies scpar;l\ed When two or three master gullies arc active on by ridges (Fig. 16.20). a coniCal volcano, widening and deepening Fi~. 16.20 Parasol ribbing on sUrlsc)'an luff·ring. west of Lake 'Abhe, Ethiopia. Photo: 1-1. Ta7.icff, 358 Volcanoes: a plancl'ary perspective themselves, there will inevitably come a point when the heads of two gullies intersect. This isolatcs a triangular, llat·surfaccd facet of the original cone. known as a p/aue:e. a term originally :ldopted by the French geographer E. i de Martonne (Fig. 16.21). Their distinctive triangular shapes also earned plalleze,~ the more I I colloquial term 'llat-irons', As erosion cOJltinues, I"alle:es get whittled away.l:lvas on the llanks of lhe \'olcano become progressively degraded, and ils summil Icvel is reduced. Ultimately, all that is left of a volcano is a gently rounded hill. Table 16.1 shows onc possible sequence ofstages in the erosional history of a cone. In regions wilh different climatic regimes, il will take dille rent periods of time for a volcano to pass through thesc sllccessivc stages. In the hyper-arid conditions of the Cetl\ral Andes it lllay lake several million years to reach stage 3: in tropical areas such as Indoncsia, it may take only 11 twenty thousand years. Fi:;. 16.21 Form:lIion of plalle:('s or 'fbI-irons' on a Necks lIlItl dykes Even after a volcano has been \·01c.1nic cone subjected 10 gullying erosion. deeply dissected. anatomically distinct land forms may remain. Feeder vents through which tion Ihal pll),S such as the Rocher Saint Michel bva reached small composite volcanoes arc often and the rocks surrounding them were of volcanic preserved long arter lhe rest of the volcano has origin played an important role ill the history of disappeared, surviving as massive pill.lrs of rock geology Table 16.1 Stages in the erosional history of a volcanic cone Stage MOI'phologieal forms Fresh, young cones, pristine lava flows and summit cralers. No glacial morames 2 Small gullil."S on flanks: lavas and summit crater discernible but degraded: cone still sharp, moraines present in glaciated a~s. J Indh'iduall:I\'a flows barely visible; no crater; well-established gullies: constructional surfaces dwindling: pltme:es initiated. 4 No 13\'3s visible; dl-.::pl)' incised gullies: pllllre:es, but original surfaces left. Considerable relief. Major U·shapcd valleys in glaciated areas 5 ]J;trdy recognizable: low relief; radial symmetry main clue to volcanic onglll. Volcanoes as landscapc forms 359 earliest inspiration in this subject. and havc cvcr sincc into parallel-sidcd depressions; wherc they cut been c1a5~;e ground to which the geological pilgrim hilS the coast line. long, dccp grooves result. Thcse a re made his way from all parts of the world. such COlllmon features of the coastline that they have acquired their own name in the local Gaelic Nicolas Desmarest (1725-1 SIS) and George language: sloe.... Poulelt Serope (1797-1876) were able to demon Dykes arc often thought of as rather minor strate that several episodes of eruption and igneous phenomena. Most arc indeed r.lIher erosion had taken plaee amongst the volc."lnoes small, less than a metre wide, There arc many ofthe Auvergne. Scrope used these observations, huge exceptions. though, In the Terti:lry dyke seemingly trivial to us today. to emphasizc the sW:lrm of north-west Britain. the Cleveland dyke continuity of geological processes. thus helping (about 100 metres widc) can be traced disconti to overturn thc prevailing 'catastrophisr view of nuously from its source in the Hebrides to thc the history of the Earth. Influenced by religious North Sea coast of Yorkshire. a distancc of belief in thc Creation. and particularly in the almost 400 km. Calculalions of ratcs ofinjection great Deluge. catastrophists had explained and cooling suggest that Ihe dyke must havc been mountain belts and the gorges that incise them as il1lrudcd very quickly. zipping across lorthern the products of single-short lived evcnts. 11 England in less than 5 days. The Cleveland dyke In Ilrit•• in. Arthur's Seal. a famous Scotlish is only one of a swarm. all of broadly the s.1me landmark which looms over the city of Edin age. which formed in response to rifting movc burgh, was the site of a Carboniferous volcano. ments rclated to the opening of the North now exhumed. Eroded necks and vents form thc Atlantic Ocean. highest points. while on the n'll1ks outlines of some of the original lava flows Can still be picked 16.4.2 Erosiotl of laun flows oul. In the United States. Ship Rock in cw Lava flows havc superlatively rough surfaccs. so Mexico jags up in an astonishing pinnacle 430 thcy arc natural traps for wind-blown dust. Their metres above the desert surface. Dykes radiating glassy outer skins also break down mpidly when out from the centrc arc beautifully displayed. exposed to watcr and air, Thus, soils quickly standing up swrkly likc walls. Ship Rock owes its form in crevices. allowing plants to colonize the soaring. perpendicular architecture to a series of flow. Obviollsly. the rate of this process depends \'crticaljoints in the breccia-filled volcanic ncck. critically on local climate, so it may vary a fcature common to other necks around thc dramatically even on a single volcano. New lava world. Interesti ngly. lhe finesl necks and puys all flows on the wct, trade-wi nd side of the Hawaiian seem to have been formed from .~mall, probably Islands arc overgrown by rain forests in only a monogenetic volcanoes. When large composite few decades. whereas those on the parched volcanoes arc eroded, compar • Volcanocs as landscape forms 361 , , ' ,. f t.• Fig. 16.24 On thc small Hcbridcan island of Eigg. Scotland. a rhyolite la\'a flow which original!) filled Fig. 16.23 Smoothl)· cun-oxl joints form the surfaces a ri\'cr valley cut in older b:ll>:llt la\as is nov. of the columns or the Gi:ml·S C:luscY. ay. Conc:I\'e presen'cd as the Scuir of Eigg. an imposing cr:lg. It and com'ex surfaces :lppear equally abundant. is a d:lssic example of volcanic in\'crsion of relief. From an 1897 skctch by Archibald Geikic. tr:lced for mallY kilometres in the 110\\ls of the Columbia River Plateau. subduing the previous humps and hollows. IIwertetl relief Obedient to the laws of physics. Where large-volume pyroclastic flows arc con· lavas flow down valleys. sometimes filling them cerned. involving thousands of cubic kilomelres completely. Distinctive landforms result if the of magm:t. lhe pre-exislillg topography may be lavas arc more resistant to erosion than the compleldy buried, leaving :111 unbroken plate:tU underlying rocks in which the valleys were cut, as many thousands of square kilometres in ex lent. often happens when lavas flow over sedimentary This situation docs not last long. Unwelded strata. The v:llley-filling lava forms a thick. pyroclastic deposits are soft and friable. and so resistant mass. while the sediments on either side they arc rapidly eroded. Deep gullies arc cuI at:lIl arc more rapidly removed. In the fullness of time. astonishing ralC as soon as the first rains fall. the original valley will be expressed as a resistant Within a few years. :1 dislinctive lopogmphy ridge of lava stllnding above the surrounding emerges. Flat-lopped relics of the original pla sediments, in which new valleys have been cut. In teau remain. separated from one another by a Ilritain, a beloved example ofi,n'ersioll of,.e/iefis dendrilic maze of narrow. vertical-sided gullies. found on the obscure Hebridean island of Eigg. (Wadis. canyons. quebradas. gulches. or gorges. where a Tertiary rhyolite lava which filled an depending on local idiom.) These gullies arc older valley now forms a 400 metre high, five often so deep and steep thai they arc mere slits. kilometer long ridge. This ridge. the famous less than a metre wide and ten or 1110re deep. so 'Scuir of Eigg' dominates the tiny island. other constricted that they arc dillicull to walk along wise renowned only because its nearest neigh {Fig. 16,25). bour is Muck. (Fig. 16.24). Inversion of reliefcan Intensive gullying or this kind is common in be seen on many volcanic cones. where flows soft rocks ofall kinds. nOI only tephra. It happens which originally Ilowed in gullies cooled to form distressingly quickly as a result of overgra7jng or thick. confined wedges. After erosion, the lavas clearing of timber in areas subject to occasional end up as resistant caps to spurs and plane;es on heavy rainstorms. Development of extremely the dissected volcano. steep (vertical) sides of gullies cuI in pyroclas1ic rocks is an expression of the fact that lhey arc 16.4.3 Erosioll of pyroclastic deposits incoherent. structurcless deposits. Unlike sedi In the short term. the topographical effeets of a mentary deposits. unweldcd pyrocl:lstic rocks heavy ash fall arc straightforward: tephra blan often lack layering. and because t hey arc unlil hi kels the pre-cxisling surface. smoothing and lled, cannot form boulders. All they can do is 362 Vokanocs: a planetary perspective Fig. 16.25 An eruption about 3000 years ago formed the Dcriba Caldera. I):.ar(ur Pro\'jncc.....'estern Sudan and ilS apron of pyroclastic fall and flow deposits. Erosion of the soft pyroclastic deposits has yielded a dendritic maze of Sleep. deep gullies. form vertical clilTs, with scree slopes of loose ignimbrite plateaux are eroded in arid env!n pumice beneath them. If the deposit is welded or ments, box canyons are common. As the edge of shows vapour phase alteration {Section 10.5,1), the plateau reccdes.l1at-topped islands ofignim· lhel1 boulders form, allowing a talus slope to brite arc left, lheir walls rising as steep, high. and develop al the foot of the cliff. forbidding as castle ramparts above talus slopes Where welding or vapour.phase alteration arc of fallen boulders. These mesas and billies make present, cooling joints and fractures also de appropriate backgrounds to Western movies. "clop; usually. but not always. perpendicular to Jointing in ignimbrites is often so perfettly Ihe original surface. On erosion. elegant sets of developed that the fracture surfaces look and feel columnar joints are sometimes exposed: not as artificially clean. They are so smooth that they commonly as in basalt lava plateaux, but more provide irresistible temptations for people to pleasing because the rocks are a warm, reddish express themselves in yrafilli. In both north and ochre colour, rather than drab basalt bbck (Fig. south America, prehistoric Native Americans 14.17). Vert iC:lI jointing accentuates the vertical used ignimbrite joint surfaces to draw pctro cliffs developed in pyroclastic rocks. Where glyphs showing animals that Ihey hunted, and - VolcoJnocs as landscape forms 363 absiraci mystical symbols (Fig. 16.26). Twen tieth-century Americans usc 1he samesurfaces for more rudimentary gralnti. , l'{mhmys In 1he Cenl ral Andcs. ignimbritcs arc exposed over huge areas at high altitudes. Prccipil;l1ion is slight in the region. taking place --' mainly inlhc fOrtn of snow which ablates rather than melling. Surface runoffis Ihcrcforc minimal. so erosiOJl by nowing water is inconsequential. By contrast. fierce winds blow from the north west for much of the year. As a consequence. much erosion takes place through aeolian pro cesses. producing a wi nd·scu!ptcd topography of yardaugs and dcnationary hollows. }'lmllmy.~ arc long. wind-eroded ridges. somewhat resembling upturned canoes. their prows facing the prevail Fig. 16.27 SPOT s..1tellite image of j'llrJOIIgs ing wind (Fi~s. 16.27-16.28). Andean ignimbrites developed in llle 4, r-million-year-old Alan:l ignimbrite on the frontier bctWl-Cli Chile and Argentina. )'(m/(mgs arc pointing into lh.: prevailing north-westerly wind. Image is :lbout 4 km across. Courll-sy of CNES. arc silicic. and so they contain generous quanti ties of quartz phenocrysts. On erosion, lhey therefore liberate the agents of their own destruc tion: quartz grains freed from their matrix and picked up by the wind rapidly abrade the remaining rock. Absolute rates of erosion h3ve never been determined. but the ·half·life· of an ignimbrite subject to aeolian erosion in the central Andes may be about a million years: after a million years, only half the initial volume rcmaIllS. Why worry about aeolian erosion rates of ignimbrites in somc obscure part of South America? One reason is that some planetary scientists believe that huge areas of Mars arc covered by ignimbrites. There is no doubt that there arc enonnous areas of superb rOrt/lIIlgs on Mars. But how did these Jtardallgs form? And what are they eroded in? Are they ignimbrites. or an older aeolian deposit? To help ,l11swer these questions. terrestrial j"ardollgs must be better understood. The high. dry. cold Andean plateau Fig. 16.26 Pre·Columbian ]>Ctroglyphs on smooth provides an exccllent tcrrestrial counterpart to joinl surract.'li on the 9-million-}"ear-old Sifon the evcn drier and colder Martian deserts. so ignimbrite. Rio Loa valley. north Chile. Llamas and funher studies of Andean j'llr(/al1{Js may throw geometric:ll symbols arc abundant. some light on an important aspect of Martian 364 Volc<1l10<'s; a plaJletary perspective Fi~. 16.28 Prow of a rlirt/lmB 50 km south of those in Fig. 16.27. Aeolian eros;ol1 is most intense within about one llletre .. of the ground; thus the = ignimbrite is mpidly undercut. and boulders faJlto Ihe valley "oor. "here Ihey arc rapidly abraded awa}'. geology. One intriguing problem is this: if (Fig. 16.29). These can form in two ways. When a ignimbrites exist on Mars. they are morc likely to pyroclastic plateau is dissected by dendritic be of mafic composition than silicic. If they arc drainage, two branches of the complex system mafic, they will lack quartz. What, then, provides cOlllmonly intersect each 01 her, isolaling a block the abrasive material to help the M:lrtian wind of the deposit which is then trimmed into a sculpt such outstanding ym'dmlgs'! pinnacle. More often. wigw:lms are formed like earth pillars: where the edge of a pl:lteau is being Wiy,mms lIIul fem rocks From C.tpp;ldocia in eroded away, resistalll blocks of lava or other Turkey to Los Alamos in New Mexico. erosion of rocks within or on lap of the deposit (pcrh:lps lag pyroclastic rocks consistcntly yields wigwams or breecias) prolec-t the underlying deposit from tent rocks: conical pinnaclcs or spires ofdazzling erosion. while the surrounding material is white rock thatray be tens of mctres ill height rapidly carved :lway. For a while, the protecting ril.:. 16.29 Tent rocks formed by erosion of the Bandelier TufT (ignimbrite) on the nanks of the Valles caldera. New Mexico. Volcanoes as landscare forms 365 boulder remains perched precariously on top of joims. On erosion. armoured joints. origilwlIy the pinnacle. but eventually it topples, leaving fractures in thc rock. stand lip as resistant walls. only the pinnacle itselr. As in other pyroclastic often sevcral metres high. In pl;lccs in Chile, these erosional phenomena, formation of wigwams walls arc so steep and thejoinling so regular that depends on the deposit being homogeneous, from a distance the weathered ignimbrite looks lacking marked horizontal or verlical variations like a ruincd city. Where hot Iluids escape to the ill strenglh. surface via pipes ralher than joints. armoured cflim/lep are produced. Fumarole mOil/ills and rjd{Je.~ Ignimbrites are Where more extensive ignimbrite ground hot when first deposited. maybe even close to water reactions took place, steam blasts may m;lgrnatic temperatures. A consequence Mthis is break through to the surface of the ignimbrite, thaI aner they arc emplaced, they sit and stew in producing fumarole vents similar to those in the their own magmatic volatiles. and in any steam Valley ofTen Thousand Smokes. On erosion. the that may be liberated from underlying ground most common expression of these fossil fumar water. Apart from causing generalized vapour oles is in myriads ofshallow depressions or pils, phase alteration. this process may causc localized better seen on iterial photographs than on the ell'ccts which have striking morphological conse ground. but in some circumstances, swelling quences on erosion. Where the ignimbrite is mounds of blisters arc exposed. These positive broken up by \'erliealjoinls. hot nuids moving up relief features arc probably the result of erosion thejointscausealteration and deposition ofsilica fCvealing differences in hitrdness around the along the joints. This may a!Tect lhe ignimbritc fumarole at dep1h within the ignimbritc, and arc around thejoinl for a few centimetres or as much not original surface reaturcs lS (Fig. 16.30). as a metre. resulting in formation of (II'mol/fell rig. 16.30 'Fumarole' mounds and ridges dc\'clop.'d on Ihe SUrf;ICC of thc CarCOlc Ignimbritc, Rio Loa valley, north Chile. Rq;ul;lr alignmcnt of Ihc ridgcs suggcsls Ihal Ihcy \\cre dC\'clopcd abovc joints in the main body of the ignimbritc. - 16.5 Topographical by-products So fa r. we ha ve considered volcanoes in isolation. usually by interfering with local drainage. On :l BUI voleanoes can also cause drast ic topographi small scale, lava flows commonly block valleys, cal changes to their surrounding landscape, impounding lakes behind a lava dam. This can 366 Volcanoes: a planetary perspective have senous consequences if the dam should 011 a larger scale, Lake Van (70 km across) ill suddenly fail. releasing large volumes of waler. Armeni:t is said to have been ponded by lavas An instructive example is Sab,lncaya vOIe'lIlO in from Nemrut volcano. And on an even larger Peru, Lavas from the north flank of this volcano scale in central Africa, Lake Kivu (looded part of once dammed the mighty Majes C:ll1yon. more the Western Rift valley when it was empoundcd thall twO thousand mclres deep, and one of the by construction of the volcanocs of the Virunga grandest in the world. A substantial lake must 10untains about five million years ago. For ha\'c existed for a time. because nat-lying sedi merly, drainage was northwards to join the Nile melllS deposited in the lake arc preserved behind via Lake Albert, When the volcanoes intcrposed the dam, which :llso forms a prominent nick themsclves. thwarted rivers initially drained into point in the canyon profile. AI some unknown and filled Lake Kivu. which cventually ovcr time, the dam was breached: there is no lake at (lowed. draining southwards at the southern end the present day. of the rift via the Ruzizi River into Lake Also in the Andes. a large debris avalanche Tanganyika. Lake Tanganyika itsclfis connected from !}arinacola volcano 13 500 years ago via the Lukuga Rivcr to the great Congo (Za'ire) blocked the existing drainage to the Pacific, River. Thus, the Kivu drainage was switched resulting in the formation of Lake Chungara. from the ile and the Mediterranean \0 the 10 km across. which at 4550 m is the highest lake Congo and Atlantic l6 (Fig. 16.31), of respL"'Ctable dimensions outside Tibet (Fig. Any map of rivers around the Rift is. ofcourse. \6.\4). of only ephemeral value. When Haroun TazielT N l'drilt Nilt N o....100 '200 300 «JJ "'" o..100 200 300..400 "'" f./lk,. Albert l.ilkl! Edward I)rain;lge Virul\g;l l.ilke MOl,lnl~;n~ "'~ from Victorill l.a ko:: Ulke l:,nganyika KiVIJ l.ilkc LIl~'e TIlrr81lrryika Tll1l8a1ryika (a) (b\ Fig, 16.31 (a) Sketch of the Nile drainage through the Western Rift before eruptions which built the Virunga Mountains. I>onding Lake Kivu. At that time, drainag.e from L:tke Tanganyika nowed northwards via the Nile into the Mediterrane:.n. (b) After formation of the Virung:t Mountains. Lake Ki\·u formed, the Ruzizi Ri\'cr drained southwards. and Lake Tanganyika drained via the Lukuga into the Congo and the Atlantic. Volcanoes as landscape forms 367 visited Lake Kivu in 1948, lava from the Kituro swell above a mantle hot-spot may inlluence volcano was streaming steadily into the lake, topography over a region thousands of kilo reshaping its northern shores, and providing a metres in extent. Radial drainage patterns incised bonanza to local boatmen in the form of shoals of as a result of hOI-spot uplifts have been mapped parboiled fish. Future eruptions along the rift 011 several continents. If we were to include the will eventually reshape drain:lge patterns once topography of the se.:l-lloors, we could conclude agaill. th.lt most all the world's landscape is of volcanic On the very largest st'ale, the topographical ongm. - Notes I. Calion, C. A. (1944). Volctmoes as landscape Ridge. In Volcanism in Hawaii. US Geol. Surv. fM/IIS. Whitcombc and Tombs, Christchurch, Prof. Pap. 1350, pp. 85-100. New :Ualand. 415 pp. 9. Martin del 1)07.7.0, A. L. (1982). Monogenelic 2. Wood. C. A. (1980). Morphomctric cvolution of VUICilllism in Sierra Chiehinautzin, Mexico. /JIII/. scori:l concs. J. VolcmllJl. Ceollwrm. Res. 7, VOICUIlO/, 45, 9-24. 387--413. 10. Greeley. R. (1977), Volcanism ofthe EaSlern Slwke 3. Wohlctz. K. Ii. and Sheridan. M. F. (1983). Rit:er Plai/!. /Jallo. NASA CR 154621. 308 pp. Hydro\'olcanic explosions II. Evolution of basal II. Oilier. C. D. Volcanoes. Australian N:uional tic tufT rings and tufT cones. Alii. J. Sci. 2.83, University Press, Canberra, 177 pp. 385--4ll 12. Scrape, G. P. (1858). Thc geology of the extinct 4. Lorenz, V. (1986). On the growth of maars and volcanol:s of Central France. John Murray. Lon diatremes and its relevance to the formation of tufT don. rings. BII/I. Volc(JtlQl. 48. 265-74. 13. Pieri, D. (1980). Martian valley: morphology, 5. Jones. J. G. (1969). Intraglacial vole-.lIloes of the distribution. age and origin. Sciellce 210, 895-7. Laurgavatn region. southwest Iceland. Q. J. Ceo/. 14. TomkiefT, S. T. (1940). Basalt lavas of lhe Giant's Soc. UJIlu. 124. 197-211. Causeway. !JIIll. Volc. 6, 89-143. 6. Milne, J. (1878). On the form of \·olcanoes. Ceo/. 15. Sheridan, M. F. (1970). Fumarolic mounds and Mag. 5, 337-45. ridges of the Bishop Tuff, California. Gool. Soc. 7. lkcker, G. F. (1885). The geometrical fonn of Amer. Bull. 81. 851--68. \'olcanic cones. Am. J. Sci. 30, 283..JJ3. 16. King. L. C. (1942). SOli/I! Africa" Scenery, 8. Moore, J. G. (1987). Subsidence of the Hawaiian pp. 153-4.