16 Volcanoes As Landscape Forms

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.

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