Chapter 27 Gorely Volcano I.T.KirsanovI. V. Melekestsev

S y non y m: Gorely khrebet, Gorelaya sopka, Pravaya Mutnov- The intensity of the volcanic activity fell between 6 and 2 thousandskaya, Vtoraya Mutnovskaya, Asacha. years ago: quiescent periods between eruptions became longer, up to

Location. Coordinates of the summit: 52°27'N, l58°07'E. hundreds of years. Comparatively rare and brief eruptions predomi-Geographic position. The volcano is situated in Southern nated, with tephra volumes of n'107 m3. Stronger eruptions that

Kamchatka 75 km southwest of Petropavlovsk-Kamchatsky. The ejected coarse cinders were but rare; one was about 5000-5500 yearsregion is a slightly uplifted and tilted westwards and southwestwards ago. The last stage of massive lava eruptions was 2000-2500 14Cplateau of Lower Quaternary basalts and andesite-basalts 800-1000 m years ago: numerous flows of a very young appearance and wonderfulabove sea 1evel. The volcano belongs to a zone of conjunction of the preservation lie immediately below the marker tephra layer of Kho-eastern horst-anticlinorium with the Tolmachevo depression. The dutka (?)-Ksudach volcanoes dated as 1700-1800 years old [Me-conjunction follows stepped sub meridional faults with vast extrusions lekestsev et al., 1987].and numerous cinder cones of variable ages. In the last 2000 years the activity of the volcano remained

Geology of the basement. The basement of the volcano comprises mainly explosive. There wf;re comparatively rare eruptions thatvolcanic and volcano-sedimentary Upper Tertiary and Lower Quater- ejected 10-20 mIn. m3 of tephra. Those were registered betweennary units ranging in composition from basalts to rhyolites. 1500 and 1900 years. about 1400 and 900-950 years ago.

Morphology and structure. The structure of the volcano consists Approximately 600-650 years ago a new period began in theof two parts: an older and a present one. The older structure is activity of the volcano: apart from the continued eruptions otshield-like, with a 13 X 12 km caldera at the centre. Basalts form the relatively low volumes of juvenile pyroclastics, there were severallower part of the pre-caldera section, andesites in the middle parts of ejections of coarse resurgent material which laid down horizonscaldera flanks and extrusions in the southern and less in the northern of explosive strata in the northern and northwestern parts of thesectors of the ancient volcano. Dacites and rhyolites are exposed foot of the volcano. One such strong explosion took place aboutin the uppermost section of cauldron flanks or form extrusions 600-650 years ago; another must have occurred in the 18th centuryin the southwestern and eastern sectors of the caldera [Kirsanov since its deposits are directly covered by the tephra of historicet al., 1980; Kirsanov, 1985]. eruptions of Mutnovsky and Gorely volcanoes. It was also the time

Ignimbrites, welded tuffs and pumices occur in the upper section of a series of lava flows erupted from the fissures that are pro-in the eastern, northern and western flanks of the caldera and com- nounced on the southwestern slope of the volcano, and from the cent-pose the entire surface of the ancient volcano, and are traced in a thick ral-vent crater.sheet from the centre to the periphery for over 15-20 km. Together It is quite possible that the phreatic eruptions of the 18th centurywith the pit of the caldera, the overall area occupied by ignimbrites heralded the last eruptive cycle of the volcano. The eruptions.is about 600 km2. They range in thickness from 5-30 m in the cauldron registered in the sections by tephra horizons occurred every 50-flanks to 300-350 m at the periphery, in particular in the margins 100 years, which is slightly more often than in the previous(upper reaches of the Karymchina, Paratunka, Zhirovaya rivers and 1500 years. Given the recorded weak eruptions whose tephra doesin Opasny ravine). The total volume of pyroclastics, together with not form independent layers, their frequency was one eruption everythose buried inside the caldera, is 100 km3 (Figs 179-180). 4-60 years. The eruptions were phreatic and phreato-magmatic.

The present structure of the volcano occupies the central part of the Description of eruptions. First dated eruptions of the volcanocaldera and comprises three merged cones that stretch in a ridge (1828, 1832, 1855 and 1869) were explosive, associated with theW-NW-wards. The absolute height of the central cone is 1829 m. central-vent craters. The 1929-1930 eruption produced periodic,There are 11 superimposed craters at the summit, and about 40 rather strong explosions that ejected ash and coarser fragmentsadventive vents with lava flows of various lengths on the slopes of rocks [Kulakov, 1935]. Broad plumes of ash-laden gases spread(Figs 181-182). east to the Pacific coast and north as far as Petropavlovsk-

Rocks composing the present volcanic structure are basaltic Kamchatsky (75 km), village of Nachiki (150 km) and even farther.and andesite-basaltic. The eruption in May, 1931 was purely explosive and central-vent.

Age and evolution. The initial stage in the history of the ancient Weak explosive eruptions from the same central-vent crater werevolcano is Early Pleistocene. It is represented by basalts, andesite- recorded in 1947, in December and January of 1961. In the last case,basalts, andesites and dacites that build up in succession a shield there was a series of explosions with gases released to heights ofvolcano with a diameter of 20-25 km. Its fragments can be observed up to 1.5-3 km.as relics of layered plateau around the source of the Opala and In June 1980, fumarolic activity of the volcano reactivated.Paratunka rivers. These deposits are covered everywhere by welded Bocche 5-10 m in diameter in the middle crater released steam jetstuffs and ignimbrites associated with cauldron subsidence at the at a rate of 200 m/sec or more. In the middle of June, explosionscentre of the volcano. Formation of the caldera and the associated ejecting clastic resurgent materials from the vents at the bottomcomplexes of rocks occurred in the Middle or early in the Upper of the crater were observed. This period seems to be a beginningPleistocene. of the eruption that was to continue with intermissions until

The present structure dates back to the Late Pleistocene. 3 July, 1981.Between 8 and 6 thousand years ago, the volcano was in a vigorous It proceeded in four phases that varied in force and durationexplosive activity with typical weak eruptions (n-l06 m3) that though it had similar eruptive events: gas-steam and phreaticoften followed one another. This was a general background for at ejections alternated with Vulcanian and mixed types of eruptions.least 8 stronger (n-l07 m3) eruptions which ejected basaltic and The Vulcanian type was manifest at the most active phases (Figsandesite-basaltic cinders. The eruptions poured out also numerous 183-187).lava flows that filled in the caldera. A new reactivation of the volcano was observed in the later

314

half of 1984 following a pronounced preparatory period. In 1985, During the eruption the streams running down the slopes of Go-a great jet of steam was observed almost continuously. The eruption rely carried water with a pH less than I -a huge amount of activehad six intensification periods: end of December 1984, January, and chemical agents shortly to be transported with atmospheric watersof February-March, middle of June-end of July, beginning of August- close to the volcanic structure.December, 1985, at the beginning and at the end of 1986 (Figs Geophysical studies. Data of ground and detailed aerial magnetic184-185). They were interrupted by weak gas and steam emissions survey characterize the volcano and the southern part of the calderato heights less than 300 m. In periods of intensification, the with a positive anomaly attributed to the present basic lava flows.height of the gas-steam column varied between 500 and 3000 m or The rest of the caldera and the surrounding territories lie mostly inmore. the region of negative magnetic field associated with ignimbrites.

The eruption of Gorely in 1980-1981 ejected minor amounts oflarge fragments, and mostly volcanic sand and ash. The products Tab I e 32of eruption occupied an area of 500 km2. In active phases of Average Chemical Composition of Rocks of Gorely Volcano,eruption, ash fell on the Pacific coast (- 50-70 km)and near the village per cent of Mass

of Paratunka (30-50 km). Its estimated volume in 1980-1981 was O-'d-- I 1 I 2 I 3 I 4 I 5 I 6 r ;.-54.1 km3 and the weight (given 1.2 g/cm3 of specific weight) was vXlues I .I ~ I " I ..I ;) I 0 I 7

-65 min. tons. Of them, 25 min. tons were juvenile and 40 mIn. tons Si02 50.82 52.26 60.38 67.63 51.68 54.87 57.38resurgent material. The heat lost with solid products was 8.4.1012 Kcal. Ti02 1.07 0.98 1.20 0.80 1.08 1.23 1.30Discharge of steam ranged from 2.4.103 to 2.7.106 kg/m3 sec. Thus, Ab03 17.01 17.65 16.16 14.69 f8.04 16.89 16.52heat output also varied from 1.5.104to 1.IOKcal/sec;during8mon ths Fe203 2.94 4.16 2.05 2.09 3.73 3.45 2.39of the eruption, the volcano released 9.5.1012 kg of steam and FeO 7.04 6.24 4.66 2.07 5.78 5.69 6.296 1.1015K I f h t MnO 0.04 0.05 0.15 0.10 0.18 0.23 0.22

.ca 0 ea. 24 MgO 7.43 5.26 2.56 1.54 5.76 3.90 2.62Th~ overall heat I~sses were ab.out 6.1~ erg, or an order of CaO 9.02 9.70 4.47 2.93 8.31 7.47, 6.15

magnitude less than In catastrophic eruptions of Bezymianny in NB20 2. 73 2.33 4.45 4.19 2.99 3.37 3.921956 and Sheveluch in 1964. K20 0.99 0.73 2.55 2.82 1.65 1.86 2.33

In the eruption of 1984, the volcano issued about I min. tons of 820- 0.13 0.14 0.20 0.04 0.25 0.33 0.30ash in December. Heat output of the eruption on 29-30 December 820' 0.19 0.12 0.85 0.30 0.57 0.54 0.44was 3.9.10' Kw [Fedotov, 1982]. P20S 0.31 0.30 0.37 0.24 0.27 0.29 0.37

In 1985 the overall heat losses were estimated at 2.2.1010 mJoules Tot a I: 99.81 99.78 99.95 99.44 100.29 100.12 100.23while the average heat output was 1.106 kW. Not e. Rocks of the ancient strucfure: I -basalts (3 an.); 2 -andesite-basalts

Products of eruptions. The ancient structure of the volcano is (I an.); 3 andesites (2 an.); 4 -dacites (3 an.); rocks of the present structure:composed of almost all rock varieties from andesite-basalts to 5 -basalts (3 an.); 6 -andesite-basalts (3 an.); 7 -andesites (2 an.). Analysesdacites. Phenocrysts account for 5-30%. In andesite-basalts, they by T.V. Dolgova, and N.R. Gusakova.

do not exceed 3-10% represented by plagioclase (48-68% An),pyroxene (a~gite) and olivine (18-20% Fa). The latter two are The negative gravity anomaly must be caused by the infillingdistributed unevenly and are often associated with magnetite. In loose pyroclastics in the caldera. The depth to the gravity centreandesites, andesite-dacites, the amount of phenocrysts varies from 3 to of the anomalous object is 2 km, mass defect 35.101S g. Part of the30%. They are represented by plagioclase with 32-56% An, rhombic anomaly may be due to a deeper object with negative excess density.(hypersthene) and monoclinal pyroxene (augite, occasionally diopsi- Variations of the anomalous gravity field within the caldera arede). Amphibole and biotite are rare. In ignimbrites and welded tuffs associated with a "dented saucer" type of structure. The volcanoof andesitic and andesite-dacitic composition, phenocrysts are repre- and the caldera are situated in the region of subsidence of the Cre-sented by plagioclase or occasionally monoclinal pyroxene. taceous basement to depths of 4 km.

The present structure of the volcano is composed of basalts, Volcanic hazard and prognosis of eruptions. The present volcano isand andesite-basalts. In the older basalts, phenocrysts contain a huge structure with II craters at the summit and 30 on the slopes.plagioclases, olivine and pyroxenes. The composition of plagioclase The slopes of the cone are covered with extensive flood lava flows.varies from 56 io 85% An, that of olivine from 15 to 30% fayalite. Tephra-chronology method has shown that there have been at leastPyroxene is represented ~y augite, occasionally by titanium-augite 3 major eruptions in the last 2000 years that ejected 10 to 20 min. m3and diopside. Younger rocks are represented by andesite-basalts. of tephra (Figs 188-192).Rocks of the volcano belong to the calc-alkaline series (Table 32). In the last 600-650 years, the Vulcanian type of eruptions were

Gas compositions and sublimates. Steam and gases were the main joined by phreatic eruptions with typical ejection of coarse clasticagents of Gorely eruptions. They rose to heights from 0.3 to 5 km resurgent material. It was deposited in a layer at the western andabove the summit as thick jets, steaming columns and eruptive clouds. northwestern foot of the volcano. Two such explosions occurredTheir plumes could be traced to distances over 50 km. The discharge 600-650 years ago and in the 18th century. It is possible that sincerate at the vent varied from 5 to 350 m/sec. Along with steam in the 18th century, a new eruptive cycle set in with eruptions everythe steam and gas mixture, there were H2S, HF, HCI and SO2. 50-100 years. Together with weak eruptions, the interval is reducedWater extracts from juvenile and resurgent material are dominated to 4-60 years. A major phreatic eruption was observed in 1980-in tIle anion part by sulfates and chlorine, and major amounts of 1981; and weaker ones in 1984-1985. The main erupted productF. Cations are dominated by Ca, AI, Mg, minor amounts of ammo- was water thrown out as steam at a rate from 5 to 15 tons/sec.nium, potassium and iron. This may suggest a stronger explosive eruption in the near future.

Fi g 182. Geologo-geomorphological scheme of Gorelyvolcanocomplled by I.T. Kirsanovusing data by I V Melekestsev and YeA. Vakin and personal observations:

1 ~ alluvial-proluvial sands of the Gorely caldera; 2 -alluvlal-proluvial and fluvio-glacial deposits of river valleys; 3 -basalts and andesite-basalts of lava flows of recenterupllons (V stage, 300-800 years ago); 4 -basalts and andesite-basalts of stageIV of the present structure of the volcano (IV stage, 800-2500 years ago); 5, 8 -ba-salts of stages II-III of the present structure (25-6 thousand years ago); 7 -olivinebasalts of cauldron faults. They reflect the initial staQe in the history of the present

structure of the volcano (over 10 thousand years ago); 8, 9 -pumic~s, lavas of extrusionsand andesitic and andeslte-dacitic flows erupted onto the surface at the end of thecauldron stage in the history of the volcano (0,); 10 -extrusions of andesites, andesite-dacites intruded along radial faults at the end of the cauldron stage of the volcano'shistory (0,-.); 11 -ignimbrites and welded tuffs of the andesitic and andesite-dacitic composition. The upper part of the section of the ancient structure of thevolcano (0,-,); 12 -ash tuffs welded to look like lava rocks, of andesite-daciticcomposition (the uppermost section of the ancient structure of the volcano (0,-,);13 -andesites and andesite-dacites of the ancient structure of the volcano (middlepart of the section 0,); 14 -basalts and andesite-basalts of the ancient structure ofthe volcano (lower part of the section, 0,); 15 -effusive-pyroclastic units of Skalistyand Dvugorby volcanoes of the andesitlc and andesite-dacitic composition (0,); da-citic extrusions on the slopes of the ancient structure of Gorely volcano (N,-O,); 17-granodiorites and diorites on the slopes of ancient Gorely (N,-O,); 18 -volcano-sedimentary (Vllyuchlnskaya suite Pg-N,), effusive, extrusive and pyroclastic unitsof Paratunka, Berezovo and Alnei series and rocks of Mutnovsky volcano (N,-O.);19 -hot springs (a), active centres in craters (b) and fumarollc fields (c) on the vol-cano; 20 -flanks of the caldera of the ancient volcano's structure; 21 -volcaniccones of the present volcano with telescopic craters; 22 -volcanic structures ofVysokaya mount (a) cinder cones of areal zones and lateral craters of the volcano (b);

23-faults; (a) -established, (b) -hypothetic