KANARÙ SALOS KARÖTASIS PASAULIO TAÖKAS
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1 PAÞINIMO ROMANTIKA m. gruodþio 6-13 d. kartu su Ðvedijos ir Suomijos studentais autorë dalyvavo geologinëje ekspedicijoje Kanarø salose. Ekspedicijai vadovavo Lundo universiteto prof. Anders Lindh. Ekspedicijos metu buvo aplankytos Lancarotës, Gran Kanarijos ir Tenerifës salos, siekiant detaliau patyrinëti jø prigimtá ir vystimosi raidà. Straipsnio autorës, geologijos magistrës, Vilniaus universiteto doktorantës Irmos Vëjelytës þurnalo skaitytojams pristatyti nereikëtø, nes jau ne vienas jos straipsnis apie tolimø ðaliø geologijà yra spausdintas G.A. puslapiuose. Kartu norisi paþymëti, kad Irma jau ne pirmà kartà randa galimybæ pasinaudoti atvirø sienø ir Pasaulio universiteto bendradarbiavimo galimybëmis, kaupdama þinias rengiamai daktaro disertacijai. Irma Vëjelytë, Vilniaus universitetas KANARÙ SALOS KARÖTASIS PASAULIO TAÖKAS Kanarø salos tai jaunas archipelagas, kurá sudaro 13 salø. Ið jø 7 (Lancarotë (Lanzarote), Fjurventjura (Fuerteventura), Gran Kanarija (Gran Canaria), Tenerifë, Gomera, La Palma, Hiero (Hierro)) gyvenamos. Salos iðsidësèiusios rytinëje Atlanto vandenyno dalyje ir prisiðliejusios prie ðiaurës vakariniø Afrikos þemyno krantø. Prieþastys, sukëlusios Kanarø vulkanizmà, nëra tiksliai nustatytos, todël ðiø salø susidarymo klausimas aktualus iki ðiø dienø. Ilgà laikà mokslininkai diskutavo, ar Kanarai susidarë ant okeaninës, ar ant kontinentinës plutos. Viena hipoteziø tvirtino, kad salos iðsidësèiusios ant vadinamosios tranzitinës (pereinamos) plutos, t.y. Afrikos kontinentinës ir Atlanto okeaninës litosferos plokðèiø pakraðèiuose. Taèiau, remdamiesi naujausiais seisminiais duomenimis, dauguma mokslininkø daro vieningà iðvadà, kad ðios vulkaninës salos susidarë ant okeaninës plutos ir pagal savo kilmæ priskiriamos vienam ið vulkaniniø salø tipui, kuris geologijoje ávardijamas karðtojo taðko terminu. Kanarø vulkaninës salos iðkilo skylant okeaninei plutai dël gilumoje tarp okeaninës plutos ir mantijos susidariusio konvekcinio mantijinio plumo, kuris yra apversto grybo formos kotas labai plonas, o kepurë plaèiai iðsiskleidusi po litosfera. Plumà sudaro iðsilydþiusi mantijos medþiaga, kuriai kylant aukðtyn susidaro magmos þidiniai. Magma, pradeginusi litosferà, á Þemës pavirðiø iðsiverþia lavos pavidalu, suformuodama ugnikalnius ið pradþiø vyksta povandeninis vulkanizmas, o pasiekus jûros lygá, jis pereina á kontinentiná. Kanarø salø vietoje plumas ilgà laikà buvo stabilus, laikësi vienoje vietoje, o okeaninë plokðtë slinko virð jo, tokiu bûdu susidarë Kanarø vulkaniniø salø grandinë. Salos driekiasi apie 350 km vakarø-rytø kryptimi, ne visai linijine trajektorija. Tokia trajektorija greièiausiai susijusi su labai maþais litosferos plokðèiø judëjimo greièiais (kai plokðèiø judëjimo greièiai didesni, okeaninës vulkaninës salos iðsidësto viena linija). Nors ðios salos yra arti viena kitos, jø reljefas, amþius ir net vystimosi istorija ryðkiai skiriasi. Todël daugiausia dëmesio mokslininkai skiria Kanarø vulkanizmo prigimties ir evoliucijos klausimams spræsti. Salos lyginamos su tokiais karðtaisiais taðkais kaip Havajai, Azorai ir pan. Karðtøjø taðkø Þemës planetoje rasta apie 50 vietø. Ðiame straipsnelyje Lancarotës, Gran Kanarijos ir Tenerifës salø ypatumai aptariami pradedant nuo seniausios. Lancarotës lavos tipø ávairovë Lancarotë viena seniausiø salyno salø. Salà nuo Afrikos kontinento skiria tik ðimtas kilometrø. Lancarotëje vulkanizmas prasidëjo prieð mln. metø atsivërus gausybei plyðiø, lava kilo á virðø ir iðsiliejo Þemës pavirðiuje. Toks intensyvus vulkanizmas pasibaigë maþdaug prieð 0,69 mln. metø. Vëliau buvo ramybës metas, kuris tæsësi iki XVIII a., kai po ilgai trukusios pertraukos m. vël prasidëjo povandeninis vulkanizmas, kurio metu ið mantijos ir okeaninës plutos á pavirðiø buvo iðkelti dideli kiekiai olivino ksenolitø. Pagal cheminæ sudëtá Lancarotës saloje vyrauja bazaltinë lava, bet pagal amþiø ði lava priklauso keturioms generacijoms: I bazaltas 15,5-12,3 mln.metø, II 2,7-1,0 mln.
2 74 PAÞINIMO ROMANTIKA Pahoechoe lava Lancarotës saloje. Pahoehoe lava in the island of Lanzarote. metø, III 0, 73-0,24 mln. metø, IV tûkst. metø ir jaunesnës. Be bazalto, dviejose vietose aptikta trachito. Saloje gerai iðsilaikæ ne tik didþiuliai ðvieþios lavos laukai, bet ir lavø tuneliai bei kûginiai vulkanai. Lancarotë tuo ir unikali, kad èia galima studijuoti jaunus, gerai iðsilaikiusius vulkanizmo darinius ir procesus. Didþiulá áspûdá palieka ávairûs lavø tipai. Vulkanologai turi nemaþai pavadinimø ávairiems lavø pavirðiams ávardyti. Lancarotëje vyraujantys lavø pavirðiai vadinami aa ir pahoehoe. Aa lavos pavirðius atvësta anksèiau negu jos vidinës dalys, todël tebejudant srautams lavos viduje bei stingstant pavirðiuje lava deformuojama ir ágyja nelygø pavirðiø bei aðtrius kampus. Aa lavos laukai panaðûs á tarsi buldozeriu sustumdytas lavos krûvas. Tokios lavos formavimuisi itin svarbûs trys veiksniai temperatûra, fluidø kiekis ir lavos srauto tëkmës greitis. Daþnai viename lavos sraute, be aa, gali susidaryti ir kitas lavos tipas pahoehoe. Pahoehoe lavos pavirðius lygus ir glotnus, todël já nesunku atskirti nuo aa. Pahoehoe lava ðioje saloje taip pat yra bazaltinës sudëties, taèiau maþesnio klampumo nei aa ir sudaro daug plonesnius sluoksnius, kuriø pavirðiø dengia apie 5 cm storio stiklo plëvelë (neiðsikristalizavusi medþiaga). Pahoehoe lavos tankis didesnis uþ aa, kadangi jos sudëtyje vyrauja stiklas, o ne iðsikristalizavæ kristalai. Ði lava juda poþeminiais tuneliais iðlaikydama ðilumà ir þemà klampumo laipsná. Iðsiliejusi Þemës pavirðiuje, ji slenka ið lëto ir atvësta pamaþu, taip susidaro susisukæs, susiraukðlëjæs lavos pavirðius, vadinamas virveliniu. Saloje gausu pavirðiuje atsiverianèiø lavos kanalø, kuriais neiðsikristalizavusi lava migravo po þeme bei iðsiliejo þemës pavirðiuje. Lavø tuneliø susidarymas po þeme priklauso nuo daugelio veiksniø nuo fiziniø ir cheminiø lavos ypatybiø, nuo iðsiverþimo greièio ir reljefo. Dar vienas ryðkus objektas ðioje saloje tai kûgio pavidalo plyðiniai vulkanai, iðsidëstæ linija vienas uþ kito. Ði ypatybë rodo regione vyravusá plyðiná vulkanizmà. Kûginiai vulkanai sudaryti ið piroklastiniø uolienø daugiausia ið vulkaniniø smulkiø daleliø (pelenø), uolienø ir stiklo nuolauþø (lapiliø), iðmestø á orà sprogimo metu. Kai kuriuose vulkanuose, pavyzdþiui, Cuervo ir Kolorado apatinëse kupolo dalyse, rasta dunito ksenolitø, iðkeltø net ið mantijos bei apatinës plutos. Piroklastiniai vulkanø iðsiverþimai itin pavojingi þmonijai. Nuo padidëjusio á orà iðmestø dujø ir dulkiø kiekio dûsta þmonës, gyvûnai, sunaikinama gamta. Vienas liudininkø kunigas Jaiza, stebëjæs 1730 m. prasidëjusá vulkanizmà Lancarotëje, savo dienoraðtyje raðë: Rugsëjo 1 d. tarp 9 ir 10 val. Þemë sprogo ir kità rytà jau iðkilo aukðti kalnai. Po keliø dienø nauji plyðiai atsivërë, ið kuriø pradþioje kaip vanduo pradëjo verþtis lava, uþtvindydama du kaimus. Rugsëjo 11 d. vël suaktyvëjo vulkanizmas ir dar vienas kaimas buvo paverstas pelenais. Ypaè didþiulá siaubà sukëlë lava, iðsiverþusi vandenyne. Didþiuliu negyvø þuvø sluoksniu buvo nuklota salos pakrantë. O spalio
3 PAÞINIMO ROMANTIKA Vulkaninë bomba, atskriejusi ið vulkano Kolorado (raudonas) Lancarotëje (jos svoris 50 tonø!). 18 d., atsivërus Þemëje dar trims naujoms angoms, iðsiverþë neátikëtinai didelis piroklastø kiekis. Nuo dujø uþduso daugybë galvijø, lavos srautai uþklojo derlingas þemes. Þmonës bëgo á kità salà, ðalia esanèià Gran Kanarijà. Pakartotinai vulkanai iðsiverþdavo iki 1736 m. Gran Kanarijos Lavos Jaunesnëje Gran Kanarijos saloje vulkanizmas prasidëjo ðiek tiek vëliau nei Lancarotëje prieð 14,5 mln. metø. Skirtingai nei Lancarotëje, èia, be bazaltinës lavos (tefritinës, ðarminio bazalto ir toleitinës), yra ir felzinës (fonolitinës, trachitinës, riolitinës). Be to, ði sala iðsiskiria giliais slëniais staèiais ðlaitais, kai kur besileidþianèiais tiesiai á vandenynà, ir veðlia augmenija. Gran Kanarija susidarë ið didþiulio skydinio vulkano, kurio didþioji apatinë dalis slypi po okeanefelininës lavos kolonos, susidariusios prieð 5,48 mln. metø joms bûdinga stulpinio skirumo struktûra. Nephelinite lava columns formed about 5.48 million years ago with typical column structure. 75 A volcanic bomb that flew from the Colorado volcano (red) in Lanzarote (it weighs about 50 tonnes!).
4 76 PAÞINIMO ROMANTIKA Tenerifës salos vaizdas ið Kanadas kalderos, èia dar galima aptikti tebeveikianèiø fumarolitø. Tenerife Island a view from Canadas Caldera, where one can detect active fumaroles. nu, o virðutinë dalis iðkilusi virð jo. Prieð 13 mln. metø, staiga iðsiverþus vulkanui, á atmosferà buvo iðmestas didelis kiekis magmos. Dar miocene iðtuðtëjus magmos kamerai (talpyklai), jos sienos nebeiðlaikë slegianèio svorio, todël vulkano stogas ákrito á iðtuðtëjusios magmos ertmæ, susiformavo kaldera. Vëliau daugiafaziai lavos ir karðtø dujø iðsiverþimai, intrudavusios uolienos (sienitai) bei kûginiø daikø spieèiai uþpildë kalderos vidø. Nukritusi ið atmosferos vulkaninë medþiaga uþklojo Þemës pavirðiø, suformuodama horizontalius piroklastinius sluoksnius (ignimbritus). Magmos kilimo kelias buvo nevienalytis (heterogeninis) ið pradþiø ji kilo radialiniais lûþiais, vëliau þiedo formos lûþiais. Tokie procesai átakojo naujø struktûrø grabenø, horstø ir pan. susidarymà. Pleistocene vulkanas pakartotinai iðsiverþë, o vëliau suiro. Vulkanizmas Gran Kanarijoje buvo nevienalaikis, todël salos vulkanines uolienas mokslininkai pagal amþiø skirsto á Güigüi (14,3 mln. metø senumo), Hogarzales (14,4 mln.metø.), Mogan (13,63-13,04 mln.metø), Fataga (12-9,85 mln.metø), Roque Nublo (5,48-2,7mln. metø) ir C fazës (3,5-3 mln. metø senumo). Itin áspûdingos Roque Nublo formacijai priklausanèios nefelininës sudëties vulkaninës uolienos su stulpinio skirumo struktûromis. Tokiø struktûrø susidarymas susijæs su lavos vësimo procesais: vëstant lavai nuo pavirðiaus gilyn, jos tûris sumaþëjo, todël susidarë plyðiai, statmeni jos tekëjimo krypèiai. Lava tapo panaði á vertikaliø daugiakampiø, daþniausiai penkiakampiø, skirtingo amþiaus stulpø kolonà. Kai kur ðie nefelininiai stulpai deformuoti, sulinkæ. Tokioms deformacijoms susidaryti átakos galëjo turëti vëlesnio lavos srauto ásiskverbimas. Ádomus geologinis objektas Martelës mara. Jos raida prasidëjo, kai magma pasiekë gruntiná vandená, kuriam staigiai ákaitus susidarë palankios sàlygos greitam vandens garavimui bei tûrio plëtimuisi. Sumaþëjus slëgiui, susikaupusios dujos ir vandens garai Þemës gelmëse sprogo ir toje vietoje susidarë plokðèiadugnis dubens formos krateris, apjuostas aukðto pylimo, supilto ið biriø vulkaninio stiklo nuolauþø (0,2-0,64 mm). Teidë aukðèiausia Kanarø virðûnë Tenerifë vienas didþiausiø þemës plotø Kanaruose. Èia iðkilusi ir aukðèiausia Kanarø salø virðûnë Teidë, kuri siekia net 3718 m. Pagal aukðtá Teidë uþima treèià vietà tarp okeaniniø vulkaniniø salø virðûniø. Uþ jà aukðtesnës tik Havajø Mauna Loa (4170 m) ir Mauna Kea (4205 m). Kai kuriø mokslininkø nuomone, Tenerifæ miocene (prieð 4 mln. metø) sudarë dvi ar trys trikampiu iðsidësèiusios atskiros salos. Prieð milijonà metø èia vandenyno gelmëse prasidëjæs vulkanizmas ir suformavo vientisà salà Tenerifæ. Vëliau ðis vulkanas suiro suformuodamas kalderà, taèiau vulkanizmas buvo aktyvus ir vëlesniais etapais. Ðioje kalderoje iðkilo dar vienas vulkanas, vëliau taip pat sugriuvæs ir suformavæs naujà kalderà, kurios dalis iðliko iki ðiø laikø ir vadinama Kanadas kaldera. Ði kaldera susidarë prieð 170 tûkst. metø, kai staigios nuoðliauþø griûtys sugriovë salos kûginæ formà ir suformavo vulkano virðûnëje didþiulæ ádubà (depresijà), apjuostà staèia siena, t.y. Kanadas kalderà. Dabar iðlikusi tik dalis ðios kalderos sienos, nes kita dalis nuslinko á Atlanto vandenynà. Ypaè áspûdingas kalderos amfiteatras atsiveria pakilus eskalatorium á 3200 m aukðtá netoli Teidës virðûnës èia dar randama tebeveikianèiø fumaroliø
5 PAÞINIMO ROMANTIKA 77 Erozijos sukurtos skulptûros Roques de Gracija daikos Tenerifëje. Volcano Cuervo formed of volcanic cinder in Lanzarote. Erosion created sculptures in Roques de Gracia dykes in Tenerife. bei sulfato ir alunito (KAl 3 (SO 4 ) 2 OH 6 ) nuosëdø. Vulkaniniø uolienø tyrimai ir geologiniø procesø paþinimas aktualûs visame pasaulyje ir reikalingi analizuojant Lietuvos kristalinio pamato kernà. Vulkanologiniai tyrimai ir konsultacijos su Summary Canary Islands a Hot Point of the World On 6 13 December 2006, the author of the present article, together with Swedish and Finnish students, had an opportunity to take part in a geological expedition to the Canary Islands under the guidance of Lund University Professor Anders Lindh. Three islands Lanzarote, Gran Canaria and Tenerife have been visited during the expedition that was arranged to study these islands, especially to elucidate their origin and development as one of hot points of the Earth. Canary Islands a young archipelago consist of 13 isles, 7 of which are populated, i.e. Lanzarote, Fuerteventura, Gran Canaria, Tenerife, Gomera, La Palma and Hiero. They are situated in the eastern part of the Atlantic Ocean close to the north-western coast of Africa (at the Sahara Desert). Factors, which caused volcanism in the Canary area, are not clear enough. Therefore, the issue of formation of these islands remains topical up to now. For a long time, scholars used to debate whether they were formed on the oceanic specialistais padeda geriau suprasti savo kraðto gelmëse slûgsanèiø metavulkaniniø (bazaltø, andezitø, dioritø, riolitø) ir subvulkaniniø (diabazø) uolienø susidarymà bei geologinæ raidà. Padëka. Nuoðirdþiai dëkoju profesoriui Anders Lindh uþ parodytus ádomius geologinius objektus ir vertingus paaiðkinimus, taip pat dëkoju Lundo universiteto Geologijos fakultetui uþ finansavimà. or continental crust. There is a hypothesis that the islands are situated on a so-called transit crust, i.e. at the margin of continental African and oceanic Atlantic lithosphere plates. The latest seismic data, however, show that these volcanic islands were formed on the oceanic crust and should be attributed to the volcanic island type the geologists call it the hot spot volcanism. The Canary Islands emerged because of the rifting of the oceanic crust due to convectional plume of the mantle formed between the oceanic crust and the mantle. This plume has a shape of a mushroom; its stem is very thin, whereas its cap is widely expanded under the lithosphere. The plume consists of mantle s molten matter that is turning up and forming sources of magma that burns a hole in the lithosphere and erupts as lava onto the Earth s surface, thus forming volcanoes. In the initial phase, this was the underwater volcanism. Later, as it emerged from water, its type became the continental one. The author would like to express her sincere gratitude to Prof. Anders Lindh for acquaintance with such interesting geological sites and valuable explanations about their origin. I am also grateful to Lund University Geology Faculty for funding my trip.
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