Fig.1

 
Fig.2

Fig.3

Fig.4

Fig.5

Opala is a large stratovolcano located in South Kamchatka about 50 km west of the Eastern volcanic front (Figs.1, 2). The volcano sits on the northern rim of a 14x12 km large Late Pleistocene caldera, which formed 39-40 ka BP. The summit of the volcano has a nice small crater. The most prominent feature at the foot of the volcano is Baranii Amphitheater - a large Novarupta-type crater filled with extrusive domes (Fig.1). This crater formed about 1500 years BP and produced 9-10 km³ of rhyolitic pumice (Fig.3) that is about 4.5 km³ of magma. Later the crater was filled with extrusive domes. There was at least one later eruption within the crater, which formed explosive craters on the surface of the extrusive domes and produced minor pumiceous tephra.

Several hazy reports of Opala's historical eruptions were known, but no real evidence of recent eruptions had been found at the slopes of the volcano (Melekestsev et al., 1991). However, our later studies have allowed to document a significant explosive eruption from the summit crater, which occurred as recently as about 300 years ago and produced rhyolitic tephra (informally known as "Ghost layer"). Earlier eruptions from Opala produced lava flows and pumiceous tephra; several large landslides have been also registered at the foot of the volcano.

The eruption from Baranii ("Sheep's) Amphitheater was one of the most voluminous explosive eruptions in Kamchatka during the Holocene (Braitseva et al., 1995,1997a; Melekestsev et al., 1991,1996). Proximal deposits are dominantly fall-out with only minor pyroclastic flow unit. The ashfall axis was directed to northeast. The tephra layer from this eruption, coded OP (Fig.3), is one of the main regional marker ash layers for Kamchatka's Holocene stratigraphy (Braitseva et al., 1997b). Its characteristic feature is presence of biotite that allows to tell it from the eastern volcanoes ashes. Tephra OP can be traced down Kamchatka river valley up to town of Kozyrevsk and also covers most of the Eastern volcanic belt north of Ksudach. The OP eruption falls within one of the all-Kamchatka intervals of intense volcanic activity (1300-2000 ¹⁴C yrs BP or cal. 0-650 AD) (Braitseva et al., 1995).

Opala is undoubtedly an active volcano posing a serious hazard to the region. For example, thickness of the compacted OP tephra 10 km downwind, near the new power station at Tolmacheva river is 70-100 cm, maximum size of lithics reaches 8 cm. Recent eruptions from Opala produced dominantly rhyolitic material that indicates a presence of living silicic chamber under the volcano.

Holocene products of Opala volcano are high potassic andesites-rhyolites. Baranii Amphitheater crater produced uniform rhyolites without any admixture of more mafic varieties. These rhyolites are very similar in composition to the most recent tephra from Opala itself ("Ghost layer") and also - enigmatically - to tephra from Chasha crater, located about 15 km to the northwest, across the fault of Tolmacheva river.

Opala volcano has been producing andesitic-dacitic lavas and tephras for most of Holocene; the last significant eruption of this kind was ab. 3500 ¹⁴C yrs BP (Fig.2). How was it possible to produce, accumulate, store and finally erupt - just in a couple of millenia - about 4.3 km³ of rhyolitic magma from the crater located on the volcano's slope, a mere 5 km from its summit? Was the OP eruptionit a part of the Opala volcano story or any special event? Why is this magma so similar to Chasha crater's one, stored 15 km away?  These are my personal questions to this volcano.      

South-western part of Opala caldera hosts wonderful peat bogs, which preserve the layers of ashes from Opala and other volcanoes.

Vera Ponomareva

Literature

Braitseva O.A., Melekestsev I.V., Ponomareva V.V., Sulerzhitsky L.D. 1995. The ages of calderas, large explosive craters and active volcanoes in the Kuril-Kamchatka region, Russia. Bull Volcanol 57/6, 383-402

Braitseva O.A., Sulerzhitsky L.D., Ponomareva V.V., Melekestsev I.V. 1997a. Geochronology of the greatest Holocene explosive eruptions in Kamchatka and their imprint on the Greenland glacier shield. Transactions (Doklady) of the Russian Academy of Sciences. Earth science sections. 352/1: 138 -

Braitseva O.A., Ponomareva V.V., L.D.Sulerzhitsky,  I.V.Melekestsev I.V., Bailey J. 1997b.  Holocene key-marker tephra layers in Kamchatka, Russia. Quaternary Research, 47: 125-139

Melekestsev I.V., Felitsyn S.B., Kirianov V.Yu. 1991. Opala volcano eruption. A.D., 500 - the largest explosive eruption on Kamchatka in our era. Volcanology and Seismology 13, 21-36.

Melekestsev I.V., Braitseva O.A., Bazanova L.I., Ponomareva V.V., and Sulerzhitsky L.D. 1996. A particular type of catastrophic explosive eruptions with reference to the Holocene subcaldera eruptions at Khangar, Khodutka  Maar, and Baraniy Amfiteatr volcanoes in Kamchatka. Volcanology and Seismology, 18, 135-160.