Post-caldera activity

Post caldera volcanic activity was generated in the caldera and on the northern rift zone. We present a small-scaled geological map focusing the caldera for better visualization (Fig. 2.28). Post caldera volcanism is divided into three sub-stages (Table 2.1: Sub-stage VI, VII, and VIII). Western half of the caldera is covered with a fresh water lake (Fig. 2.28: Nemrut lake), limiting observation of the post-caldera activity in only the eastern part of the caldera. There is also a hot lake at the foot of the northern caldera wall which is fed by hot sources (Fig. 2.28: Ilığ lake).

Sub-stage VI (30 -12 ka)

This period is represented by comenditic intra-caldera lava flows and domes (Table 2.1; Fig. 2.28). Planar flow banding is evident in these lava flows (Fig.

2.29a); holohyaline, obsidian facies units, occurrence of vesicular pumiceous zones are common (Çubukçu, 2008). In addition, intra-caldera lavas bear benmoreitic enclaves (Fig. 2.29b; Çubukçu, 2008). Clues of local hydrothermal alteration is evident especially in the lava dome forming Göl hill (Figs. 2.28 and 2.29c) and in the lavas encircling the elongated maar at south of Göl hill (Fig.

2.28). Hydrothermal alteration is marked with a strong devitrification. Former obsidian flow is marked with relict, angular glasses, surrounded with a devitrified casing around Göl hill (Fig. 2.29c, d). In larger scale, alteration is typical with reddish, brownish oxidation colors in the caldera (Fig. 2.29c).

Sub-stage VII (12 ka - AD)

Intra-caldera phreatomagmatic - phreatic activity is dominant in this sub-stage (Table 2.1). Moreover, there are multitudinous maars in the caldera; their products are stratigraphically interbedded, consequently the distinction of their products is not easy. They cover most of the former intra-caldera comenditic lava flows (Fig.

2.28). Products of the Big maar (Fig. 2.28) are widespread in the caldera, plaqued on the western shore of Nemrut Lake. They also surmount the caldera rim and are deposited on the NE, E and SE flanks of the volcano.

Figure 2.28. Geological map of Nemrut caldera.

Figure 2.29. Post-caldera lava flows and hydrothermal alteration. a) Flow bending in lava flow, b) Benmoreitic enclave in obsidian, c) Hydrothermal alteration on Göl hill, d) Devitrified obsidian flow on Göl hill.

Big maar continued its activity with a comenditic lava flow through west. Deposits of a latter maar (south of Göl hill) partially cover the comenditic flow of Big maar.

Base surge deposits are typical with dune and anti-dune structures, bomb sags (Fig.2.30a, b), breadcrust bombs (Fig.2.30c) and cross bedding (Fig.2.30d). At the northeastern caldera rim, products of Big maar presents a very local, whitish – yellowish – dark brown colored sequence (Fig. 2.30e). Especially, yellowish and dark brown colored horizons of this sequence bear large (up to 25 – 30 cm) pumices and lithics. At first glance, these horizons may be confused with a plinian fall (i.e. NeFB3); however, there are large obsidian, basalt and most importantly, ignimbrite clasts in the brown horizon.

Figure 2.30. Base surge deposits and their components in the caldera. a, b) Bomb-sag structures, c) Large breadcrust bomb on the tuff ring, d) Cross-bedding in the base surge deposits, e) Proximal phreatomagmatic deposits with large clasts.

Nemrut Rift zone (Sub-stage VIII; AD - Today)

Latest activities took place in the historical times. This sub-stage is represented by rift zone activity (1441 – <1597 AD) on the northern flank of Nemrut volcano.

Karakhanian et al. (2002) reported an historical event, which occurred around Tatvan town (Appendix-A); however, precise location of this event is still a matter of debate. Recent activity will be discussed in the subsequent chapters.

The rift activity occurred between Nemrut plain and Kantaşı hill (Fig. 2.28); the ridge along the rift and its surrounding is covered with Kantaşı ignimbrite.

Throughout the ridge, rifting cracked the welded Kantaşı ignimbrites; lava poured and flown over them. The rift zone is slightly bended from NNW to N and continues throughout ~5 km. We observed paralleling fault cracks at the southern section of the zone; the cracks are relatively narrow (Fig. 2.31a) when compared to middle (Fig. 2.26b) and northern sections (Fig. 2.31b) of the rift zone.

Figure 2.31. Nemrut rift zone at the northern section of the volcano. a) Narrower rifting cracks closer to the caldera and b) northern section of the rift zone with a larger crack and domal obsidian knob in the middle. For the scale please note the presence of three members of our team in red circle.

Except the southernmost part of the rift zone, lava activity is obvious in the rift. The activity in the rift is observed as knobs of obsidians (Fig. 2.31b); furthermore, there are locations where the crack is totally filled with obsidians and basalts (Fig. 2.32a, c). Breadcrust bombs are found along the rim of the rift (Fig. 2.32b).

Figure 2.32. Nemrut rift zone and comenditic products. a) Rift crack filled with comenditic obsidians, b) Breadcrust bomb found at the rim of the rift, c) Basaltic lava pooled at the draining spot. For the scale please note the presence of the member of our team in red circle.

In the crater of the cone forming Kantaşı hill, there is a comenditic lava lake (Fig.

2.28) from which lava poured and flown through north (extent: 1 km) and south (extent: 230 m) (Figs. 2.26a and 2.33a).

The most spectacular feature of the rift zone is the bimodal activity. Synchronously with the comenditic flows, basalt flows were effective at the rift zone (Fig. 2.33b).

Şerefhan (1597) witnessed and well delineated the basaltic activity. He invigilated lava fountains for repeating years and described the lava flows (Aydar et al., 2003;

Appendix-A). Originating from the rift, two (aa-like) basaltic flows are obvious throughout the eastern flank and their length reaches up to 890 m and 1.3 km respectively (Figs. 2.28 and 2.33c). Another basaltic flow originates from the southern side of the Kantaşı hill and extends 1.2 km to the east (Figs. 2.28 and 2.26a). Two branches of basaltic lavas are overlain by obsidian flow at the northern side of Kantaşı hill (Figs. 2.28, 2.1 and 2.33b). Their lengths are 130 and 780 m, respectively. Thicknesses of the obsidian flows may reach up to ~10 m, while basaltic flows are thinner than ~2 m.

Figure 2.33. Products of bimodal rift zone activity. a) Obsidian flown throughout north flank of Kantaşı hill and b) underlying basalt flow at the northern front of this obsidian flow. For the scale please note the presence of the member of our team in red circle. c) Basaltic flows originated from the rift, flown through the eastern flank of the ridge. Please note the flow channel indicated with white pointed line.