BICICLETERO UND

Maar volcanoes and tuff rings are notoriously difficult to distinguish in some cases, due to the potential for a crater to evolve through both styles. However, in general, the following characteristics are more frequently possessed by maar volcanoes:

 A high percentage of country rock fragments in the ejected tuff. This is due to the diatreme initiating at depth, expelling large quantities of country rock to be included in the ejecta ring as xenoliths (Lorenz, 1973; Francis and Oppenheimer, 2004; Carey and Houghton, 2010).

 Maar volcanoes are usually negative relief landforms. The development of the diatreme at depth expels country rock and creates unstable vent walls which periodically collapse in on themselves, creating a deep pit. When the magma source recedes, the crater can collapse even further into the empty chamber. It is common to see the country rock in the crater wall of a maar volcano due to these eruption processes (Lorenz, 1973; 1986; 2003).

 Crater lakes are common occurrences in maar volcanoes due to the low or negative elevation. If the conditions are right in the lake, diatoms will flourish and their frustules will settle out of suspension to form diatomite. The lake infill deposits can be protected for long periods of time as the surrounding ejecta ring provides shelter against weathering and erosion processes.

Tuff rings generally possess subtly different characteristics:

 A high percentage of fragmented juvenile material compared to country rock lithics. Eruptions that form tuff rings tend to initiate at the surface or at shallow depths. The diatreme does not migrate downwards and therefore country rock lithics are poorly represented in the ejecta.

 Tuff ring explosions generally create a shallow crater with a thick ejecta ring that has roughly equal inner and outer slope angles (Cas and Wright,

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1987). The crater is often at the same elevation or higher than the surrounding environment.

 Tuff rings often form nested scoria cones and effusive flows when surface water runs out (Cas, 1989).

6.7.1 Is the Onewhero crater a maar or a tuff ring?

The Onewhero Volcanic Complex is commonly referred to as the Onewhero tuff ring although the crater is characterised by features commonly observed at both maar volcanoes and tuff rings (Briggs et al., 1994; Gibson, 2011). The crater has a diameter that ranges from 2 – 2.5 km wide with an elliptic shape. This is quite large for a maar, but larger maar craters do exist (Jordan et al., 2013; Otterloo et al., 2013; Blackfield et al., 2014).

The ejecta ring that encompasses the Onewhero crater is composed of a mixture of ash, fragmented scoria, olivine crystals, and country rock lithics. In a lithological investigation by Gibson (2011), it was found that only 5 – 20% of the ejecta ring was composed of country rock material, which is low for a maar-forming eruption (Figure 6.15).

Gibson (2011) also noted inner slope angles of 14 – 25° and outer slope angles of 6 – 7°. These slope angles are consistent with a maar-formed ejecta ring, but this is not a fair representation, as the tuff ring at Onewhero has had more than 0.88 ± 0.06 Myr of ejecta ring deterioration. The elevation of the crater floor ranges from about 99 – 111 m, due to inferred Quaternary displacement across the nearby Waikato fault. The elevation of the topography surrounding the crater ranges from 100 – 200 m. Even if the initial eruption occurred at close to sea-level, the two-dimensional gravity modelling revealed an eruption crater that was up to 120 m deep before sedimentary infill. It was shown earlier in this investigation (section 3.2.5) by the presence of diatomaceous sediment found in the crater that a lake formed some time during the post-eruption period. The ejecta ring slope angles, negative elevation of the original crater floor, and presence of a crater lake are consistent with a maar- forming eruption.

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On a geological map for the Onewhero region (Waterhouse, 1978) an outcrop of Whaingaroa Siltstone appears on the inside of the tuff ring in the southwest of the crater. If this is reliable, it means that much of the crater floor is below the original surface. This is probably the best physical evidence for confirming whether or not a phreatomagmatic landform is a maar volcano or a tuff ring. If Waterhouse (1978) is correct in his observations I am confident in proposing that the Onewhero crater is a maar until further evidence to the contrary.

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Figure 6.15: Physical components of the Onewhero (a.) and Kellyville (b.) tuff rings (Gibson, 2011).

6.7.2 Is the Kellyville crater a maar or a tuff ring?

The Kellyville Volcanic Complex is commonly referred to as the Kellyville tuff ring, although the crater has features consistent with both a maar and a tuff ring (Briggs et al., 1994; Gibson, 2011). The crater has a diameter that ranges from about 1 – 1.5 km wide.

The base of the ejecta ring at Kellyville is dominated (80%) by juvenile material (Figure 6.15). There is a trend of increasing crystal and lithic content up through the tuff deposits with the crystal and lithic content increasing markedly to the point where lithics make up almost 50% of the tuff ring (Gibson, 2011). This could be due to the diatreme deepening and incorporating more country rock. The percentage of lithics in the Kellyville tuff is far more consistent with a maar-forming eruption than that of the Onewhero tuff.

Gibson (2011) noted an inner slope angle of 10° and outer slope angles of 7 – 9°. These angles are very similar, and this, combined with a minimum tuff ring height of 92 m, is compelling evidence for the presence of a tuff ring.

The argument for the presence of a maar is just as strong. A crater lake was present sometime after the eruption, as confirmed by an outcrop of pure diatomite. Colchester (1968) also identified the Mercer Sandstone in outcrop, lying unconformably beneath the pyroclastic deposits. The two-dimensional gravity modelling revealed a crater-fill of basalt that reaches a thickness greater than 60 m. This suggests that the eruption cut considerably into the pre-eruptive surface. These three pieces of evidence are all consistent with maar-forming eruptions.

The evidence for the presence of a maar or a tuff ring is inconclusive. I believe that the Kellyville crater has maar-like subsurface features but it also has tuff ring-like surface features. In this investigation I have referred to the crater in Mercer as the Kellyville maar.

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