Rocks that tell a story
Volcanic and hydrothermal rocks are found on both Earth and Mars. Where water heated by volcanism occurs on Earth we find life.
Was this the same on Mars back in its distant past, when water flowed on the surface and volcanoes were active?
Rocks collected from Rotokawa Geothermal Area
Volcaniclastic breccia showing hydrothermal alteration (red iron oxide mineral). These rocks have been collected from the Rotokawa geothermal area. Breccias in geothermal areas generally form by landslides or hydrothermal explosions.
Sinter from Pyrite Pool
Spicular (needle-like) and scalloped sinter formed under acidic conditions, from Pyrite Pool, Rotokawa Lagoon. Sampled from Rotokawa geothermal area.
Rocks collected from Tikitere
Breccias in geothermal areas generally form by landslides or hydrothermal explosions
Coarse volcaniclastic sandstone, cemented with hydrothermal silica (Left)
Silica-charged hydrothermal fluids percolated through coarse volcaniclastic sands,at Tikitere(Hell’s Gate geothermal area) cementing them and turning them to a sandstone.
Volcaniclastic breccia with pumice clasts (grey) and sulfur-rich cement (Right)
Presence of sulfur suggests ancient fumarole activity in the formation of the rock
Rocks collected from Mangatete Quarry
Zeolitised diatomite – fine; volcaniclastic gravel – coarse (Left)
Greenish boundary has been silicified by hydrothermal silica, from Mangatete Quarry
The coarse deposit in the rock sample might be pyroclastic, which means hot sedimentary debris of volcanic origin derived directly from a volcanic eruption. Pyroclastic flows from Mt Vesuvius in Italy buried Pompeii and Herculeum during the Roman Empire.
Sinter = siliceous hot spring deposit. 9,400 years old (Drake et al., 2014). Wavy laminated to packed fragmental sinter texture, indicating growth of cyanobacterial microbial mats on bottom or surface of a hot water stream, with broken silicified mat sheets torn up in storms and deposited in point bars of the geothermally influenced creek.
Look for eye-shaped features along the side of the deposit – these are bubbles of photosynthetic (oxygen) gas that were silicified inside the mat, flatted by the stream action and continued growth (layers) of mats. Top of Mangatete Quarry.
The diatomite at Mangatete represents silicified lake sediments of the Huka Formation (Late Quaternary age, est. 50-60kyrs; kyrs=thousands of years in this area, cf. Brathwaite, 2003); it formed from diatoms in a quiet lake setting and the sediments were later zeolitised by circulating subterranean hydrothermal fluids after the lake sediments were deposited (Brathwaite, 2003). The diatomite is used for kitty litter and other products (Blue Pacific Minerals) because of the unique absorptive properties of its clinoptilolite zeolite – see: https://www.bpmnz.co.nz/en/minerals/zeolite/
The gravel washed into the ancient lake during a high energy event – volcanic eruption or storm, probably the former.
Braithwaite, R. L. (2003). Geological and mineralogical characterization of zeolites in lacustrine tuffs, Ngakuru, Taupo Volcanic Zone, New Zealand. Clays and Clay Minerals, 51(6), 589-598.
Extensional volcanic arc settings commonly host long-lived or ephemeral lakes that are formed by either structurally controlled subsidence, subsidence following explosive eruptions or by volcanic eruptions blocking water outflows (Manville et al. 2007). These lakes are depocentres for extra- or intra-basinal pyroclastic deposits (e.g. Cas et al. 1990, 2001; Nelson & Lister 1995; Manville 2001).
The central Taupo Volcanic Zone (TVZ) hosts large and deep lakes (<1–45 km long; <40–185 m deep). Explosive caldera-forming eruptions in the TVZ generated current Lake Taupo and Lake Rotorua and volcanic damming formed Lake Rotoiti, Lake Tarawera and Lake Okataina (Manville et al. 2007). The position, thickness and orientation of lacustrine deposits assigned to the Huka Falls Formation (HFF; Grindley 1965) define the distribution of ancient Lake Huka, a precursor of Lake Taupo (Smith et al. 1993; Manville & Wilson 2004; Rosenberg et al. 2009a; Bignall et al. 2010).
Lakes and marine basins within or close to volcanic centres are depocentres for pyroclastic deposits and serve as a record of eruptive activity (e.g. Manville 2001). In some cases, the [coarse, pumice-rich] pyroclastic deposits that punctuate the thin fine-grained background [lake] sedimentation are thick, massive to graded, pumice-rich, density current deposits fed directly from volcanic eruptions. The source of these eruptions can be from either relatively deep (≥150 m) subaqueous vents or hot pyroclastic flows traversing the shoreline from a subaerial vent (Cas & Wright 1991; White 2000; Allen & McPhie 2009; Allen et al. 2012). Identifying the vent setting for thick, pumice-rich density current deposits can be problematic as the material from both sources mixes turbulently with water and is transported in water-supported mass flows producing similar deposits. Additionally, deposits may also be poorly preserved due to reworking events, hydrothermal alteration and segmented uplift and exposure. Detailed lithological examination is a key method for determining transport and depositional processes as well as for inferring eruption conditions of pyroclastic deposits (e.g. Cas & Wright 1987; McPhie et al. 1993). Pumice rounding, lithic clast type and clast distribution are important attributes that enable the two different origins to be identified (Allen & McPhie 2009; Allen et al. 2012).
Drake, B. D., Campbell, K. A., Rowland, J. V., Guido, D. M., Browne, P. R., & Rae, A. (2014). Evolution of a dynamic paleo-hydrothermal system at Mangatete, Taupo Volcanic Zone, New Zealand. Journal of volcanology and geothermal research, 282, 19-35.