| Title | Black Terrace, Rotomahana |
|---|---|
| Authors | P. White, A. Lorrey, J-M Woolley, P. Vidanovich and T. Dronfield |
| Year | 2020 |
| Conference | New Zealand Geothermal Workshop |
| Keywords | Black Terrace, Rotomahana, sinter, GPR, passive seismic tomography |
| Abstract | Prior to 1886 there was widespread thermal activity around Lake Rotomahana. In addition to the Pink and White terraces, there were smaller sinter terraces, including one known as Black Terrace, or Te Ngawha a Te Tuhi. All of those features were either ruined or buried during the 1886 Tarawera eruption. The former locations of the Pink and White terraces are now beneath the enlarged Lake Rotomahana, but the Black Terrace site to the northwest is outside the former and current lake. About two months after the Tarawera eruption, a hydrothermal eruption at the location of Black Terrace threw out rocks and mud, and the crater grew to ~180 m in diameter over two days before activity gradually waned over the next few weeks. Being surrounded by hills covered in unconsolidated eruption debris, this crater was rapidly infilled over subsequent years. It is no longer obvious on maps, aerial photos, or in the field. Through a combination of geological and geophysical techniques, we believe that we have relocated the buried Black Terrace Crater, although it remains to be seen whether any sinter is preserved at depth. Proximity to the crater is indicated by boulders (up to 1.5m diameter) that overlie Rotomahana Mud near the site. Those boulders are mostly rhyolite, but include several fragments of silica sinter. There are indications from LIDAR of a partially preserved crater rim deposit around the inferred southern edge of the crater. Ground Penetrating Radar (GPR) revealed possible normal fault features close to where the western crater rim was inferred to lie, and features that might represent a buried, partial ejecta apron to the west of the buried crater rim. Passive seismic tomography data revealed a lateral east-west change, with hard reflectors located 35-50m below the ground surface within the bounds of where the crater floor is inferred to lie. |