The Pamphlet Collection of Sir Robert Stout: Volume 67
Records of Previous Eruptions and Earthquakes
Records of Previous Eruptions and Earthquakes.
Volcanic Ash.—Near Ateamuri, on the River Waikato, pumice-sands are seen capping high flat-topped hills, and sections on the Taupo Road show two ash-deposits divided by a chocolate-coloured band representing an old surface-soil. The older of the two is pumice-ash with quartz, and contains masses of pumice as well as angular pieces of rhyolite and, very rarely, black scoria. It is of considerable thickness, but the bottom is not exposed. The upper ash, above the old surface-soil, is 10ft. page 8 thick in places, although usually less, and it can he traced north to Lichfield, where it thins out. It is composed of pumice-sand containing angular fragments of pumice and of compact rocks up to 1in. or 1½in. in diameter. These rocks are chiefly variously-coloured rhyolites; more rarely porphyritic basalt with olivine. At the base, just above the surface-soil, there is often a thin band of grey, very fine pumice-ash with small quantities of quartz. It is from½n. to¼in. thick, and is very like the fine ash lately thrown out of Mount Tarawera. The coarse pumice-ash contains fragments of carbonized wood, and shows no appearance of getting finer upward.
At Galatea, on the banks of the Rangitaiki River, overlying the liparites, there is a pumice-ash with quartz, very like the newer ash at Ateamuri and also containing carbonized wood. It is, however, covered by 2ft. to 4ft. of loose, coarse rhyolitic sand.
At Lake Rotorua the fine pumice-ash, already mentioned as occurring in the cliffs at Te Ngae, contains no quartz, while the beds below it are composed largely of quartz grains. This is probably owing to the ash having fallen into the lake and the quartz grains having sunk rapidly, while the lighter pumice remained in suspension for some time.
All the beds at Earthquake Flat are pyroclastic, but in the upper part, above the fumarole-clays, we have a fine pumice-dust (No. 3) overlying coarser beds much as at Te Ngae, and we cannot doubt but that this dust is the finer portion of the ash which fell into still water, and consequently settled after the coarser part.
* Lyell: "Principles of Geology," vol. 2, p. 127.
No scoria cones were known in the district, but we were informed by Mr. Frazer, of Taheke, that on the south side of Mount Tarawera, in a line with Rotomahana, there used to be a hill about 400 yards long, 200 yards wide, and 120 yards high, rising up from a flat, with a crater-like hole on the top and hundreds of boulders of obsidian lying about it. The inside of the crater sloped and was covered with fern.
Dr. von Hochstetter also mentions that Rotomakariri, near Rotomahana, had curious circular coves which reminded him of the tuff-craters near Auckland;* and he said that the solfatara of Ruahine, near Tikitere, had the appearance of being an active crater.† Viewed with our present knowledge, we may well believe that the crateriform springs of the White and Pink Terraces, of Rotopunamu and other places on Rotomahana, commenced with explosions like those which happened on the 10th of last June.
* "New Zealand," p. 419, and fig.
† "New Zealand," p. 430, and fig.