Lake Rotorua

Early morning on Monday, 28th November 2016, people in the Rotorua suburb Ohinemutu next to Lake Rotorua were torn from sleep by a series of loud thuds, followed by a spraying sound. Half asleep, some of them managed to witness a rare spectacle. Out on the lake a few geyser-like eruptions between 7 m and 30 m (23-98 ft) height took place, probably driven by accumulated steam from rock layers only some metres underneath the bottom. Even if such events are rare, they leave no doubt about the system's volcanic nature.
The Rotorua Volcano erupted for the last time less than 25,000 years ago, while the last major eruption happened around 240,000 years ago. When the emptied magma chamber collapsed after the major eruption, a caldera of 22 km (14 mi) in diameter was formed and later mostly occupied by a lake, now called Lake Rotorua. Still today many thermal springs and fumaroles on the bottom and the shore discharge into the lake and enrich the water with elemental sulfur, sulfur compounds and other volcanic emissions. Besides the hot springs around the lake, the thermal activity becomes also apparent by ancient sinter deposits and a yellowish colouration of the lake water, especially on windy days. Two more or less regularly active geysers are known in the area, and a further few springs have shown one-time spouting activity.

Sinter deposits at Lake Rotorua
Ancient sinter deposits at Lake Rotorua's Sulfur Bay

Lake Rotorua's shallow Sulphur Bay, located immediately east of downtown Rotorua, is surrounded by extensive, bare areas peppered with hot springs and fumaroles. These are called Te Arikiroa (The Sulfur Flat) and are accessible from Rotorua on a hiking trail. For safety reasons the trail always keeps a quite large distance to the thermal features, though.

Te Arikiroa / Sulfur Flat
Te Arikiroa / Sulfur Flat, Lake Rotorua with steaming thermal features far in the background

Every now and then small waterbodies next to the path become visible which are collecting in- or outflowing thermal waters, often bluish clouded by silica particles.

Te Arikiroa / Sulfur Flat
Te Arikiroa / Sulfur Flat

Close to the town centre of Rotorua a section of the shore is named Sulphur Point due to the frequent occurence of sulfur deposits, muddy hot springs and fumaroles. Here above all Cameron's Laughing Gas Pool catches the eye, simply because it is the largest spring on location. The name implies that this pool emits laughing gas (nitrous oxide), but that is not the case. Instead, a mixture of hydrogen sulfide and carbon dioxide ascends as small bubbles to the surface of the spring. On bathers, however, these toxic gases in high concentrations can have the same effect as laughing gas, causing euphoric reactions accompanied by nearly uncontrollable fits of laughter. Despite the risk of poisoning and fainting the water of Cameron's Laughing Gas Pool was used with some precautionary measures for therapeutic applications in the late 19th century.

Cameron's Laughing Gas Pool
Cameron's Laughing Gas Pool

Even downtown Rotorua hosts many hot springs at close distance to the lake shore. The most popular thermal areas here are Government Gardens and Kuirau Park. Due to a tight schedule we only visited Government Gardens to see the hot springs around the famous Rotorua Bath House from 1908.

Rotorua Bath House
Rotorua Bath House in Government Gardens

Diagonally in front of the impressive, Elizabethan style building, Whangapīpiro (evil smelling place) captivates with its clear alkaline water and the massive sinter rim. Over long periods of time Māori used it as cooking pool. Later it got the second name Rachel Spring after Madam Rachel, an English cosmetician. Whangapīpiro's activity varies between quietness, overflow or boiling, but as far as known it has never shown geyser eruptions. The small pump house in background is still in use to pipe its water to the Polynesian Spa.

Whangapipiro / Rachel Spring
Whangapīpiro / Rachel Spring

At a short distance west of Whangapīpiro once a deep chasm full of boiling water was located. The Ngāti Whakaue called it Ōruawhata and it served them as a burial place where the remains of their warriors could rest in peace, out of reach for their enemies. Already in the 19th century the crater was filled in for unknown reasons. In 1889 French-born Engineer Camille Malfroy constructed a pool system with three artificial geysers only a few feet northwest of the former Ōruawhata, using its piped thermal water. Malfroy's Geysers rather gave the impression of water fountains in an European palace garden than of a natural spring, whereas the mechanics behind the 12 m high playing geysers was very clever and way ahead of his time. After Malfroy's death in 1897 the system deteriorated to a great extent and therefore it has been largely removed. Today, only a small crater with slightly boiling water and two dead pipes is left. As a matter of some surprise, in 2009 and 2014 it turned out that this crater hosts one of only two repeatedly playing geysers in the north of Rotorua, even if the rare eruptions from the crater spring are only up to 1.5 m (5 ft) high (see S.C. Pearson-Grant, B.J. Scott, E.K. Mroczek, D.J. Graham, Rotorua Surface Feature Monitoring Data Review: 2008-2014, GNS Science Consultancy Report 2015/124).

Malfroy's Geyser
Malfroy's Geyser (or actually its more natural looking successor)

The other persistent geyser of north Rotorua is called Little Waikite and is located on private ground in the already mentioned Māori village Ohinemutu. According to the cited monitoring report, Little Waikite Geyser was observed to erupt intermittently up to a height of 1 m (3 ft) on each visit between 2008 and 2014.
Further named hot springs in vicinity of Lake Rotorua are Porahi (Ohinemutu), Postmaster's Pool and Stop Bank Spring (Ngapuna area) and Soda Spring, Mayor's Mouth, Tarewa Spring and Kuirau Lake (Kuirau Park). In Kuirau Park, on 26th January 2001 the unnamed muddy hot pool 721, around 3 m (10 ft) in diameter, erupted unexpectedly up to 100 m (330 ft). The hydrothermal explosion blew open a crater 10-12 m (33-40 ft) in diameter, throwing 1 m (3 ft) diameter blocks 50 m (165 ft) away. Fortunately, nobody came to any harm. This was only one of several similar examples demonstrating impressively that geothermal activity in Rotorua is not only a blessing but may also be a curse.

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