Located in the center of Northern Island, the lake Rotomahana has an area of 8km² with a maximum depth of 112.4m. It is located about twenty kilometers from Rotorua, at the foot of the volcano Tarawera. The geography of the lake was strongly altered during a strong eruption of the Tarawera in 1886. The volcano, as well as the lake is located in the caldera of Okataina

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The Rotomahana lake within the caldera

Rotomahana geology setting

Around the lake, it is possible to observe three types of rocks:

– conglomerates composed of volcanic detrital rocks

– volcanic ash from volcanic eruptions

– rhyolite. The rhyolite is a volcanic magmatic rock of fairly clear color: pink or gray and sometimes blue. The minerals visible to the naked eye within the rock are: quartz, feldspar and biotite

Rotomahana Lake Formation

The lake was formed at the same time as the volcano Tarawera during the appearance of the caldera of Okataina. The caldera is a collapse forming a crater resulting from an extreme and strong explosion during a volcanic eruption. When the gas escapes, it will carry with it a large quantity of ash and magmatic debris in a strong volcanic explosion in the form of a fiery cloud. The magmatic chamber being empty in its upper part, it will undergo a collapse of the earth’s crust forming a crater called caldera.

From this caldera of Okataina will appear rhyolitic domes. At 1111m high, Tarawera volcano is the highest of the three Wahanga, Ruawahia and Tarawera domes. Its formation comes from the volcanic activity of the “Taupo Volcanic Zone” (TPZ).

Tarawera formation

900 years ago, the eruption of the Kaharoa volcano began with a strong explosion. This explosion made it possible to unclog the rising duct of the magma of the previous volcanic debris.

This will cause the pulling up of a magma which will put on itself on the already existing dome (a). A second explosion will accumulate ashes and debris on the dome (b). The second will also allow to unclog the new ducts of rise of the magma which goes the evacuation of the magma allowing to make a new layer on the dome (c).

It is thus possible to define the formation of the volcano Tarawera as an accumulation of lava and debris on several layers making it possible to raise the dome.

In 1886 the Tarawa erupted killing less than 150 people. This resulted in the destruction of one of New Zealand’s wonders, the “Pink and White Terraces”. Hot water containing large amounts of calcium bicarbonate precipitated calcium carbonate, leaving large white layers of limestone and travertine on the hills, forming natural ponds and terraces. The white terraces were the largest and considered more beautiful than the pink terraces. They were considered by some to be the eighth wonder of the world and were the main tourist attraction of the country

Hiking around the lake

There is a 15 km hike to the Tarawera, Wahanga and Ruawahia Mountains.

At the end of the way, after about 3-4 hours of walking, we arrive at a campsite with thermal springs. These thermal springs are within the lake. A real pleasure to feel the fresh water of the lake and the warm water from the springs on his body after a hike.

To return, you have two options, either take the way back, or take the boat taxi. With the boat taxi you will be able to reach other sources which are not accessible by th way. But there are still plenty of springs available around the lake which are beautiful.

For all definitions: go to this page.

For the time scale is here!

Source

“The field guide to New Zealand Geology, An introduction to rocks, minerals and fossils”; Jocelyn Thrnton, Pinguin Books

“In search of ancient New-Zealand”; Hamish Campbell & Gerard Hutching; Pinguins books