Sulphur Bay provides a living example of the geothermal and volcanic processes that once formed the now-destroyed White Terraces. Through the interaction of sulphur-rich gases, acidic hot springs, and silica deposition, visitors can witness firsthand how volcanic activity sculpts mineral landscapes. This earthcache offers insight into the dynamic balance between mineral formation and erosion, illustrating how natural forces continue to shape and reshape the Earth’s surface over time.
Sulphur Bay is located on the southern edge of Lake Rotorua, within the Rotorua Caldera, a volcanic depression formed by a massive eruption around 240,000 years ago. It lies within the Taupō Volcanic Zone (TVZ), New Zealand’s most volcanically and geothermally active region, shaped by the subduction of the Pacific Plate beneath the Australian Plate. This tectonic setting creates a thin crust, high heat flow, and an abundance of faults that allow geothermal fluids to rise to the surface (GNS Science).
Beneath Sulphur Bay, magma provides the heat that drives hydrothermal circulation. Surface water percolates down through fractures, is heated, and becomes chemically enriched with dissolved minerals—especially silica (SiO₂)—and volcanic gases such as hydrogen sulphide (H₂S) and sulphur dioxide (SO₂). As these hot fluids return to the surface and cool, the silica begins to precipitate, forming siliceous sinter terraces. These deposits can create flat terraces, mounds, or pool rims and are chemically similar to those that once formed the iconic Pink and White Terraces (Mountain & Wood, 1988).
Siliceous sinter is a form of amorphous opaline silica, which precipitates under near-neutral to slightly acidic conditions when supersaturated geothermal water cools. These deposits are often white or creamy in appearance, though microbial mats and mineral staining can introduce yellows, oranges, and greens. In addition to silica, elemental sulphur also deposits around fumaroles and acidic pools as gases oxidize at the surface, forming vivid yellow crusts and contributing to ongoing acid-sulphate alteration of the surrounding rocks.
However, these formations are highly susceptible to erosion and degradation over time. Unlike more stable rock formations, siliceous sinter is porous, friable, and chemically unstable, especially in fluctuating temperature and moisture conditions. Erosion at Sulphur Bay is driven by several key processes:
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Chemical weathering: Acidic waters, particularly those enriched in sulphuric acid, continue to dissolve and weaken sinter deposits and surrounding rocks (Fournier & Rowe, 1966).
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Hydraulic erosion: Rainfall and geothermal runoff can physically erode terraces and crusts, particularly where flow paths concentrate energy.
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Steam and condensation: Cycles of condensation and evaporation around fumaroles contribute to mineral dissolution and redeposition, altering surface features regularly.
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Thermal stress: Daily and seasonal temperature fluctuations cause expansion and contraction in fragile sinter, promoting cracking and flaking over time.
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Microbial and biological activity: Thermophilic microorganisms can both stabilize and destabilize deposits, depending on biofilm characteristics and mineral-trapping efficiency (Jones et al., 2001).
This constant interplay of deposition and erosion results in a landscape that is always in flux—new terraces form while older ones degrade. This dynamic is reflected in sinter systems worldwide, including those at Yellowstone (USA), Iceland, and former sinter fields in New Zealand. At Sulphur Bay, these natural forces provide a rare chance to observe sinter growth and decay in real-time, offering a living analog to how the White and Pink Terraces developed over thousands of years and how they may have been gradually eroding even before their abrupt destruction in the 1886 Tarawera eruption.
Image credit: Waimangu Volcanic Valley (Pink & White Terraces)
At the site of this Earthcache, you are able to observe layers where the mineral formation and erosion has occured, seeing how the natural forces have continued to shape and reshape the Earth’s surface over time. You’ll see a fascinating geothermal landscape featuring steaming vents, bubbling mud pools, and colorful mineral deposits along the shoreline. Look closely at the edges of the pools and terraces to observe unique textures and vibrant hues created by natural geothermal processes. As you explore, you’ll notice how the landscape is continuously changing—where minerals build up in some areas and erode away in others—offering a rare glimpse into the dynamic forces shaping this volcanic environment.
Please respect this area and obey signs as this is an active geo-thermal area. There is no need to leave the path to access and complete this Earthcache. Please actively supervise children.
References used to create this Earthcache (and for further reading):
GNS Science
Geothermal Energy in New Zealand - NZ Ministry for the Environment
Pink & White Terraces - UNESCO World Heritage
The Growth of Siliceous Sinter Deposits Around High-Temperature Hot Springs - Science Direct (Mountain & Wood)
Hot Springs, Mud Pools and Geysers - Te Ara Encyclopedia NZ
Microbial biofacies in hot-spring sinters: a model based on Ohaaki Pool, North Island, New Zealand - Science Direct (Jones et., al.)
To log this Earthcache:
1. Post a photo of yourself (or your GPS or other identifying object) with the view over Sulphur Bay in the background.
2. Observing Silica Deposition: Look carefully at the edges of the geothermal pools or terraces.
Describe the texture and color of the mineral deposits you see. What mineral is likely forming these deposits, and how is it created?
3. Signs of Erosion or Change: Choose an area with visible mineral deposits or sinter formations.
Describe any signs of erosion or weathering, such as flaking, cracks, or runoff patterns. What natural processes might be causing this breakdown?
4. Learning from Sulphur Bay: The White Terraces were created by similar geothermal and volcanic processes.
Explain how observing Sulphur Bay helps you understand how the White Terraces formed, evolved, or were lost.
You do not need to wait for a reply before logging this cache.