Andrew Scaplen's Geopostcards

The Mudpots of Rotorua (Wai-o-Tapu)

The mudpots are a very hot geothermal area and can reach temperatures around 800º C. The heat comes from the Hikurangi Trough where subduction is occurring. This heats up water underneath the surface of the earth, which rises to the surface over the colder water because the hot water is less dense in comparison to the hot water. This unique geothermal activity produces constant bubbling, but periodic violent eruptions have been known to occur. The hot water from the subduction turns the ash and glass into mud, which bubbles to the surface in order to release the heat. The mud is made from bentenite and smectite, two groups of clay minerals. The glass and ash form clay when they come in contact with the hot water. This expandable matter forms the mud pools when the mud accumulates because the mixture of the glass in the clay and the hot water is unstable. Once the mud cools, it leaves a thin layer of hardened clay at the surface and around the edges of the mudpots. The mud has a greasy feel to it and is used by the cosmetic industry to create mud masks that dry out facial pores, leaving smooth, clear skin. The sulfur emerging from the bubbling mudpots leaves an unpleasant smell similar to rotten eggs. Currently the mud level is particularly low, but the thickness of the mud usually changes with seasonal changes.

Kura Tawhiti Conservation Area

The rocks near Castle Rock are made up of limestone, a sedimentary rock formed by the deposition of sediment layers of calcium and carbon over a long period of time. The rocks were deposited their by glacial movements from the last ice age. We know that they must have come from the last ice age because the soft limestone would not have survived multiple glacial advances. This gives them the age of approximately 12,000 year old (post glacial). The layers of the rocks are chemically weathered, forming a scalloped appearance. There are broken joints where water has percolated into the rock ad corrosion of the inside of the rock has occurred. Rain water seeping into the ground gradually weathered the bedrock forming cracks and joints and once this bedrock became exposed, particularly during the ice age, the freezing water expanded in the cracks and accelerated the weathering process. The ice molded topography also shows evidence of glaciation such as glacial groves and smoothed topography. You can also see signs of glaciation in the rocks from the striations in the soft limestone formed by moving glaciers that have formed abrasions. The repeated freezing and thawing of ice has formed an accumulation of water where constant weathering is occurring cyclically. Some ways to show that this rock is limestone without chemically testing it in the lab is the abundance of fossils which are very common to limestone. Another easy way to tell that this is limestone in the field is that limestone easily dissolves in an acid, so by putting some acidic (low pH) solution on the rock, if the rock fizzes, you know it is limestone.