Lisa Angotti's Geo Postcards

The above photo, taken at Wai-o-tapu Geothermal Field in Rotorua on the North Island of New Zealand, shows an area called the Primrose Terraces. The millions of small terraces, which appear as small 'steps' in the sediment, are formed when water overflows from the Champagne Pool, another nearby formation. The Champagne Pool is a collection of hot (74 degrees Celsius) water filling a 62 metre deep explosion crater lined with sinter, a chemical sediment or crust deposited by a mineral spring (Random House Unabridged Dictionary). This sinter dissolves into the hot water as high-concentration silica. When the water from the Champagne Pool overflows, spilling over the area now known as the Primrose Terraces, it cools as it travels, and much of this silica precipitates out as sinter, coating the landscape and forming these terraces. The Primrose Terraces cover about 3 acres, making them the largest in the southern hemisphere, and they have been forming for about 700 years since the formation of the Champagne Pool.






The photo above shows an interesting rock type known as pumice. This rock is an extremely light and porous piece of volcanic debris, formed when volcanic lava cools quickly, leaving large pockets of air trapped inside. The lava hardens around these air pockets, leaving large holes in the rocks formed. Because they have so many air pockets, the rocks are extremely light. For example, the pumice in the picture above, larger than the palm of my hand, weighed little more than a sponge of the same size. Because pumice is so light and has a very low density (due to the large amount of air pockets in the volcanic material), pumice floats (see picture below).

The above photo is of the same piece of pumice floating in Lake Taupo. Because the pumice floats, it is easily transported by water movement, even at very low velocities. Because of this unique feature of this rock type, locations of pumice deposits are often used to identify locations of former water movement. In the case of Lake Taupo, pumice deposits far offshore from the current location of the lake are one of the factors used in identifying terraces which mark former lake levels.