North America. At its height during the most recent ice age, the lake would have been about deep. Prominent old shore lines, called strandlines by geologists, can be seen west of the Lake. Mono Lake is in a geologically active area at the north end of the Mono–Inyo Craters volcanic chain and is close to Long Valley Caldera. Volcanic activity continues in the Mono Lake vicinity: the most recent eruption occurred 350 years ago, resulting in the formation of Paoha Island. Panum Crater (on the south shore of the lake) is an example of a combined rhyolite dome and cinder cone. Tufa towers Many columns of limestone rise above the surface of Mono Lake. These limestone towers consist primarily of calcium carbonate minerals such as calcite (CaCO3). This type of limestone rock is referred to as tufa, which is a term used for limestone that forms in low to moderate temperatures. Tufa tower formation Mono Lake is a highly alkaline lake, or soda lake. Alkalinity is a measure of how many bases are in a solution, and how well the solution can neutralize acids. Carbonate (CO32-) and bicarbonate (HCO3−) are both bases. Hence, Mono Lake has a very high content of dissolved inorganic carbon. Through supply of calcium ions (Ca2+), the water will precipitate carbonate-minerals such as calcite (CaCO3). Subsurface waters enter the bottom of Mono Lake through small springs. High concentrations of dissolved calcium ions in these subsurface waters cause huge amounts of calcite to precipitate around the spring orifices. The tufa originally formed at the bottom of the lake. It took many decades or even centuries to form the well-recognized tufa towers. When lake levels fell, the tufa towers came to rise above the water surface and stand as the majestic pillars seen today (see Lake Level History for more information). Tufa morphology Description of the Mono Lake tufa dates back to the 1880s, when Edward S. Dana and Israel C. Russell made the first systematic descriptions of the Mono Lake tufa. The tufa occurs as "modern" tufa towers. However, there are tufa sections from old shorelines, when the lake levels were higher. These pioneering works in tufa morphology are still referred to by researchers today and were confirmed by James R. Dunn in 1953. The tufa types can roughly be divided into three main categories based on morphology: Lithoid tufa - massive and porous with a rock-like