Worlds Deepest Hot Spring

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1. As you peer into the spring look at the bottom of the pool what type of rock do you see.

2. Is there a mixture of Tuffa and Travertine in the area?

3. Looking at the water what color does the spring appear to be is there multiple colors and what does that tell you about water temp and how its heated?

The first deposits are called the Dakota Sandstone. This sandstone is a tough, compact rock, yellowish in color on fresh breaks, reddish on long exposed surfaces.

As the sea deepened, 2000 – 4000 feet of Mancos Shale was deposited. In some places the shale is crisp and flaky, and there are some beds of limey sandstone. Because the Mancos Shale was laid down in oxygen poor waters, organic matter that fell to the bottom did not oxidize; this is why the shale is darkly colored. The seashore temporarily retreated. Rocks of the Mesa Verde Group were laid down on land when the sea went out. The first was the Point Lookout Sandstone. Then came the Menefee Sandstone. During Menefee time, in ponds and swamps near the sea shore vegetation grew, died and was buried to become coal. The sea advanced westward again, and the Menefee was overlain by the Cliff House Sandstone, the uppermost member of the Mesaverde Group. The Cliff House Sandstone is another beach and near-shore deposit similar to the Dakota, and was overlain in turn, as the water deepened, by the Lewis Shale. The Lewis was deposited in the same conditions as the Mancos, and accordingly looks just the same. A layman would be hard pressed to distinguish one from the other.

A slow-motion cataclysm named the Laramide Orogeny began in Jurassic times west of the center of the continent, but in the north-south band that includes Colorado, it was not felt until the end of the Cretaceous Period. Compression created by continental drift pushed up highlands including the San Rafael Swell, the Kaibab Uplift and the Monument Upwarp. Those were soon followed by the San Juan Dome and the Rocky Mountains.

By the Early Tertiary, the San Juan Mountains were 20,000 feet higher than the San Juan and Paradox Basins on the south and west, also products of the orogeny, and erosion had exposed Precambrian rocks in the peaks. As the land rose, of course, the sea retreated. volcano 485 b Another period of uplift followed, raising the Colorado Plateau another 5000 feet, and all hell broke out in the San Juan Mountains. About 40 million years ago volcanoes began belching ash and lava over the area. Ten million years later, the eastern San Juans were a vast volcanic plateau dotted by stratovolcanoes erupting ash at intervals to further bury the landscape. As the action moved to the south and west, eruptions became more cataclysmic. Enormous blasts of rhyolitic ash were followed by the collapse of the volcanoes into calderas ten to twenty miles wide above the emptied magma chambers.

The mother of all volcanoes erupted 27.8 million years ago, creating what is believed to be Earth’s largest caldera. The La Garita volcano seems to have erupted over the course of less than a week as much as 3,000 cubic miles of magma. This material, the Fish Canyon Tuff, makes up the La Garita Mountains and is found over a wide area north of Pagosa Springs. The “kill-zone” of this eruption probably extended well into Kansas. The resultant caldera is a rough oval about 22 by 50 miles wide, extending from Saguache Park across Creede and south into the Weminuche Wilderness west of Wolf Creek Pass. Fish Canyon Tuff can be viewed along Highway 160 on the pass. Unlike the much younger Valle caldera near Los Alamos, New Mexico, where you can see part of the crater’s walls, the La Garita caldera was obscured by subsequent volcanism, which continued over the next ten million years until the San Juans became quiescent.

Because it is mostly buried, it has taken geologists about thirty years to discover the true size of the La Garita Caldera; the southernmost portion was only identified in 1995. Following the volcanism, erosion cut deep valleys and canyons into the tuff and lava. Then, during the Pleistocene, a period of continental glaciation, ice caps formed over much of the San Juans. When the ice melted about 15,000 years ago, the mountains were left with over-steepened valleys where landslides and avalanches are common.

Hot springs are found where there has been geologically recent volcanism. There are several in the San Juans. The one at Pagosa Springs is the largest. Here, heavily mineralized waters reach the surface at more than 150 degrees. The water contains hydrogen sulfide, which accounts for the rotten egg odor you sometimes smell when you come into town. It also contains a large amount of dissolved silica [quartz] that precipitates as tufa when the water evaporates. [Tufa is similar to travertine.] You can see a part of these deposits across the river from the municipal parking lot. Man-made structures obscure the fact that the buildup of spring deposits was responsible for the bend in the river here. Although the spring discharges water at a rate of about 700 gallons per minute, it is used in the pools at the motels and to heat several buildings in town, so it no longer overflows to deposit more tufa along the river.

Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) mineral particles or rock fragments. Typically quartz and feldspar; lithic fragments are also common. Other minerals may be found in particularly mature sandstone. Most sandstone is composed of quartz or feldspar because they are the most resistant minerals to weathering processes at the Earth's surface, as seen in Bowen's reaction series. Like uncemented sand, sandstone may be any color due to impurities within the minerals, but the most common colors are tan, brown, yellow, red, grey, pink, white, and black. Since sandstone beds often form highly visible cliffs and other topographic features, certain colors of sandstone have been strongly identified with certain regions. Rock formations that are primarily composed of sandstone usually allow the percolation of water and other fluids and are porous enough to store large quantities, making them valuable aquifers and petroleum reservoirs. Fine-grained aquifers, such as sandstones, are better able to filter out pollutants from the surface than are rocks with cracks and crevices, such as limestone or other rocks fractured by seismic activity. Quartz-bearing sandstone can be converted into quartzite through metamorphism, usually related to tectonic compression within orogenic belts.

Shale is a fine-grained, clastic sedimentary rock composed of mud that is a mix of flakes of clay minerals and tiny fragments (silt-sized particles) of other minerals, especially quartz and calcite. The ratio of clay to other minerals is variable. Shale is characterized by breaks along thin laminae or parallel layering or bedding less than one centimeter in thickness, called fissility. Mudstones, on the other hand, are similar in composition but do not show the fissility.

Modern tufa is formed from alkaline waters, supersaturated with calcite. On emergence, waters degas CO2 due to the lower atmospheric pCO2 , resulting in an increase in pH. Since carbonate solubility decreases with increased pH, precipitation is induced. Supersaturation may be enhanced by factors leading to a reduction in pCO2, for example increased air-water interactions at waterfalls may be important, as may photosynthesis. Calcite is the dominant mineral precipitate found; however, the polymorph aragonite is also found.

Modern travertine is formed from geothermally heated supersaturated alkaline waters, with raised pCO2 . On emergence, waters degas CO2 due to the lower atmospheric pCO2, resulting in an increase in pH. Since carbonate solubility decreases with increased pH, precipitation is induced. Precipitation may be enhanced by factors leading to a reduction in pCO2, for example increased air-water interactions at waterfalls may be important, as may photosynthesis. Precipitation may also be enhanced by evaporation in some springs. Both calcite and aragonite are found in hot spring travertines; aragonite is preferentially precipitated when temperatures are hot, while calcite dominates when temperatures are cooler. When pure and fine, travertine is white, but often it is brown to yellow due to impurities

Now a little about hot springs themselves. Let's start at the center of a hot spring, a brilliant aquamarine. The center of the spring is just above its underground water source, and it's where temperatures are the highest — up to 189 degrees Fahrenheit (or 87 degrees Celsius), Smithsonian reported. There, the water is too hot for most microbial growth. It is, therefore, mostly clear water. The center of the spring is blue for the same reason that the sky is blue: When sunlight hits the water's surface, the light scatters, and blue light scatters the most, meaning that's what reflects back to your eyes. The hot spring's water cools as it spreads farther from the source, and that, in turn, changes the bacteria that can live in it.

Water issuing from a hot spring is heated geothermally, that is, with heat produced from the Earth's mantle. In general, the temperature of rocks within the earth increases with depth. The rate of temperature increase with depth is known as the geothermal gradient. If water percolates deeply enough into the crust, it will be heated as it comes into contact with hot rocks. The water from hot springs in non-volcanic areas is heated in this manner. Steam Crepuscular rays at Mammoth Hot Springs In active volcanic zones, water may be heated by coming into contact with magma (molten rock). The high temperature gradient near magma may cause water to be heated enough that it boils or becomes superheated.