A map of Mormon Lake and surrounding geologic features. The m.y. marks are age of rock in millions of years. Ball is on the downside of the faults. Base map by National Geographic.
This view is west from sheet lava flows above the fault scarp to Mormon Lake, Bass Point and Mormon Mountain, circa 1987.
Courtesy
A map of Mormon Lake and surrounding geologic features. The m.y. marks are age of rock in millions of years. Ball is on the downside of the faults. Base map by National Geographic.
Arizona has a lot of lakes. When full, some lakes are among the largest in the country. Almost all of Arizona’s lakes are in valleys or canyons that are blocked on the downstream end by dams. Only a handful of lakes, maybe just two, are in basins formed by natural processes. Mormon Lake is one of these natural lakes; Stoneman Lake is also a natural lake.
Geologists classify Mormon Lake as a volcano-tectonic depression. That means the lake basin is bound by volcanoes and geologic structures such as faults. Mormon Lake is on the small end of the scale of such features.
The east side of the lake basin is confined by a fault scarp up to 240 feet high that truncates horizontal sheet lava flows. Fault scarps along the narrow trough at the north end of the basin truncate sheet lavas and lava flows from Pine Grove Hill cinder cone. Movements on the east and north bounding faults between six million years ago (m.y.), the age of the sheet lavas, and 3.9 m.y., the age of a cinder cone that overlies the fault south of the lake, established over half of the basin’s perimeter.
The south and southwest sides of the lake basin are bound by a northwest-trending row of three overlapping shield volcanoes that are topped by cinder cones. After the shield volcanoes erupted around 4.5 to 4.1 m.y. the south and southwest sides of the basin were defined. At this time there was no enclosed basin, and water could flow out, either west to Oak Creek if it could find a way through the volcanic field, or, more likely, north to Walnut Creek.
Mormon Mountain completed the encirclement of the basin when it erupted at 3.1 m.y. The large lava dome closed the basin on the west side, and Mormon Lake started to fill.
Mormon Lake has not filled to capacity and overflowed during the years since settlers arrived in the area in the late 19th century. The lake might have overflowed in the distant past, however, probably during the wet glacial periods. A possible spillway at the north end of the narrow trough is a shallow gap in the rim at about 7,180 feet elevation, the lowest on the basin's perimeter. This gap is only about 200 feet long across the divide to the north side where a deep gully is cut in the side of Walnut Creek.
Mormon Lake village, at 7,120 feet elevation, would be under about 60 feet of water if the lake filled and overflowed into the headwater of Walnut Creek. Several factors might account for historical water levels that stay below the rim: 1: A small catchment area that supplies insufficient inflow to fill the basin. 2: A leaky bottom that loses water down the faults. 3: Climate change to drier conditions.
Water levels of the lake rise and fall with seasonal weather and long-term climate changes. In the past the lake has dried up, and westerly winds scoured the lake floor, as witnessed by dust deposits (loess) on the sheet lava flows east of the basin. In modern times the lower parts of Mormon Lake village have flooded.
If the climate scientists are correct about their predictions for a warmer and drier climate in the southwest, the property owners in Mormon Lake village might feel like holding off on buying flood insurance.
Richard Holm is an emeritus professor at Northern Arizona University.
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