The present day serenity of Sloan Canyon National Conservation Area contrasts to its violent volcanic past. Sloan Canyon contains one of the best records of volcanic activity in the Las Vegas area and contains at least eight volcanoes that erupted from time to time millions of years ago. Volcanoes that have not erupted for a while, but which may erupt again, are considered dormant. Volcanoes that have not erupted for thousands of years are called extinct. All the volcanoes at Sloan Canyon are extinct.
Petroglyph Canyon is recognized for the abundance of petroglyphs in this area. Petroglyphs contain designs that have been scratched, etched, or pecked into a rock surface by early Native Americans. The Petroglyph Canyon area is also home to 13 million year-old volcanoes of the Sloan Volcanic Section. The Sloan Volcanic Section consists of four volcanoes called Mount Hanna, Mt. Ian, Mount Sutor, and the Center Mountain Dome, and covers an area of 30 square miles. Petrogylph Canyon cuts through the Mt. Sutor volcano, composed of many coalescing volcanic domes. These volcanic domes are composed of viscous (sticky) lava that squeezes out of a volcanic vent like toothpaste. The lava rarely flows but stands above the vent and forms a volcanic dome. Domes can be over a mile in diameter and may be several thousand feet high. Good examples of volcanic domes which are modern are found in the Mono Craters just north of Bishop, California, and in the summit crater of Mt. St. Helens in Washington State. These volcanic domes are similar to the Mt. Sutor volcanic dome complex.
Rocks in Sloan Canyon have a rich volcanic past, and are classified as igneous. Igneous rocks (derived from the Latin word igneus, meaning “of fire”) are one of the three rock types, the others being sedimentary and metamorphic rock. Igneous rocks are formed through the cooling and solidification of magma (molten rock) either below the surface as intrusive rocks, or on the surface as extrusive rocks. Dacite is the most abundant type of rock in this area. Dacite is a fine-grained extrusive igneous rock. It is generally gray in color, but may range from nearly white to black. It may also vary from brown to reddish brown, depending on the amount of iron oxide in the rock. Dacite contains black mica (biotite) and white feldspar (plagioclase). When biotite oxidizes, it takes on a golden sheen under direct sunlight and is referred to as brassy biotite.
If you look carefully on your hike up the canyon,you may observe areas of vertical banding. The vertical banding reflects flow patterns in the rock, and in general suggests that magma in the domes was emplaced vertically.
As well as vertical flow banding, in places the rock is also cut by vertical fractures. These fractures are formed by cooling or later tectonic forces and commonly cut across flow banding. The vertical fractures are more common in the hard glassy rock that cooled rapidly. These fractures are seen in the petroglyph gallery area.
Another prominent geologic feature is called rock stripes. They are parallel lines of large stones, and intervening stripes of dominantly finer material oriented down steep slopes. They were formed during a wetter and colder period during the Pleistocene Epoch and are due to frost action. The rocks are covered with desert varnish, also called desert patina which is a dark brown color.
Hidden Valley is unique and differs from other areas in Sloan because of the unique geological features. It contains dikes, breccia, columnar and tectonic joints, and spheroidal weathering. These features are the result of the numerous volcanoes that occurred here long ago.
There are a number of dikes
located throughout this area. Dikes are sheet-like bodies of magma that cut through layers of rock. Dikes may form as vents in volcanic cones and are often vertically oriented.
Breccia, which is the Italian word for broken rock, can be seen at various places along this trail. Breccia is a rock similar to a conglomerate that can form in many ways. Volcanic Breccia in this area is formed by the crumbling of volcanic domes. Domes are very steep- sided and unstable and they tend to slump and sometimes explosively disintegrate. These processes result in a variety of rock fragments cemented together into a fine-grained groundmass called breccia.
Both columnar and tectonic joints are also seen here along the trail to Petroglyph Canyon. Columnar joints are formed through the cooling of hot rock masses, particularly lava. Tectonic joints or cracks form in hard rock that is stretched so greatly to the point that its brittle strength is exceeded. After this happens, the rock breaks into several pieces either at right angles or at some angle to the direction of stretching.
Another prominent geological feature to look for is spheroidal weathering. The process of spheroidal weathering is slower than other common types of weathering. Spheroidal weathering is a type of chemical weathering that creates rounded boulders. The rocks form at great temperatures such as 760 degrees Celcius and usually three sets of joints develop. When the rocks cool down and water flows through the rocks, the acid in the water combine with the minerals, and attack the angles of the rocks. Spheroidal weathering causes planes on the rock where water flow can penetrate from three directions and leads to the formation of rounded corners.