SP Crater, located approximately 25 miles north of Flagstaff, Arizona, is classified as a Cinder Cone volcano. It is situated within the San Francisco Volcanic Field, a region containing hundreds of volcanoes. Its well-preserved, symmetrical shape and dark, distinctive lava flow make it a prime example of this type of volcano. The formation of SP Crater is linked to the explosive style of eruption that characterizes all cinder cone volcanoes.
Defining the Cinder Cone Volcano
A cinder cone, also known as a scoria cone, represents the simplest and most common type of volcano. These volcanoes are constructed almost entirely from loose pyroclastic fragments, which are chunks of magma that solidify in the air. The primary material is scoria, a dark, vesicular, or bubble-filled, igneous rock that resembles cinders, giving the cone its name.
The formation process begins with a single, typically cylindrical vent through which gas-charged, basaltic magma is violently ejected. This explosive release, often described as a fire fountain, breaks the molten rock into small pieces that cool quickly as they fly through the air. These fragments of cinders and scoria rain down around the vent, accumulating to form a steep, conical hill.
Cinder cones are characterized by their steep slopes, which typically rest at the angle of repose for loose material, ranging between 30 and 40 degrees. They are relatively small compared to massive shield or stratovolcanoes, rarely rising more than a few hundred meters above their surroundings. Most cinder cones have a bowl-shaped crater at their summit, marking the location of the central vent.
These eruptions are typically short-lived, with the volcano forming rapidly during a single, sustained eruptive episode. Once the gas pressure dissipates, the eruptive activity usually ceases, giving cinder cones a comparatively brief lifespan in geological terms. This short duration means they are highly susceptible to erosion over time, making fresh-looking examples like SP Crater particularly notable.
SP Crater’s Specific Geological Characteristics
SP Crater stands out in the San Francisco Volcanic Field due to its remarkably pristine appearance. The cone itself rises approximately 820 feet (250 meters) above the surrounding plains, culminating in an almost perfectly symmetrical form. Its summit features a deep, bowl-shaped crater that measures roughly 400 meters across and about 60 meters deep.
The volcano is composed of basaltic andesite material, with the black scoria and cinders forming the steep flanks of the cone. A notable feature is the agglutinate rim that caps the summit, where volcanic fragments were still partially molten when they landed and fused together. This hardened, welded rim has helped the structure resist erosion, contributing to its sharp, youthful outline despite an age estimated to be around 5,500 to 6,000 years.
Most distinctively, a massive, dark lava flow extends approximately 4.3 miles (7 kilometers) northward from the base of the cone. This flow, known as the “SP flow,” is a blocky, rough ‘a’ā lava type, which formed when the low-viscosity basaltic lava erupted. The lava appears to have issued from the same vent area, likely pouring out from a breach at the base of the cone after the initial explosive phase created the structure.
The stark contrast between the symmetrical black cone and the long, dark, unvegetated lava field makes SP Crater a prominent landmark in the region. The relationship between the cone and the flow is complex, with some geochemical data suggesting the flow slightly predated the cone’s final formation, where the cone overlaps the lava field without deformation. This interplay of explosive cinders and effusive lava flows is a common sequence in the life cycle of many cinder cone volcanoes.