Amboy Crater is classified as a cinder cone volcano. This type of volcano is characterized by its simple, symmetrical structure, built primarily from fragmented volcanic material rather than continuous lava flows.
Amboy Crater’s Specific Classification and Structure
Amboy Crater is a classic, nearly symmetrical cinder cone. It is also known as a scoria cone, named for the loose, fragmented pyroclastic materials (ash and cinders) ejected during an explosive eruption. The accumulation of these fragments around the central vent leads to the cone’s signature steep slopes, which typically rest at angles between 30 and 40 degrees due to the angle of repose of the loose debris.
The structure rises approximately 250 feet (76 meters) above the surrounding desert floor, reaching an overall elevation of 944 feet (288 meters) above sea level. At its base, the cone measures about 1,500 feet (457 meters) in diameter, with a prominent bowl-shaped crater at its summit. A notable feature is a break, or breach, on the western side of the cone, which is where basaltic lava flows escaped and poured out across the plain during the final stages of the eruption.
The composition of the cone is primarily basaltic cinders, which are dark, vesicular (bubbly) fragments of rock that cooled quickly mid-air. These materials accumulated in layers, giving the cone a nested structure that suggests at least four distinct eruptive episodes over a short period.
Geological Context and Timeline of Activity
The volcano is situated in the eastern Mojave Desert of California, within the modern-day Mojave Trails National Monument. Amboy Crater is the source vent for the larger Amboy Lava Field, a dark expanse of basaltic rock that covers an area of approximately 27 square miles (70 square kilometers). This lava field is composed of low-viscosity basaltic flows, including both blocky ‘a’ā and ropy pahoehoe types, which spread out over the flat desert terrain.
Geological dating places the primary period of activity for Amboy Crater in the late Pleistocene epoch. Cosmogenic 36Cl dating methods applied to the volcanic deposits estimate the cone’s age to be approximately 79,000 years old, with a margin of error of about 5,000 years.
The last major activity is considered to have occurred around that time, and the crater is currently classified as dormant. The area’s volcanism is linked to the extensional tectonics of the Basin and Range Province, where the stretching of the Earth’s crust facilitates the rise of magma.
How Cinder Cone Volcanoes Form
Cinder cone volcanoes are formed through a specific process driven by gas-rich magma. The formation begins when molten rock, typically basaltic in composition, containing a substantial amount of dissolved volcanic gas, rises toward the Earth’s surface. As this magma approaches the vent, the sudden drop in pressure causes the dissolved gas to expand explosively.
This forceful expansion shreds the molten rock into a spray of liquid fragments, launching them into the air. As these fiery pieces arc through the atmosphere, they cool rapidly and solidify into lightweight, porous fragments known as cinders or scoria. These cinders then fall back to the ground, accumulating around the vent in a symmetrical fashion to build the cone.
The majority of cinder cones are considered monogenetic, meaning they result from a single, relatively short-lived eruptive episode that can last from weeks to a few years. This brevity in their eruptive history is the primary reason cinder cones are generally small, rarely exceeding 1,000 feet in height. After the explosive phase, a final, less-gassy batch of magma may emerge, often burrowing through the base of the cone to form the surrounding lava field, as was the case with Amboy Crater.