At nearly 1,000 years young, Sunset Crater volcano is a geological infant, the latest development in a series of eruptions that have taken place here over the past 6 million years. It is only a small part of the impressive San Francisco Volcanic field of northern Arizona.
From this point, three different types of volcanoes are visible. What makes a volcano tall or short, steep or gentle, or an eruption more or less violent? Mostly the magma’s stickiness or viscosity, which is determined by its chemical composition and gas content.
Cinder cones, like Sunset crater and most of the 6000 or so volcanoes in this field, tend to be cone shaped with sides not steeper than 33 degrees. A cinder cone is literally a pile of loose fragments. It is easily eroded and will change shape, becoming less steep as it ages.
Meghann M. Vance, Northern Arizona University Anthropology Laboratories