Cinder cones are a common type of volcano. They are relatively small and composed of basalt, a dark fluid lava that is rich in iron and magnesium. The basalt is also scoriaceous (frothy), as the magma is typically rich in gass. The eruption starts out as fountains of fire with frothy basalt cinders falling out around the vent to build up into a cone. Other material is usually incorporated into the ascending magma, so big crystals and even bits of the surrounding rock (the “country rock”) are also ejected with the cinders. Occasionally a large blob of lava, a so-called "bomb," will be thrown out as well. Commonly the last eruptive activity is a flow of basalt lava that pushes its way out the base of the cone through the loose cinders. Mexico's Paricutín, which erupted in the 20th century, is a modern example.
Although not completely symmetric, due to the vagaries of the winds blowing the cinders around, cinder cones are usually not markedly asymmetric, and are often one-off events. Easy Chair Crater is unusual, however, in its asymmetric form. It is not just oval, but has the northeast rim some 550 feet higher, which gives it the chair-like form for which it is named.
This seems to be a case where a second eruption did occur almost on top of the original one. The older crater is the higher part to the northeast, while the younger is in the lower part to the southeast.
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