The skyline of Cima Dome is interrupted by the conical outlines of dozens of remarkably well-preserved volcanic cinder cones and black basalt lava flows. The earliest began about 7.6 million years ago and eruptions continued until at least 10,000 years ago, near the end of the most recent ice age.
Unlike the violently explosive eruptions that created the rocks of Hole-in-the-wall, cinder cones form when lava erupts as relatively benign liquid fountains. As lava is spewed through the air, it solidifies instantly, often preserving bubbles created by escaping gasses. If an eruption of this type continues long enough, fragments accumulate to form a cinder cone.