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The volcanic cones were formed 130000 years ago near the end of a series of eruptions. A crack in the Earth's surface, called a fissure, more than 8 km long and a few km deep, opened up and spewed out magma in a "curtain of fire". Most of the lava from the many eruptions flowed toward the Rio Grande, forming a cap over the softer sandy soils. As cooling lava blocked the fissure, small openings or vents formed. From these vents came accumulations of ash, cinder and lava, creating the cinder cones we see today.