Igneous activity associated with the extension occurred from 12 to 4 Ma. Both intrusive (plutonic/solidified underground) and extrusive (volcanic/solidified above ground) igneous rocks were created. Basaltic magma followed fault lines to the surface and erupted as cinder cones and lava flows. Some volcanic rocks were re-worked by hydrothermal systems to form colorful rocks and concentrated mineral formations, such as boron-rich minerals like borax; a Pliocene-aged example is the -thick Artist Drive Formation. Gold and silver ores were also concentrated by mineralizing fluids from igneous intrusions. Other times, heat from magma migrating close to the surface would superheat overlaying groundwater until it exploded, not unlike an exploding pressure-cooker, creating blowout craters and tuff rings. One example of such a feature is the roughly 2000-year-old and deep Ubehebe Crater (photo) in the northern part of the park; nearby smaller craters may be less than 200 to 300 years old.