Geologically, the volcanic activity that took place in Arizona during the last four million years is very young. Those volcanic fields that are less than four million years old should probably be considered dormant, but not extinct. Volcanoes do not erupt without giving some type of warning. The land surface might even bulge slightly. Shallow magma heats ground water, commonly enough to cause it to boil.
Hot springs are common.
In addition, gases such as carbon dioxide that are associated with magma might bubble from the ground and, if sufficiently concentrated, even cause plants to die. Although swarms of small earthquakes have occurred near the Grand Canyon, seismologists do not attribute them to the movement of magma. No evidence suggests that magma is currently moving toward the surface in Arizona. The river originally flowed from the upper right toward the lower left. The red line marks the contact between basalt and the underlying bedrock.