The Cascade Range consists of the hills and mountains north of Chico and well into Washington State. The range, a volcanic arc, developed by subduction of the San Juan De Fuca and Gorda plates underneath the North American plate. Water is dragged in along with oceanic crust, lowering the melting temperature of subducting rocks. This creates magmatic bodies that are less dense than surrounding continental rock and travel up towards the surface. Where they break the surface, volcanoes are formed. As with most volcano arcs that develop by subduction, the cascades are generally intermediate (andesetic) in composition. This means that lava flows tend to be very viscous (thick) and can lead to explosive events due to trapped, pressurized gas. Mount Lassen is a plugdome volcano, while Mount Shasta is a composite cone or stratrovolcano. These types tend to erupt violently, as was illustrated by Mount Saint Helens in 1980.
If Mount Lassen ever erupts, it is unlikely that Chico will be affected by anything other than ash fall. Contrary to some legends, the prodigious supply of rocks surrounding Chico that have been used to build long-lasting walls are not from a volcanic eruption. Read more about this in Geology.
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