Plate 1 has pictures of typical rocks from Sonoma County. Magma may flow across the surface as lava, commonly trapping gas seen as round holes in dark rocks of the Sonoma Mountains. The fine-grained white rocks around the Petrified Forest trees is ash or tuff that fell from the air after a nearby volcanic explosion. Welded tuff is a hard rock formed by ash clouds which sweep down the sides of volcanos and are welded by hot glass. This resistant rock forms the heights of Mount Saint Helena, exploded from a volcano 2 million years ago.
Plutonic rock cools slowly deep within the earth so the crystals are larger.
Salt and pepper textured granite and diorite are typical rock types on Bodega Head, west of the San Andreas Fault. Rock at the surface is exposed to the weather, and can break down into solid fragments or be dissolved or changed by weak acid in water. Light colored sand on the beach at Bodega Head comes from the minerals in the granite bedrock loosened by the weather and blasted by storm waves. Rain, running water, wind, glaciers and landslides can move solid and dissolved pieces of rock down slope to be deposited in the ocean, in lakes or rivers and in sand dunes. These surface processes commonly form layers of loose sediment which become solid sedimentary rock.