Round Top dike

Now walk around the left side of the water tower, and carefully observe the rock face above you. Note the varying textures of the rock, such as the spheroidal structures and the crumbly, gravely folded areas. In front of the interpretive sign is a vertical band of dark rock. (There is a substantial amount of lichen and buckwheat growing here, so try to find spots where fresh rock is exposed.) This dark, basalt band is one of the most interesting features of the park, particularly to a vulcanologist: it represents a dike, the path that upwelling magma once took through the interior of this volcano. You are standing in the caldera of the volcano, and the dike represents one of the major artery. It is very rare to have such a close view as this. The rocks surrounding the dike include gravelly mudstones, tuff-breccias (consolidated volcanic ash and rubble), and more basalt. Some of the volcanic rocks contain rounded structures with concentric layers. These formed via spheroidal weathering, where chemical weathering and temperature fluctuations created a layered-onion appearance. All of these volcanic rocks are part of the Moraga Formation. A key feature of volcanic rocks is their small grain size--try to see if you can pick out individual crystal grains on exposed rock faces, and consider how their size compares to those you may have seen on plutonic rocks, such as granite (think about the big crystals in a granite countertop). Why do you think volcanic rocks have smaller grain sizes than plutonic rocks like granite which formed from magma that cooled and solidified underground?