Stop #2

Figure #4: This figure shows the location of Stop #2, it is at the base of the volcanic neck.
Above Figure #4: This figure shows the location of Stop #2, it is at the base of the volcanic neck.

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Calavera Hills Volcanic Plug and Neck

Stop #2 (33 degrees 10’ 8” N and 117 degrees 17’ 5” W)

Stop #2 is midway up the hill and at the base of the Calavera Hills Volcanic Plug. The rocks that comprise the “volcanic neck” are very resistant to weathering relative to the surrounding material. So the volcanic neck is what remains of the volcano after the surrounding sediment and pyroclastic material have been stripped away. It is composed of a fine-grained (aphanitic), “mouse gray” rock that is dacitic in composition.

This is a close-up of the Calavera Hills volcanic neck. Note the medium grey material in the center. This is the exposed volcanic neck.

Above Picture: Stop #2, this is the Calavera Hills volcanic neck material.

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Volcanic Neck

The mineral grains of this unit cannot be seen with the naked eye. The rocks here are “mouse gray” and considered intermediate in color. It probably has a similar mineralogy as the rock unit at Stop #1 but the cooling history is very different. These rocks were expelled out of the volcano and are very commonly associated with subduction zones as in the Cascade and Andes mountain ranges. In summary, this unit has an aphanitic texture, is an extrusive igneous rock with a composition of dacite. Below are a series of photographs showing the fine-grained dacitic volcanic unit.

This is the first of a series of 2 pictures that shows the gray fine-grained (aphanitic) unit. This one is from a distance.

This is the first of a series of 2 pictures that show the gray fine-grained (aphanitic) unit. This one is a close-up.

Above Pictures: These 2 pictures show the fine-grained dacitic unit at a distance and then as a close-up. The orange you see in this picture is localized iron staining.

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Columnar Jointing

Notice the near vertical, long, linear features near the top of the ridge. This is called columnar jointing. The columns form due to stress as the lava cools from the base upward and from the top downward. The columnar joints are generally six-sided as you can see in the last 2 pictures of this sequence. Here are a series of pictures showing this feature.

This is a series of 3 pictures that show a feature called columnar jointing. This is a a distance.

This is a series of 3 pictures that show a feature called columnar jointing. This is closer.

This is a series of 3 pictures that show a feature called columnar jointing.This is a close-up.

Above Pictures: This is a series of 3 pictures that show a feature called columnar jointing.

Along the road on the way to Stop #3 we see an exposure of Eocene Santiago Formation that is seen being crosscut by the Miocene Dacite Volcanic Plug.

Above Picture: The gray material of the Miocene Dacite Volcanic Plug material (left) crosscutting the tan Eocene Santiago Formation (right).