Stop #2

Calavera Hills Volcanic Dome

---Crosscutting Relationships---

 

An aerial view of the Calavera Hills volcanic dome with fieldtrip stops

Stop #2 is at the base of the volcanic dome. (Google Earth)

 

 

Volcanic Dome Crosscutting the Santiago Formation

Heres a drone overview of Stop 2, I would suggest you watch it before and after reviewing this stop. The volcanic unit crosscuts the Eocene Santiago Formation on both sides of the dome and each side has a baked zone where hot volcanic material contacts the Santiago Formation. The long vertical lines in the interior of the dome are columnar joints. These features are shown in more detail below. What does the principle of crosscutting relationships tell us about the relative age of the rocks here?

The Calavera Hills Volcanic Dome

The Calavera Hills Volcanic Dome.

 

East side of dome volcanic unit crosscutting Santiago fm East side of dome volcanic unit crosscutting Santiago fm

The east side showing the gray material of the volcanic unit (right) crosscutting the white to tan Eocene Santiago Formation left). The baked zone is not as well developed on this side.

 

West side of dome volcanic unit crosscutting Santiago fmWest side of dome volcanic unit crosscutting Santiago fm

                                                         The west side showing the gray material of the volcanic unit (left) crosscutting the white to tan Eocene Santiago Formation (right). You can see the baked edges in the Santiago Formation where it goes from white on the right to a dark brown as you approach the contact. The baked zone may be mixed with a paleosol.

 

 

 

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 six-sided as you can see in the last 2 pictures of this sequence.

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.

Columnar jointing in the volcanic unit

 

 

 

Volcanic Dome Rock Unit

These rocks were extruded giving this unit an aphanitic/fine grain texture and a composition of andesite. The mineral grains of this unit cannot be seen with the naked eye.

 

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.

Fine-grained andesite the orange you see in 2nd photo is localized iron staining.

 

 

Walking from Stop 2 to Stop 3.

The trail goes by exposures of white to light tan Santiago Formation and on the walking path you see a very light colored surface. As you progress down the trail, the path becomes redish orange in color and you begin seeing outcrops of tonalite. The tonalite outcrops are very weathered, appear in redish orange hues and show evidence of spheroidal weathering. You can see the subtle color change in the soils on the trail in the google map image. 

Exposure of white Santiago fm on path to stop 3Exposure of orange Green Valley Tonalte on path to stop 3

Santiago Formation and Green Valley Tonalite exposures 

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