University of California Press University of California Press
Geology of the Pinnacles National Monument


Why do the breccias occur only on the west side of the rhyolite? An immediate answer to this question cannot be given. More than one hypothesis is probably correct. Several possibilities present themselves, as follows:

1. Tilting of the entire block, followed by erosion and stream sapping
2. Dipping fissure, causing oblique projection of ejecta
3. Crater rim lower or formed of weaker material on one side
4. Pyroclastics removed from east side by faulting

The faults bounding the volcanic rocks have not been exactly evaluated but the presence of 45- and 50-degree dips indicates a westward tilting of at least a few degrees. Tyrrell notes that

Fine volcanic "ash" will stand at an angle of 30 to 35 degrees while the coarser scoria and slag masses may attain an angle of 40 to 45 degrees.45

45Tyrrell, G. W., Volcanoes, p. 27, 1931.

The window of granite within the rhyolite to the south of Chalone Mountain seems to limit this tilt through faulting to not more than 10 degrees. A glance at the section on the areal map will demonstrate the possibility of removal through erosion of the pyroclastic deposits which may have lain along the east side of the central mass. The close proximity of Chalone Creek and Chalone fault to the line of contributing foci has caused removal of more material on the east side than on the west. The relative importance of the other agencies can only be determined at present by conjecture.

The large amount of accidental material (granite) comprising the major part of one of the beds on the north side of North Chalone is rather impressive. Its presence on one horizon speaks eloquently of a period of violent subterranean explosions or movement along the contributing fissure, so that huge blocks (up to 6 feet or more) of granite were loosened and thrown out to the almost complete exclusion of rhyolitic material for some time.

The total length of activity at the Pinnacles cannot be accurately determined, but violent eruptions were repeated over a rather long period, as indicated by the vast thickness of pyroclastic deposits.

Fig. 11. West-dipping beds of volcanic breccias. This is North Chalone Peak and his material has been derived from the vent shown in figure 10.


By calculating the fault displacements as closely as possible, it is apparent that approximately 3 cubic miles of volcanic material still remain in the Pinnacles area.

It is apparent from the section A—A' that at one time the vents must have been enclosed to a much higher elevation than at present. Even the outermost pyroclastic deposits probably sloped upward and finally intersected the rim of the old crater. By projecting the bedding planes of the volcanic ejecta toward the vents we may conclude that the original volume of ejecta was certainly two or three times the amount now remaining. The total volume of the extrusives is therefore considered to have been from 6 to 10 cubic miles.

Even these figures, however, do not allow for much fine material or "ash" which would be carried away by wind and water. It is not impossible that the "ash" may have amounted to even more in volume than the larger fragments which settled nearer their source. Miocene sediments are noted for a high content of fine volcanic material and the Pinnacles craters may have made important contributions to these beds.

The maximum thickness of volcanic breccias and other pyroclastic deposits occurs in section A—A' through the Little Pinnacles as shown on the areal map and amounts to 4500 feet. From this section the volcanic deposits thin to both north and south. It is probable that the original thickness of fragmental volcanics never greatly exceeded this amount.

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Last Updated: 8-Jan-2007