MODE OF EMPLACEMENT OF THE VOLCANIC ROCKS
Most of the dikes and sills evidently fill tension cracks developed in the granitic complex. As noted on the map, these cracks generally follow the northwest-southeast structural trend of the California Coast Ranges. They are almost exactly parallel with the San Andreas rift, which lies less than 6 miles to the northeast of the Pinnacles.
The large central body of massive and sheet rhyolite is closely related to the material filling these tension cracks, both in age and in composition. The central body, however, differs in structure and texture since it was formed by surface flows and by the accumulation at the surface of a mass which was exuded in too viscous a condition to move far from the surface outlet. A process similar to that mentioned by Williams36 for the formation of volcanic domes is postulated, except that, at the Pinnacles, the material came up through a long fissure and an elongated dome or ridge was formed.
This volcanic material was deposited on a slightly irregular granitic floor, as is evidenced by the masses of rhyolitic lava which appear outside the main fault to the west. Part of the lava to the south, however, issued from local openings and is not connected with the central mass. Rapid cooling on the cold granitic surface caused the viscous magma to solidify rapidly as glass. Some of the later flows are also formed of obsidian and pitchstone through rapid cooling.
Although most of the lavas are rhyolitic, differentiation of the magma caused a rather large amount of andesite and basalt to be poured out. Along the mountain ridge between South Chalone and Pyramid Peak, vertical flow lines in otherwise more or less massive rhyolite indicate one of the important fissures through which the massive volcanics were extruded.
Toward the close of the volcanic period, greater viscosity of the lava, perhaps resulting from loss of volatiles and reduced temperature, caused partial and temporary stoppage of the vents and fissures, but periodic gas and steam explosions caused this solidified material to be fractured and thrown out on the sides of the ridge in large quantity. Much fresh magma was also thrown out at the same time. At least five foci of volcanic eruption have been detected in the solid rhyolitic mass. These are characterized by more or less circular masses of volcanic tuff within nonfragmental rhyolite. Vertical bedding is noticeable in these tuffs and, in general, the surrounding lava flows dip away in all directions, although the lava may be so massive as to give little indication of this phenomenon. Erosion characteristically carves conical spires from the vent tuff. The spires are particularly numerous toward the periphery of the vents, where they tend to lean slightly inward.
It will be noted from the map that the largest center of eruption is that of South Chalone. Other centers follow a more or less north-south trend. Several zones, not shown on the map, were noted, which probably contributed fragmental volcanic material, but evidence was incomplete. Toward the north, one or more ancient craters may be obscured by the covering of later Tertiary detritus on the northeast side of the Chalone Creek fault.
Concerning pyroclastic rocks, Anderson37 notes that volcanic breccias may have the following origin:
At the Pinnacles National Monument, post-Miocene erosion has removed part of the evidence for the origin of the Pinnacles Formation. However, it seems worthwhile to review the foregoing outline and to determine, if possible, which of the agencies noted could have caused the great thickness of pyroclasts.
Following the outline, the various possibilities will be taken up in order.
Crumbling of a dome.Extrusion of the viscous rhyolitic magma of the Pinnacles area through a fissure or several fissures almost certainly formed domes38 or their equivalent in the form of a steep-sided ridge of volcanic rock. Crumbling of this ridge would form a mantle of talus and probably accounts for at least part of the volcanic breccia of the Pinnacles.
Intrusions.The production of pépérites through the intrusion of magma into wet sediments may be dismissed from consideration.
Friction breccias.Anderson notes that
A certain amount of this action may have taken place, but it would be impossible to differentiate such breccias from those derived by process A.
Crumbling of advancing lava flows.This has contributed to the fragmental volcanics under discussion. Rather thin layers, which owe their origin to this phenomenon, are intercalated between more typical breccias, especially in the vicinity of the Chalone Peaks. It is impossible to differentiate exactly, however, the amount of brecciation resulting from increasing viscosity of the flow from that incurred by flowing over and enveloping fragments on the one hand, and receiving fragmental showers from above on the other.
A variety of lava types in most of the samples examined leads the writer to believe that a rather large amount of foreign matter has been enveloped in the lava flow
Vulcanian eruptions.Anderson says that
Peléan eruptions (nuées ardentes).Anderson further says that
In the Pelean eruption a black cloud composed of solid, liquid, and gaseous material rises from the volcanic opening. Gravity causes the cloud to settle rapidly but the gases and occluded steam act as a propelling agent which causes the material to be carried rapidly down the slope. The magma is nearly always rich in dissolved gas and forms large amounts of pumice.
Vesicular fragments found on the sides of North Chalone Peak are formed by the weathering out of glass blebs from a network of more resistant rock. The almost complete absence of pumice suggests that this type of explosion was less important than the Vulcanian described above.
Ultra-vulcanian eruptions.This type of eruption, characterized by steam explosions and the absence of new ejecta, has been defined as ultra-vulcanian by Mercalli,42 and semivolcanic by Lacroix.43
Concerning these three types of eruption, it would be difficult to determine which was the most important contributor to the Pinnacles breccias, although those of vulcanian and ultra-vulcanian type were undoubtedly more important than the nuées ardentes.
Dry avalanches.Perret, as an eyewitness to the disastrous eruption of Vesuvius of 1906, graphically mentions in his notes that
Conclusions.The pyroclastic deposits of the Pinnacles Formation are essentially angular, and not water worn. It is believed that only a few degrees of tilting have taken place after deposition, so that most of the dip of these beds is original, and furthermore it is unnecessary to postulate transportation of more than 2 or 3 miles for any of the material of these beds with the possible exception of that at the extreme northern part of the area.
Had these breccias been deposited as a mud flow with any degree of liquidity, the foregoing conditions would have been different. Fragments would have been more rounded, the material would have come to repose at much lower angles, and the detritus would have been scattered over a far wider area, even though it is conceded that erosion has removed much of the material to the west of the Pinnacles fault. We may therefore omit a discussion of "Breccias transported by water" as not strictly applying to the problem at hand. Moisture may have helped to compact the detrital material as it lay on the steep slopes of the volcanic ridge, but certainly there was an insufficient amount of moisture to form a pasty mass which would flow at low angles.
Briefly, we may conclude that the thick pyroclastic deposits of the Pinnacles were formed by several different agencies. Crumbling of the original volcanic ridge formed a rather deep mantle of talus. To this was added the products of vulcanian and ultra-vulcanian eruptions from vents along the ridge, especially from the vent of South Chalone Peak. Many flows followed explosive eruptions and enveloped pyroclastic fragments and some flows crumbled into breccias as they advanced.
Avalanches and the natural angle of repose for fragmental material kept the sides of the ridge at angles ranging from 30 to 40 degrees. The climate was probably arid and the ejected material cemented before it could be carried off as a mud flow.
Last Updated: 8-Jan-2007