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Geology of the Pinnacles National Monument


Chief interest in the Pinnacles region centers about the volcanic rocks, since it is the erosion of these that has produced the unusual and scenic effects for which the National Monument is famous. They range from rhyolites through andesites to basalts and occur typically as thick beds of fragmental ejecta, although flows as well as dikes and sills are abundant. Rhyolites are much more abundant than the more basic types, probably comprising 80 per cent or more of the total. The chief volcanic mass occupies an area approximately 7 miles long in a north-south direction by 2 miles wide, although numerous dikes and sills are exposed outside this area.


Within the principal volcanic area the distinction between intrusives and extrusives cannot usually be drawn, so that, under this heading, the main emphasis will be laid on the dikes and sills which are intruded into the granitic mass. It will be noted from the map that the dikes generally follow the dominant structural trend of the Coast Ranges, that is, northwest to southeast. It seems that the first expression of volcanic activity within the region was the intrusion of dikes and sills, several of them of large size. Continued activity developed central vents from which issued abundant fragmental material and sporadic flows.

The material in most of the dikes and sills may be classified as rhyolite porphyry25 although some petrographers might refer to them as quartz porphyry or liparite. Several more basic types are found and will be described later. The minerals of the rhyolite vary slightly from place to place and the texture differs with the size of the intrusion and proximity to the margins. That the width of an intrusion does not always determine its grain, however, is shown by the fact that a dike in the southern part of the area and at least 1000 feet thick is aphanitic throughout.

25Johannsen, Albert, Petrography, vol. 2, p. 265, 1932.

Typical examples of the rhyolite porphyry are white to gray in color and contain abundant phenocrysts of glassy quartz and feldspar up to several millimeters in diameter. Black shiny biotite in minute flakes is also abundant. One unusually persistent dike at the north edge of the fragmental material is nearly pure white in color and of aphanitic texture, except for scattered cubes of limonite, after pyrite, which average 1 mm. on a face. Some of the larger dikes are rather coarsely porphyritic and approach granite porphyry. One dike contains euhedral crystals of orthoclase up to 2 cm. in length.

Microscopically, a typical example of the dike rock is porphyritic with light grayish brown micro-spherulites in a glassy matrix, although there is some birefringent material in irregular areas. Phenocrysts of clear, glassy, embayed quartz, ranging in size from 2 to 3 mm., compose from 15 to 20 per cent of the whole, and euhedral prisms of glassy sanidine with included biotite and apatite together with albite and oligoclase comprise most of the remaining phenocrysts. Hexagonal flakes of greenish brown mica are abundant.

One of the most striking features of the rhyolite is the abundance of albite and since it is one of the most abundant feldspars in nearly all the rocks examined we may consider the rhyolites to be essentially rich in soda. This probably holds true also for the aphanitic types. Biotite is present in most of the dike rocks and may amount to 10 or 15 per cent of the total volume. It is shiny black in the hand specimen but greenish brown under the microscope, and occurs in small hexagonal flakes. Neither hornblende nor augite was observed in any of the typically acid rocks, although they occur in the more basic types. Sericite is frequently found as an alteration product of the feldspars.

Magnetite is sparingly present, locally forming the rims of biotite flakes as a reaction product. Colorless and pinkish zircons are rather common and apatite is almost universally present, especially as needle-like inclusions in the quartz and feldspar. Titanite is present in some of the dike rocks. Pyrite is common, especially in the darker rocks.

Where the road crosses Stonewall Creek, in section 16, a white hyaline rhyolite, showing vertical flow lines, contains elongated amygdales of calcite.


As already noted, these are less abundant than the rhyolites. They may occur as individual dikes of some persistence, or as members of composite dikes, which are common in the northwest part of the area. The more basic intrusives, however, usually occur separately.

In composite dikes, it may be difficult to determine which of the two types was the first to be intruded, but usually the evidence points to an earlier age for the rhyolite. Figure 6 (p. 11) shows a contact toward the center of a composite dike in which fragments of rhyolite porphyry have been incorporated by later dacite. In some composite dikes, the darker member has mineral constituents which would still cause it to be named rhyolite porphyry or quartz latite.

A typical specimen of dacite is grayish black in color and contains visible phenocrysts of quartz and feldspar. Microscopically, the rock is porphyritic with a microcrystalline groundmass which is trachytic in parts. Rounded quartz phenocrysts up to 5 mm. account for from 5 to 10 per cent of the rock, and andesine-labradorite in euhedral crystals is the most common feldspar. Biotite seems to be the only ferromagnesian mineral, although alteration may have removed traces of others. Magnetite, apatite, pyrite, and titanite are minor accessories.

Another type which forms several prominent dikes, notably a strong dike on the west side of the Pinnacles through the exact center of section 9, is a grayish, olive-green rock, megascopically of fine, even texture. It is a rather typical propylite or hydrothermally altered andesite, carrying epidote, calcite, chlorite, serpentine, leucoxene, hematite, and sericite. The original feldspars are altered beyond recognition.

A number of dikes and sills were recognized in the area exposing massive and sheet rhyolite, by their columnar jointing and porphyritic textures, but they are typically absent from the deposits of pyroclastics. Intrusives should not be confused, however, with lateral eruptions, which will be mentioned later. Dikes and sills are more resistant to weathering than the granitic rocks and the finer grained intrusives often form well-defined ridges.


Under this division will be discussed the large central mass of volcanic rocks which is chiefly composed of rhyolitic surface flows or simply of massive rhyolite without apparent structure. Volcanic glass is especially abundant over much of the area. The main mass of rhyolite is definitely older than the pyroclasts, although in the northern part of the area the pyroclastic rocks have been covered by a still later lava now of rather small areal extent. Obsidian and rhyolite from lateral eruptions occur sporadically along the west side of the fragmental volcanics and were probably contemporaneous with them, but these are of minor importance.

The main mass of extrusives does not differ in composition from the dikes and sills already discussed, but the textures are finer as a result of more rapid cooling. Typical material is light gray, pinkish, vermilion, or green in color; microgranular, so that only an occasional flake of biotite or quartz may be distinguished with the hand lens; and has a well-developed flow structure. Specimens of rhyolite with unusually well-developed flow structure have even been mistaken for petrified wood.

Microscopically, this rhyolite is finely porphyritic and shows a considerable variation in groundmass texture, but the mineral composition is rather constant. In many samples, the groundmass is microspherulitic, the microspherulites alternating with fluidal bands of glassy material. Fluidal lines may be absent and an aggregate of minute quartz and feldspar grains, lath-shaped, squarish, or irregular, contains a noticeable amount of colorless, interstitial glass. The refractive index of the feldspars is usually less than that of balsam, indicating sanidine and albite. Since the coarser grained rocks are all soda rich, it may be supposed that these aphanitic types are also soda rich. Magnetite is sparingly present, also zircon. Biotite and rare flakes of colorless mica are the only important characterizing accessories.

Volcanic glass occurs in thick masses, especially in the vicinity of the old vents and near the granite contacts where the lava poured out on a cold surface. Where fresh, it is generally black or dark colored, and has a sharp conchoidal or rough fracture, but it becomes porcellanous and pale green upon partial devitrification. Thin seams and stringers of opal are usually developed. Some specimens of fresh glass are perlitic and others show a "bird's-eye" structure of slightly elongated black glass blebs in a greenish, hyaline matrix. Smooth curved surfaces may be developed through pressure.

Much of the glass has a refractive index of less than 1.5 (a typical specimen has n=l.496) which indicates a silica content of more than 70 per cent.26

26George, W. O., Jour. Geol., vol. 32, p. 353-372, 1924.

The opaline material described above suggests a final hot-spring period following the more active volcanic period.


In addition to the typical rhyolitic extrusives which are the most abundant and important, there are also andesites and basalts as well as gradational types. Unfortunately, most of the outcrops of andesitic rock are so much altered that exact determination is impossible.

In the northern part of the area, as much as 600 feet of basic material (andesites and basalts) occur toward the middle of the lava series. They are conformable with thin rhyolite flows above and grade down into less basic rocks and rhyolitic glass. In the central and southern part of the area, a considerable amount of altered andesite appears toward the base of the rhyolitic mass. On the south side of South Chalone Peak, several areas of reddish, ferruginous, volcanic rock stand out in sharp color contrast from the surrounding gray rhyolite and are usually more basic, although part of this iron is probably secondary.

One highly ferruginous rock from south of Chalone is fine grained and deep reddish brown. Under the microscope, it shows a hyalopilitic texture. Though much altered, the chief feldspar is recognizable as andesine-labradorite, and augite in small grains is scattered throughout the groundmass. Biotite is apparently an original constituent but is too altered in the specimen examined to determine its optical properties. Hematite composes approximately 20 per cent of the rock. The type is here termed a much altered, ferruginous, augite andesite.

At the east side of the main volcanic mass and toward the base occurs a chocolate-brown type which carries rather large, scattered amygdales of white calcite. Although a propylite, the rock was a porphyritic andesite before alteration, with a cryptocrystalline to trachytic groundmass.

A second type of amygdaloidal andesite occurs along the trail a short distance northwest of Pinnacles Lodge. The groundmass is in large part of brown glass with scattered laths of andesine and minute grains of colorless augite. There are also phenocrysts of colorless augite up to 1/2 mm. in diameter. Thirty per cent or more of the rock is made up of flattened amygdales filled with penninite and minor carbonates, giving the rock a peculiar green spotted appearance.

Toward the extreme north end of the Pinnacles National Monument, and some 400 feet structurally below the rhyohite-breccia contact, occur several hundred feet of rather basic flows, which are conformable to the overlying breccias. The more basic of these flows may be correctly termed basalt. Megascopically it is grayish black and fine grained, with minute grains of yellowish green olivine. Microscopically, it is finely porphyritic with a hyalopilitic groundmass, generously sprinkled with minute octahedra of magnetite and grains of augite. The feldspar laths of the groundmass are of andesine-labradorite, and phenocrysts include colorless augite, olivine, and nonpleochroic hypersthene. The olivine is partly altered to serpentine.


Pyroclastic material occurs in large volume and covers almost the entire area set aside for the Pinnacles National Monument. Characteristic erosion of these deposits has formed the scenic pinnacles, spires, cliffs, and crags of the region. Fragmental material includes agglomerate, breccias, and tuffs. Because of the variation in interpretation of terms describing volcanic ejecta, the terms proposed by Wentworth and Williams27 will be followed as closely as possible.

27Wentworth, C. K., and Williams, Howel, Report of the Committee on Sedimentation, Nat. Research Council Bull. No. 89, 1932.

Fig. 7. Typical volcanic breccia from the west side of Hawkins' Peak.


Using under this term the rather liberal definition proposed by Tyrrell,28 we might include most of the volcanic ejecta of the Pinnacles National Monument. Tyrrell defines agglomerate as

Compacted and indurated accumulations of scoria and blocks, cemented by ash and its decomposition products, are called agglomerate. This material naturally forms within or near the volcanic focus, and is the principal filling of ancient volcanic vents. Crater avalanche debris, and the deposits of incandescent clouds, may also be compacted to agglomerate.

28Tyrrell, G. W., Volcanoes, p. 65, 1931.

According to Wentworth and Williams, a more restricted definition includes only

contemporaneous pyroclastic rocks containing a predominance of rounded or subangular fragments greater than 32 mm. in diameter, lying in an ash or tuff matrix and usually localized within volcanic necks (vent agglomerates) or at a short distance therefrom. The form of the fragments is in no way determined by the action of running water, as in volcanic conglomerate, but is a primary feature determined during the actual eruption.29

29Op. cit., pp. 45 and 46.

With this more restricted definition in mind, it is impossible to include more than a minor part of the fragmental rocks, since most debris greater than 32 mm. diameter was thrown out in a solid condition rather than a semi-plastic one. The deposits should be included under volcanic breccias. Fragmental material with rather large, rounded bombs may be observed, however, along the branch of Chalone Creek north of Hawkins' Peak.

Fig. 8. View due north from near the top of Hawkins' Peak, showing massive bedding and inclination of volcanic breccias.


According to Wentworth and Williams, breccia includes

more or less indurated pyroclastic rocks consisting chiefly of angular ejecta 32 mm. or more in diameter. If the fine tuff matrix be abundant, the term tuff-breccia seems appropriate.30

30Op. cit., pp. 45 and 46.

By far the major part of the "Pinnacles Formation" should be called a breccia. (Less important types will be specifically described later.) The volcanic breccias are massively bedded (see figs. 2 and 8) and of surprising uniformity in the northern portion of the area, but they have suffered from avalanches and lateral eruptions in the southern part. Dips range from 20 to 50 degrees with 35 degrees as an average figure and are inclined generally west, although dips become northerly toward the north part of the area. Bedding planes are often rather poorly defined and the attitude can best be determined at a considerable distance from the outcrop. Individual beds may range from a few feet to several hundred feet in thickness. Close to the source the breccias are interbedded with many thin flows of rhyolitic glass, tuff, and lapilli-tuff, but become purer breccias as the distance from the source in creases.

Constituents of the volcanic breccias are generally cognate, and vary in size from volcanic ash to fragments or blocks up to 10 feet or more in diameter. Average fragments range from 1 to 3 inches in diameter with a sufficient amount of finer material to serve as interstitial filling. None of the fragments are water-worn, although the corners of many have been rounded by attrition from moving down steep slopes, or by attrition of the fragments in a dry state at the time of explosion. Sorting is generally lacking except for a rough stratification resulting from different periods of eruption.

Fig. 9. Typical vent tuff from the summit of South Chalone Peak.

The volcanic breccias are essentially composed of light gray to pinkish, fine grained, rhyolitic fragments, both massive and banded, in a matrix of the same composition. Cementation of the fragmental material has been sufficient to form solid rock formations, which stand in vertical cliffs several hundred feet high. Concerning this process Tyrrell writes that

Fragmental volcanic products are easily compacted and hardened into firm rock. Many of them are largely composed of glassy matter which is chemically unstable. Under the action of infiltration waters, and the free acids of volcanic origin which they retain, the particles break down with the formation of secondary compounds, binding the materials together into a firm compact rock. . . . pozzolana, a volcanic tuff of the Naples district, which when mixed with lime forms a hydraulic cement that will set under water, was first used by the Romans.31

31Op. cit., pp. 62 and 63, 1931.

The fragmental volcanic deposits may also be locally opalized, as previously mentioned.

Besides cognate material, which forms more than 90 per cent of the pyroclastic detritus, there are sporadic blocks and fragments of accidental32 material, which at one locality (northwest corner of section 15, on the side of Chalone Peaks) form the chief constituents of a bed 100 feet thick. Blocks of granite up to 6 feet in diameter occur in great abundance and may be traced for a considerable distance along the strike. These blocks are rounded and are partly cemented with arkosic material as though the granitic blocks had been exposed to weathering before being covered with a protective blanket of rhyolitic ejecta.

32"Accidental" is here used as "an adjective . . . to designate pyroclastic materials derived from volcanic rocks, non-consanguineous with the magma involved during the eruption, or from other igneous, metamorphic or sedimentary rocks through which the vent was developed" (Wentworth and Williams, op. cit., pp. 45-47, 1932).

Granite is the only accidental material found among the ejecta and this is not surprising in view of the fact that the granite was almost completely denuded of overlying sediments and metamorphic rocks before volcanic activity took place.


A very common pyroclastic rock composing the upper half of the bedded material of North Chalone should be defined as a lapilli-tuff. Lapilli-tuff, according to Wentworth and Williams, is an indurated deposit essentially made up of lapilli in a fine tuff matrix. Lapilli may be essential, accessory, and accidental ejecta, ranging mostly from 32 to 4 mm. in diameter. The lapilli tuffs of North Chalone are composed almost entirely of essential and accessory fragments ("accessory" signifying pyroclastic materials derived from previously solidified volcanic rocks of consanguineous origin). Judged by the rounded and somewhat glassy edges of part of these fragments, they must have been thrown out in a semiplastic condition. The coarser of these lapilli tuffs should be styled a true agglomerate, but the greater number of the fragments in most beds are less than 32 mm. in diameter, so that the term lapilli-tuff is considered more appropriate.

A representative specimen contains pink, purplish, gray, and greenish angular or subangular fragments averaging 15 mm. in diameter, firmly cemented in a yellowish gray tuff matrix. All the fragments are aphanitic and many show banding. Bedding is massive and a few thin flows of pink rhyolite are intercalated.

Lapilli-tuffs also occur to the north of Chalone Peaks but in much less volume.

Pyroclastic deposits of minor importance include several thin beds of fine vitric tuff interbedded with the volcanic breccia, especially toward the base of the series. These contain glass shards with a small percentage of quartz and feldspar. Some of the light colored and fine grained dike rocks may be confused with the fine vitric tuffs if field relations and microscopic textures are not carefully studied. Fine grained dike rocks usually show fluidal lines and are free from the broken crystals and glass shards of the tuffs.

Other types have resulted from the hardening and cracking of viscous rhyolitic lava during flow (flow breccia or crackle breccia); from the flowing of liquid lava over fragmental material; from the dropping of ejecta into lava at the time of flow; and from a combination of all these processes.

The volcanic breccias are conformable to the rhyolite flows on the east but are in fault contact with the quartz diorite on the west. They presumably lie unconformably on a more or less peneplained granitic surface and certainly covered at one time a much larger area than they do at present. Erosion has not yet exposed the depositional contact of granite and fragmental volcanics on the west side, although a partly cemented outwash of volcanic detritus, overlying the granite, occurs in several places and may be mistaken for an actual contact; for example, at the headwaters of Stonewall Creek.

Erosion has removed almost all traces of tuffs and breccias from the west side of the Pinnacles fault. Information on the extent of this deposit as well as on the extent of the finer volcanic dust or "ash" deposits prior to erosion, is entirely lacking.

Toward the north the relation of the volcanic breccias to the underlying rocks is somewhat obscure but there is apparently a gradation downward from fragmental volcanic ejecta into granitic detritus. This contact is in large part obscured by Pleistocene deposits of land-laid volcanic detritus washed from the central mass.

Fig. 10. View showing erosional features of vent tuff occurring on the southwest side of South Chalone Peak. The verticality of the vent material is in sharp contrast with bedded volcanic rocks to the left.


The age of the volcanic rocks is somewhat uncertain. Abundant fragments of typical Pinnacles rhyolite and volcanic breccia occur within the fanglomerate material underlying the diatomaceous shales. These fanglomerates are considered to be Temblor in age, so that much of the volcanism, if not all, occurred prior to upper Temblor time. Fragments of typical Pinnacles rhyolite are lacking from conglomerates immediately overlying the granite, which are considered to be Vaqueros or lower Temblor in age.

Kerr and Schenck found lava flows of both Vaqueros and Temblor age but considered the volcanics of the Pinnacles to belong to the Temblor age.

Lavas in this vicinity [5 miles north of Pinnacles] are probably a continuation of the rhyolite that occurs typically in the Pinnacles National Monument, and they lie stratigraphically above the Miocene sediments.33

33Kerr and Schenck, op. cit., p. 474, 1925.

Reed notes that

The Temblor formation is noted for the vast quantity of volcanic rocks associated with it in some areas. . . .34 The importance of the Temblor as a period of volcanic activity, and the importance for historical geology of a correct correlation and interpretation of Temblor volcanic rocks, have been recently discussed by H. S. and H. R. Gale ("Miocene Volcanism," a paper read before the Cordileran section, Geol. Soc. America, March 7, 1931).

34Reed, R. D., Geology of California, pp. 175-176, 1933.

Fairbanks finds that

Within the San Luis Quadrangle, there are two distinct types of rhyolite. Both of them occur in the basal portion of the Monterey shale and probably belong to the same period of volcanic activity. The ash and tuffs, with one local flow, were laid down early in the history of the formation, before the enormous thickness of siliceous shales had been deposited. They mark a period of eruption during the deposition of an apparently conformable series of sediments.35

35Fairbanks, H. W., U. S. Geol. Survey Folio 101, p. 7, 1904.

He later notes that the rhyolite is rich in soda.

The writer believes that volcanism began at the close of Vaqueros time and continued into that of lower Temblor. The Temblor volcanic period is recognized over much of California. The dikes, sills, and massive and sheet rhyolite of the central mass are approximately of the same age. The pyroclastic deposits are later, although the time difference was not great and flows of some magnitude occurred from time to time, even after volcanism had entered upon its explosive stage.

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