SURFACE STRUCTURE OF LITTLE GLASS MOUNTAIN
Jon N. Fink
The purpose of these two stops is to observe and interpret the structural relations exposed at the surface of a very young rhyolitic Obsidian flow. Before surface structures can be discussed however, the flow stratigraphy and the original attitudes of foliations must be determined. Unfortunately these Holocene obsidian flows do not have any well exposed cross sections so we are limited to observations of flow fronts and to comparisons with older, more dissected flows. At these stops we will first define the stratigraphy and discuss the attitudes of foliations as seen in flow fronts; then we will go onto the upper flow surface and see the variety of structural relations exposed there.
Stop #1 - Northeast lobe, Little Glass Mountain
Before we can interpret the deformed flow structure we must be able to recognize the undeformed structure. This stop offers an opportunity to observe the different units which make up the flow stratigraphy. First we note the layer of tephra which covers the ground surrounding the flow. Isopach maps indicate that the source of this tephra is under Little Glass Mountain (Heiken, 1978).
If we look at blocks within the talus pile we can distinguish 3 principal types based on color and vesicularity: black glassy obsidian, brown to greenish grey coarsely vesicular pumice, and whitish grey finely vesicular pumice. These three rock types have uniform chemical composition (73% SiO2) and all exhibit flow layering, but they differ in density, with the obsidian having the highest. In the flow front above the talus pile these three rock types appear as coherent units, with foliations generally conforming to the contacts between units.
Both in the flow fronts and on the upper flow surface the coarsely and finely vesicular pumice units are nearly always separated by obsidian; they do not appear in conformable contact. Furthermore, the coarse pumice nearly always underlies the obsidian, so that the apparent stratigraphic sequence is (upward): tephra, coarsely vesicular pumice, obsidian, finely vesicular pumice. Above the tephra and on top of the flow are breccias comprised of blocks of the other three flow units (Figure 1).
In older more dissected rhyolite and rhyolitic obsidian flows, foliations near the base are generally horizontal whereas those near the top are more nearly vertical. The depth of the transition from vertical to horizontal flow-layering varies from flow to flow and within a given flow, however it generally lies within the top 25% of the flow. Thus within the stratigraphy seen in Little Glass Mountain the contact between the coarse pumice and obsidian layers may be considered to be originally horizontal. Mapping the orientations of this contact on the flow surface indicates the deformation. Figure 2 shows the three units exposed in m fold within the flow front. This fold and others like it are also exposed on the upper flow surface.
About 20 m south of this fold is an anticline of coarse pumice which appears to be dissected by a valley 3-4 m deep. Later we will see that these valleys which trend perpendicular to the flow front are fractures which commonly bisect anticlines cored by coarse pumice. If we climb onto the flow through this fracture and climb the northern wall, we will be able to see much of the area covered by the map in Figure 3.
The most prominent structure in this part of the map area is a fracture about 2 m deep which cuts through coarse pumice. One can easily see that the foliation patterns in the two opposing walls correspond exactly. In addition, the entire coarse pumice area is enclosed by an area of obsidian, which in turn is surrounded by fine pumice. The foliations within the coarse pumice and its contact with the obsidian define an anticlinal structure which plunges to the northwest (into the flow).
20 m to the north is a series of several more of these plunging, fractured, coarsely vesicular pumice anticlines. The obsidian adjacent to each anticline gets pushed laterally away from the fracture plane, and where two fractures parallel each other the obsidian and fine pumice may get sandwiched into a tight synclinal configuration. In Figure 3, these synclinal axes parallel to the fractures are marked by heavy dots. Notice that the separation or spreading across each fracture increases toward the flow front, suggesting that these fractures form, at least in part, in response to circumferential spreading near the front of an advancing flow lobe.
The coarsely vesicular pumice bordering each of the fractures is folded, with fold axes perpendicular to the fracture axes. This creates a pattern of coarse pumice domes. In figure 3, the fold axes are indicated by heavy lines with arrows, whereas the fracture axes are marked by cross bars. These folds generally lie perpendicular to the flow direction and are interpreted to be surface folds caused by compression, similar to the ropes on pahoehoe basalt flows (Fink and Fletcher 1978).
The basal position and relatively low density of the coarsely vesicular pumice form the basis for gravity instability within the flow structure. The instability can give rise to regularly spaced diapirs of coarse pumice. On aerial photographs of both Big and Little Glass Mountains, such regularly spaced areas can be clearly seen. Depending on their position relative to the flow front these areas of coarse pumice may be subjected to subsequent folding or fracturing or both, resulting in the complex structural pattern observed on this portion of Little Glass Mountain.
Stop #2 - Northwest lobe, Little Glass Mountain
At this locality we will sse similar structures as before, but here fractures are more developed. Although the entire stratigraphic sequence occurs here, we will concentrate on structures found within a large area of coarsely vesicular pumice. This area has a broad domal structure and is bordered by obsidian.
The first structure is located about 2/3 up the flow front, 20 m south of the small parking area. Here coarse pumice and obsidian with very large vesicles (over 10 cm in diameter) form an overturned fold, whose axial plane plunges into the flow (Figure 4). Stretching of vesicles indicates the extension direction associated with folding. Mineralization of the vesicles occurred prior to this folding as the coatings are also stretched. Within this fold the obsidian drapes over the coarse pumice, and higher up in the front the obsidian forms a syncline cored by finely vesicular pumice. These folds with axes parallel to the flow front continue oo the upper flow surface.
Climbing to any high point on the upper surface one can see several 3-5 m deep fractures trending normal to the flow front. Here again, examination of the foliation patterns in the opposing walls shows the large amount of lateral separation associated with these fractures, up to 40 m in some cases. The obsidian and fine pumice which stratigraphically overlie the coarse pumice are generally upturned around the margins of this area, but within the area they have been sandwiched into tight synclines and even "subducted" between adjacent fractures. Although the cumulative extension associated with these fractures is over 100 m, this "subduction" could conserve the volume of the flow. Figure 5 is an interpretation of the interaction between upward rising coarse pumice and downward propagating fractures.
Folds with axes normal to the flow direction can also be seen in this area, and these extend laterally into the higher ridges to the south. Detailed examination of the structure of the highest of these ridges (Figure 7) shows that it also is interrupted by a series of subparallel fractures, with separation increasing toward the flow front, again due to extension associated with radial expansion of the flow.
Aerial photographs of this lobe (Figure 8) show several other areas of coarse pumice and obsidian with irregular spacings along the flow direction. These have been interpreted as diapirs and the spacing has been related to the stratigraphic thicknesses and viscosities of the different textural units.
In summary, Little Glass Mountain, being nearly erosion-free, offers a rare opportunity to observe the surface structure of a rhyolitic obsidian flow just as it appeared upon cooling. The structural relations seen here could only be detected on older flows with great difficulty because erosion and vegetation prevent continuous examination of foliation patterns, and subsequent alteration obscures the textural differences originally present.
Fink, J. H., 1978, Surface structures on obsidian flows: PhD thesis, Stanford University, 175 p.
Fink, J. H., Gravity instability in the Little Glass Mountain rhyolitic obsidian flow, northern California: Tectonophysics, in press.
Fink, J. H., Surface folding on rhyolite flows: Geology, in press.
Fink, J. M., and Fletcher, 1978, Ropy Pahoehoe: Surface folding of a viscous fluid: Journal of Volcanology and Geothermal Research, v. 4, p. 151-170.
Heiken, G. 1978, Plinian type eruptions in the Medicine Lake Highland, California and the nature of the underlying magma: Journal of Volcanology and Geothermal Research, v. 4, p. 375-402.
Last Updated: 28-Mar-2006