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Wendell A. Duffield

Publications and source records attributed to Wendell A. Duffield.

29 records · Page 2Linked to original sources
USGS1977

Some characteristics of Pele's hair

Pele's hair is a filamentous variety of brown sideromelane glass that forms during eruption of basaltic lava. Strands of Pele's hair form from droplets of lava that are spun or stretched into filaments during quenching, and others may form as chilled streamers of lava. Common elongate vesicles, sometimes twisted, indicate extreme stretching and twisting during hair formation. Hair diameter ranges from about 1 to 300 micrometres. Refractive index of hairs decreases with hair diameter and is most probably a function of the process of formation rather than chemical composition. Masses of Pele's hair form natural spun-glass filters that trap small particles and serve as sites for sublimate deposition. Such deposition may begin even while hair is falling to the ground through an eruption fume cloud. Sublimates include carbonates, sulfates, sulfur, and less commonly hydrocarbons, thus complicating the interpretation of volatiles in Pele's hair in terms of original magmatic constituents. Vesicles, which provide the most nearly pure samples of magmatic volatiles, contain mostly H 2 O and CO 2 .

Journal of Research of the U.S. Geological Survey↗
USGS1975

Chemical compositions of Kilauea east-rift lava, 1968–1971

The major element chemical compositions of lava from four eruptions on the east rift zone of Kilauea between August 1968 and October 1971 reflect three petrologic processes: Production of chemically distinct batches of magma in the mantle. Separation of olivine, augite, and plagioclase from liquid during flow in the rift-zone conduits. Mixing of different magmas during ascent to the surface. Chemically none of the four Kilauea east-rift eruptions matches the preceding summit eruption in Halemaumau that ended in July 1968. The Mauna Ulu eruption, May 1969 to October 1971 (the last of flie east-rift eruptions), can be divided into five olivine-controlled and chemically distinct variants. Three of these characterize the first seven months of the eruption and are closest in composition to the 1967–8 Halemaumau eruption. Variants 4 and 5 were erupted later and have compositions that are distinctly different from that of the 1967–8 eruption. Major differences are higher Al 2 O 3 (0·15–0·23 per cent), and lower K 2 O (0·07–0·10 per cent) and TiO 2 (0·12–0·23 per cent) in variants 4 and 5 at the same MgO content. Some lavas from eruptions in August and October 1968 and February 1969, have olivine-controlled magma compositions that are identical to mixtures of Mauna Ulu variants 1–3 and the 1967–8 composition. This observation fits an hypothesis advanced earlier by T. L. Wright and R. S. Fiske that magmas in the central magma chamber become mixed with magmas in the rift zone and can be identified as mixing components of rift eruption magmas before they appear as distinctive magmas in summit eruptions. Lavas representing mixing of olivine-controlled magma with differentiated magma were erupted in October 1968, February 1969, and in May and December 1969. The changes in amount of K 2 O and TiO 2 during the latter part of the 1969–71 Mauna Ulu eruption are the reverse of the overall secular change in composition of Kilauea summit lavas from pre-1750 through 1967–8. The K 2 O and TiO 2 contents of the latest overflows during the 1969–71 Mauna Ulu eruption (April 1971) are comparable to that of lava erupted at Kilauea summit prior to 1750. The changing chemistry of Kilauea magma is found to be of use as a ‘tracer’ in the complex Kilauea conduit system. Application of these data to older lava sequences is difficul because of the complexity of the processes controlling lava composition and the absence of detailed information about the time-space chemical variation during individual eruptions.

Hawaii↗
USGS1975

Geology of the Sierra Foothills Melange and adjacent areas, Amador County, California

Detailed outcrop mapping in the western Sierra foothills of Amador County, Calif., has resulted in some major changes in the interpretation of stratigraphy and structure. The Amador Group was originally defined at its type locality on the south bank of the Cosumnes River in Amador County to include the Cosumnes Formation and the conformably overlying Logtown Ridge Formation, but the new data indicate that the lower boundary of the type Logtown Ridge should be located 600 m farther west (downsection) than originally designated and that this boundary is a fault. The strata that were originally called the Cosumnes Formation are part of a lithologically diverse assemblage of tectonically intermixed rocks that constitute a newly recognized melange and thus are not a formational rock-stratigraphic unit as the earlier workers believed. Thus, the names Cosumnes Formation and Amador Group are both inappropriate in their type area and are abandoned. The Logtown Ridge Formation is here divided into four members, some of which cross what earlier was considered to be a formational boundary of the Logtown Ridge with overlapping pyroclastic strata. The outcrop mapping requires additional changes, although of lesser importance, in the identification and correlation of other Mesozoic rocks in Amador County. The newly recognized melange forms a 4-km-wide belt underlying the Logtown Ridge Formation. In addition to the type section of the abandoned Cosumnes Formation and scattered fault-bounded blocks of strata of Cosumnes lithology, the melange comprises rocks heretofore mapped as "western belt of Calaveras Formation," considered to be of Paleozoic age. Single clasts of this huge tectonic breccia range from a few centimeters to a few kilometers in maximum dimension. Distinctive strata are generally disrupted, and pervasive shearing is common. In the absence of fossils, no age of original deposition can be assigned to any clast or matrix of the melange, but on the basis of indirect structural evidence, the intermixing that formed the melange probably took place during the Late Jurassic or before, and therefore the now sheared and faulted strata must originally have been at least this old. Available data are ambiguous but suggest that rocks were intermixed to form the melange when the strata were horizontal or nearly so. Similarly, the overlying Logtown Ridge and Mariposa Formations were faulted when these rocks were essentially horizontal. The entire section was subsequently tilted to its present, nearly vertical position. Traditional syntheses of the tectonic history of the Sierra foothills argue that the faults there have always been steeply dipping. Although this may be true for some faults, the new interpretation suggests that most faulting occurred before the section was steeply tilted. Neither suggestion can yet be proved, but we maintain that the highly deformed rocks mapped in Amador County represent primarily the effects of subduction at a continental margin, possibly augmented by gravity tectonics in a trough of sediment accumulation there. On the basis of the ages of affected strata, this period of subduction was Late Jurassic but possibly began at an earlier time. If this interpretation of the melange in Amador County is correct, a belt of similarly deformed rocks should extend far beyond the limits of the study area.

California↗
USGS1973

Reinterpretation of the boundary between the Cosumnes and Logtown Ridge Formations, Amador County, California

Recent detailed geologic mapping in the Sierran foothills reveals that rocks previously included in the Jurassic Amador Group must be redefined. The term “Amador Group” was applied by Taliaferro and Clark to a section of epiclastic metasedimentary rocks (the Cosumnes Formation) and the seemingly conformable overlying metavolcanic rocks (the Logtown Ridge Formation). New structural and stratigraphic evidence indicates that at their type localities on the banks of the Cosumnes River the boundary between the two formations should be relocated about 610 m downsection from the position shown by Clark. This change removes all known paleontological control on the age of the Cosumnes Formation. Structural relations show that the type Cosumnes and Logtown Ridge Formations are in fault contact at the Cosumnes River. Rocks of the Cosumnes Formation are now grouped with a complex unit of megabreccia that includes other strata previously termed the “western belt” of the Calaveras Formation. The megabreccia formed, at least partly, sometime between late Paleozoic and Late Jurassic times, but rocks in the megabreccia, including the Cosumnes Formation, could be older than late Paleozoic. The term “Amador Group” is herein abandoned.

California↗
USGS1973

A photogeologic method for determining the direction of horizontal dilation from patterns of en echelon fracturing

The direction of horizontal dilation in areas characterized by tensional tectonics can be determined from a statistical study of en echelon patterns of fracturing observed on aerial photographs. Relative, to a north-south dilation, nearly all directions of zones of dextral (right-lateral) en echelon fractures lie in the northeast quadrant, while those of sinistral (left-lateral) en echelon fractures concentrate in the northwest quadrant. Statistically, directions of the two types of zones define unimodal frequency curves that intersect at about 90 degrees to, and thus define the direction of, applied dilation. The method has been field tested twice and is believed to be suitable for any area characterized by (1) generally unidirectional horizontal strain and (2) an adequate population of geologically contemporaneous fissures, including roughly equal development of sinistral and dextral en echelon arrays. Where exposures and aerial photograph coverage are adequate, the direction can be determined quickly to within about 10 degrees accuracy without fieldwork. The method should be useful for preliminary structural studies, especially in inaccessible areas.

Journal of Research of the U.S. Geological Survey↗