Geology topics

Drew T. Downs

Publications and source records attributed to Drew T. Downs.

42 records · Page 3Linked to original sources

USGS2019

Eruption age and duration of the ~9 km3 Burney Mountain dacite dome complex, northern California

At ~9 km3, the six dacite domes of Burney Mountain (db1–db6) constitute the most voluminous Quaternary dome complex in the Cascades volcanic arc. Whole-rock geochemistry, electron microprobe, and petrographic data indicate that the domes are magmatically related, which, when integrated with geomorphology and stratigraphy, indicate early (db1, db2, db3) and late (db4, db5, db6) erupted groups. We present 40Ar/39Ar ages of 271.9±4.6 ka (db1), 280.8±8.2 and 281.7±6.8 ka (db2), and 290.2±6.0 ka (db3) along with a previous age of 280±12 ka (db1). These ages scatter over 20 kyr, whereas remanent magnetic directions are similar between 53.3–59.0° inclination and 352.7–355.9° declination. The latter dataset indicates that the dacite domes were emplaced over a geologically brief time interval; not thousands of years. Crystal-size distribution patterns of plagioclase were used to calculate residence times, which we use to infer the duration over which the eruptions likely occurred. Three slopes represent three populations of plagioclase crystals (fine-grained groundmass, coarse-grained groundmass, phenocrysts). A commonly used growth rate for plagioclase in dacitic magmas (10-10 mm/s) yields 9–10 years of growth for the coarse-grained groundmass (early erupted domes of db1, db2, db3), whereas plagioclase in the fine-grained groundmass (late erupted domes of db4, db5, db6) grew over 4–5 years. All plagioclase phenocrysts have apparent residence times of 26–36 years; however, they contain high An>70 resorbed cores with sieve textures, which have euhedral, lower An<65 overgrowth rims. Similarities in chemistry between groundmass plagioclase and phenocryst overgrowth rims indicate that they grew concurrently, and we therefore propose that both have similar residence times. Thus, the Burney Mountain dacite dome complex was emplaced during a single eruptive episode over the course of years to decades at 281.1±4.8 ka (weighted mean age).

California↗

USGS2018

Reconstructing lava flow emplacement histories with rheological and morphological analyses: the Harrat Rahat volcanic field, Kingdom of Saudi Arabia

Mafic volcanic fields are widespread, but few have erupted in historic times, providing limited observations of the magnitudes, dynamics, and timescales of lava flow emplacement in these settings. To expand our knowledge of effusive mafic eruptions, we must evaluate solidified flows to discern syn-eruptive conditions. The Harrat Rahat volcanic field in western Saudi Arabia offers a good opportunity for this, with a historical eruption in 1256 CE and many well-preserved prehistoric flows. We combine historical observations and rheological and morphological analyses of the youngest flows with analytical models to reconstruct eruptive histories and lava flow emplacement conditions in Harrat Rahat. Petrologic analysis of samples for emplacement temperatures and crystallinities shows cooling trends from vent to toe of ~ 1140 to ~ 1090 °C at rates of 2–7 °C km −1 , crystallinities increasing from 0.5 to 60%, and apparent viscosities increasing from 10 2 to 10 9 Pa s. High-resolution topographic data facilitates quantitative analysis of morphology and interpolation of pre-eruptive surfaces to measure flow thicknesses, channels, and levees, and enables calculation of eruptive volumes. Analytical models relating flow morphology to emplacement conditions are applied to estimate effusion rates. Within the suite of studied flows, volume estimates range from 0.07 to 0.42 km 3 dense rock equivalent, with effusion rates on the order of 10 to 100 s of m 3 s −1 and durations from 1 to 15 weeks. These integrated analyses quantify past lava flow emplacement conditions and dynamics in Harrat Rahat, improving our understanding and observations of fundamental parameters and controls of effusive eruptions in mafic volcanic fields.

Harrat Rahat↗

USGS2018

Timescales of magmatic differentiation from alkali basalt to trachyte within the Harrat Rahat volcanic field, Kingdom of Saudi Arabia

A fundamental goal of igneous petrology is to quantify the duration of time required to produce evolved magmas following influx of basalt into the crust. However, in many cases, complex field relations and/or the presence of a long-lived magmatic system make it difficult to assess how basaltic inputs relate to more evolved magmas, therefore, precluding calculation of meaningful timescales. Here, we present field relations, geochemistry, 40 Ar/ 39 Ar ages, and 36 Cl ages for volcanic rocks from the Harrat Rahat volcanic field, Saudi Arabia. These data document a systematic and repeated temporal progression from alkali basalt to trachyte for the youngest eruptives. From ~ 150 to ~ 17 ka the following eruptive sequence occurred four times: (1) alkali basalt, (2) hawaiite, mugearite, or benmoreite, and (3) trachyte. We interpret each eruptive sequence to result from injection of basalt into the crust, and its subsequent differentiation and eruption of progressively evolved magmas. We use the interval time between successive eruptions within a given sequence to calculate the duration of time required to produce trachyte from alkali basalt. Differentiation from alkali basalt to intermediate compositions (hawaiite, mugearite, benmoreite) took ≤ 2 kyr on average. Differentiation from intermediate compositions to trachyte took a maximum of 6.6 ± 3.5 to 22.5 ± 1.6 kyr. Thus, the total duration of differentiation was ~ 9 to ~ 25 kyr. Timescales presented here are insensitive to processes evoked to drive differentiation because they are based solely on the ages and compositions of eruptive products from a system characterized by a simple, repeated differentiation sequence.

Harrat Rahat volcanic field↗

USGS2018

Volcanic history of the northernmost part of the Harrat Rahat volcanic field, Saudi Arabia

We present a detailed geologic investigation of Pleistocene to Holocene mafic volcanism within the northernmost part of the Harrat Rahat volcanic field, proximal to the city of Al-Madinah, Saudi Arabia. Our study area covers ∼570 km 2 , and encompasses lava flows, scoria cones, and shield volcanoes of 32 mapped eruptive units consisting of continental, intraplate alkalic and tholeiitic basalts, hawaiites, and a mugearite that erupted from at least 1014 ± 14 ka to a single Holocene event at 1256 A.D. Typical lava flows are roughly 10–15 km long, although they reach nearly 23 km, 1–3 km wide, and ∼10 m thick. The majority of eruptives in our study area erupted ca. 400–340 ka and ca. 180–100 ka. Despite small individual volumes (<1 km 3 dense rock equivalent), each unit resulted from eruption of a distinct magma batch that was influenced by clinopyroxene, olivine, and plagioclase fractionation. Some of these units are interpreted to have undergone magma mixing prior to eruption. Combining our age determinations, geochemistry, and paleomagnetic data sets indicates that several eruptions were temporally and/or spatially clustered. Aligned scoria cones and elongate vent edifices were constructed atop fissure vent systems that reflect the stress field controlling dike ascent through the middle to upper crust.

Geosphere↗

USGS2017

Age of the youngest volcanism at Eagle Lake, northeastern California— 40 Ar/ 39 Ar and paleomagnetic results

The age of the youngest volcanism at Eagle Lake, California, was investigated using stratigraphic, paleomagnetic, and 40 Ar/ 39 Ar techniques. The three youngest volcanic lava flows at Eagle Lake yielded ages of 130.0±5.1, 127.5±3.2 and 123.6±18.7 ka, and are statistically indistinguishable. Paleomagnetic results demonstrate that two of the lava flows are very closely spaced in time, whereas the third is different by centuries to at most a few millennia. These results indicate that the basalt lava flows at Eagle Lake are not Holocene in age, and were erupted during an episode of volcanism at about 130–125 ka that is unlikely to have spanned more than a few thousand years. Thus, the short-term potential for subsequent volcanism at Eagle Lake is considered low.

California↗

USGS2016

Mihi Breccia: A stack of lacustrine sediments and subaqueous pyroclastic flows within the Taupo Volcanic Zone, New Zealand

The Taupo Volcanic Zone (TVZ), New Zealand, encompasses a wide variety of arc-related strata, although most of its small-volume (non-caldera-forming) eruptions are poorly-exposed and extensively hydrothermally altered. The Mihi Breccia is a stratigraphic sequence consisting of interbedded rhyolitic pyroclastic flows and lacustrine sediments with eruption ages of 281 ± 18 to at least 239 ± 6 ka (uncertainties at 2σ). In contrast to other small-volume rhyolitic eruptions within the TVZ, Mihi Breccia is relatively well-exposed within the Paeroa fault block, and contains minimal hydrothermal alteration. Pyroclastic flow characteristics and textures including: 1) breadcrusted juvenile clasts, 2) lack of welding, 3) abundant ash-rich matrix, 4) lack of fiamme and eutaxitic textures, 5) lack of thermal oxidation colors, 6) lack of cooling joints, 7) exclusive lacustrine sediment lithic clasts, and 8) interbedding with lacustrine sediments, all indicating that Mihi Breccia strata originated in a paleo-lake system. This ephemeral paleo-lake system is inferred to have lasted for > 50 kyr (based on Mihi Breccia age constraints), and referred to as Huka Lake. Mihi Breccia pyroclastic flow juvenile clast geochemistry and petrography correspond with similar-aged (264 ± 8, 263 ± 10, and 247 ± 4 ka) intra-caldera rhyolite domes filling the Reporoa caldera (source of the 281 ± 81 Kaingaroa Formation ignimbrite). These exposed intra-caldera rhyolite domes (as well as geophysically inferred subsurface domes) are proposed to be source vents for the Mihi Breccia pyroclastic flows. Soft-sediment deformation associated with Mihi Breccia strata indicate either seismic shock, rapid sediment loading during pyroclastic flow emplacement, or both. Thus, the Mihi Breccia reflects a prolonged series of subaqueous rhyolite dome building and associated pyroclastic flows, accompanied by seismic activity, emplaced into a large paleo-lake system within the TVZ.

Taupo Volcanic Zone↗