Geologic evidence for a magma chamber beneath Newberry Volcano, Oregon

Abstract

At Newberry Volcano, central Oregon, more than 0.5 m.y. of magmatic activity, including caldera collapse and renewed caldera‐filling volcanism, has created a structural and thermal chimney that channels magma ascent. Holocene rhyolitic eruptions (1) have been confined mainly within the caldera in an area 5 km in diameter, (2) have been very similar in chemical composition, phenocryst mineralogy, and eruptive style, and (3) have occurred as recently as 1300 years ago, with repose periods of 2000–3000 years between eruptions. Holocene basaltic andesite eruptions are widespread on the flanks but are excluded from the area of rhyolitic volcanism. Basaltic andesite in fissures at the edge of the rhyolite area has silicic inclusions and shows mixed basalt‐rhyolite magma relations. These geologic relations and the high geothermal gradient that characterizes the lower part of a drill hole in the caldera (U.S. Geological Survey Newberry 2) indicate that a rhyolitic magma chamber has existed beneath the caldera throughout the Holocene. Its longevity probably is a result of intermittent underplating by basaltic magma.

Upper crustal structure of Newberry Volcano from P‐wave tomography and finite difference waveform modeling

“…Silicic eruptions mainly occur within the caldera, such as the Big Obsidian Flow (1300 years old), and mafic eruptions occur outside the caldera, such as the Northwest Rift Zone eruptions of 7,000 years ago [ Mckay et al , 2009]. To cause this pattern, rising mafic magmas are thought to encounter a central, silicic magma storage system beneath the caldera [ MacLeod and Sherrod , 1988]. The fresh mafic magma intrudes and underplates the silicic magma, causing it to erupt in the caldera, but elsewhere the unobstructed mafic magma erupts at the surface.…”

Multiple origins of obsidian pyroclasts and implications for changes in the dynamics of the 1300 B.P. eruption of Newberry Volcano, USA

“…The most recent, called the Big Obsidian Eruptive Period, occurred about 1300 B.P. (Kuehn 2002;MacLeod and Sherrod 1988).…”

“…1). These three phases are compositionally indistinguishable and appear to have erupted from the same vent (MacLeod and Sherrod 1988). A reworked ash layer that underlies the Newberry Pumice may also be part of the Big Obsidian Eruptive Period, suggesting that there was minor explosive activity prior to the climactic eruption (Kuehn 2002).…”

“…A reworked ash layer that underlies the Newberry Pumice may also be part of the Big Obsidian Eruptive Period, suggesting that there was minor explosive activity prior to the climactic eruption (Kuehn 2002). The total erupted volume is about 0.2 km 3 dense rock equivalent (DRE) divided approximately equally between the obsidian lava and pyroclastic fall deposits, with only 0.002 km 3 DRE in the pyroclastic flow deposit (MacLeod and Sherrod 1988). The total tephra volume of 0.1 km 3 is typical of deposits from supblinian eruptions (VEI=4).…”

Field-trip guide to the geologic highlights of Newberry Volcano, Oregon

“…Geothermal drilling by private industry (Swanberg and others, 1988) followed and has continued intermittently (Frone and others, 2014). Geologic and geophysical work that began at the volcano in the 1970s resulted in a variety of studies published in a special issue of the Journal of Geophysical Research (MacLeod and Sherrod, 1988). These studies, together with geologic mapping by MacLeod and others (1995), provided a basic framework for understanding the volcano.…”