Newberry Volcano - Geology

Geology

The Newberry Volcano lies at the northwest end of both the Brothers Fault Zone, and the High Lava Plains (area of blue lines on diagram at right) of volcanic features in central Oregon that have been developing since about 16 Ma. Volcanism in the High Lava Plains is bimodal, having both basaltic (mafic) and rhyolitic (silicic) types of lava. Some of the rhyolites show a progression in ages from about 10 Ma in the east to the geologically recent Newberry Caldera in the west. This track, and its implication of a moving source, roughly mirrors the similar track of the Yellowstone Hotspot from the McDermitt Caldera on the Oregon—Nevada border, on which basis the Newberry volcanism has been attributed to a so-called hotspot (a plume of hot material arising from the earth's mantle). But whereas the Yellowstone Hotspot reflects the motion of the North American plate across a hotspot, the Newberry track does not. Although the cause of the intense regional volcanism around 16 Ma is hotly debated, the age progression across the High Lava Plains is now attributed to a reservoir of hot material that expanded over time, and the linearity of the trend attributed to exploitation of a deep, pre-existing weakness in the crust associated with the Brothers Fault Zone.

A great deal of volcanic activity has occurred on Newberry's shield, which itself has one of the largest collections of cinder cones, domes, lava flows, and fissures in the world—so much so that local residents call these parasitic vents the "Paulina Mountains", thinking of them as a separate mountain range. Most of the cinder cones are 200 to 400 feet (60 to 120 m) high and have shallow saucer-shaped summit craters. They are typically surrounded by basalt or andesite that erupted from their bases forming large lava beds. The northern flank holds three distinct lava tube systems that formed in pāhoehoe: the Horse Lava Tube System, Arnold Lava Tube System, and the Lava Top Butte basalt. On the northwest flank of the volcano is Lava Butte, located next to Highway 97 south of Bend, is a good example of this kind of cinder cone and an ʻaʻā lava bed. There are also about 20 rhyolite domes or fissures on the eastern, southern, and western flanks. Larger examples include 580,000-year-old McKay Butte on the west flank, 80,000-year-old China Hat and 850,000-year-old East Butte on the far eastern base.

During the late Pleistocene and Holocene periods, there have been six eruptive episodes. Four of them rhyolitic, on the eastern half of the caldera, and two basaltic, on the flanks.

• South Obsidian Eruptive Episode: An estimated 12,000 years ago, an obsidian dome and related obsidian flow erupted in the southeast part of the caldera.
• East Rim Eruptive Episode: About 11,200 years ago (10,000 14C years B.P.), mafic cinders, scoria, spatter, and lava flows erupted from a fissure on the east rim of the caldera.
• Interlake Eruptive Episode: A series of rhyolitic eruptions began in the caldera approximately 7,300 years ago (6,200 14C years B.P.). They produced a widespread phreatomagmatic pumiceous tephra deposit, obsidian flows, large and small pumice cones, and a pumice ring. This eruptive episode probably lasted for about 200 years.
• Northwest Rift Eruptive Episode: About 7,000 years ago (6,100 14C years B.P.), basaltic andesite lava and cinder cones erupted from extensive fissure vents on the northwest and south flanks of Newberry. Spatter and cinders also erupted from a fissure on the north caldera wall. The lava flows range up to 5.6 miles (9 km) long and are more voluminous at lower elevations. This eruptive episode probably lasted for less than 50 years.
• East Lake Eruptive Episode: About 3,500 years ago, obsidian flows and associated pumice deposits in the caldera erupted from caldera ring fractures.
• Big Obsidian Eruptive Episode: About 480 A.D. (1,470 14C years B.P.), a three-part sequence of rhyolitic eruptions began which included an air-fall tephra, ash-flow tephra, and an obsidian flow from a common vent at the base of the south caldera wall. The initial Plinian eruption 480 A.D. (1,580 14C years B.P.) produced the Newberry pumice fall deposit which blanketed the east flank of the volcano and areas to the east. About 210 years later, the Paulina Lake ash flow (1,310 14C years B.P.) spread from near the south caldera wall to Paulina Lake. The final phase of the eruption produced the Big Obsidian Flow, which covers 1.1 square miles (2.8 km2).

Newberry's highest point is located 1,500 feet (460 m) above the southern caldera floor on Paulina Peak, which is just one peak on the caldera rim. The volcano's south flank descends into the basaltic flatlands of central Oregon.