Geology Reference
In-Depth Information
A cinder cone just south of Bend, Lava Butte has a well-developed crater on top and a vent which
extruded lava on the south side (photo courtesy of Oregon Department of Geology and Mineral
Industries).
Crater at the summit. Covering 500 square miles, the
volcano is among the largest Quaternary volcanoes in
the United States. The caldera was caused by collapse
or sinking of the center of the mountain in somewhat
the same manner that the caldera formed at Crater
Lake. Today the central caldera is occupied by East
and Paulina lakes which are separated by cinder cones
and a massive obsidian flow.
With a comparatively short eruptive life lasting
around 1/2 million years, the active period of Newberry
volcano spans that of Crater Lake 70 miles to the
southwest. About 500,000 years ago, a huge shield
volcano, 4,000 feet above the surrounding plateau, was
built by successive flows of basalt and silica-rich
rhyolite interspersed with layers of volcanic ash, dust,
and consolidated tuffs. The summit of the volcano may
have collapsed at least twice after two large eruptions
to create the caldera. Continued intermitent activity
produced small domes, lava flows, and obsidian 300,000
to 400,000 years ago that was followed by a huge ash
eruption about 1600 years ago. Most recent activity
began 2,000 years ago when lava discharged from a
fracture on the south rim of the caldera spreading
about 1 1/2 miles toward the center. This striking Big
Obsidian flow is the largest of several young flows in
the crater producing over 1/2 cubic miles of frothy
pumice, ash, and obsidian as well as the dome marking
the vent. A distinctive 6,900 year old layer of pumice
and ash from ancient Mt. Mazama is laminated in
among Newberry's own eruptive debris. Near the end
of the cycle more than a half dozen separate obsidian
flows, interlayered with a volcanic froth or pumice,
spread out within the caldera.
The long outer slopes of the volcano are
sprinkled with over 400 parasitic cinder cones, some in
regular lines, indicating alignment along fractures.
Nineteen rhyolitic domes, flows, small explosion
