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The Lassen region has been volcanically active for about 3 million years. Lassen Peak and
nearby volcanic domes are the most recently active parts of the Lassen "volcanic center", which
began to erupt about 600,000 years ago. From 600,000 to 400,000 yars ago, eruptions built a
large conical volcano, often referred to as "Brokeoff Volcano" or "Mount Tehama", which was
roughly the size of Mout St. Helens, Washington. Later, this volcano became inactive and was
mostly eroded away, leaving remnants that include Brokeoff Mountain, Mount Conard, Mount
Diller, and Diamond Peak.
Subsequent eruptions from the Lassen volcanic center have formed more than 30 steep-sided, mound-shaped accumulations of volcanic rock, called "lava domes". Eruptions about 27,000 years ago formed Lassen Peak, one of the largest lava domes on Earth. When Lassen Peak formed, it looked much like the nearby 1,100-year-old Chaos Crags domes, with steep sides covered by angular rock talus. However, from 25,000 to 18,000 years ago, during the last ice age, Lassen Peak's shape was significantly altered by glacial erosion. Lassen Peak and its neighboring lava domes are not typical, conical "stratovolcanoes" like Mount Shasta or Mount Rainier. Those large volcanoes were formed by repeated eruptions of lava and ash from a central summit vent over tens of thousands of years. The Lassen dome field, in contrast, is an example of a volcanic area that erupts lava from numerous individual vents, each of which is active for a few years to a few decades and usually does not erupt again. The composition of the molten rock (magma) that feeds volcanism in the Lassen area ranges widely in its content of silica (SiO2). When high-silica (dacite) magma rises to the Earth's surface, it can erupt explosively to produce ash clouds and pyroclastic flows. Dacite magma extruded nonexplosively forms lava domes, because it is too viscous to flow far away from its source. Low-silica (basalt) magma is more fluid and usually erupts less explosively than dacite magma. Eruptions of basalt magma typically produce elongate lava flows, as well as build cinder cones (piles of small frothy lava fragments or "cinders") around volcanic vents. In the past 50,000 years, at least seven major episodes of dacitic volcanism produced lava domes and pyroclastic deposits in the Lassen area, and another five episodes produced basaltic and andesitic (silica content between basalt and dacite) lava flows. In addition, about 20 smaller volcanoes erupted basaltic lavas in the larger region surrounding the Lassen volcanic center. |
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