The history of the Lassen volcanic center then proceeds differently.
From there 310,000 years , the activity involves the north side of Tehama / Brokeoff volcano , where more silicic eruptions produce the Lassen dome field , a heart of dacitic domes, surrounded by an arc of hybrid andesitic flows
Dacite domes are divided into two sequences after the age of eruptions :
The Bumpass sequence, of 300,000 to 190,000 years , has produced a group of 15 dacitic to rhyodacitic lava domes in the southern part of the field.
Eagle Peak sequence , of 70,000 years to the present day , is the mother of more than seven dacitic to rhyodacitic lava domes, lava flows and pyroclastic deposits in the northern part of the field . This sequence includes the formation of two prominent structures: the Lassen Peak, 27,000 years old , and Chaos Crags 1,100 years old , and the products of the five oldest eruptions.
Similarly andesitic flows are divided into two groups: the older Twin Lakes sequence , dated from 315,000 to 240,000 years , and Twin Lakes youngest sequence , 90,000 years to the present day . Clustered cones and lava flows are located primarily on the central plateau of Lassen Volcanic Park and around the Lassen dome field .
It should be noted that these various sequences are characterized by a series of lava formed by a continuous process of magma mixing , producing variable rock units in composition and appearance.
Furthermore, no volcanic activity is dated to the period from 190,000 to 90,000 years.
Eruptions at Lassen Volcanic Center in the last 70,000 years / Time line - click on photos to enlarge - doc . USGS
The two most recent marking the volcanic eruptions center :
- Around the year 1666, a period of activity of several hundred years built the complex Cinder Cone .
- The apical Lassen Peak eruptions from 1914 to 1917, are the latest episode of activity center.
They are considered part of the young sequence Twin Lakes.
The Lassen Peak seen from Lake Helen - caps of black dacite rocks emerge from the clearer dome - photo Lee Siebert 1972
More in detail , the eruption of Chaos Crags :
The development of Chaos Crags happened during a typical basaltic volcanic cycle , dated there 1100 1000 years.
The initial activity formed a cinder cone , set up two pyroclastic flows and grows a dome that has corked the vent.
After an interval of about 70 years , the dome was pulverized by an explosion , which caused a pyroclastic flow and ash fall, visible in the drainages of Manzanita and Lost Creek located within 10 km. Chaos Crags.
This eruption was followed by the growth of five domes , three presented avalanches collapse of the dome. Chaos Jumbles was formed 700 years later when the domes collapsed in a series of three avalanches of blocks. (Wood & Kienle )
The morphology of the domes has hardly been modified by erosional processes.
The eruption of Cinder cone :
Cinder Cone, 215 meters high , is located northeast of Lassen Peak.
Its eruption in 1666 is responsible for five lava basalto - andesitic and andesitic lava flows, forming two cinder cones and ash fallout over an area covering 200-300 km ² ... all this in a short time .
The remains of the original cone are still visible on the south side of the larger of the two cones currently present (see the lava flows map ) .
Lassen Volcanic Park - the double edge crater of Cinder Cone - the scale is given by a man in the center - photo BelsKr2004
Map of the Cinder Cone's lava flows : Fantastic lava beds and Painted Dunes lava flows - YC : young scoria cone - OC : old scoria cone - USGS