a national monument by President Theodore Roosevelt to protect the area for "future generations to study and enjoy". Roosevelt had been told that the area was actually extinct in terms of eruptive activity. Despite assurance from Native Americans in the area that the mountain was indeed active, settlers continued to think that Lassen Peak was extinct and "dead" and so continued to settle nearby. The natives continued to declare that "one day the mountain would blow itself to pieces". In May 1914, almost exactly seven years later, the volcano began a large explosive eruption sequence. More than a hundred eruptions of varying size took place over the next seven years, attracting national interest and the designation of national park in 1916. Geography and geology The state of California is located on the Pacific Ring of Fire, an area of highly active tectonic activity that includes seismicity and volcanism. Brokeoff and the other volcanoes near Lassen Peak were produced by subduction of the oceanic Juan de Fuca Plate under the North American Plate. Brokeoff Volcano represents the Pleistocene component of the Lassen volcanic complex, an area of active volcanism for more than three million years. It formed during a period of eruptive activity approximately 600,000 years ago on a series of faults; the largest of these eruptions was about 50 times as powerful as the 1980 eruption of Mount St. Helens. During the Pleistocene, constantly renewed lava of andesite flowed from the central vent of the volcano. The eruption of overlapping andesitic lava flows accumulated to build the volcano. By the time it reached its peak size, the volcano was immense, measuring at its base between and wide and more than tall. Activity at Brokeoff lasted for approximately 200,000 years. Eruptions varied from ejection of pyroclastic material to andesitic lava flows built primarily of feldspar, pyroxene, and olivine. Soon after the formation of Brokeoff, other, smaller volcanoes began to form. Shield volcanoes like Raker Peak, Red Mountain, Prospect Peak, and Mount Harkness appeared. At this point, the Pleistocene was coming to an end. One additional and final vent formed on the northeastern flank of the volcano, erupting glassy dacitic flows. Over time, the volcano became dilapidated from extensive erosion and hydrothermal alteration, and was eroded by creeks and glaciers. Later activity built more than thirty other cones known as the Lassen Domes. Today the most active volcano in the area is