Mount St. Helens
Castle Creek and Sugar Bowl eruptive periods

The next eruptive period, the Castle Creek period, began about 400 BC, and is characterized by a change in the composition of St. Helens' lava, with the addition of olivine and basalt. The pre-1980 summit cone started to form during the Castle Creek period. Significant lava flows in addition to the previously much more common fragmented and pulverized lavas and rocks (tephra) distinguished this period. Large lava flows of andesite and basalt covered parts of the mountain, including one around the year 100 BC that traveled all the way into the Lewis and Kalama river valleys. Others, such as Cave Basalt (known for its system of lava tubes), flowed up to 9 miles (14 km) from their vents. During the first century, mudflows moved 30 miles (50 km) down the Toutle and Kalama river valleys and may have reached the Columbia River. Another 400 years of dormancy ensued.

The Sugar Bowl eruptive period was short and markedly different from other periods in Mount St. Helens history. It produced the only unequivocal laterally directed blast known from Mount St. Helens before the 1980 eruptions. During Sugar Bowl time, the volcano first erupted quietly to produce a dome, then erupted violently at least twice producing a small volume of tephra, directed-blast deposits, pyroclastic flows, and lahars. East Dome, a small hypersthene-homblende dacite dome on the east slope of the volcano, was likely formed around the Sugar Bowl period. Formation of East Dome was preceded by an explosive eruption.
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