Mount St. Helens has been the most active and most explosive volcano in the Cascade Range during the past 4,500 years.
Mount St. Helens has been the most
active and most explosive volcano in the Cascade Range during the past
4,500 years.
More than 100 layers of volcanic
ejecta, or tephra have been
found around Mount St. Helens. These testify to its restless nature.
These layers are exposed in river
valleys, and in the steep walls of the crater
They record a turbulent history
which stretches back 40,000 years.
Layers of volcanic rocks scattered
across the landscape read like loose pages in a history book. These
layers help geologists to reconstruct a volcano's life history. The
geologists study clues hidden in each layer and map the order in which
the layers were deposited.
Geologists use this approach to
divide the history book of Mount St. Helens into nine "eruptive periods".
Each period was marked by episodic activity lasting from a few decades to
a few thousand years.
During this time, lava domes,...
lava flows,...
hot pyroclastic flows,...
mudflows,...
and ash were repeatedly
produced.
These active periods were
separated by quiet times. These times lasted from less than a century to as much as
a few thousand years.
But based on its past behavior,
Mount St. Helens is likely to remain active for several decades. During
the current active period, it may produce additional explosive
eruptions.
Return to:
Mount St. Helens & Other
Volcanoes
Geological History
Before the Eruption
Mount St. Helens
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