batholith. Widespread dormancy period The West Cascades period came to an end 17 million years ago when the Columbia River flood basalts began erupting in eastern Washington and Oregon. For a reason unknown to scientists the initiation of the flood basalts seemingly caused a significant dip in volcanic activity in the cascade chain lasting for over 8 million years. During this time the volcanoes were stripped down to their cores by weathering and erosion because they were not active enough to rebuild. This low point lasted from 17 to 9 million years ago and came to end when the Columbia flood basalts waned. High Cascades period As production of the Columbia River flood basalts slowed 9 million years ago the Cascade volcanoes became active again. The volcanic arc also drifted further east to its present location. When the Columbia basalts stopped entirely 6 million years ago the Cascades of central Oregon spectacularly flared up. This flare up lasted between 6.25 and 5.45 million years ago and is known as the Deschutes Formation. During this 800,000 year span around 400 to 675 km3 of pyroclastic material was expelled in 78 distinct eruptions. It has been hypothesized that a heightened flux of basalt, possibly induced by tectonic slab-rollback, was focused beneath the volcanic arc and into the shallow crust by minor amounts of crustal extension. This extension allowed for the high flux of basalt to be stored at shallow levels beneath a new arc locus within fertile crust, resulting in the silica-rich volcanism we see in the Deschutes Formation. After this pulse of activity the cascades retreated to the levels of activity we are more familiar with today. For the remaining 5 or so million years the ancestors of many of the modern day Cascade volcanoes were built. Around half a million years ago a generation of older volcanoes died and many of the stratovolcanoes that we see today began their growth such as Glacier Peak and Mt. Shasta (600,000 years ago), Mt. Rainier and Mt. Hood (500,000 years ago), Mt. Adams (450,000 years ago), and Mt. Mazama (420,000 years ago). Modern arc The volcanoes of the Cascade Arc share some general characteristics, but each has its own unique geological traits and history. Lassen Peak in California, which last erupted in 1917, is the southernmost historically active volcano in the arc, while the Mount Meager massif in British Columbia, which erupted about