MJC Rocks of Geotripper correctly identified our mystery mountain as Lassen Peak (sometimes incorrectly called Mt. Lassen). In my first WITW post, I didn't want to say "20th century" eruption because if you Google: 20th century eruption ash nevada - Lassen Peak comes up 2nd and 4th (without the ash it comes up 4th), hence my geologically vague "fairly recent" time descriptor.
Lassen Peak, the southernmost volcano in the Cascade Range, is about 70 km (about 45 miles) northeast of Red Bluff, CA, and about 120 km (about 75 miles) southeast of Mt. Shasta. Lassen Peak's most recent eruptive cycle began in 1914 and lasted until 1921. The largest eruption was on May 22, 1915, with locally devastating pyroclastic flows and lahars, a large ash plume, and ash falling as far east as Elko, Nevada.
The generalized map above, from the USGS - Cascades Volcano Observatory site, shows the volcanic deposits resulting from the eruptions of May, 1915, along with the tree blow-down line, which outlines an area now simply called the Devastated Area. A better version of this map and a more detailed description of events can be found in the USGS Fact Sheet 173-98. You can find other good references and links at the CVO Menu page for Lassen Peak and the Lassen Volcanic National Park website.
Brokeoff Mountain, above, is part of an older, larger stratovolcano sometimes called Mt. Tehama, a volcano similar to today's Mt. Shasta, and almost the same size as Mt. St. Helens before it blew up in May, 1980. Mt. Tehama began erupting 600,000 years ago and continued erupting until about 400,000 years ago, when a collapse caldera formed, similar to today's Crater Lake, without the lake.
On Highway 89, between the south entrance to the park and Lassen Peak itself, you can drive right through the Sulphur Works hydrothermal area, which features hot springs, fumaroles, and bubbling mudpits. Sulphur Works is thought to be part of the central vent area of old Mt. Tehama. In winter, you can cross-country ski up the highway to this area (and elsewhere). For a really great hydrothermal tour, hike down the Bumpass Hell Trail to the Bumpass Hell hydrothermal area. They say that the area is named after a settler who fell into a boiling pool, but people have been wondering about the pronunciation for a long time (bum-pass or bump-ass?).
Above, an example of the volcanic rocks you can see on Highway 89 going north (and switchbacking around a lot) toward Lassen Peak.
Highway 89 is usually open from sometime in June until sometime in October or later. The first time I drove through the park, I was coming south from a family-type get-together in Oregon. We drove over the pass from north to south and stopped at the old ski chalet, which has since been torn down in order to build a real visitor's center. They had some great "Go Climb a Volcano" T-shirts, so we all got one! Currently, you can support the park by going to the online store and buying green wristbands. They also have some great books and posters, inlcuding a poster of the 1915 eruption.
And here's a view of the "backside" of Lassen Peak, from the parking lot in the Devasted Area on the northeast side of the mountain. If you continue driving north on Highway 89, there are other stops to make, one at Subway Cave, an old lava tube - and if you continue on to Mt. Shasta, you can stop at Burney Falls.
UPDATE 18Jun2010: Lutz has a great photo of the Sulphur Works from 2007 at geoberg.de.
UPDATE 22Sept2012: The above photos were taken in October, 2006.