(You should read about the more mature and seasoned volcanoes of Pennsylvania here.)
Mt. Lassen National Park and volcanic far-northern California is a part of the country most people don't think about - even most people in the rest of California. California is big. It has a whole top half that people forget about. From the Golden Gate Bridge it's 340 miles to the Oregon border.
Lassen is in the Cascades, not the Sierras. The Sierras extend north to about Susanville (the border is the red line on the map - border zones of any kind are always interesting in their own right.) The Cascades begin there and go northward just barely into British Columbia, a product of the subduction of the Juan de Fuca plate (red arrow pointing at the submarine ridge marking the southern plate boundary.) Our first stop on this trip (the middle circle on the map) was therefore the the southernmost Cascade. I used to argue that the often-overlooked Sutter Buttes in California's Central Valley (the southernmost circle) were the southernmost Cascade, but among geologists (see here for Hausback et al's article) this seems to be more of a settled matter today so I must defer to the experts.
The area has more geologic and cultural history than any such empty quarter has a right to. Lassen last erupted in 1916, and Ishi was the last uncontacted Native American, a story that's really worth reading, both fascinating and of course sad.
Recent eruptions aside, Mt. Lassen is a good volcano for families. I've literally lost count of the number of times I've been at the top, but it's also a hike, not a climb, and it's not a death march by mountain hiking standards. That said, it's 2.5 miles to the top of average 15% slope, starting at 8,500' ending in at 10,500', so it can be annoying.
As you can see above from my awesome mountaineering moves (I call this the reverse glissade) the top is cool and all (there's usually a lot of snow algae) and there are steam vents but the best part of the park is Bumpass Hell, a steaming, boiling, sulfurous, Io-like bubbling volcanic waste in the middle of the park, down the slope of the mountain.
(More here.) They close the trail to it for much of the year because of snow and ice hazards, because they don't want MORE people falling into the boiling acidic mud and cooking their leg (seriously. That's what happened to the eponymous Mr. Bumpass. If you thought "Bumpass Hell" wasn't family friendly, would you prefer "Cooked Human Leg Thermal Area"?) And the rangers REALLY get sick of people asking when the trail will open, so to prevent this they put the data from past years right on the website. I knew we would be going the first weekend of August. Through July, the trail remained closed. I was getting nervous. So I did what any family man would do, which is plug the opening-date data into a linear regression:
I don't know how I missed these after 25 years living in this state. First, maybe not a necessary point for anyone else besides me: they are NOT McCloud Falls, with which I often confused them in days past. You can swim and wade in McCloud Falls if you can tolerate cold water, but Burney Falls is colder still, remaining at a toasty 42 to 48 degrees Fahrenheit. As we descended into the valley the temperature dropped by at least 20F in 0.3 miles. I was trying to figure out on the map what stream or river the falls were on, and the answer is - they are the beginning of a river. If you look at the top of the falls, you can see the water is coming directly out of the ground. Definitely the most unique and I think most esthetically pleasing falls I've ever seen.
Below: it's good to see that visitors to the park are so considerate that they risk a fine for vandalism by carving an underline into the sign, emphasizing the strength of the waterfall, so as to warn would-be swimmers of the danger.
THE LAVA BEDS
Lava Beds National Park is the greatest national park you've never heard of. Great for families, full of "actionable" geology, native history, and two of the less-proud chapters of American history. The Lava Beds and neighboring Tulelake really are smack up against the Oregon border. It's inland high desert, so it doesn't look much like what people expect the area would. (You want redwood trees? Go to the coast. Redwood National Park is a mere jaunt of 286 miles to the west.)
The Lava Tubes, Cinder Cones, and Ice Caves
There are hundreds of lava tubes in the park, caves formed by the movement of lava underground. Some of them are big - you could drive a truck through them. Others are small and you spend most of your time inside crouching. There are some developed for tourism, many of them within 1/4-1/2 mile of the park. By "developed" I mean, one of them has lights, and others just have a ladder leading down into the darkness. The undeveloped ones are holes in the ground you crawl into. I thought this would be a tough sell for a young kid, but after initial hesitation, this was by far the favorite part of the trip. Sentinel Cave in particular was pretty cool. There are lava flows and cinder cones throughout the park as well. You don't have to be a geologist to see them and understand what formed them.
Credit above to NPS, and below to Grant Sinclair at wanderfilledlife.com.
Above: the interior of a cave without light (Duh. Although, it really is an actual photo.) Below: with light (Sentinel Cave.)
(Above: a collapsed cave.) We also visited a cave outside the park that has year-round ice in it (the aptly named Jot Dean Ice Cave) but it wasn't quite as nifty as I remembered - no River Alf, no floating hair, and not a flashing eye to be found. Maybe because I forgot to bring any milk of paradise, or honeydew to dine on. You can see a couple tubes just peeking above the surface in the pictures. I've read that just in the Lava Beds park proper they discover on average six new caves per year, and there are doubtless many more to be discovered in the surrounding national forest.
Above image inside the cave from Steven Johnson's Flickr page. Below, landscape around the cave, and below that, a very ominous sign. "Other occupants"?
Any trip that doesn't have a (positive) surprise is suboptimal, and for this one, it was the fantastic dark-sky talk given by a ranger at the Lava Beds. I really can't recommend it enough. Learning about the stories behind the constellations (not just the Greek ones), and seeing shooting stars and Milky Way so thick against the sky you imagine you can reach up and feel its texture.
The History - A Stolen Home, and an Unwanted Home
There are petroglyphs carved into the sandstone of a cliff face, dating from when there was a lake there. Of course we don't know what they all mean; you can imagine people fishing a thousand years ago, sitting on tule mats and making some of the dots to count up their catch, others to tell a story or protect against disease or snakes. Some of them are definitely snakes. Amazing to think of how different we are from the people who made these signs, and wonder whether in the distant future someone will try to understand what we write today.
Credit above to Sierra Nevada Geotourism, and below to Wiki.
And we are different from the people who made these symbols, because the Modoc people were removed from their land at gunpoint. Their first encounter with white people was with Fremont's men, marching south, who mistook them for their traditional enemies, the Klamath, who they'd split off from six hundred years prior. (Imagine aliens landed on Earth in Russia, the Russian army shot at them, the aliens took off and landed again in the U.S. and attacked us to get revenge. That's what it was like.) Twenty-five years later the army came to remove the Modoc, the Modoc realized the land they had been marched off to was terrible, escaped and went back to the Lava Beds - where they used collapsed, un-roofed caves as a place for trench warfare. Two hundred men, women and children held off two thousand Union troops for six months, at one point making the mistake of thinking if they killed the Union chief (General Canby) that the Union would withdraw. The Modoc War only ended when the Union cut off their water supply, beginning the process of draining most of the lake that their answers fished on and made petroglyphs next to. (First photo below of one of the trenches in Captain Jack's Stronghold credit tripadvisor.)
Just outside the park is the monument to Tulelake internment camp, where almost 30,000 Japanese-Americans - those considered "troublemakers" among interned citizens - were sent during World War II. George Takei was here as a boy. I'm torn about this. It should be recognized and we should remember it and teach our kids about it so we don't make the same mistakes in the future. But somehow, once something like this is cleaned up and formally recognized, there's an artifical safety and sterility to the place, like it's behind glass (e.g., see the better-known Manzanar internment camp in the eastern Sierra.) The first time I saw it, I only knew where to look by looking it up ahead of time, and it was just piles of cinder blocks and rusting barbed wire, with no interpretive plaques to filter and protect us from reality. Image credit to discovernikkei.org.
They were cold this year. There was a bit of wading. Definitely no swimming. You want to see people being happy swimming, check out the video. (Narrator: they were not happy.)
Well there it is man! Looming over most of the proceedings, and especially impressive when approached from the east via Route 89 (image credit newsfromnativecalifornia.com.) Please also enjoy this historical image of of this dashing and, dare I say, rougueish fellow conquering the mountain ca. 2003.
 You might understand the attraction to the idea of the Sutter Buttes as the southernmost Cascade - if you look back up at the map, there's a smaller ridge parallel to the southern boundary of the modern JDF plate, south of it, that comes ashore at Point Arenas and would pass immediately underneath the Buttes on the way to the northwest corner of Lake Tahoe. The Sutter Buttes are about 1.5 MA old, newer than the Cascades, but about the same age as the coastal volcanoes like Mt. Konocti at Clearlake, and the volcanics NW of Tahoe like Granite Chief and Red Star Ridge along the Western States Trail.)
 It's actually kind of amazing there were only 217" there on April 1, given California's historic rain and snow year - although the further north you went, the less remarkable it got. People in Washington State we visited were surprised to hear how wet it had been.)