You might say this past month’s VVVc theme has been lava domes, but ironically it was for a completely different reason that I decided to have you all come find me this week at Lassen Peak, one of the biggest lava domes in the world.
The volcano lies in northern California, and is the southernmost active volcano in the Cascades Volcanic Arc. The Cascade volcanoes, like other volcanoes around the world that appear near the edges of tectonic plates, are the result of oceanic plates (in this case, the Juan de Fuca and Gorda plates) slowly sliding up under the edge of continental plates (here, the North American plate). As the water-logged oceanic crust is forced downward below the continental crust, the presence of that water lowers the melting point of the rock above. Remember, the rock of the lithosphere is very hot, but normally under extreme pressure, thus making it solid. With the introduction of water, the hot rock melts and becomes magma, and the magma begins to rise to the surface and, if it is ambitious enough, becomes a volcanic eruption.
Lasen Peak is actually the new kid on the block, geologically speaking. Between 3 and 4 million years ago, the plateau upon which Lassen stands was built up by lahars and lava flows from ancient nearby volcanoes. Those flows were initially basalt, but later consisted of more viscous andesitic and dacitic lavas. Then, around 600,000 years ago, a stratovolcano called Tehama was born. Tehama reached a height of 11,000 feet (3,400 m), but collapsed to form a wide caldera around 350,000 years ago. There were other smaller volcanoes constructed around the area after that, but none so large as Tehama.
Around 31,000 years ago, a new vent opened on collapsed Tehama’s northeast slope and erupted flows of fluid dacite lava that accumulated to a thickness of 1,500 feet (460 m). The eroded remnants of those flows now appear as black glassy columnar lava formations in the area surrounding Lassen Peak. The dome itself rose up abruptly, perhaps in three or four years, piling more and more thick lava upon itself and constantly sloughing away its crust until it was encircled by heaps of talus, or fragments of broken rock. The dome was glaciated for a period of 7,000 years though, so much of its original craggy shape has been altered. As a youth, Lassen Peak may have resembled its younger lava-dome brother, Chaos Crags.
Many other volcanic edifices are scattered all around Lassen Peak and bear such colourful names as Sunflower Flat (a dome, ironically enough), Sulfur Works and Devil’s Kitchen (thermal vents), Bumpass Mountain (named after a 19th century prospector, not a bump on a backside), and Vulcan’s Castle (a logical designation). One peak called Brokeoff Mountain lies south of Lassen, and is the most visible remains of the ancient Tehama volcano.
Lassen is most well known for its surprise awakening in May of 1914 with a series of phreatic steam eruptions which created a fresh crater atop the old dome. Then almost a year later, the characteristics of the eruption changed and large blocks of lava were seen tumbling from the summit. A new lava dome soon was observed growing within the crater, quickly filling it. Less than a week later, an explosion shattered the new lava dome, showering the surrounding summit with the domes hot remnants and generating huge lahars, especially to the northeast. That night, slightly hotter dacite lava overflowed the new crater and traveled down the northeast and west flanks of the volcano. Then, on 22 May, 1915 the “big one” happened. A great explosion initiated a 30,000 ft (10,000 m) eruptive column of ash, generating pyroclastic flows and new lahars, devastating the northeast flank a second time. This area is now aptly named The Devastated Area and has remained relatively barren due to its nutrient-poor and porous soil.
Excluding the 1980 eruption of St Helens, the Lassen eruption is the most recent eruption in the 48 contiguous United States. It was also one of the first eruptions of its time to be widely documented in photography, both by the area press as well as one B. F. Loomis, a local landowner and amateur photographer whose large collection of eruption images offered new insights into our understanding of how eruptions work.
A few steam explosions occurred afterward in 1917, but since then, aside from thermal venting and hot springs, the volcano has been silent. However, it is one of the Decade Volcanoes due to its likelyhood of future explosive activity as well as proximity to many large West Coast populations.
If you want to learn much more about Lassen Peak, this National Park Service PDF entitled Lassen Volcanic National Park Geologic Resources Inventory Report may have a daunting title, but is packed with fascinating geological information and photos of the diverse volcanism of the area.
But if you maybe just want to look at a majestic (though lumpy) lava dome as it passes through the seasons, there is a webcam. THIS is what initially brought me here. This gorgeous vista paints such a serene picture of an area that outwardly might seem almost too quiet..until you know its geological history.
This week Jesper found me almost instantly and picks up 5 points. Leslie, after a fun-filled tour of the entire Cascades range, still was here in time to pick up 3 points. Thanks for playing..and be sure to be here around 11pm Monday (That’s 4am Tuesday UTC) for a new adventure at a new volcano. 🙂