Alaska is a wonderland. The harsh winters make the land difficult to live in, and in consequence much of the wilderness seems hardly touched by human hand. No need for rewilding here – it is wild enough to begin with! Amidst the wilderness are the most majestic mountains of North America, and some of its most impressive volcanoes. Among these are the Wrangell mountains. Anywhere else, mountains such as these would dominate the region. Here, they live in the shadow of other, better known mountains. The Wrangell mountains are a volcanic region, build up relatively recently by eruptions. Mount Wrangell itself, over 4317 meters tall, remains an active volcano. But underneath runs a deeper history, and lies an older volcanic field. The Wrangell Mountains are build on an ancient volcanic history, which played an important part not only in the building of America, but in the evolution of the Earth.
The Wrangell-St. Elias National Park is the largest protected wilderness in the United States. To give it a European scale, this park is larger than Switzerland. The Wrangell Mountains take up about a fifth of the park. To the southwest are the St Elias mountains, extending into Canada. All the mountains here are volcanic, and almost all are impressively high. Mount Bona, in the adjacent St Elias mountain, is the tallest US volcano, beating such giants as Mauna Loa. It is a bit of a cheat, though, since only the summits are volcanic. The volcanoes grew on top of an already highly elevated range. They are among the tallest volcanoes in the world not by their own effort, but because they grew on the shoulders of giants.
The individual Wrangell volcanoes are described very well in the Guide to the Volcanoes of the Western Wrangell Mountains, Alaska-Wrangell-St. Elias National Park and Preserve, by Donald Richter and collaborators, published by that most open-access publisher of all, the USGS. (It is USGS Bulletin 2072, https://pubs.usgs.gov/bul/2072/report.pdf.) The information in this post comes to a large extent from that highly recommended source.
Volcanoes in cold storage
The oldest volcano in the Wrangell mountains is also its highest peak, Mount Blackburn; at 4,996 m it is just shy of the 5 km mark. It was named after the US senator Joseph Blackburn, well-known in his time but now remembered only because of the mountain. The naming was a bit of a long-service award for his public career. Mount Blackburn is rather older than the senator, though. It began to grow 5 million years ago with a granite intrusion. Volcanic activity came later, and the shield began to build. Around 3.4 million years ago the 5-km wide caldera collapsed. Andesite lava was ejected from the circular caldera fault at this time. These are the only Blackburn lava flows which have survived the subsequent erosion. After the caldera collapsed, the volcanic activity ceased.
Now the activity moved 50 kilometers to the north, where around 3 million years ago Skookum Creek Volcano began to grow. It started with basaltic lavas, followed by much larger andesitic flows. In its heyday, Skookum developed a caldera 14 km across. Activity ceased 2 million years ago; most of the mountain has since eroded away. The caldera rim disappeared in its entirety, and only the lava flows erupted within the caldera remain. The mountain is 2,172 m tall, and is only a shadow of its older self.
After Skookum became extinct, activity shifted southwest, to Tanada and Jarvis. Tanada is around 1.5 million years old, and like the other volcanoes here it experienced caldera collapse and died. Jarvis was active at the same time but grew larger. It is still over 4 km tall but is now deeply eroded. There may have been two overlapping calderas but this is not fully clear. Both Jarvis and Tanada became inactive 1 million years ago. Capital Mountain became briefly active 1 million years ago but activity here ceased within 100,000 years.
Now a new series of giants began to grow, starting about 800,000 years ago. This happened to the west of the previous activity. The first stirrings were at Mount Sanford where lava flows from three separate centres build up smaller shields. The three centres were approximately aligned north-south and may have been part of a rift. The northernmost centre, uniquely, erupted a rarity, a rhyolite flow extending almost 20 kilometers to the north. The eruption then moved to the centre where it build up an imposing shield which incorporates the three older volcanoes. Sometime between 300,000 and 100,000 years activity here ceased. It has left a 4,949 meter high mountain with a spectacular 1.5-mile-high rock face on the south face. There may be a filled-in caldera but this is not clear from the structure.
The westernmost peak of the Wrangell Mountains is Mount Drum. It too is volcanic, and probably formed between 600,000 and 200,000 years ago. It is significantly lower than the other peaks, at a measly 3,661 meters, but this is not for want of trying. The bulk of the mountain was destroyed in an explosion that happened between 200,000 and 100,000 years ago. The current Mount Drum is only one side of the original mountain; the other side is Snider Peak. As much as 100 km3 may have been removed in the explosion. The last and presumably final eruption was between 125,000 and 150,000 years ago.
The last of the gang of three is Mount Wrangell itself, which is the only currently active volcano in the Wrangell Volcanic Zone. It too is a large shield volcano, build up by andesitic eruptions. Its current volume is about 900 km3. The mountain started to build up by 600,000 years ago. The youngest lava flows are about 50,000 years old. A cone next to Wrangell, called Mount Zanetti, may be only 25,000 years old. And there has been more recent activity.
The recent activity is shown in reports of eruptions in 1784, 1884-5, and 1900. However, the one in 1784 is highly dubious, in part because the mountain wasn’t discovered until 1819. The eruption is mentioned in a book on the geology of Alaska by the Estonian mineralogist Constantin Grewink, and is a second-hand mention of an eruption from ‘Chetchina’. There is a river of about this name near Wrangell, and therefore this eruption was subsequently identified as Mount Wrangell. However, Grewink himself assigns it to the Andreanof Islands, and it is now usually assumed to refer to Great Sitkin.
The 1884/5 eruption is also considered unconfirmed. This eruption comes from a report by a local prospector, John Bremner, who was about 30 miles from the mountain. It was a typical Wrangell winter, as is evident from his journal: ‘Clear but very cold the floor of my cabin is frose two foot from the fire and I thought I had made it almost air tight so you see I am in no danger of melting with the heat.’ (www.avo.alaska.edu). He reports seeing smoke from the mountain in December 1884 and on February 3 a very large column of smoke and ‘hurling imense stones hundreds of feet high in the air’ as well as a rumbling noise. Other people only reported the smoke. There may have been a phreatomagmatic explosion but the doubt comes from the fact that he was 30 miles away. At that distance, the ‘hundreds of feet’ corresponds to 0.1 degree, a fifth of the diameter of the full Moon: it is hard to see how he could have seen such detail as the ‘immense stones’.
That leaves us with the 1900 eruption. This was not directly observed but inferred: It comes from a report in the Galveston Daily News of June 10, 1902: “In June, 1900, I observed a black patch several miles in extent on the southern slope of the mountain, extending from the summit down to perhaps 11,000 feet elevation. As it was covered with snow when the mountain was next seen, about the last of September, it seems probable that the bare area was due to material thrown out by the volcano.” Another report from 1899 mentions an eruptive plume and a lava stream, although there is no confirmation of any lava. It is sometimes suggested that this may have been smoke from the fumarole patch on the southwestern flank of Mount Wrangell. However, it appears that such activity is not that unusual, and more dark patches stemming from phreatic explosions have been reported since.
Mount Wrangell has an impressive summit caldera which measures 4 by 6 km, filled with 1-km thick ice. Along the caldera rim are three further smaller craters, imaginatively called: Mount Wrangell Crater, North Crater, and East Crater. North Crater appears to be the source of the occasional explosions. This crater has significant ground heat. It lost almost 90% of its ice between 1965 and 1984, at a time when the other craters showed no change in ice cover. This followed the 1964 Good Friday Alaskan super-quake (M9). Interestingly, the 1900 eruption was shortly after the September 3, 1899 Yakutat earthquake (M8). After this quake, there are some reports of increasing smoke coming from the summit. The possibility exists that these quakes opened up pathways for underground hot water.
These are the main volcanoes of the Wrangell Volcanics. Some smaller peaks are not included, and it is conceivable that some further, minor eruption centres were buried under the developing shields.
The plot below shows the time line of the Wrangell Volcanics. It began with a granite intrusion 5 million years ago, which developed into volcanic eruptions a million years later. Since that time, the activity has moved around, building one mountain after another but rarely more than one at a time.
Mud, glorious mud
Immediately west of the Wrangell Mountains is the Copper River valley, known for its large copper deposits. This area has mud volcanoes, where mounts of soil a kilometer wide and 100 meters high have been created by warm springs. They have the appearance of small shields. The 1 meter deep pits at their peaks produce warm muddy water and methane gas. The mounts are surrounded by dead trees, killed by the salty mud after activity here increased following the 1964 quake.
A 50-cm thick layer of white ash can be found in areas surrounding the Wrangell Mountains. It is only just underneath the surface and clearly represents a recent event. Two events, in fact. There are two main layers, one to the north and one to the east, and radiocarbon dating of wood buried by the ash shows that the northern layer is older, and was deposited between AD 150 and 500. The eastern lobe covering a part of Canada came later, in an eruption several times larger than the first one. Tephra found in Greenland’s ice has given us a precise date: this eruption happened in the winter of AD 853 (a date 50 years earlier is also in the literature but is now out of date). Tephra from the explosion has been found as far east as Poland.
The two pyroclastic flows together amounted to a 50km3 eruption. These were two serious eruptions; the second eruption was large enough to cause the local population to migrate south and temporarily vacate the region.
It turns out that they came not from the Wrangell Mountains but from the other half of the park, the St Elias Mountains. This is a much older mountain chain, extending to the coast. The highest volcano of the US is here, called Mount Bona. The White Ash eruptions came from its lower, secondary peak, Mount Churchill. This is clearly a volcano with a serious temper. East of its current summit is a caldera 4 by 3 km across: this is likely the source of the ash, and probably the location of its summit before these events.
Mount Churchill may have had a third, much smaller explosion 300 years ago which gave rise to a tephra deposit. But there is no evidence of older eruptions similar to the 853 event. This was a double one-off event. Churchill remains a riddle wrapped in a mystery inside an enigma.
The Wrangell Volcanic Zone is not overly active. The eruptions seem to come in bursts, voluminous when they happen, quiet at other times. The volcanoes grow very large: clearly when a magma chamber has formed, it stays in place for a long time. The volcanoes all grow as a shield, and develop a caldera at the end of their evolution. These are not (strongly) explosive, but form because magma is withdrawn from the magma chamber. Where the magma goes is less clear! The calderas collapse to over a kilometer in depth, but much of this is filed in with basaltic flows which probably come from the ring fault. There are some similarities here with the formation of volcanic mares on the Moon. The magma is basaltic although there are differences from normal subduction zones. And one Wrangell volcano has produced this ultimate rarity, a rhyolitic voluminous lava flow.
Mount Churchill seems an exception, as it has produced at least two rhyolitic explosive eruption during the holocene. It is an offspring from the Wrangell Volcanic Zone, located away from the other volcanoes. The closest Wrangell volcanoes to it have been extinct for several million years.
The volcanic zone may be in the process of dying. There has been no large eruption for the past 200,000 years, and only a small region remains active. There is westward drift of the activity over time, and perhaps a new volcano will develop where the mud volcanoes are located. But there is no indication for this.
The source of the Wrangell volcanics is clearly the subduction of the Pacific plate underneath Alaska. In this area, this plate subducts towards the northwest at a rate of 5 cm per year. The volcanic activity in the Wrangell Volcanic Zone has over time also moved northwest, although at a rather slower rate of 10 km per million years or 1 cm per year. The subducted plate begins to melt when it reaches a depth of 100-120 km, called the Wadati-Benioff Zone. The precise depth depends on the temperature and how wet the subducted rock is. At the point where the melt initiates, the newly formed magma begins to percolate upwards. At the surface, a volcano begins to grow. Obviously, these volcanoes are not near the subduction trench, but some distance down stream – how far depends on the angle of subduction. For shallow subduction the volcanoes will be at considerable distance, whilst for steeper subduction they are closer. The Wrangell mountains are further from the coast and one may expect a shallower angle of subduction.
However, things are a bit more complex. The Pacific plate subducts along the entire Aleutian arc, but volcanic activity is not the same everywhere. The arc of volcanoes ends west of Anchorage. Further east, there is a length of 400 kilometer without volcanoes, so unexpected that it has a name: it is called the Denali volcanic gap. The Wrangell Mountains are immediately east of this gap, and are a bit in-land, as a clearly separate mountain range. There is more here than meets the eye.
This is the first part of the Wrangell post. Come back next week to read about reluctant subduction, building America, and the flood that brought the rain.
Albert, January 2020