The Wrangell Mountains are an unexpected treasure in a land of riches. Here are shield volcanoes higher than Mauna Loa, which have grown up over the past 5 million years. It stands alone amidst the crowd. From the Wrangell Mountains, other mountains ranges can be seen in every direction, but none of those are currently volcanic. Apart from Mount Churchill next door, the nearest other volcanoes are hundreds miles distant on either side. What is special about the Wrangell Mountains? Can we solve their mystery?
A few years ago the USGS produced a brilliant poster showing the earthquakes and faults of Alaska. (Here is the high resolution version.) It is a scary reminder of how much geological pressure Alaska is under. The locals may be in some doubt about global warming (probably because to them it sounds too good to be true), but they cannot ignore global deformation.
There are two very notable regions of activity. All along the northern Pacific rim, from Russia to Canada, runs a deep subduction trench, called the Aleutian megathrust. Here, the Pacific plate is meeting its end. And it is a sticky end: as the plate sinks and moves under Alaska, the top keeps getting stuck on the overlying continental mass. After a while the push from behind gets too much, and the stress suddenly releases. The earthquakes this generates are among the largest in the world. The M9.2 in 1964 is the second largest earthquake reported since the 19th century. At the eastern point the fault splits in two and turns south. From here on it is no longer a subduction trench: it morphs into a transform fault, called the Queen Charlotte Fault. It is named after the wife of mad King George III: whether she would appreciate having a fault named after her is debatable!
The second notable fault is the Denali Fault which curves in an arc across southern Alaska. It is a transform fault, with the southern side moving towards the west. The Denali fault continues to curve into Canada, but it seems it may also be attaching itself to the Queen Charlotte fault: when in 2002 the Denali fault ruptured in a M7.9 quake (which did surprisingly little damage as it hit a wilderness area), at the eastern end the rupture followed the Totschunda fault towards the southeast, and diverted from the Denali fault.
The other notable feature of southern Alaska are its mountains. The southern part is seriously mountainous, with a large number of different ranges. There are coastal ranges, in-land ranges, the Alaska range (closely attached to the Denali fault), and our topic of interest, the Wrangell Mountains. Coastal ranges are typical for subduction zones, where the continental crust is horizontally compressed. But the in-land ranges also need an explanation. The answers are at sea.
Yakutat: Pacific survivor
The pressure that Alaska is under comes from the Pacific ocean. Alaska has become involved in a battle for domination of this ocean, and it is the victim of the final skirmishes. Nowadays, the Pacific ocean is dominated by a single plate, called (of course) the Pacific Plate. This is the plate that is subducting underneath the Aleutian arc: it covers the ocean from Alaska to near Antarctica, and from California to PNG. In the southeast, the Nacza plate is holding out, and in the west the Australia plate and the Philippine plate claim part of the Pacific ocean. But these are small fry compared to the giant, the largest of all the Earth’s plates. The Pacific plate covers 20% of the Earth’s surface! Perhaps this is the largest plate there has ever been. But the domination is not yet total. Around the edges are some micro plates which maintain a pressured independence. Along the Americas, these include the Cocos plate, the Rivera plate and the Juan de Fuca plate. They are fragments of a different time.
For in the past, the Pacific ocean was a much more varied place. There were plates of all shapes and sizes which made up this magnificent and magnificently diverse ocean. But one day, somewhere in the middle, the Pacific plate began to form. It grew and grew, and over time (lots of time) squeezed out all competitors, leaving only small remnants which are now little more than microplates. These microplates are finding themselves pushed against, or even under, the neighbouring continents.
One of these microplates plates is found in the northeast corner of the Gulf of Alaska. There is little of it left: it covers a small region between the Pacific and the coast, smaller than the Netherlands. This survivor -barely- is called the Yakutat block. It consists of oceanic basalt, covered with several kilometers of sediment. The plate is under severe pressure from the Pacific plate, and is moving at 4.5 cm/yr into Alaska. This is an astonishingly high rate, about the speed India had when it slammed into Asia.
The USGS earthquake plot shows the impact of the Yakutat microplate. Pay close attention: this is the current mover and shaker of Alaska. The shaking is literally true: it causes huge earthquakes: even the 1964 M9.2 superquake is related to the Yakutat. The moving is also true: the areas south of the Denali fault are being pushed out of the way, towards the east. The continent is crumbling under the onslaught: the impact has pushed up the St Elias, Chugach and Tatkeena mountains.
Clearly, the giant Pacific plate has been expecting the Yakutat plate to meekly surrender, subduct and get lost. Seismological surveys have shown that part of the Yakutat plate has been lost, and has slid to its grave underneath Alaska. But the Yakutat microplate is too young and too buoyant to give in easily, and it is using teenage energy to run its own extinction rebellion. Directly ahead of the Yakutat plate are the St Elias Mountains, but here there is only a small section of subducted plate. The subducted plate is mainly found off to the side, underneath the Chugach Mountains, due east of Anchorage. And even this lost part is not behaving like a normal, dipping subduction zone. Because of the rebellion against subduction, where it dives under Alaska the plate stays close to the surface, moving nearly horizontal at a depth of about 40 km. It is doing a ‘slab subduction’. In the end this rebellion fails: eventually it does go down. The depth begins to increase underneath the Talkeetna Mountains, and finally the Yakutat plate dives down steeply when it reaches the Denali fault. Here it also seems to bend to the northeast, for reasons that are not entirely obvious. While the Yakutat docked, the Pacific Plate behind slowly changed its direction of motion, from north to more northwesterly. The bend at the end of the subducted part of the Yakutat could be from that change in direction.
(An alternative model poses that the part north of the Chugach mountains is not actually the Yakutat plate but belongs to a now-defunct plate, the Kula plate.)
The flat subduction explains the lack of volcanic activity in the Denali gap, west of the Wrangell Mountain. The subducting plate does not go deep enough to generate magma.
Where did the Yakutat plate come from? Clearly, it didn’t form where it is found now. The precise origin is not known. The Pacific plate moves along the North American coast, and this suggests the Yakutat came from that direction.
A suggestion is that it formed at a triple point. At one time, there was a plate in the North Pacific called the Kula plate. It is long gone, subducted out of existence underneath the Arctic ocean and Alaska. There was a triple point between the Kula, Farallon and Pacific Plate. It is not uncommon that a re-arrangement in such a region leaves a bit of oceanic crust unattached to any of the three plates, and becomes a microplate.
If this is the case here, it would happened some 50 million years ago, off the coast of Northern California or Oregon. After that, the Yakutat began to move north, in the process rotating as well. It arrived near Alaska between 10 and 25 million years ago.
Alaska would never be the same again. This was a mini-India colliding with a mini-Asia. It formed mountains not quite up to Himalayan standards, but which have few equals. These mountains can keep their heads up.
Building the Wrangell Mountains
In this way, the Yakutat microplate is blamed for raising the St Elias, Chugach and Tatkeena Mountains. But whether it also grew the Wrangell Mountains is less clear. There is one claim in favour and one against. In its favour, the oldest volcanism in the Wrangell/St Elias Mountains is dated to 26.3 million years ago, which is around the time when the Yakutat plate arrived in Alaskan waters. The oldest volcanic flows are found at the Alaska-Yukon border, closest to where the Yakutat arrived. The current Wrangell mountains formed 5 million years ago: all the older flows are towards the St Elias mountains. Over time, the volcanism moved northwest, more or less in accordance with the movement of the attacking plate.
But the Yakutat has an alibi. It is located to the south, off the coast, and the subducted part is further west. The Yakutat never came to the Wrangell Mountains. Like Antigonish, it just wasn’t there:
Yesterday, upon the stair,
I met a man who wasn’t there!
He wasn’t there again today,
Oh how I wish he’d go away
The timing has also be questioned. The arrival of Yakutat in Alaska has been dated by the oldest volcanics, and this introduces a circular argument into the geology. The model above (albeit an old one) has the arrival considerably later. Can we charge another guilty party with the wilfull formation of Mount Wrangell?
Seismological studies have revealed a very weak Wadata-Benioff zone just south of the Wrangell mountains, dipping to the northeast. It shows up as a zone of a few earthquakes, around 50 km deep. It does not continue west or east: it is an isolated zone, much like the Wrangell mountains themselves. Something must have been subducted underneath, but it is not clear what. Currently, the Aleutian trench ends before it reached Canada, and the Queen Charlotte fault is a transform fault. But in the past the Pacific plate moved in a slightly different direction, and there would have been a subduction zone along this fault. It was overrun by the Yakutat plate but may have become inactive before that time. Is the subducted area seen underneath the Wrangell Mountains perhaps from this zone? In that case, was it attached to the Yakutat plate or did it belong to the Pacific Plate? But in the latter case, shouldn’t the volcanism and mountain building be older than it is?
Raising the basin, building the model
There is another hint to what went on, and this comes from the peculiar lava found in the Wrangell Mountains. These are thick flows ranging from basaltic andesite to dacite, with only a few rhyolite eruptions. The lava composition remained the same over time. The flat, far ranging flows build up large shields. The magma clearly derived from an oceanic plate, not continental crust, and this shows that there is subducted oceanic plate underneath the Wrangell Mountains. But the volcanics is not typical for subduction zones.
It has been argued that any subduction in this region may have stopped. Much of the plate motion may now be taken up by the Transition fault, on the southern side of the Yakutat. If more of the motion is transform, this lessens the pressure on Yakutat and the continent beyond. One version of this model links the lack of Wrangell eruptions over the past 100,000 years to this cessation of compression. That seems a bit optimistic, given that there have been many such interruptions of volcanic eruptions in this region. 100,000 years is just too short to make strong geological claims.
But this does point at another possibility. The type of eruptions are more typical of an extension basin than of a volcanic arc. And if you take a map of Alaska and use tipex to remove the Wrangell Mountains, what is left is a basin between the St Elias Mountains on the coast and the Alaska range further north. (To the younger readers, tipex is an analog, liquid eraser, popular with a previous generation of students. It works brilliantly on a phone screen as well but only once.) Could these mountains have grown up in a basin, hiding the evidence by their sheer size?
Where could a basin come from? After all, a collision zone is a region of compression, and basins come from extension. The two don’t go together well. Straight subduction can form basins: it typically gives a raised area from compression, a depression behind it called the fore-arc basin, a volcanic arc, and possible another basin behind the arc. But this region is too complex for that.
Extension can also come from bends in transform faults, and the The Denali fault is such a system. The 2002 earthquake followed the Totschunda fault, and this gave even more of a bend than the curvature of the Denali fault itself. North of this fault, there is no movement and therefore no problem. To the south, the crust moves west through the bend. The pressure on Alaska was bad enough to begin with, but now there is rotation as well, a shear. Such curved motion can cause extension. The Wrangell volcanics is perhaps the result of this: the extension can have caused decompression melt in the Wadata-Benioff zone lurking below. It is perhaps not easy to think of some of the highest mountains in North America as having formed in a subsidence basin!
Other models have been proposed. One paper poses that an ancient spreading ridge was subducted underneath Alaska and although extinct, the ridge topography, now underground, has caused the activity. This seems an unnecessary complication. If the Yakutat plate originates from near a triple point, it forms still relatively young, warm oceanic crust. This melts more easily than the typical subducting oceanic plate, and generates the type of magma found in the Wrangell Mountains. This is perhaps the easiest model, where the intermittent nature of the eruptions is because the shallow subduction of this rebellious plate only barely reaches the depth where melt can occur.
It is surprising that more is not known of an area that has caused one of the largest eruptions in North America since Ilopongo. There is not yet a unified model that explains all. We are still seeing science in action, trying out models to see which one fits.
There are still mysteries to be resolved about the Wrangell Mountains. It is in some way a scar of the battle for the ownership of the Pacific ocean. But exactly how this scar came to be is unresolved. And it is tied into another mystery. Because all around the Wrangell Mountains are the remains of a much older volcanic field, one that has become caught up in another mystery. Come back next week, to read about the raising of Alaska, and the rain that lasted a million years.
Albert, January 2020