The Wrangell Mountains are Alaska’s most secretive volcanoes. We have looked at what they are (post I) and how they may have formed (post II). This area of Alaska has suffered the brunt of the most recent continental collision on Earth, and the Wrangell Mountains are an immense scar from this monumental accident. But there is something more hiding in the landscape. On either side of the Wrangell Mountains, both north and south, there is a strip of land which looks different. It has strange and unexpected fossils, corals from a tropical ocean far warmer than Alaska should have been. Go further away from the mountains, and the fossils disappear again. What is a sliver of the tropics doing in Alaska? In this third post, we will take a look behind the scenes and delve into history. It will take us to the final secret of the Wrangell Mountains.
Once these tropical fossils had been discovered, similar rocks were found elsewhere in North America. They follow the Pacific coast line, but with gaps. Vancouver Island is at the far end of their distributions: it seems to consist for 50% of these rocks, although the fossils are slightly different to those found at the Wrangell Mountains. The age of the fossil corals far exceeds that of the Wrangell Mountains. Clearly, the mountains grew on top of a much older land. But the younger was known first, and thus the elder was named after the younger: this strip of ancient tropical heritage is now called the Wrangellian terrane. Delve down into this terrane, and a thick layer of greenstone appears. The greenstone comes from an oceanic flood basalt, ten times as old as the proverbial mountains, which happened while much of the region was below sea level. The greenstone is not the oldest layer of the Wrangellian terrane: below the thick basalt is even older material, apparently coming from an island arc. Where was this arc? Why was it later flooded by a flood basalt? And how did this tropical sea end up, of all places, in Alaska?
It is all tied up with the way Alaska formed. Just like the birds that come to visit here every summer, Alaska is itself a migrant. In fact, it is a melting pot of many migrants. This is a complex place, an amalgamation of many different parts with many different histories. We need to go a long way back to disentangle the events that lead to the raising of Alaska.
The story begins more than 400 million years ago. The one constant in our world is change, and indeed many things were different. But some were not. The various countries around the Arctic, with all their disagreements and wars, have a common heritage. The plates that they call home possibly were always close together. At times they even exchanged parts. But Alaska wasn’t there. The rocks that were to become the future core of Alaska at this time belonged to Scandinavia: they were part of the Baltic plate. Still, plates will be plates. They split, drifted, collided, crumpled, stuck, and split again, in a dance of continental tango, akin to the ever-shifting alliances in Poul Anderson’s story Tau Zero, of travellers on a journey with no end. Alaska was born out of this dance.
One of the many collisions between the close group of plates formed a mountain range that can still be traced around the Arctic. It is most prominent in North Greenland and Ellesmere Island, but there is evidence elsewhere around the Arctic, from Canada to Siberia. It is called the Ellesmere orogeny. Exactly what collided with what is lost in time: this was 350 million years ago and the trail has grown rather cold. For a while, this collision may have unified some of the continents which now encircle the Arctic ocean. Of course, it wasn’t the Arctic at the time. The tango took place much further south.
But in the dance of the plates, all partnerships are temporary. The Arctic ocean began to form, in a way that is not fully understood. Some sea floor spreading was involved, but it was ultra-slow. There was also rifting caused by plates rotating away from each other. We don’t really know how much was spreading and how much rotation, but the effect was to form the Canada basin, separating North America from Siberia.
Parts of the Ellesmere mountains now began to move west along a transform fault, traveling over 2000 kilometers. Following the sideways motion, the separated part also rotated by 90 degrees. It slightly overshot, but when it finally came to a halt North America held on to its newly acquired appendage. This transported region became the northernmost part of Alaska. At the time it may have been a marine shelf, with a series of low-lying islands. To the south was the sea.
Alaska’s Brooks range was next to form. This area was part of the transported terrane, and as such can make a claim to be part of the Alaskan heartland. Now it came under stress from an approaching plate to the south, perhaps involving subduction. The approach occurred between 140 and 120 million years ago: it pushed up a mountain range and squeezed out an ocean basin in the process. Some oceanic crust belonging to that basin was squeezed up rather than down: it can still be found still in central Alaska. The approaching plate is given the name Koyukuk, and the collision added the Yukon-Koyukuk Terrane to Alaska.
After this, things became more complex. Another terrane was added to Alaska: it is called the Yukon–Tanana Terrane and nowadays it covers much of central Alaska. This was clearly another microcontinent that docked. The new terrane attached itself to North America along the Tintina fault; the southern edge of the Yukon-Tanana Terrane is formed by the Denali fault.
The Yukon-Tanana Terrane docked with North America 150 to 200 million years ago, not in Alaska but further south. After attaching itself, the terrane moved north along the Tintina transform fault to finally arrive in Alaska. The dates put the attachment to North America before the formation of the Brooks mountains, but the arrival in Alaska was after. As it arrived, it pushed the Yukon-Koyukuk terrane to the west.
Just above the Tintina Fault, in a tiny triangle along the Canadian border, is the only part of Alaska that is actually original North American! Everything else in Alaska came from elsewhere. This triangle is, to the best of my knowledge, uninhabited. The nearest metropole is the twin town of Eagle City and Eagle Village, 30 km to the south, with a total reported population of 162. Every Alaskan lives on borrowed crust! America was build by migrants, but in Alaska the land itself formed that way.
And finally, it was time for Wrangellia to arrive. This was already a complex entity: Wrangellia consists of a variety of different terranes which had merged before arrival at Alaska. Like the terranes that preceded it, it came from the south and west, and like the Yukon-Tanana Terrane, it attached itself to North America first, and slid up to the north after that.
After Wrangellia came the Yakutat plate, which is still in the process of docking. This platelet also moved north, but was not strongly attached to North America and instead collided head-on with Alaska.
Clearly, Alaska is a hard place! It consists of many different terranes, arriving here one after the other over a period of several hundred million years. In fact, I have left out several of these fragments. A geological map of Alaska looks truly psychedelic. This was the place where all these platelets wanted to be, like Isolde searching for Tristan, or Beren for Luthien. But it raises questions. Why were there so many platelets along the North American margin? Where did they all come from? And what attracted them to Alaska? These are difficult questions to answer.
But an answer to the last question is beginning to appear. All these platelets were going to where they came from. They were going home.
The basement rocks in these wandering terranes is ancient. The oldest rocks are more than 500 million years old, possibly much older. Over time they experienced volcanism and stress, and collected debris in the form of sediments and fossils. These can be studied to look for similarities with other regions. The sediments and similarity in some fossil remains suggest that the terranes spend significant lengths of time close to the west coast of North America, in the nearby ocean. But the older rocks tell different stories. Their relations are not with America, and not with the south or west, but with regions elsewhere around the Arctic. They connect to Baltica, and it seems many of the terranes started out as fragments of that Scandinavian plate.
In one of several major collisions in this part of the world, Baltica had merged with North America. This collision formed the Caledonian and Appalachian mountains. But the wandering terranes had split off from Baltica before those events, and while the continents came together to form Laurentia, the shedded fragments were on their own journeys. One younger terrane seems linked to the Ural mountains, and was part of the Permian collision between Europe and Siberia. But this fragment too ended up adrift in the open ocean.
Around the time of the Baltica-North America collision, the lost fragments got caught up in a rapid westward drift. They moved around Laurentia, and into the Pacific ocean (Panthalassa). On the way they had moved around the Ellesmere fragment that was on the road to become northern Alaska. In a way, it was an early negotiation of the future Northwest Passage. The encircling of Laurentia was completed by drifting eastward to move close to the North America coast. Their journey finally ended when the old fragments of Baltica reassembled themselves and build up Alaska.
There is a long-standing affinity between North America, Siberia, and Northern Europe. Vitus Bering comes to mind: he was a Danish cartographer exploring Alaska for the Russian navy, thus bringing together all three of the Arctic continental plates in his own life story. And now it seems that Alaska itself was made by European refugees, all traveling different paths to find their home. It is a true American story. People still move to Alaska to make it their home – these terranes moved there to re-assemble theirs. All of Alaska, with the exception of that tiny area where no one lives, has an exotic heritage.
But Wrangellia has its own story to tell. In Alaska, the Wrangellia terrane forms an east-west belt, 45 km wide; it is the end point of a 2500 kilometers long range extending along the Canadian coast. And unlike other Alaskan territorial migrants, it did not come from Scandinavia. It came from Oceania.
Wrangellia did meet other migrants along the way. This complex region contains other terranes, swept up by Wrangellia long before its arrival at Alaska. The largest of these swept-up pieces is the Alexander terrane. It is much older than Wrangellia itself: Alexander is more than half a billion years old. Like most of the building blocks of Alaska, there is a Baltica connection; Alexander formed at a subduction zone along Baltica, moved away and joined the migration of platelets towards Laurentia. But it didn’t make it. It was intercepted mid-ocean.
In contrast, Wrangellia, the one Alaskan terrane not related to the current Arctic, appears to have formed as an island arc, roughly 300 million years ago. This was near the equator, in a tropical ocean. The southern part may be a bit older, at 370 million years; as many as four separate arcs may have come together to form Wrangellia. It merged with Alexander very soon after its formation, also around 300 million years ago. Alexander was perhaps sucked in by the subduction zone that caused the Wrangellia volcanic arc, overrode it and became attached. It was just a road accident.
For a while, the combined microcontinent had peace, as it drifted around in a tropical ocean. The area had sunk below the water line, perhaps with some remaining islands. But this existence was shattered 232 million years ago. The now oceanic plateau began a rapid rise as heat came from below. Eruptions started and layer after layer of basalt flowed. Initially the eruptions were below sea level, and formed pillow lavas, especially at the southern end of Wrangellia, but soon the area rose above the water. The basaltic eruptions continued for a million years. And this was no Iceland: it erupted several times more lava than Iceland ever has, and it did so ten times faster. The eruptions were intense, at 1 km3 per year every year. Normally events like this happen in pulses, and the eruption rates during a pulse could have been several times higher still – imagine a Laki event every other year! When it ended, a volume of at least 1 million km3 had been erupted, and all of Wrangellia was buried under several kilometers of basalt. Reconstructions put this flood basalt eruption in the northern tropics, in the eastern Panthalassa ocean, not too far from Laurentia.
There has been some discussion on how long this flood basalt eruption really lasted. There may have been more than one phase. Uranium dating has shown dates from 227 to 233 million years ago, and these may indicate different pulses. The end date is well determined, but the start date is more uncertain. The million year duration may have been underestimated. However, it was short enough that the lava flows do not show magnetic reversals.
There are few dykes in the basaltic layers, which suggests this was not a rifting event. This, together with the evidence of rapid uplift before the eruption, shows the hallmark of a fast rising mantle plume. Wrangellia was in the wrong place at the wrong time. It was just another accident in what was already an accident-prone plate, this wandering Jew of the oceans, a flying Dutchman doomed to drift from storm to storm.
The microplate, loaded with its large igneous province, now moved towards North America; it was presumably pulled in by a subduction zone between the two. It arrived around 120 million years ago. This was the third accident suffered by the poor plate plate! Whilst Wrangellia attached itself to the continent, a thrust zone developed with mountain building and granite intrusions on the North American side. This was the largest piece of crust to join North American since the Jurassic. After the docking, the subduction zone re-established itself southwest of the combined terrane.
The long sliver, perhaps somewhat similar in appearance to the Baja California, now started to move north along a transform fault, the ancestor to the Denali fault. The amount of travel is disputed: one model has it move north by 3000 km, another model only by 1000 km. In either case, 50 million years ago (or perhaps a bit earlier) the vanguard of Wrangellia finally arrived in Alaska, after a journey of a quarter of a billion years.
The defining feature of the Wrangellia terrane remains its layer of ancient flood basalt. This now forms the Nikolai Greenstone, and is found on both the south and north side of the Wrangell Mountains, and presumably underneath the mountains as well. It is also seen on the south flank of the eastern Alaska Range.
This vast volcanic field extends even further: it surfaces as far west as the Alaska Peninsula, and as far south as Vancouver Island, albeit under a different name: the Karmutsen Formation. The image shows how dominant this layer is. It reaches a maximum thickness of over 6 kilometers! Wherever this thick sequence of greenstone is found, you can be sure to be looking at rocks from Wrangellia.
The thick scar tells the story of what can happen to an innocent bystander. Ignoring the Lurking maxim (‘don’t be there’), it managed to step on the toes of one of the world’s major events. The event is now named after this bystander: the Wrangellia Large Igneous Province. And by giving it its name, Wrangellia also took the blame for the consequences.
The million years of rain
The Triassic was not the best time to be alive. The continents had now come together to form Pangea: they were bunched up, surrounded by the world ocean (Panthalassa). At one side of Pangea there was an intrusion by the developing paleo-Tethys ocean, more or less where nowadays China is located. The climate had turned hot and dry. This is not uncommon: supercontinents tend to cause hothouse climates with high CO2 levels. The desert age culminated in the Carnian, between 237 and 227 million years ago. Rocks from this time are found in many places across the world: they are often dull red, dried and baked.
Over time things slowly improved, as the paleo-Tethys began to split Pangea in half, but especially the interior of Pangea was a hostile place. Wrangellia, not being part of Pangea, knew nothing about this. But this was the world into which Wrangellia erupted its flood basalt.
In the middle of this age of the desert, 232 million years ago, things briefly changed. It can be seen in rocks around the world, often as a grey stripe that runs through the red desert rocks. It tells a story of a much wetter world. In the midst of the desert, the rains had come. Across Europe, conditions changed. Drought-tolerant plants were replaced by ones adapted to more humid conditions, and dolomite formation shows that the water table had become much higher. The Great Wet is seen in rocks in Morocco, Nova Scotia, Colorado, China, India and Papua New Guinea, amongst others. New rivers formed, extending deltas and depositing sediments into the sea. And the deep sea was not safe: carbonates stopped forming, indicative of a change in pH of the water. This was a world-wide change. The change was not identical everywhere: whereas Pangea became wet, Gondwana may have seen some drier regions. The change seem to have started quite suddenly, but it ended more slowly over a period of perhaps several million years. After that, the desert returned and the world quietly forgot about this wet interlude, the million years of rain.
The cause appears to have been volcanic. This is indicated by two things: an increase in temperature by a few degrees (perhaps 4 to 7 C), and an increase in the ‘light’ isotope of carbon. Both indicate CO2 being added to the atmosphere, of non-biological origin. The only such event known from this time which is large enough is the Wrangellian flood basalt eruption. The time fits, and Wrangellia is now seen as the prime suspect for causing the million years of rain. It was its flood that caused the rain. These rainy years are now called, with all the creativity of scientific writing, the CPE: the Carnian Pluvial Episode.
Evolution saw its chance. This was 20 million years after the Permian-Triassic extinction which had devastated the world. Very little had been left, and in the harsh, arid world of the Triassic life had been slow to re-establish itself. Coal deposits show a long hiatus. Still, life can be hard to stop and in spite of the conditions and the terrible start, species had eventually become more diverse and grown in abundance. And now, suddenly, conditions became benign. Not everywhere: especially the seas show an extinction event, perhaps not unexpected after a subsea flood basalt. But elsewhere, life saw an opportunity. It exploded into a huge diversity, turning the Earth into a massive tropical rain forest. To give just one example: amber became widespread at this time.
Similar explosions of diversity can be seen in our world. The tropical rain forests and the tropical reef are home to much of our own world’s diversity. Both of these ecological systems are very young. The Amazon rain forest, for instance, did not exist 20,000 years ago. When it formed, ecological niches suddenly appeared everywhere, completely unoccupied. It gave every mutation a chance to survive and develop. The huge diversity of the fish species of Lake Malawi also arose from its youth. And the same happened during the million years of rain.
One group of animals in particular benefitted. Early dinosaurs had appeared some 10 million years earlier, but they were limited in number, and remained small and largely insignificant. Now they went for it. The first true dinosaur fossil footprints date from the million years of rain. Before the rains, dinosaurs accounted for less than 5% of faunal fossils. Afterwards, 90% of fossilized fauna were dinosaurs. Both the ancestors of Triceratops and Tyrannosaurus Rex were now abroad. The dinosaurs came out as clear winners, and when the conditions deteriorated again, they remained on top. It would remain this way for the next 150 million years. Dinosaurs ruled while Wrangellia drifted, merged with North America and moved north. It took a sudden worldwide catastrophe, 10 million years before Wrangellia reached Alaska, to finally topple them.
This part of the story of the million years of rain seems eerily echoed in Charles Dickens’s novel on legacy, “Bleak House”:
As much mud in the streets as if the waters had but newly retired from the face of the earth, and it would not be wonderful to meet a Megalosaurus, forty feet long or so, waddling like an elephantine lizard up Holborn Hill.
But the complex land of Wrangellia has one more story to tell. For another group also first appeared during the rains, although they would need to wait a long while for their chance to rule the world. The oldest known fossils of mammals date from the million years of rain. In a way, we too are a gift from Wrangellia.
Of course, every story has its critics. The evidence for a rainy period is very strong. Whether it affected everywhere, and whether it was episodic or continuous, is still a matter for discussion. And what was the role of the peculiar paleo-Thethys ocean in this? A lot of the evidence for rain comes for regions surrounding this ocean. The relation of the rain to the Wrangellia flood basalt could do with more evidence. It comes from a lack of other strong candidates, and from the agreement in time. Perhaps another qualifying volcanic event could still be found. But at the moment, Wrangellia is the prime suspect as the cause of the million years of rain.
Other parts of the story also have caused disputes. Some argue that the platelets that docked with North America had originally come from North America itself: they returned home. It is hard to be sure. The argument is based on similarity with other regions in the northwest of the US. However, this leaves the strong relation to Baltica unexplained. An elegant solution would be if those US regions themselves also came from Baltica. Science dissects and rebuilds the story lines with each new discovery. Like the world, the story changes every time we tell it. But the story still needs to be told.
We have come a long way. From a little known but majestic volcanic range in an arctic wilderness we have travelled through a continental collision which is still affecting Alaska today, shaking Anchorage just a year ago. But we have gone further, and found that much of Alaska came from northern Europe, outcasts from a time of turmoil. And the rocks on which the Wrangell volcanoes were build turned out to derive themselves from another age of turmoil, one which changed the world. Both the age of the dinosaurs and the age of the mammals can be traced back to them. We ourselves are the final secret of the Wrangell Mountains.
Albert, January 2020