As with all things in geological time, the history of Alaska starts with plate tectonics. Plate tectonics is, to geology, what evolution is to biology – the grand theory that, so far at least, does the best job of explaining our natural world. Tectonics is something that wasn’t widely known when I was a kid and I have been able to watch it grow in complexity and acceptance until, today, geology explains our continents and landforms admirably.
As an example, something I found fascinating as I read about Alaskan geology was that the copper ore that was mined out of the Kennecott Copper Mine (now in Wrangell St. Elias National Park) is of the exact same composition and located in the exact same kind of rock structures as was mined from Isle Royale and the Kewenaw Peninsula in and around Lake Superior, although separated by thousands of miles.
If you assume the world looks exactly like it does now, that phenomena is something very difficult to explain – Certainly nothing human moved all that rock over there and buried it underneath hundreds of feet of other rock. But with a theory of plate tectonics, you can visualize that massive continents are constantly in motion across the surface of the earth, colliding with each other and, in some instances, breaking up into smaller pieces. Those smaller pieces can then, because they are of a different mass and composition, migrate along a different path and end up in some other location.
Plate tectonics is now taught even in elementary school science classes and – except for a few ‘flat-earth’ types – is now widely accepted. We understand that the earth’s crust is composed of a dozen or so major plates which move according to their own dynamics across the surface of the earth. If plates have been in constant motion, then that means that we can’t envision the layout of the continents as they are now. Their shapes and locations could have been, and actually were dramatically different than they are now. In fact, we have pretty good evidence that at times past most or all of the plates were combined into a single mass – Gondwanaland. Only later did this massive landmass break up into smaller plates.
Two of those plates are the North American and the Pacific plate. The former is an ancient landmass that forms the basis for our continent. The latter is the plate underneath the Pacific Ocean. One is a continental plate ‘floating’ relatively independently across the earth’s surface, while ocean plates are, typically, heavier and, under water. Because of these different compositions, they react differently to dynamic forces and, as a result, move in different directions relative to each other.
The North American plate is, generally, moving westerly while the Pacific plate is moving sort of northeasterly. And, of course, that means they are colliding. As plate tectonics teaches us, the result of this collision is that the lower and heavier Pacific plate is being ‘subducted’ under the North American plate.
One effect of this collision between the Pacific and North American plates is mountain building. Just like when two cars collide, there is significant crumpling, breakage, and distortion when two plates collide. A very common result is the formation of mountain chains. One of the earliest mountains to form was the Brooks range, during the Mesozoic era, about 125 million years ago. (Gates of the Arctic National Park is built around the Brooks Range.) Later, the compression caused by the push of the Pacific plate from the south, also formed the mountains of south-central Alaska, including the Wrangell Mountains, and the Alaska Range, home of Alaska’s, and North America’s, two tallest mountains. Denali, formerly known as Mt. McKinley is the tallest mountain on the continent. St. Elias mountain is the second largest mountain in Alaska and is part of the world’s tallest coastal range. (Denali and Wrangell St. Elias are two parks on our itinerary.). Smaller mountain ranges, all formed in pretty much the same manner, form the backbones for the two parks on the Alaskan Peninsula (Lake Clark and Katmai) as well as the coastal ranges in the panhandle giving it the isolation it is so famous for.
Another effect of this ‘subduction’ is that the rocks of the Pacific plate, as they fall under the continental crust, are compressed and heated to an extreme degree. At a certain point, determined by the local physics, the heat and pressure can actually liquify the rock and turn it into magma. Again, depending on what the composition of the continental crust above it is, the magma may very well rush to the surface and create various kinds of havoc, known as volcanoes.
Alaska has extensive evidence of past volcanic activity. The major rock formations making up the bulk of Denali’s massif is a volcanic pluton, which is an intrusion of volcanic rock pushed up into the surrounding crust. It doesn’t necessarily break the surface, but is of igneous, molten origins. Later, after fully hardening, in mountain building events, the volcanic plug was pushed through the surface to become the tallest mountain in North America, and one of the starkest relief changes (from valley floor to mountain peak) in the world. There are plenty of other examples in Alaskan geology of igneous rocks showing the role of volcanic activity dating back millions of years.
Significantly, the southern part of Alaska forms part of the Pacific ‘Ring of Fire’ – a semi-circle of violent volcanic and earthquake activity around the Pacific Ocean rim. Alaska is the home of several violent volcanic and earthquake eruptions. In 1912, the second largest known volcanic eruption ever to occur, blew out a basin in what is now Katmai National Park. The effects of the ash dispersal, four times the amount of Mount St. Helens in 1980, was to reduce the average temperature of the earth by nearly 2 degrees Fahrenheit for two years. Originally, scientists thought it was the Katmai volcano that blew its top, but it was later determined that the eruption actually came from a new outlet, Novaerupta, about eight miles away, but was so massive that it drained all the magma from Katmai, causing it to collapse as well.
If that wasn’t enough, in 1964, recorded history’s second most powerful earthquake occurred on Good Friday. Deep below the subduction point, the earthquake was so powerful that 112 people were killed in Anchorage and the tsunami it generated wiped out multiple coastal villages and towns. In the 1900s more than 60 volcanic eruptions and/or earthquakes were recorded in volcanoes all along the Alaskan peninsula, including several in and around Lake Clark National Park and extending down the peninsula and along the Aleutian Island chain for a thousand miles. Aniakchak National Monument is a classic volcano caldera that is on our itinerary. The activity persists and few of these volcanoes are considered dead. Mount Wrangell itself is an active volcano, last erupting in 1930.
While it is the movement of the two big plates, the Pacific and the North American, that provide most of the context for Alaska’s geology, the composition and structure of Alaska’s land mass is exceedingly complex. In addition to the mountain building and volcanism, there is also a very complex history of terrane accretion. A ‘terrane’ can be though of as a ‘mini-plate’. Instead of being continent size, a terrane is a much smaller land mass that also floats along the earth’s crust mostly independently. Most of them are island size or possibly slivers of larger land masses that have fallen off their parent plate. It turns out that more than half-dozen of these mini-plates have slammed into southern Alaska.
When these smaller terranes slam into a larger plate, parts of them might be subducted underneath the North American plate, following the path of the Pacific plate. But some of the surface structures are sort of peeled off the top of the terrane and consolidate with the North American plate. This has happened multiple times, each time adding a fault line and a chunk of land that is composed of, potentially, a very different type of rock. Thus geologists have found layers of rocks with fossils from radically different parts of the world. Many of these terranes, for example, have tropical plant fossils, coal, or peat deposits, none of which originated at the Alaskan latitudes. (Of course, the original land mass of the North American plate that became Alaska started out at the equator and later drifted into the Arctic areas.). If you look closely at the circular arrangement of the mountains of southern Alaska, you can see that they likely arose, in part, from the pressure applied when the terrane that became the Kenai peninsula slammed into Alaska just east of the Cook Inlet.
The result of all this terrane accretion is that Alaska has a very complex arrangement of rock layers and fault lines which contribute to a rich geology. This terrane accretion is especially evident in southeastern Alaska (the panhandle) where gaps between terranes have been filled in with ocean waters creating the inside passage and the complex structure of the Alexander Archipelago. This is especially evident in Glacier National Park and, on the other side of the Gulf of Alaska, Kenai Fjords National Park.
With mountain building, of course, also comes erosion and there has been plenty of that. Kobuk Valley National Park surrounds the Kobuk River which drains part of the Brooks Mountains. After Alaska ended up so far north, the mountains ended up channeling arctic winds which were particularly dry. The lack of moisture, coupled with the cold temperatures ends up freezing the soil. Frozen soil does not erode down to fine silt and dirt, but rather stops at the size of sand grains. As a result of the wind and the presence of persistent sand, the Kobuk Sand Dunes were created and while not as big as at Great Sand Dunes in Colorado, are amazing none the less because of their unusual location above the arctic circle.
Perhaps the most impressive erosion, and a feature that Alaska is known for, is the work of glaciers. From U-shaped valleys where glaciers used to be, to ice fields occupying hundreds of square miles, to alpine glaciers in the taller mountains, to calving glaciers at the oceans edge – Alaska has them all. Although quickly receding, we still expect to see calving in the bays of Glacier Bay and the Fjords of Kenai. Our flightsees in Wrangell-St. Elias and the Brooks Range should expose us to high-mountain cirque glaciers as well as massive piedmont ice fields where multiple alpine glaciers merge into slower-moving flows that go on for miles. And, of course, Lake Clark and Lake Naknek (in Katmai) are glacially-sculpted lakes, similar in origin to the Great Lakes. Even in the far north, where ocean born moisture is minimal, we will see the effects of continent-size glaciation from thousands of years ago.
I can’t pretend to understand Alaskan geology the way a scientist does, but my senses tell me that Alaska has an awful lot going on. Hopefully I can appreciate just a small part of it. Can’t wait to go…