And an ice age would cause a lot of problems besides just nuclear waste! Icelandic glaciers like this one have released damaging floods, but nothing on the scale of those generated by Glacial Lake Missoula. Nothing of the magnitude that occurred in the late Pleistocene. The Glacial Lake Missoula flooding—and other floods we know about that occurred around the same time in geological history—were associated with extremely large ice sheets.
When you have large ice sheets, you have not just the water in the ice but big lakes that those ice sheets dam up. So there's a lot of water available for producing the volume that such megafloods would entail. Today, we only have two big ice sheets on Earth, and both of them almost completely fill landmasses—Greenland and Antarctica. So you don't have so-called ice marginal lakes, though in Antarctica you do have some subglacial lakes. That brings up another issue: could big subglacial lakes release floods?
Doesn't look like that's the case today, except with much smaller glaciers like those in Iceland. There you can get subglacial lakes, particularly associated with volcanism, that release floods. But neither the glaciers nor the lakes are nearly as big as anything we saw in the past. We had huge floods around many of the big ice sheets, such as the Laurentide, which blanketed much of arctic Canada east of the Rocky Mountains and southward to the Great Lakes. Huge megafloods were associated with its surrounding lakes, particularly Glacial Lake Agassiz, which covered much of central Canada centered around Winnipeg.
That released floods both to the south through the Mississippi River system and also under the ice out through the straits that enter Hudson Bay. There were also massive megafloods in Asia. Some of them were associated with big ice sheets that blocked the rivers that currently flow north from Russia into the Arctic Ocean. Others were associated with mountain areas, such as the Altai Mountains, the Sayan Mountains, and some of the mountains around Lake Baikal.
All of these are in southern Siberian Russia, along its borders with Kazakhstan and China and Mongolia. These mountain floods were comparable in magnitude, though perhaps not in volume, to the Missoula floods.
Billions of years ago, Mars hosted megafloods that dwarfed those that formed the scablands. Courtesy NASA. There are always puzzling things. For one thing, many of the processes that occurred during the cataclysmic flooding are of a scale that we can't reproduce in the laboratory, and they're difficult to evaluate theoretically because of not having that kind of experimental test.
There are features of erosion and sediment transport, for instance, that we don't fully understand. We have hypotheses for most things we see, but we still have to see how effective these hypotheses are. We try to advance this by finding other places and other examples where we can get some kind of constraint on how those processes operated. So these other megaflood areas I mentioned are important in helping us understand cataclysmic glacial breakout floods as a general phenomenon, not something unique, say, to Missoula.
We're finding them all the time. Most interesting and puzzling are those we're finding on other planets. We know from work over the past 30 years that Mars, for instance, has had even bigger floods on its surface than we see in the Channeled Scabland. Those floods seem to have been both similar to and somewhat different from the Missoula floods. In many cases they involved much more water and even larger flood-eroded landscapes than in the Channeled Scabland.
We could spend this whole interview talking about Mars, because there's so much that is not fully understood there, and we only have incomplete information about it.
One reason I study the Channeled Scabland is that it's helpful in understanding Mars as well as what we might find on other planets in the future, not just in our own solar system but beyond.
Yes, and they're in surprising places. I think some of the most puzzling landscapes are those that developed beneath the big ice sheets. We have a few such landscapes today under Antarctica and Greenland. Also puzzling are ones that were exhumed from beneath the big ice sheets in British Columbia, which was covered by the Cordilleran ice sheet, and in much of eastern North America where the big Laurentide ice sheet was. Those landscapes are not very well understood because we don't really have a modern analogue to them.
In geology, we try to understand things to a large extent by a sophisticated use of analogy, where we have features that are being formed by processes today or in some way that we can get a handle on how those processes operated long ago. Where we understand the relationships and then we try to extrapolate that into unknown landscapes.
The big ice sheets of the last ice age were in many cases warm-based ice sheets. That is, there was a lot of water underneath the glaciers. The landscapes that form there are a complex combination of ice and water processes involving meltwater and high pressures. There are all kinds of bizarre features on those landscapes that we don't fully understand. That's one category. Each thundering torrent of water, ice, and debris raced across the Rathdrum Prairie and into eastern Washington, stripping away tons of soil and rock and carving a region known as the Channeled Scabland.
All the water released from glacial Lake Missoula had to pass through Wallula Gap, where the Columbia River turns sharply west towards the Pacific. As flood waters backed up behind this narrow break in the Horse Heaven Hills, they filled up vast areas of the Walla Walla, Pasco, and Yakima valleys creating a short-lived lake, Lake Lewis, which reached depths of more than feet deep. Eventually, floodwaters forced their way through additional choke points in the Columbia River Gorge and Kalama Gap, filling the Columbia River Gorge and Willamette valleys until ultimately reaching the coast and spreading out for hundreds of miles along the floor of the Pacific Ocean.
Ice Age flood sites in Washington State Parks. The huge lake may have emptied in as little as two or three days. Over a period of years the glacier would advance, once again blocking the river, and the dam and the lake would form again. This process was repeated scores of times, until the ice sheet ceased its advance and receded to the north at the end of the Ice Age. It is assumed that the same processes would have occurred earlier during other glacial advances throughout the Ice Age, although most of the evidence for the earlier events may have been removed by the flooding that occurred during the last glacial advance.
The floods built gravel bars as tall as feet and moved boulders weighing many tons as if they were pebbles. Some of the eroded material was deposited along the path of the floods, but most of the eroded material was carried out onto the floor of the Pacific Ocean, where extensive deposits of flood sediment have recently been identified hundreds of miles from the current mouth of the Columbia River.
In the following years the account was refined, as evidence of more than one flood was discovered. It is now established that there were large numbers of Ice Age floods that swept across the US Northwest, and some of them were among the largest and most powerful floods that have ever occurred on Earth.
Early settlers in eastern Washington State recognized that the landscapes were different than those seen in other places but were not sure why. A closer examination of the features in the Basin led one geologist, J Harlen Bretz, to propose that only a sudden cataclysmic flood, on a scale never before considered possible, could account for the phenomenal size and distinctive characteristics of the landforms.
This radical idea was not well received by fellow geologists, and a long-running scientific dispute followed. Ultimately his extensive field work, plus additional research by others, conclusively established that many extraordinarily huge and powerful Ice Age floods had shaped the region. It was in that Bretz published the first in a series of scientific papers in which he proposed that the severely eroded Channeled Scabland, Dry Falls, and other immense geologic features had been formed by huge, powerful floods that had swept through the Columbia Basin during the Ice Age.
0コメント