Thursday, March 29, 2012

The Great Pleistocene Ice Sheets

I sat in on a scientific talk at the Cascades Volcano Observatory today, and was reminded of some old and learned a number of new, interesting things (interesting if you live in the Pacific Northwest, that is):

1. There are two kinds of glaciers:

(a) Alpine glaciers are the ones derived from snow falling on a high mountain, accumulating as ice, not melting much because higher elevations are always colder, and thus forming slow "ice rivers" down the flanks of the mountains. These are so common in Alaska, Canada, and the Pacific Northwest that many of them even have names - especially when they are on volcanoes. In fact, there was a dog-fight going on in 2004 about what to name the crescent-shaped glacier forming behind the 1980-86 dome of Mount St Helens. One party wanted to call it "Crater Glacier", while another wanted to call it "Loowit Glacier" (if you follow that blue link, you will come to an amazing Native American myth).

The glacier's name all became moot when the volcano erupted again in October 2004 and split the 200-meter-thick (600' deep) glacier into an "east arm" and a "west arm". I was one of the last four people to walk between the two arms' northern meeting-point in August 2007, when I was helping a geophysics team make some geoelectrical soundings there. The 10-month winter closed in shortly afterwards, and by 2008 the two 25-meter-thick (75' deep) glacier "toes" had merged. One of my geoelectrical sounding stations, that "saw" down at least 950 meters (~1,000 yards), is now under the merged glacier, and can never be reoccupied again. At least in my lifetime, anyway.

(b) Continental Ice Sheets are the monster ice sheets that form pretty much like Alpine glaciers - but form over land that is constantly being snowed on and not warming up enough to melt away. In North America these have waxed and waned for millions of years, the last set melting back only about 10,000 - 17,000 years ago (depending on where we are talking about) to expose what we now call New York, and Vancouver, and Chicago... and a place called Canada. In northern Europe, these great ice sheets grew to be at least 3 kilometers (~2 miles) thick, and so heavy that they actually depressed the crust of the Earth by hundreds of meters (the Earth's crust is plastic over long stretches of time).

2. Glaciers 'majorly' change the face of the land. Because of climate change, virtually all glaciers remaining have been retreating for close to a century - longer in some cases. They leave behind several very distinctive land-forms:

(a) Weirdly U-shaped valleys. They are "weird" to someone who has worked most of his life in a desert, anyway. These valleys are the living proof that something very big scraped and scoured all the loose edges free of...

(b) Huge piles of boulders and rocks. These huge piles now form distinctive land-forms called "glacier moraines" - typically long, skinny ridges of rocks and bounders. The Hill Cumorah is a classic example of a glacial moraine. Huge rocks (called "glacier erratics" for fairly obvious reasons) have been identified with a source terrain that can be literally hundreds of miles north of where they now lie. If you ever drive through the northern half of the United States, you will often see huge, house-sized boulders sitting in the middle of a field somewhere. All the smaller ones have been bulldozed aside by the farmers, leaving an odd distribution of only the ones to big to move. These typically came from Canada.

3. There is a Pacific Decadal Oscillation going on: this is a roughly 10-year cycle of cold and wet periods interspersed with warm and dry periods, and they roughly oscillate in 10-year high/low cycles in the Pacific Northwest. It's often hard to sort these out in the short term, because they mix with the ENSO (El Nino-Southern Oscillation) events that most people have heard of. We are currently in a cooler-wetter part of a PDO cycle, which can easily fool certain people who can't accept that climate change is going on - because some glaciers are actually increasing a bit (Mount St Helens' still un-named glacier[s] is an example). If you look at the plots of glacier length over a 60 - 100 year scale however (when we had grandparents with a scientific bent who recorded such things), you will see a long, steady decline with 1 - 3 year increase 'spikes' on it.

4. Continental Ice Sheets can also make major, long-term changes in rivers. The Alaska Panhandle (the southern "tail" of Alaska) is an "accreted terrain" - that means that island arcs of volcanoes forming in the Pacific ocean migrated over time (or the North American continent encroached on it, from another frame of view) until they smashed up against the continental margins. Then another one came and smashed up against the first. And then another. Geologists have dated some of these by radiometric means back to the Jurassic era, up to 200 million years ago. Same thing happened to the Pacific Northwest: what my house is built on wasn't originally even part of the North American continent, but some island paradise (or volcanic hell-hole).

Now think of your kitchen baking experience. If you keep kneading bread in one direction, then bake it, you will see inside it that it has a certain texture - a preferred way to split apart, for example. This becomes really obvious if you sprinkle cinnamon and sugar on one surface and then roll it up: you've created a naturally weak zone just like a sedimentary layer made of mud that becomes a shale millions of years later. Bingo: a preferred place for a fault to happen if you have tectonic forces at work. Ever see a pile of really, really beat-up, shattered rock? This is often a quick way to identify shale from a distance even before you can lay a hand (or hand-lens) on it.

In the Alaska Panhandle and the Pacific Northwest, the "incoming" oceanic terrains came from the west-southwest... this means that the "texture" trends north-northwest (perpendicular to the "smashing force"). If you are going to have valleys and ridges, they will align this direction: the "softer" rocks weather out preferentially, while the tougher rocks become the ridges. At one time all the rivers in the Pacific Northwest DID align north-northwest. The Frasier River in British Columbia and the Skagit River north of Seattle are examples.

However, when the great continental ice sheet came grinding and crushing down from the north, it blocked these rivers and dammed them up. This caused great, long lakes to form until they got so deep that they started spilling over one of the lower points of the bounding ridges. You know what water breaking through a hoed row in your garden does: it rapidly widens the opening. Thus the Skagit River, which once flowed northwards, made its way steadily westward (a lake at a time) and now opens out to Puget Sound north of Seattle. Early geologists couldn't figure out why a river would cut through a landscape instead of follow that landscape. No one understood this until they understood continental ice sheets.

For a different reason (the blockage was not ice, but HUGE amounts of flood basalt magma) huge lakes formed in what is now the interior of the northwestern United States. Bonneville Salt Flats? After a long time Lake Bonneville drained westward, leaving the salty, flat bottom behind. The Columbia River? It finally broke through a lower point in the dam basalt (pun intended) and ripped open the Columbia River Gorge, and now they all drain to the Pacific Ocean.

And all this just because water freezes.

Incidentally, there are all sorts of other wide-scale interactions involved:

(a) For example, when northern Europe and North America and Asia were covered with huge ice sheets, the oceans had quite a bit less water. What you may now refer to as "Virginia" or "Florida" actually extended at least 50 kilometers farther eastwards. Native Americans' ancestors cooked mussels and clams on beach dunes that are now 7 meters (~20 feet) under the sea.

(b) While all this was going on, today's deepest and driest desert, the 'Rub al-Khali (Empty Quarter) was quite literally THE Land of Ten Thousand Lakes (sorry, Reo). I have a photo of myself standing next to what looks like the skull and horns of one really different-looking water buffalo - an antique bovid that roamed in those lakes. I have personally dug up fresh-water shells from the desolate desert floor there. This is a region where "normal" humidity can get down to 2% - you wake up every hour, all night long, with your nostrils on fire, and you MUST drink and "snuff" water or you will end up with a bloody nose. Everyone sleeps on their sleeping bags in the Empty Quarter, and no one sleeps very well.

(c) Ocean currents were totally different: the Gulf Stream... wasn't. For that matter, if the Gulf Stream somehow was ever blocked, you can wave goodbye to London and the rest of Europe, because don't look behind you but a huge ice-wall is fast approaching. This has actually happened a number of times in the last several millions of years.


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