When you put marshmallows into a cup of hot cocoa, what   happens to them?  Even if you drop the marshmallows from some height above the cup, they float to the surface (after splashing cocoa—“hot chocolate” to some of you—on the countertop), don’t they?  They float because they are “lighter” than the hot, liquid cocoa.  For marshmallows, this is because they contain a large percentage of air.  A boat also floats in a lake because it contains a large volume of air (e.g., a solid concrete block sinks in water, while a hollow concrete canoe floats—college engineering students actually have contests and races to prove this). 

Now, what will happen if we melt some butter in the microwave and pour it into the cup of hot cocoa and marshmallows?  First, we just ruined a perfectly good cup of hot cocoa.  But second, the melted butter (sorry, melted heart-healthy Smart Balance in our case) also floats on the top of the hot cocoa and flows beneath the marshmallows.  If you don’t believe this works, try it for yourself.  But, be sure to clear it with your wife first as mine is pretty particular about what I do in “her” RV kitchen! 

If you have some Peeps left over from Easter, you could use them instead of marshmallows.  They’re made the same way and were just going to get thrown in the trash anyway because no one actually eats Peeps, do they?  And, besides, the yellow color would contrast nicely with the hot cocoa.  Come to think of it, I should have “borrowed” my grandsons’ Peeps for my experiment.  I could have saved those perfectly good marshmallows for our favorite afternoon snack—microwave s’mores. 

You are probably wondering how hot cocoa, marshmallows, and melted butter relate to the geology along the Alaska Highway.  These foods are acting just like the mantle, continental crust, and oceanic crust of our earth where the North American (continental) Plate is drifting westward over the Pacific (oceanic) Plate (see photo below).  The hot cocoa is the earth’s hot mantle, the source of the heat in our volcanoes and geysers and the energy driving the “convection cells” that move the earth’s plates around, separating them or crashing them into each other.  The lightweight marshmallows are the continental crust (continents and large islands of the earth) that floats on top of the mantle.  The melted butter is the oceanic crust that also floats on the hot cocoa, but flows under (“subducts”) when it meets a marshmallow (continent).

 What happens to our marshmallows if we leave them in the hot cocoa without letting it cool too much (or drinking it too fast)?  First, the bottom of the marshmallows starts getting soft and gooey, then will actually begin melting and dissolving into the cocoa, right?  As it melts, it gets smaller and smaller until it becomes a mini-marshmallow and then may dissolve completely.

Because the mantle is hot, the bottom of the continental crust also heats up and begins melting.  If the melting continental crust finds a deep crack to flow through and is able to reach the surface, we may see an explosive eruption of light-colored “rhyolitic” (gooey and sticky like melted marshmallow) ash and lava.  A good example would be the explosive 1991 eruption of Mt. Pinatubo on the big Philippine island of Luzon (photo from: http://pubs.usgs.gov/fs/1997/fs113-97/ ).  Three days after this USGS ash cloud photo was taken, the volcano exploded in the second largest eruption on earth in the 20th century.

If melted oceanic crust travels through a crack to the surface, we may see a red-hot, semi-liquid, slow-flowing “basaltic” river of lava (kitchen analogy: molasses), as at Kilauea on the Big Island of Hawaii. 

Why do continents drift ("float") on the mantle?   You have all seen the periodic table of elements sometime in your past. Remember that some elements, like hydrogen, are much lighter (have a lower atomic weight) than other elements, like lead.   That's why we put hydrogen in balloons to make them float in air (air=78% nitrogen + 21% oxygen).   And why, pre-EPA, we used lead weights to sink fishhooks in our favorite fishing hole (and why a "Lead" Zeppelin is a flying impossibility, even filled with hydrogen!).

Rocks that primarily contain lower atomic-weight elelments like silicon (At. Wt.=28), aluminum (27), magnesium (24), and carbon (12) will be "lighter" than rocks containing higher atomic-weight elements like iron (56), manganese (55), and titanium (53).   Although iron is found in most rocks (that's why many are "rust-colored" in hues of orange, red, and maroon), the oceanic crust and mantle are very iron-rich and the continental crust is much more silicon-rich.  

If it helps to visualize this, find a clean plastic dish small enough to weigh on your kitchen scale.  If your wife makes her own jewelry, fill the dish with as many small glass beads as you can get in it, level with the top, and weigh it (don't break the scale!).  Now, do the same thing with as many very small finishing nails or tacks as you can pack/shake in, level with the top of the dish and weigh it.  If you packed the nails in tightly enough, the weight of the dish of nails should be much greater than the weight of the dish of beads. 

Now why did we just do this?  Because glass is almost pure silicon and small nails are nearly pure iron, so the iron should weigh more than the silicon, right?  Atomic weight 56 = 2 x atomic weight 28 or nails are twice as heavy as glass beads (if the amount of air is the same in both dishes!).  So, silica-rich continental crust (marshmallow) floats on the iron-rich mantle (hot cocoa) and rides over ("subducts") the slightly less iron-rich oceanic crust (melted butter).

Next time: Is it "terrain" or "terrane"?  Which are we seeing in Canada and Alaska?  The short answer is "both" but I'll discuss the very significant difference next.

Want to see more photos of Mt. McKinley glaciers?  I forgot to define a few terms relating to mountain and tidewater glaciers a couple of posts back.  So, this post will attempt to rectify that omission.

Tarns are small lakes that fill cirques, the circular snow accumulation bowls on mountain slopes.  An arête is a sharp ridge of rock that is left between two cirques that are being eroded toward each other.  Arêtes may be seen on many peaks along the Icefields Parkway in Banff and Jasper NP and on the slopes of Mt. McKinley and other mountains in the Alaska Range.  My first 2000 photo below shows a classic bowl-shaped cirque on Mt. McKinley surrounded on three sides by arêtes.  The second photo, taken from slightly above the ridge, shows how just narrow and steep one of Mt. McKinley’s arêtes can become.  The photo also clearly shows the fracture (“joint”) pattern of the very hard, erosion-resistant granite that forms Mt. McKinley.

 

 

A horn is a sharp-pointed, pyramidal mountain peak formed by the“headward erosion” of several cirques. The Matterhorn in Switzerland is the most obvious and most famous example.  But my wife’s 2007 photo of cloud-shrouded Mount Saint Nicolas in Glacier NP, Montana is also a great example, with four cirques, one even containing a tarn, surrounding its peak and creating its classic pyramid shape.

 

I mentioned earlier that glaciers are very effective bulldozers and can excavate large volumes of rock as they move.  When a glacier flows down a “V”-shaped mountain valley originally formed by stream erosion over many thousands, or even millions of years, it can quickly widen and deepen it into a classic “U”-shaped glacial valley, sometimes in only a few hundred to thousand years.  This gouging out of rock by ice is exactly like using a piece of coarse sandpaper on a newly cut 2x4.  The sandpaper quickly smoothes the end of the lumber, but, due to the coarseness of its grit, it may also knock splinters off the corners.  And, just like coarse sandpaper produces large quantities of sawdust, the glacier produces huge volumes of “rock dust” (actually called “rock flour,” but known to geologists as “silt”).

The glacier picks up everything from sand to boulders and carries them along, frozen into the bottom ice.  The sand and gravel may smooth and polish or groove ("striate") the bedrock surface over which the glacier is flowing, but the boulders may knock off huge chunks where they hit an ice-wedged fracture (joint).  The glacier can also erode by “ice plucking,” where water freezes in those joints and, over time, loosens the rock sufficiently that it can fall onto, or is picked up by, the glacier.

What happens to all that rock debris that the glacier creates, incorporates into its ice, or pushes ahead as it moves downslope?  That debris is partly carried along by the ice and partly carried away from the ice by glacial meltwater streams.  The part carried by the ice is a random mix of every size rock particle.  The part carried by the streams is “sorted” by the running water (just like a sluice box sorts out placer gold from sand).  First, the boulders are likely not moved much by the water, the cobbles are dropped out a short distance downstream from the toe of the glacier, and the sand is carried greater distances downstream. 

Obviously, a tidewater glacier can drop all these particle sizes directly into the ocean.  But, in a mountain glacier terminating some distance from the coast, only the very fine “rock flour” may reach tidewater.  Because this silt-sized sediment is so fine and so easily agitated by waves and currents, it may take years to completely settle out, so may move far out to sea.  My 2000 photo below shows a tan-colored “rock flour” plume being discharged into the deep green water of the Lynn Canal near Haines.  The plume is coming from the Katzehin River, which originates as meltwater from Meade Glacier.  This glacier is actually located east of the Lynn Canal in the Coast Range, about 25 miles southeast of Skagway along the Alaska-British Columbia border.

 

This same rock flour/silt is found in Lake Louise and Moraine Lake in Banff NP and at many glacier-fed lakes like Portage in Alaska.  In those lakes, however, the water is a turquoise blue that attracts every photographer who sees it on a sunny day.  My photo is probably unusual in that regard, as few photographers are attracted to "dirty water," even if they know the cause!

Next time: How do plate tectonics and continental drift really work?  A kitchen experiment involving hot cocoa (hot chocolate), marshmallows, and melted butter provides an analogy that is easy to understand, but difficult to swallow! 

Imagine ice two miles thick covering your town!  If you are from the Great Lakes region, the Dakotas, Montana, Idaho, Washington, or most of Canada, that may well have been true of your region during the Pleistocene ice ages.  During the past 2-2.5 million years, perhaps as many as 10 distinct ice ages have occurred.  But each ice age didn’t last 1/10 of that time because there were warmer “interglacial” periods between each glacial advance.  Why are these ice ages important in understanding the geology along the Alaska Highway and its approaches?  Because nearly all the landforms you are seeing along your route—mountains, hills, river valleys, lakes, waterfalls, fjords, sand dunes, remaining glaciers—were modified by, or actually created by, this ice, its melt-water, or its winds.

During the Pleistocene, so much of the earth’s water accumulated in the thick “ice sheets” as snow and ice that sea level around the entire globe was as much as 450 feet lower than it is today.  This lower ocean exposed the Bering Sea “land bridge” that connected North America to Siberia and the remainder of the Eurasian continental land mass.  Also, at the times of peak glaciation, only the highest peaks in the Canadian Rockies were exposed above the ice.  This thick ice had a bulldozing effect many times greater than any glacial erosion that we see from mountain glaciers today.  So, even the hard bedrock of the Rockies shows the erosive effects all this ice.

Lobes of the ice sheets dammed up huge rivers like the Columbia, creating gigantic lakes, with names like Glacial Lake Missoula and Glacial Lake Columbia.  When these ice dams broke, as they did multiple times, the resulting floods (“jokulhlaups”—Icelandic, not Swiss!) gouged out great valleys (e.g., Grand Coulee) and stripped the soil and loose rock off the lava flows of eastern Washington (the “channeled scablands”).  In addition, the constant cold winds coming off these miles-high sheets of ice picked up sand and silt from the outwash plains at their edges, creating sand dunes, as at Carcross, YT, and in the Kobuk Valley National Park, AK, and windblown silt/rock flour-covered (“loess”) hills, like the Palouse of eastern Washington.

Features created directly by the ice include moraines and till (discussed in the previous post), drumlins, eskers, kettles, kames, tarns, arêtes, and horns.  Drumlins are rounded hills of till smoothed by the passing ice, with a steeper slope in the direction of movement.  Eskers are long, narrow, snake-shaped ridges of gravel that were formed by water flowing beneath the ice of a glacier or ice sheet. 

Kettles are small, generally circular lakes or ponds created when a chunk of gravel-covered glacial ice, left behind by a retreating glacier, melts and the surface collapses.  When I was investigating a proposed damsite in Devil’s Canyon on the Susitna River, our helicopter flew daily over hundreds of kettle ponds located between the Denali Highway and Devil’s Canyon.  These ponds contained huge numbers of grayling, but most had probably never been fished because the ponds were too small to land a float plane.  In my 2000 aerial photo below, the kettle ponds are clearly visible in the outwash plain below Ruth Glacier. 

Kettle ponds are often associated with kames, which are holes in the glacier filled with gravel by streams flowing across the surface of the ice.  When the glacier retreats, the gravel is left as a small hill. 

Also visible in the photo to the left are the areas bare of trees.  These grassy areas are not due to clear-cutting by loggers (they are all down in the Alaska Panhandle, not in Denali National Park).  They are the result of an extremely high water-table combined with one last feature left from the ice ages that no Alaska RVer will soon forget—permafrost.  Permafrost is just short-hand for “permanently frozen ground” (does anyone remember how to take short-hand?).  Soil that was frozen during the last ice age has, in Northern Canada, Alaska, and parts of the Rocky Mountains, never thawed out.  This frozen soil, on which only a thin surface layer thaws each year, creates the roller-coaster frost heaves that are the bane of RVers throughout the Yukon and Alaska.  All those little red or yellow flags that you soon come to love to see, but hate driving past, are due to glacial geology!

I don't know if it still exists in Fairbanks, but in the early1970's, I visited a "permafrost tunnel" operated by the University of Alaska and the U.S. Army Cold Regions Research Laboratory.  After we passed through an airlock designed to prevent thawing, we walked into a tunnel excavated in frozen Pleistocene gravel, sand, and silt.  Being an engineer, I felt very strange walking under boulders the size of a small car hanging in the ceiling with no visible means of support.  There were also interestingly shaped, dark-colored ice lenses in the walls and ceiling.  It was a tour I'll never forget, so not everything about permafrost is bad!

Next time: Tarns, arêtes, and horns should have been discussed in the last post because they are related to mountain glaciers, not ice sheets.  So, they will be discussed in the next post.

A Glacial Primer

Regardless of the route you use to get to Alaska   and to circle through the central part of that state, you cannot miss seeing glaciers.  There are glaciers in Banff and Jasper NP in Alberta; near Stewart-Hyder in BC; around Juneau, Haines, and Skagway for those traveling on the AMH ferries, along the Glenn Highway; near Valdez, Portage, and Seward; in Glacier Bay, Kenai Fjords, and College Fjord; and on the slopes of Mt. McKinley.  There are many types of glaciers, but the named glaciers along your routes are generally either "mountain glaciers" or "tidewater glaciers." 

Both mountain glaciers and tidewater glaciers may be tongues of larger, mountaintop glaciers called "icefields."  The large icefield that feeds the famed Athabasca Glacier in Jasper National Park is called the Columbia Icefield.  The icefield that feeds the Exit mountain glacier at Seward and the tidewater glaciers of Kenai Fjords National Park is called the Harding Icefield, named for the 1920's U.S. President who died of a heart attack on the way home from his only trip to Alaska.

Mountain glaciers consist of many parts, most of them originally named by the Swiss and some of which are shown in my 2000 photo taken from the air of Ruth Glacier.  Ruth Glacier is located on the southern flank of Mt. McKinley in Denali National Park.  A "cirque" is a bowl-shaped snow accumulation area that can be a source of snow and ice for a mountain glacier.  Because all glaciers act as very efficient rock excavators, they break rocks off valley walls as they move downslope.  This rock is called a "lateral (ie., side) moraine."  The rocks being bulldozed off the bottom of the glacial valley, pushed ahead of the toe, and left there when it begins to retreat are called a "terminal moraine."  And, when two glaciers flowing down adjacent valleys meet and join, their adjacent lateral morains also join forming a "medial moraine."  A "hanging glacier" is a steep wall of ice flowing out of a cirque or hanging valley, just like Yosemite Falls flows out of a hanging valley located above the U-shaped, glaciated Yosemite Valley.

If large quantities of rock fall from the cliffs of an oversteepened, U-shaped glacial valley onto the surface of a glacier and the glacier begins to retreat, the glacier can become completely covered by the rock debris. This is sometimes called an "ice-cored moraine" because it looks like a moraine even though it is actually still a moving glacier. The lower parts of Dome Glacier in Jasper National Park and Ruth Glacier in Denali National Park are ice-cored moraines.  My 2000 photo of Ruth Glacier on the right clearly shows both lateral moraines, the medial moraine, the ice-cored moraine, and an amazingly cloudless Mt. McKinley.

Remember, a mountain glacier is just a river of ice instead of a river of water and moves downslope by gravity flow just like a river of water moves downstream.   Therefore, "right" and "left" on a glacier are just as they are on a river--the right bank/lateral moraine is to your right if you are looking downstream or toward the "toe" (as opposed to "head") of a glacier.

Finally, the random mix of silt ("rock flour"), sand, gravel, pebbles, cobbles, and boulders dropped by a glacier when it melts ("retreats") is called "till."  And the better-sorted rock flour, sand, and gravel moved downstream by streams of glacial meltwater gushing from the toe of the glacier are called "glacial outwash," as shown in my photo of the meandering meltwater streams and glacial outwash plain below the toe of Ruth Glacier.

Next time: A little about some slightly larger glaciers--ones that covered most of Canada and much of the U.S., reached depths that left only the highest peaks of the Canadian Rockies exposed above the ice, and disappeared only 14,000 years ago.

Reflections on the “history of geology” (not “geologic history”):  Fifty years ago   this year, I began my first college geology course.  By the time I graduated, I had still not studied, nor even heard of, plate tectonics, continental drift, sea-floor spreading, mid-oceanic ridges, subduction zones, Pangaea, or iridium layers.  We had heard no explanation for why the eastern coastline of South America looked like it should fit perfectly into the west coast of Africa on every globe.  Not until 1969 did I read about some of those topics, not in one of my graduate school courses, but in an issue of Scientific American that was devoted to new discoveries in oceanography. 

So, concepts that every 5 year-old dinosaur fanatic or adult viewer of PBS or TLC can now discuss in detail were completely unknown to most geology students in the 1960s.  As a college senior, I used a textbook for a course in Regional Geology that was originally published in 1929.  I even wrote a term paper for the course on a “geosyncline,” the now 100-year old concept that predated plate tectonics as a global/regional explanation for the formation of mountain ranges.  Meetings of the Geological Society of America, the American Association of Petroleum Geologists, and the Society of Exploration Geophysicists were still heatedly arguing during much of the 1970’s about the validity of those new concepts. 

 
Only over the past 30 years have all the pieces of the global plate tectonic puzzle fallen into place.  Now the explanation seems so logical that most people visiting a museum of natural history "to see the dinosaurs” don’t have a second thought when they see videos of black smokers along the Mid-Atlantic Ridge or the Pacific Plate being subducted beneath the North American Plate.  Yet all those advancements in the science of geology occurred during my professional career.  Amazing!

What brought these memories flooding back was a used paperback book I just bought on Amazon.com for our trip to Alaska.  The book is entitled “Banff National Park: How Nature Carved Its Splendor” and contains road-logs and photos of the geology of Canada's most famous national park.  The problem is that the book was published in 1977 and never mentions plate tectonics or continental drift.  Page 22 even shows diagrams of the geosyncline that formed the Canadian Rockies.  The following page clearly states: “For reasons we do not yet understand, the trough area was then severely compressed so that the rocks in it were folded and broken.”  Now, folks, the Lesson for Today: 1) don’t buy geology books from the 1970s expecting to find explanations based on plate tectonics; 2) don’t buy a more recent book on geology that doesn’t have the terms “plate tectonics” or “continental drift” in the table of contents or index, and 3) not all Amazon used books are worth even $2.39 + $3.99 for shipping!

Why belabor the newness of plate tectonics and continental drift?  Because all of the bedrock geology you are seeing on your Alaska trip is explained by these recent concepts.  You are traveling to Alaska along the old western margin of the North American Plate.  In some areas, you will be driving on an ancient “island arc” the size of Japan that crashed* into that Plate.  When you look at your Alaska map and see the long Alaska Peninsula curving away to the southwest and transforming into the Aleutian Islands, you are seeing a modern volcanic “island arc” sitting on the boundary between two plates. And, when you reach Homer and view Mt. Redoubt, Mt. Iliamna, and Augustine Volcano across Cook Inlet, you are looking at part of the “Ring of Fire.”  That ring is the circle of volcanoes marking plate boundaries that surrounds the entire Pacific Ocean and causes the earthquakes that frequent Japan, Chile, Indonesia, Nicaragua, and, yes, Alaska.   When you visit Anchorage, Seward, Portage, Kodiak, or Valdez, you will be seeing cities and towns nearly destroyed by the Good Friday Earthquake of 1964 and its subsequent tsunamis. 

 

When you drive the Icefields Parkway through Banff and Jasper National Parks, you will be viewing a deck of cards pushed eastward for the past 70-100 million years, sliding over one another due to the pressure from crumpling island arcs riding up over the edge of the westward-drifting North American Plate.  When you visit Denali National Park and attempt to view Mt. McKinley through the clouds, you will be looking at the result of one plate ramming another at an angle that pushed up the highest mountain on our continent.  So, you are seeing geology that had no good geological explanation until the theory of plate tectonics became widely accepted.

* “Crash” is a relative term.  Think of two fully loaded semi-tractor/trailer trucks in a head-on collision where you throw away the stopwatch and pull out a calendar.  At a rate of an inch a year, it may take several decades for the two trucks to finally come to a complete stop!  We are talking geologic time, not human time, in the collisions of tectonic plates.

For all "Roll Call Alaska 2013" RVers:

 

We RVers are now, or soon will be, driving through some of the most interesting, visible, and photogenic geology in North America.  We will see some of earth’s oldest sedimentary rock around Banff and Jasper and some of the youngest igneous rock in the active volcanoes of the Alaska Peninsula, visible across Cook Inlet from Anchorage, Kenai, and Homer.  And in between those two bookends is just about every other age and type of geology that exists.  If you are interested in the mountain ranges, river valleys, glaciers, fjords, and volcanoes along your route and wondered why they are there, you are asking questions about geology.

We are leaving Denver in mid-June and are planning to cross the border at Osoyoos about 10 days later.  Then we plan to use the Milepost's "Central Access Route" to Kamloops, the "Western Access Route" to Prince George, and the Yellowhead Highway and Stewart-Cassiar Highway to Watson Lake.  We will travel the Alaska Highway to the South Klondike Highway, then on to Skagway and Haines (by ferry), the Haines Highway, then the Tok Cutoff and Richardson Highway to Valdez.  From there, we will go to Anchorage and the Kenai, up to Talkeetna, Denali, and Fairbanks and home through Dawson City, Whitehorse, Dawson Creek, Jasper, and Banff. 

I realize that we are going at a later date and in the opposite direction from many of you. So you may have already passed through some of the areas that we will not see until August or September.  But I have worked in Alaska on and off since 1972 and we visited Banff and Jasper just a few years ago.  So, if you have questions, I may know or can find the answer, even if I have not visited it yet this year.  And, if you are returning from Alaska via Valdez, the Tok Cutoff, Haines, Skagway, and the Cassiar-Yellowhead route, I will have already traveled through that area. 

If there are other geologists on the "Roll Call Alaska 2013" list, feel free to add your knowledge to the blog.  For those non-geologists who have questions, please feel free to attach your photos of interesting geologic features that you see to your responses on the blog or to personal messages to "fanrgs" on RV.net.  By the time all of us return from Alaska, maybe we will have put together a usable geologic road log for next year's Alaska Highway travelers--something that doesn't seem to exist on-line or in libraries in 2013.