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.