On the Road Again - Post 5

The Grand Tetons are the youngest of the Rocky Mountains and they are considered the most photogenic. Happily, this is not further evidence of our youth-obsessed culture. It is instead because there are no foothills to obstruct the view of the craggy, snow-dotted peaks as they soar 13,000 feet above sea level and 7000 feet above the Jackson Hole valley. And while the ravishing Tetons are mere babes at 10-13 million years old (as compared to the Rockies at 50 million years old), the rocks of which these youthful beauties are made are among the oldest on earth.

The tale of why there are no foothills leading up to the Grand Tetons and how 2.5-billion-year-old Precambrian rocks ended up rising above the Wyoming plain a mere 13 million years ago is a great illustration of the impacts of three of geology's money players: tectonic plates, glaciers, and erosion. Even if you aren't agog about geology the way I am, stick with me on this. It's a great story.

The earth is comprised of layers around a solid center core. The inner core is estimated to be about 12,000 degrees Fahrenheit. The rock gets progressively cooler and more solid toward the outside. The stuff between the core and the earth's crust is called the mantle, and it comprises the bulk of the planet. The mantle is rock, but it is neither solid nor liquid. It constantly moves and flows, but super-slowly, like gooey taffy or warm plastic. The thin layer of hard rock that consists of the crust and the upper mantle is called the lithosphere. We're still talking pretty hot rock where the mantle meets the lithosphere: over 700 degrees F.

Thanks to the spinning of the planet, the flowing of the mantle under the lithosphere, and differential temperatures all over the earth, the lithosphere is and has always been under a lot of pressure. It responded by breaking up into big puzzle pieces called tectonic plates. Driven by the heat of the circulating, semi-molten mantle underneath, these plates move around, nudging, grating, grinding and occasionally crashing against one another. Sometimes they even ride up on or slide underneath each other. For example, the Pacific Plate has been crowding the North American Plate for eons; these plates meet along the coast of California at the San Andreas fault. You know what happens there.

Between 120 million and 55 million years ago, a plate called the Farallon collided with the North American Plate. The Farallon got pushed under our plate, down into the mantle, and it stuck. This is what allowed the Rockies to be formed so far inland. When the sub-plate fell down into the mantle, a lot of liquid hot magma (as Dr. Evil would say) welled up and caused severe volcanic activity. This created more mountains and it also left a lot of residual heat in the lithosphere.

The heat caused the North American Plate to stretch, spread back out and crack. Since the land was being stretched from east to west, the cracks formed north to south. Cracks are faults and they angle deep into the earth's crust. The Teton fault was thus formed, about 13 million years ago. As the land continued to stretch, the fault pulled apart. One side (Jackson Hole) slid down along the fault, and the other side (the glorious mountain range) rose up. The side that dropped down displaced some of the underlying mantle, which further pushed up the rising side.

The differential is huge - the mountains rise about one foot for every four feet the valley drops. The current differential is about 27,000 feet from mountain top to valley floor. (Stay tuned for why the numbers don't quite seem to work.) And these forces are all still at work: whenever the two chunks of crust move along the fault, there is an earthquake. The Tetons are the product of 13 million years of earthquakes, the last huge one about 4800 years ago.

There are no foothills leading up to the Grand Teton range because the valley sank. The Precambrian rocks of which the young mountains are made were meandering along, minding their own business, in the mantle under the lithosphere until they got displaced by that sinking valley. These ancient rocks are gneiss and granite, metamorphic and igneous rocks (respectively) that are very hard once they cool. They erode at a much slower rate than the sedimentary rocks like shale, limestone, sandstone and siltstone that used to comprise the whole region and still comprise the bulk of Jackson Hole. The softer sedimentary rocks that originally rose up from the earth along the fault line, pushed up by the sinking valley and the harder metamorphic rocks, have eroded completely away but for the ever-shrinking top hat of sandstone Mt. Moran wears to this day.

Where did they go? This post has gotten too long, so I'll give short shrift to my favorite geologic phenomenon of all: glaciers. During the big Ice Age, which began 2 million years ago, thick ice covered the entire region (along with most of the rest of the northern parts of the world). Smaller ice ages followed, the most recent between 50,000-14,000 years ago. The Grand Tetons were buried several times under thousands of feet of ice for tens of thousands of years. Whenever the earth warmed up, the glaciers receded; their movement created the topography we see today.

Glaciers are the big kahuna of sculpting erosion. They bulldoze, carve, scour, polish and striate everything in their path. They also carry along - and then, when they melt, dump - unimaginable tons of eroded material. This is why the Jackson Hole we see is only 7000 or so feet (and not the full differential) lower than the tips of the Tetons: glacial debris filled up the other 20,000 feet.

I'm dying to go into more detail about glaciation, but I fear I might have exhausted even the patience and interest of those who've stuck with me this far. So I'll stop here and leave you with pictures.


The following picture isn't very good, but I offer it to show you the enormous scale of the place. I took the shot with 18x magnification and still the herd of enormous elk in the middle looks like nothing more than a thin, slightly bumpy brown line. The elk were in a standoff with a bear hoping to separate a tasty baby from its mother.