Everything You See Is Moving

14 08 2009
DeadHorse

In the fullness of geologic time, even the tallest mountains crumble to the sea.

This is the process of erosion, where rock is pulverized into sediment and transported far away from its original source.  Eventually the sediment gets deposited in a new location, buried, and then baked into new rock by the immense heat and pressure of the Earth’s interior.  This rock may then be uplifted and eroded again, continuing the geologic cycle that has been slowly and inexorably churning up the crust of this tiny dot we call a home ever since its birth four and a half billion years ago.

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Many things can erode rock.  Wind can do it.  The rock can do it to itself, if it becomes so steep it triggers a landslide.  But by far the dominant force of erosion on planet Earth is water.

Water has a number of weapons in its arsenal when it comes to destroying rock.  The most powerful is its solid form, ice.  We may refer to things like waiting in line at the DMV as “glacially slow”, but this is a misnomer.  Geologically speaking, glaciers are actually extremely fast.  In a matter of millennia- a mere blink of an eye in geologic terms- a mountain glacier will push inexorably through an alpine valley, driving its rocky enemies in terror before its advancing wrath like a giant bulldozer.  All of the rock it scrapes up gets piled at the front of the glacier in something called a terminal moraine- because it marks where the glacier terminates- and when the glacier finally melts this pile gets left behind as a giant dirt wall across the floor of the valley.

P1020359

This is the leading edge of Fox Glacier, on the South Island of New Zealand. The large pile of broken rock at its base is the terminal moraine.

Continental glaciers do the same thing as mountain glaciers, only more so.  Rather than being confined to high-altitude valleys, continental glaciers- as their name implies- cover entire continents with a moving sheet of ice over a mile thick.  Most of Antarctica and Greenland are presently covered with continental glaciers, and only twelve thousand years ago large portions of North America and Europe were too.  Long Island is just one of the many moraines left behind by the great North American ice sheet; Cape Cod is another.

But ice doesn’t get to have all the fun, of course.  Liquid water is also a powerful force of erosion.  It all starts when rain falls on a mountainside.  That rain flows into swift-moving streams, which move rapidly down the steep face of the mountains.  As the water descends, small streams merge into medium-sized tributaries, which join into large rivers.  As the river approaches the sea, the slope of the land begins to level out- often as a result of the past action of that very river- and the water slows down.  At all stages the water has the ability to chemically dissolve ions in the rock it flows over (the continuous input of these ions is how the sea becomes salty), but more important than the chemical action of the polar water molecules is the brute physical force of a fluid in motion.

The physical force of flowing water increases the faster the water flows, so swift mountain streams have the greatest erosive power.  This is why older mountain ranges like the Appalachians are smooth and rounded while new ones like the Himalayas are steep and craggy:  steep slopes get eroded the fastest.  Water going down a waterfall can break off whole boulders; but slow-moving water can barely hold up a sand grain.

niagara

Niagara Falls. Viewed from above, you can easily see how the force of the water is cutting into the cliff face. In fact, since the Falls formed roughly 12,000 years ago, they have retreated about 7 miles.

bayou

Muddy deposits are being created in this slow-moving bayou of the Mississippi delta.

As rivers approach the sea they slow down, and begin to deposit the sediment they picked up when they were moving faster.  However, they don’t deposit the sediment evenly.  If there is any bend in the river- any bend at all, no matter how slight- water moves faster around the outside and slower around the inside.  This means sediment gets eroded from the outside and deposited on the inside, making the bend sharper.  Over time, this causes old rivers to meander like the village drunkard in the wee hours of the morning.  If meanders get too steep they can pinch off, forming oxbow lakes.  Every once in a while a river floods, filling its floodplain with fresh sediment.  Over time a mature river will change course many times, always seeking the steepest slope and filling it with sediment, until eventually, like a giant belt sander, it flattens a region hundreds of miles square.  A large portion of the American south has been flattened in this way by the Mississippi River.

meander

This is a diagram of a meandering river taken from the St. Ivo School Geography Department.

Erosion and deposition determines the shape of the terrain we see around us on the Earth’s surface.  Often, differences in the underlying rock determine the course erosion takes.  Some rocks, like schist, are stronger in two dimensions than the third.  This is because schist contains several varieties of the mineral mica, whose atoms are arranged like sheets of loose leaf paper.  And like the kingdom which was lost for want of a nail, the arrangement of these tiny atoms can control the shape of islands and peninsulas many miles long.  This is the case in Casco Bay in Maine.  Other times, two types of rock of different hardness will be in close proximity to each other, and this difference in hardness shows up dramatically in topography.  If hard rock is deposited inside soft rock it will stick out, like at Devil’s Tower in Wyoming; if soft rock is deposited inside hard rock it will be indented, like the Giant Steps on Bailey Island, Maine.

HS foliation

This is a map of part of Casco Bay in Maine, overlain with a closeup of a schist outcrop on Bailey Island and a diagram of the crystal structure of mica. For scale, the outcrop in the picture is a little bit taller than I am.

devils tower

Basalt is harder than sandstone, so Devil’s Tower sticks out. However…

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…Basalt is softer than schist, so the Giant Steps are indented.

All around us the Earth is dancing.  Rock, water, and wind are slowly performing a brilliant and beautiful ballet down through the millennia.  Over millions of years mountains are ground into dust, and from the dust new mountains are built.  The graceful motions of the geologic cycle never stop, yet most of us go about our lives completely unaware of it.  The greatest dance troupe of all time is performing a masterpiece before our very eyes, yet because it is so slow we ignore it.

So the next time you pause to admire the stern majesty of a mountain range or the lush green of a valley, just remember:  everything you see is moving.  Under the force of wind, water, and the inexorable push of the tectonic plates, the entire surface of our planet is in motion.  It dances to a beat far deeper than anything we can experience in our fleeting lifespans; yet still it is not quite beyond us.  By practicing science we can perceive a small part of the great dance, and more importantly, we can come to appreciate just how big the dance is, and how much of it still lies hidden from us.

-Mike

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