Excerpts from Jim Conrad's
from the May 22, 2005 Newsletter issued from the Sierra Nevada foothills
somewhat east of Placerville, California, USA
THE SIERRA NEVADAS, VERSION TWO
Very unusual for this time of year, this has been a chilly, rainy week in the foothills. It hasn't rained much more than it might during the winter, but what rain we had added itself to snow-melt from above, and that caused worries about flooding. On Thursday some of us hiked to the bottom of the canyon just to gawk at the rampaging river. From above we can't see the river, but we'd been hearing its powerful roar and we knew it must be something to see.
In fact, around here it's impossible to ignore the lay of the land and the matter of geology. At some point even the most indifferent person has to wonder "Why is that enormous canyon next to us?" or "Why do so many rock strata in the canyon's wall stand vertically instead of lying horizontally like in most places?" or "Why do the Sierra Nevadas rise so majestically to the east, while the broad Central Valley lies just to the west?" or "Why are there so many earthquakes around here?"
In the 1960s when I was working toward a degree in geology, only about ten minutes of just one of the hundreds of geology lectures I attended dealt with the theory of plate tectonics, or continental drift. At that time most college profs considered the idea that continents could float atop an earthly core of magma as too outrageous to seriously consider.
Now the science of plate tectonics very neatly answers the above questions. Now I can stand next to my trailer and visualize myself as riding atop a massive, 350-mile-long block of granite plowing into the western side of Nevada.
Well, here's a simplified outline of the story:
400 million years ago, long BEFORE the Earth's landmasses crammed together to form the supercontinent of Pangea, a coastline of the land that would someday constitute North America lay where today Nevada and Utah are found. Then the floor of the Pacific Ocean began grinding beneath the North American Plate. Over millions of years, unimaginably intense buckling and fracturing caused rock to melt and molten magma from deep within the Earth to work toward the surface and sometimes erupt as volcanoes.
As a result of all this friction, buckling and fracturing, about 150 million years ago, off the future North American western coastline, a volcanically active "island arc" appeared -- in size and shape probably similar to Japan and Indonesia today, which are considered to be undergoing a similar process. Over enormous periods of time the island arc off North America's coastline slammed into the continent, fusing with it. Then North America's Pacific coastline was maybe 50 miles west of were I am now and mountains were formed, but they eroded down. This story is told in greater detail here.
About 75 million years ago more of the ocean's crust was thrown up against the new coastline and was added to it as well, extending North America's coastline even farther into the Pacific. Mountain ranges with volcanoes formed along the new coast but they also eroded down during the next tens of millions of years.
Finally about 5 million years ago a new period of mountain building began that included the rising of our current Sierra Nevadas to the east, and the Coast Range, which on a clear day I can just see across the Central Valley to the west.
The Pacific Floor continues grinding beneath the North American Plate, so the grinding, fracturing and magma-making has not stopped. A few hundred miles north of here Mt. St. Helen is currently building a lava dome inside its crater. Small earthquakes not making the news except locally are occurring regularly in California where the plates grind together, and no one thinks the earthquakes will soon end.
If you have Microsoft PowerPoint installed on your computer you can download a well illustrated presentation outlining the above story in detail here.
You can learn all about continental drift and geological processes associated with it at the Berkeley University Plate Tectonics Site at www.ucmp.berkeley.edu/geology/tectonics.html, where you can even view an animation showing how the continents have drifted during the last 750 million years!