And Now . . . The Mountain.
Sunday July 5, 2009
Probably the biggest erosional landform is the most familiar: the mountain. The great majority of the familiar-type mountains were carved, not built. The other kind of mountain is depositional: the volcano. We tend to think of volcanoes as special, but there are uncounted thousands of them hidden on the ocean floor. They're mostly small, but they're everywhere, so common that they have their own category: seamounts.
So it isn't clear to me which type of mountain is more typical, volcano mountains or mountain mountains. But the existence of mountains poses a puzzle: to be carved by erosion, the rocks must have been lifted first. What kind of forces in the planet do that? We had no real answer to the mountain problem, just some poorly supported hypotheses, until less than 60 years ago. And the answer we found explains the existence of volcanoes, too.
The mountain problem
Gallery of peaks
Other erosional landforms
About volcanism
Mount St. John, Napa Valley — Geology Guide photo
Comments
In certain tectonically active areas, I would argue that tectonic control is more important than erosion in shaping mountains.
For example, consider the mountains of the Basin and Range Province of the western United States, which John McPhee famously compared to “an army of caterpillars, marching south to Mexico.” In general, each of these mountain ranges is bounded by one precipitously steep side, where the mountain has been lifted up by a major, north-south-trending normal fault, and one more gentle-sloped side, where Tertiary volcanic rocks dip down slope (where not eroded away), indicating that the range has been tilted on its side, sticking up like a broken floorboard. Certainly erosion wears down the slopes quite a bit from the steep angles of the fault surface and tilting Tertiary strata, and major canyons have been carved which separate each mountain range into individual peaks, but the overall shape comes from active tectonics, not erosion.
This contrasts greatly with places like the Appalachians, where erosion is king, and every mountain represents a more resistant rock type that has managed to outlast many millions of years of erosion.
Living in tectonically active California, I’ve learned to expect that any mountain (or even hill) that rises abruptly from its surroundings (or any alluviated valley lower than its surroundings, for that matter) may be the product of active faulting or folding. Two examples: Mount Diablo (east of San Francisco Bay) results from compression where two strike slip faults are offset. Wheeler Ridge, by I-5 just north of the Grapevine, is one of a whole series of actively growing anticlinal folds generated by compression along the west side of the San Joaquin Valley. Yes, erosion has carved gaps* which break this ridge into separate hills, but the overall shape is unmistakable. (*one of these antecedent stream gaps is host to a major pumping station on the California Aqueduct; wish I could illustrate here . . .)
So I advocate for three categories of mountains: erosion-dominated mountains, tectonically-dominated mountains, and deposition-dominated mountains (volcanoes, mostly . . . ).
California definitely has good examples of tectonic landforms. Actual mountain mountains of straight tectonic origin are scarce. You can point to the Kettleman Hills, for instance, which are too young to have been carved extensively. Those are too low to be mountains. The Basin and Range ranges, gorgeous and impressive as they are, are not raw tectonic blocks. They have been eroded for millions of years. Erosion is steady and always wins, eventually, against tectonism or volcanism.
But as you say, around California it seems like every hill is a sign of something active going on that’s holding it up.
Erosion certainly wins every time. I’m sure you’re right that there’s no such thing as a 100% raw tectonic mountain. But it’s rare to find 100% pure anorthite, too, yet it makes a useful end-member for the classification of feldspars.