Pinnacles of volcanic tuff
Near Silverjack Lake, Colorado
Colorado's San Juan Mountains, in the southwest corner of the state, should not be considered part of the Rockies.
The Rocky Mountains were formed during the Laramide Orogeny, an event 70 to 40 million years ago in which the Farallon Plate collided with the North American Plate where California is now. Like two cars in a head-on collision, the western half of North America was crumpled. The Rockies are the largest of the crumples.
Then the east edge of the Farallon Plate began sinking. Ocean plates are heavier than the Hot Stuff they're floating on, so this happens all the time. This allowed the Farallon to start sliding under North America (it wanted to continue moving eastward) as it started melting, with the east edge drooping down into the Hot Stuff.
The east edge was melting and drooping faster than the plate was moving eastward, so this opened a gap between the plates, and North America had to stretch westward to cover it. Instead of straightening out the crumples, North America started thinning out in other places -- much like pizza dough will stretch unevenly and develop thin areas and holes when you try to stretch it.
And magma began moving up through the thin spots. In some places, it never broke through to the surface, creating the numerous laccoliths in eastern Utah and western Colorado. In others, huge volcanoes erupted, forming among others the Jemez Mountains of New Mexico and the San Juans of Colorado.
This happened well after the Rockies were formed -- somewhere around 30 to 20 million years ago. And, of course, they're made of very different material than what the Rockies are made of.
Sometimes volcanoes produce lava, and sometimes they produce ash. Here, they produced ash, thousands of feet thick. When it settled, it was still hot enough that the grains of silica welded to each other, forming a type of rock called tuff.
Tuff isn't very tough; it erodes easily and strangely, tending to form pinnacles and sharp-edged mountain peaks. And these pinnacles near Colorado's extremely scenic Silverjack Reservoir are a prime example. You'll see these throughout the San Juan Volcanic Field, from Gunnison all the way south to the New Mexico state line around Durango; from Ridgeway all the way east to the San Louis Valley.
Click the photo, then click on "Photo Details" (at the bottom of this long description) to see the geotag. This is a wonderful place to visit and camp when the aspens are in color. #geology #colorado