My family & I traveled this weekend from Denver to Mesa Verde Nat. Park in Colorado. Along the way, no doubt sparked by the tedium of driving, we talked about what we saw whizzing by outside the windows.
The central & northern Colorado rockies are very sharp & defined while the Sangre de Christo range in the south is much softer appears much more highly erroded. My understanding, from one of these road-side displays, is that this difference is because the southern ranges are volcanic in nature while the ones in the north were formed from upthrusts & fault blocks and other fun tactonics.
OK, no problem.
Then I read about how the northern rockies are all this ingeous rock, granite, etc.
Well, “Igneous == Fire == Volcanic” says the back of my brain. What’s the diff between this & the Sangre de Christos in the south? If the origin of the rocks is all volcanic, what’s with the south that they weather so differently? Obvously, the mountains on the south are much “looser” looking, no seriously large blocks of rock - more like gigantic gravel piles. Still, if lava piled up, why didn’t it make large blocks of rock?
Are these “volcanic” southern ranges cooled volcano cones? Why no distinctive volcano shape? Any volcanic activity left in these buggers?
The return trip was up the western plains then back across on I-70 to Denver. There’s miles & miles of yellowish-white soil on the western side of Colorado occaisionally smeared with black and red. I think the base is mostly gypsum but what’s with the colors? Also what process lays down thousands of square miles of this ugly stuff.
There seems to be companies digging into it for some strange region. What are they after?
I don’t know anything specifically about the geology of the Sangre de Cristo range, but I can give you a little bit of (grossly oversimplified) info on why different rocks weather differently. Igneous rocks do not all have the same chemistry, and that’s what affects their weathering characteristics. Rocks like granite that are very rich in quartz tend to be very resistant, because quartz (silica) doesn’t react with water. Rocks like basalt and gabbro that have more minerals that are higher in iron, magnesium and potassium, such as amphibole, pyroxene and potassic feldspar do react with water and thus will weather much more readily. When you see large blocks of rock, that is usually the result of weathering, as water and plant roots work their way into small cracks and make them larger, eventually causing the rock to break up.
Volcanoes are not always cone-shaped. Andesitic (rock that is more quartz-rich, melts at lower temperature) volcanoes – the Cascades in the PNW are in this category – erupt magma that is more viscous, and their eruptions are usually much more explosive than basaltic (less quartz, higher temperature) volcanoes such as the Hawaiian chain, and so you don’t get that nice symmetrical cone shape. I’m pretty sure the southwestern mountains are extinct, but the northwestern ones are still very much active.
Perhaps someone with more specific knowledge of the region will be along to help you with info on what exactly is being dug out on the CO plains, as I really have no idea there.
This website says the Sangre de Cristo range is a fault block - not volcanic.
I took my “volcanic” information from a roadsign in a place called “Slumgullion Pass”, a Colorado site of a massive landslide. Perhaps the volcanic activity wasn’t as extensive as I thought.
The top link has more links to lots of pictures. Very nice country.
Not only am I a petrologist, but I spent three weeks in May doing geology in the Sangre de Cristos!
Clarification #1:
Volcanic rocks are igneous, but all igneous rocks are not volcanic.
Like that granite you mentioned. Igneous, absolutely, but it represents magma that never made it to the surface–and probably didn’t even make it within several km’s of the surface!
Clarification #2:
The Sangre de Cristos are mostly metamorphic and igneous rocks that have been faulted up; there are volcanics in the Sangres, but they’re volumetrically minor. One exception that springs to mind is the Latir Range, which is dominated by a large caldera. But, as Geobabe pointed out, not all volcanoes look like Hollywood/Cascade-style volcanoes. You could spend your life in a caldera and never know you were in a volcano! Add to this that the volcano is old (27 Ma) and quite eroded, and you get something that has a lot of igneous rocks, but looks nothing like a volcano.
Clarification #3:
Most of the volcanism in that area isn’t in the Sangres, but in the San Juan range (which is nuttin’ but several overlapping calderas), and the Taos Plateau, which consists of several extensive flood basalt flows extruded from S. Colorado and a good number of rhyolite and dacite domes (e.g., San Antonio Dome) sitting atop these basalts.