FishMan473 said:
I thought most of those 'mountains' in the Phoenix area were made of granite, since they were the cores of ancient mountains/volcanos. I haven't ridden all the trails in the area, but South Mountain, Estralla, and McDowell all seem to have pretty course soil that does well after rain.
Now that the thread has been completely hijacked, so what makes Dacite different from granite? Is it just the color, granite is more pale with more pink feldspar? Are there any other significant differences? I've never really understood the way igneous rocks are classified.
I mainly study rivers and the impacts of dams but I will try to answer your questions and continue to thread hijack.
Igneous rocks are subdivided by: 1) the size of the mineral grains and 2) the chemistry or mineralogy of the rock. Mineral grain size depends on how quickly the magma cools and crystallizes. Thus, granites, diorites, and gabbros are rocks the formed deep and cooled slowly. Rhyolites, Dacites, Andesites, and Basalts are fine-grained because they were exposed to air on the earth's surface. If a magma is cooled very quickly , particularly a rhyolite magma, it may not have a chance to crystallize at all and instead will form a natural glass, obsidian (that is there is no organized crystalline structure).
Then there is chemistry which direclty affects the types of minerals that will crystallize. At the mafic end of the spectrum (ie. basalt) the minerals are likely to be olivine and pyroxene, minerals rich in iron and magnesium, and a form of plagioclase feldspar. Hence the rock is dark colored (basalts and gabbros). At the felsic end, the rock is likely to contain feldspars rich in calcium and sodium, and quartz. These rocks are light colored (rhyolites and granites). Of course there are exceptions to this crude scheme but in general it holds true. So, you are correct granite has more pink feldspar than diorite which contains less quartz and has more mafic minerals. It is also finer grained because it ventured forth onto the surface as a lava flows Granites are often associated with batholiths, huge bodies of intrusive material that in many places form the cores or root of mountain ranges. Dacite on the other hand is more often associated with volcanism, like the SF Peaks, a stratovolcano, and the lava domes of Dry Lake Hills and Elden Mountain.
So the types of soils formed from weathering igneous rocks (which applies to sedimentary and metamorphic rocks as well) depend on the stability of the minerals present, the bonds holding them together, and the percentage of quartz (very stable). Since feldspar is a key mineral in most rock types it weathers into one kind of clay and other silcate minerals into other types of clay with varying abilities to soak up water (clays are "sheet" silcates so you can think of them as pages of a book and water spreads the pages apart and makes them cleavable-slippery).
As for the mountains of Phoenix there is a lot of variety. The fables Superstition Mountains are an enormous caldera complex that erupted violently some 40 million years ago. Lots of dacitic ash-flow tuffs. The Mazatzal Mountains are way old Precambrian sedimentary rocks that got uplifted and thrust faulted to the surface during the Mazatzal orogoeny (1.6 billion years ago and are possibly the oldest rocks in AZ). Orogeny means mtn building and plutonism was also associated with this such as the granites around Payson and Prescott, the McDowell Mountains and Sierra Estrella. South Mountain and the White Tank Mountains are metamorphic core complexes composed of intrusive or metamorphic rocks which rose to the surface as the region got pulled apart during development of the basin and range province 10-25 million years ago (hence many of the ranges are parallel with large basins separating them). These ranges have lots of foliation (the rocks got stretched and hosed over) and the rocks are gneisses and mylonites. They have more gentle peaks than the jagged peaks elsewhere in the phoenix basin. Camelback Mountain is composed of Tertiary sedimentary rocks overlying Precambrian granite.
AZ is indeed a cool state geologically speaking.