It's Garys fault!

[nachoman]

I few notes about northern Arizona geology from my memory...

The "cinders" in the picture above are being quarried from a "cinder cone" volcano. These volcanoes are very common in the Flagstaff area as part of the San Francisco Volcanic Field. The cinders are used primarily for road traction in the winter and possibly mixed with concrete or asphalt as aggregate. I have seen some along the BNSF mainline so I know it has been used for railroad ballast, but I would think it is much too light to be used exclusively. It would have an advantage of draining extremely well. I think "cinder blocks" are made from the cinder waste from coal power plants, and not from volcanic cinders.

The "cinders" are a type of rock called "scoria" that was ejected during the eruption and were mostly solidified by the time they fell to the ground. They range in size from sand grains to the size of a small car, and are composed primarily of volcanic glass and plagioclase feldspar. Scoria is mostly void space - gas bubbles. I am not sure what causes the red and black color, but it may be due to slight mineralogical differences of the magma, the presence of water or other volcanic gasses that alter the color, the temperature of the eruption, or post-eruption weathering. The red color is not due to an "Iron ore", because technically an ore has to be in a sufficient quantity to be extracted usefully and economically. There is iron in the scoria, but it is in a form that is not easily extracted in quantity. Cinder cones are usually loose piles of cinders, making climbing them extremely difficult.

San Francisco Mountain (Which Humphrey's peak is the highest peak) is a large stratovolcano that erupted primarily Andesitic lava. The rocks are generally Andesite with some volcanic ash and pyroclastic layers intermixed. The andesite is medium to dark gray to sometimes pinkish. I don't know if it has ever been used for ballast in the area. Some of the ash and pumice from this volcano has been used as a lightweight additive to concrete (especially in Glen Canyon Dam).

The other red rocks in the area are all sedimentary. In the Sedona area, they are part of the "Schnebly Hill Formation". This formation is a mixture of muddy sandstones and sandy mudstones. The red color arose from the staining by iron oxides after the sediment deposition. The red color is in the cement that holds the grains together, not the grains themselves, and is not in sufficient quantities to be extracted as an ore. In the Grand Canyon, the primary red rocks you see are the Supai group - another mixture of mudstones, sandstones, and limestones, and the Redwall Limestone. Again, these rocks were stained red by iron oxides after deposition. I am not aware of any place where these rocks were mined in sufficient quantity for their metallic minerals. It is also unlikely they were ever used as ballast around Flagstaff because they all lie far beneath the surface or are exposed at lower elevations.

Further north and east of Flagstaff, other sedimentary rocks are red color, especially the Moenkopi formation near Flagstaff and parts of the Chinle formation. These formations are primarily mudstones and shales and would make poor ballast - but provide petrified wood and some nice flagstone. Above them are many sandstone layers, some of which are also red.

There is Granite in Arizona, but that is in the transition zone of central Arizona or the mountains in the Basin and Range of southern Arizona. Usually, this granite is very old and crumbly, and would not do well for countertops or curling stones. But, because it crumbles easily it is often used for landscape gravel (and likely ballast) as either decomposed granite or crushed granite.