How tall were the San Francisco Peaks?

San Francisco Peaks

San Francisco Peaks, photo by me.

The six tallest peaks in Arizona, the San Francisco Peaks, were, half a million years or so ago, the slopes of one much larger shield volcano. I’ve heard from various sources (while I was living there) that it was over 15,000 or even 16,000 feet tall. This would have made it comfortably the tallest mountain in the continental United States, even at the short end of that estimation.

Personally, I’m also curious how much of an impact that much larger and higher peak had on the climate in the region, given that would have translated to vastly more precipitation, and probably year-round melt-off from permanent snowcaps. That’s a bit ambitious a project for a Friday blog, however.

For now, we’re going to stick to the question of how to tall were the San Francisco Peaks before they erupted?

There’s very little official information out there, surprisingly. As you can maybe expect, a cataclysmic explosion or two, followed by glaciation of the peak and caldera during the Pleistocene, probably complicate the scientific determination of the mountain’s height.

So I thought we’d try something fairly unscientific.

The Unscientific Sciency Bits

Shield volcanoes tend to rest at something called the maximum angle of repose. Simply put, since they’re made by piling molten goop out of a hole in the top, that goop tends to slide downhill at a fairly predictable angle. Magma varies from region-to-region a surprising degree, so the exact angle of repose is variable. This bit’s fairly scientific, but here’s where we’re going to get a little fanciful.

I went and looked at pictures until I found an intact shield volcano that looked like it would line up with the mountains of the San Francisco Peaks. I picked Mount Fuji, and it didn’t just line up, it lined up perfectly, so, as unscientific guesses go, San Francisco Mountain looked a lot like Mount Fuji before it went and blew its top off.

Mount Fuji

If anyone knows the correct photo credit for this, let me know. Reverse search just gives me a load of stolen versions.

Just to reiterate, this bit is guess work.

From there, though, we can make a pretty easy mathematical guess at the original height of San Francisco Mountain, assuming our hypothesis, “It had an angle of repose similar to Mount Fuji,” is correct.

San Francisco peaks, how tall, before eruption, height

Sidenote: Mount Fuji is actually 300 feet shorter than modern day Mount Humphrey, but rises from sea level, so it actually has twice the prominence!

Though the altitude of the tallest peak is 12,635′ the prominence (distance from base to peak) is only just over 6,000′.

The difference between the silhouette of the current peaks to the theoretical peak is roughly half the current prominence–we could be more exact, but there’s no point on account of this being guesswork–putting ancient Mount San Francisco at roughly 15,500′.

I think you could safely assume an error in the range of 1000′ but, in either case, this does lend weight to the idea that Mount San Francisco in Arizona was, long ago, the tallest mountain in the Continental United States. From its peak, on a clear day, the distance to horizon would have been 120-125 miles. Someone standing on top would have been able to see into the Grand Canyon, the Phoenix Valley, and prominent peaks in Utah, Nevada, California, and possibly some of the mountains of Colorado and New Mexico, as well.

Would’ve been something to see!

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