Let’s go on a Sunday morning train ride (paying particular attention to the mountains):
Note: You might have difficulty seeing this video here; it worked when I scheduled the post, but now you may have to watch it on YouTube: sorry for any inconveniences!
What’s so special about those mountains, besides their incredible beauty?
They are part of a large igneous province (LIP). This particular LIP has the added distinction of containing a lot of silica, and so is known as a silicic large igneous province, or SLIP.
You might have heard of flood basalts. These are areas on the planet where molten rock has flowed out of the ground in huge quantities. In the US Pacific Northwest, the Columbia River flows through a good example of such a LIP.
In SLIPs, there is a lot of silica in that magma, making things very sticky and very explosive. There is also a huge amount of material–enough, for example, to construct those mountains in the video above.
While true lava flows do exist in the Sierra Madre Occidental, most of the underground magma in this region exploded out of the ground as ignimbrite.
Those hot ash flows were sort of like this pyroclastic density current on Italy’s Mt. Etna . . .
. . . but on a much larger scale.
The Sierra Madre Occidental SLIP has an estimated volume of as much as 300,000 to 400,000 km3 (72,000 to 96,000 cubic miles).
To put that in perspective:
- The flank collapse at Mount St. Helens in 1980 contained about 2.5 km3 of rock, with an additional 1.3 km3 of ash spread out over the Pacific Northwest
- The plinian/ultraplinian eruption of Mount Pinatuba in 1991 produced somewhat more than 10 km3 of ash
- The biggest eruption on the planet since the last ice age ended, at Tambora in 1815, produced a little more than 50 km3
- The largest known eruption in the last 2 million years, at Toba about 74,000 years ago, had a volume of a little over 2500 km3
There just is no comparison between a single volcanic eruption–not even a supereruption like Toba’s–and a large igneous province event.
The Mount St. Helens debris avalanche ruined part of a national forest. The Sierra Madre Occidental SLIP emplacement buried much of the western part of the Mexican subcontinent.
It didn’t appear overnight, but it was quick in geologic terms.
Most of the formation is still unmapped, but experts think it likely that these mountains formed as humongous ash flows erupted, probably through calderas and fissures in the ground, in two pulses:
- Over 200,000 km3 was blown into place throughout what is now the Sierra Madre SLIP between roughly 32 and 28 million years ago
- Another 50,000 to 100,000 km3 was erupted, much of it in the southern part of the SLIP, between 24 and 20 million years ago
This might be a good time to look at today’s video again: life here certainly wasn’t as pleasant back in the day when the Sierra Madre was under construction!
What caused this huge eruption, and will it happen again?
This SLIP emplacement happened at the same time and was part of the somewhat better known ignimbrite flare-up in the southwestern US, tens of millions of years ago. It all was associated with a complex situation involving plate tectonics and the formation of the Rocky Mountains, which have a very peculiar location on the North American continent:
A slight correction: it was actually the Farallon plate, not the Pacific plate, that underwent this flat subduction. That plate is almost all gone today; also, the angle of subduction deepened again.
So we’re not going to have another ignimbrite apocalypse any time soon.
Instead, we have beautiful scenery to enjoy, because of this ancient catastrophic event.
Circulating water in these volcanic rocks has also concentrated valuable ores, especially silver, gold, copper, and molybdenum, so there’s that, too.
More than 90% of the Sierra Madre Occidental has yet to be mapped. Questions remain.
Geologists are particularly curious about how such huge volumes of magma formed and how they erupted.
The land broke apart and the Gulf of California appeared towards the end of the Sierra Madre SLIP emplacement. This complicates the picture, although continental rifting like that is typical of SLIPs.
The more geoscientists can learn here, the better prepared they will be to understand other parts of the world where people now live atop huge ash flow deposits.
Yes, this is a lead-in to next week’s Sunday Morning Volcano . . .
Featured image: Comisión Mexicana de Filmacione, CC BY-SA 2.0
Aguirre-Díaz, G. J., and Labarthe-Hernández, G. 2003. Fissure ignimbrites: Fissure-source origin for voluminous ignimbrites of the Sierra Madre Occidental and its relationship with Basin and Range faulting. Geology, 31(9): 773-776. (Abstract only)
Aguirre-Díaz, G. J.; Labarthe-Hernández, G.; Tristán-González, M.; Nieto-Obregón, J.; and Gutiérrez-Palomares, I. 2008. The ignimbrite flare-up and graben calderas of the Sierra Madre Occidental, México. Developments in Volcanology, 10: 143-180. (Abstract only)
Bryan, S. 2005. Silicic large igneous provinces.
Ferrari, L.; Valencia-Moreno, M.; and Bryan, S. 2007. Magmatism and tectonics of the Sierra Madre Occidental and its relation with the evolution of the western margin of North America. Geological Society of America Special Papers, 422: 1-39.
Murray, B. P.; Busby, C. J.; Ferrari, L.; and Solari, L. A. 2013. Synvolcanic crustal extension during the mid-Cenozoic ignimbrite flare-up in the northern Sierra Madre Occidental, Mexico: Evidence from the Guazapares Mining District region, western Chihuahua. Geosphere, 9(5): 1201-1235.