Tuesday, May 6, 2008

Peavine (is not a volcano)

Because of the recent blog hits I've gotten by people searching for things like "is Peavine a volcano," I'd like to reiterate and strongly emphasize that Peavine is NOT A VOLCANO.

Peavine Peak, locally known as Peavine, is a smallish Basin-and-Range mountain about 7 miles northwest of downtown Reno, NV. It is clearly visible from most of the Truckee Meadows, the north-south-trending valley "where the Truckee runs through," the valley that contains the cities of Reno and Sparks. Unlike many basin-and-range mountains, Peavine's range-front fault is oriented in a northwest-southeast direction, and is down-to-the northeast. The steep, normal-faulted northeastern side of Peavine is seen on the left side of the photo below, which was taken from Highway 395 near Bordertown, looking more or less south. Also seen below, Peavine's southwestern side (seen to the right of the peak in this photo) is shallower, as though the original surface - prior to uplift and slight tilting to the southwest by faulting - consisted of low rolling hills.


Below, you can see the steep northeastern side of Peavine as seen from Highway 395, looking about southwest from about the Red Rock or Stead exit.

The photo below shows Peavine as seen from the Truckee Meadows, from Highway 395 south of The Spaghetti Bowl looking to the north or northwest. In this photo, the steep northeastern side is to the right, and the shallower southwestern side is to the left. Mogul would be off the picture to the left.



Rocks on Peavine consist in part of the metavolcanic rocks of the Peavine sequence. These metavolcanic rocks are relatively old - Mesozoic, somewhere in the 70 to 250 million-year-old range. They consist of various kinds of volcanic rocks that have been metamorphosed, mostly to greenschist facies. As a report from the NBMG says, "These rocks tell of a time when this area was similar geologically to the Cascades of Oregon and Washington today, with frequent volcanic eruptions due to a subducting tectonic plate to the west." The volcanoes that created the Peavine sequence rocks are long, long gone. These old, metamorphosed volcanic rocks include metamorphosed felsic to intermediate flows and related plugs, metamorphosed felsic ash-flow and air-fall tuffs, and metamorphosed volcaniclastic conglomerates and sandstones.

Also, if you live in the Reno area, remember the local saying: don't plant until the snow is off Peavine. It isn't really spring until that happens.

References:
NBMG, Earth Science Week 2001 Field Trip #1, cited above.

NBMG Map 4Gg - Geologic map of the Verdi Quadrangle, Bell and Garside, 1987.

NBMG Map 4Ag - Geologic map of the Reno Quadrangle, Bonham and Bingler, 1973.

6 comments:

Chuck said...

Are they the volcanic equivalents to the Sierra Nevada batholith?

Silver Fox said...

I'm not really sure about that - would have to dig a little deeper. I think it's likely that the metavolcanic rocks are at least broadly correlative. The batholith is mostly Cretaceous and Jurassic, I think, with lesser amounts of Triassic intrusions. I think the Peavine sequence is mostly Triassic to Jurassic, so it might correlate with early plutons of the Sierra Nevada batholith.

Here's a couple references:
http://www.colorado.edu/GeolSci/Resources/WUSTectonics/SierraBatholith/mesozoic.htm
http://gsa.confex.com/gsa/2002AM/finalprogram/abstract_41273.htm

And maybe Geotripper knows a little more about this (Under the Volcano, and all!).

MJC Rocks said...

Geotripper here...you've got me on Peavine Mountain, even though I went to UN Reno. I think your comments are about right. The bulk of the Sierra intrusives are late Jurassic and Cretaceous in age. Some of the metamorphic rocks in the Sierra predate these, and others include volcanic centers that were active calderas during the intrusion of the batholith.

Silver Fox said...

Overall, I'm thinking that the metavolcanic rocks of the Sierra Nevada and of western Nevada (the batholith is broken into pieces by the Basin and Range) - the metavolcs are mostly older than the batholith, and the volcanic centers related to the batholith are not metamorphosed - but I can't find a really good reference about that, and I don't have a CDMG "Geology of Northern California" Bulletin 190 handy. I may not have one at all!

Anonymous said...

You are so far off in your claim that Peavine Peak is not a volcano it's not even funny. It is listed with the USGS and the CVO as an extinct volcano with an eruptive history dating back as recently as 65 million years. Do your research before misinforming the web.

Silver Fox said...

Dear Anon, the website you mention quotes the NBMG Report that I quoted above, and says nothing about an "extinct volcano" at Peavine Peak. In fact, Cascade-type stratovolcanoes of whatever age (250 million years old to a minimum of 70 million years old) were responsible for depositing the volcanic rocks one can now see on Peavine Peak and on the mountain below the peak. One can either drive up the Basin-and-Range mountain to see these very old volcanic rocks, or one can hike up to see them. I've done both.

Peavine Peak's shape was formed by post-17-Ma Basin-and-Range faulting, not by the very old volcanism. The actual volcanic vents that erupted the 70 to 250 Ma Peavine sequence volcanic rocks might possibly be identified by detailed geologic mapping, provided that those vents have not been eroded in the geologic eras since their eruption.

CVO says "These rocks tell of a time when this area was similar geologically to the Cascades of Oregon and Washington today, with frequent volcanic eruptions due to a subducting tectonic plate to the west." That is the exact quote I referenced above, which is from the NBMG Report I also referenced above.

CVO also says, "Peavine Peak rose relative to the adjacent valleys along range-front faults that have been active over millions of years."

If you notice carefully, the list at the top of this CVO webpage lists places such as Carson City and Great Basin National Park, neither of which are extinct or even ancient, long-eroded volcanoes. The webpage is an excellent source to find out more about Nevada's volcanic history, and to find out the location of some fairly recent to very ancient volcanic rocks. This CVO page shows a map of Nevada's major volcanic areas.

Volcanic rocks are rocks that originated from volcanoes. They are not equilavent to volcanoes. That was the entire point of this post.