Back when I was a kid, I had a fascination with rocks. I still sort of do to an extent. I use to collect them, and am thinking of starting again. I typically would if I saw a rock collection at a store.
I like purple amythist (I think, it's spiky) and a kind of golden brown non spiky thing. I also have a rock shaped like a man with a big nose like the man from the war peeping over the wall - his name escapes me - it's in profile, coloured in and next to the amythist on my mantelpiece.
As my signature suggests, I am a geology nerd (AKA "studying geology in college"). I LOVE rocks, especially igneous rocks. In my dormatory, I have a chloritic schist that has been doubly-folded during a collisional event involving the Yukon-Tanana Terrane up in Alaska. Right now I am studing all the ins, outs and inbetweens of volcanics that erupted during the 2006 eruption of Augustine Volcano. Looking at the thin sections under a petrographic microscope is quite stimmy, especially when there are orthopyroxene-clinopyroxene reaction rims -- beautiful colors. At the University of Alaska Fairbanks, I have access to the Bulliten of Volcanolgy, the Journal of Volcanology and Geothermal Research among other high-level journals about this topic. They provide good reading!
If anyone has volcano questions, you know where to turn to (me)
~CGK
I still have a big collection of rocks from the days when this was my interest. Every now and then if i find one interesting enough I will pick it up.
If anyone has volcano questions, you know where to turn to (me)
~CGK
I have a question. when was the last volcano eruption around Yosemite N.P. and what kind of volcano where or are they?
I have a few volcanic rocks from that area and I would like to know an estimated age.
3cl,
Based on my research of the area, the last activity directly involving a magmatic body coming to the surface was roughly 100-230 years ago in the Mono Lake Volcanic Field (MLVF)and formed a lava dome in the vicinity of Pahoa Island (Stine, in Baily et. al., 1989).
Since the region around Mono Lake has a very complicated volcanic past with many eruptive episodes spanning many hundreds of thousands of years, it is hard for me to judge the geologic age of your particular samples.
Various volcanic features dot the area around Mono Lake, including lava domes, cinder cones and a large, 15 by 30 kilometer (mostly obscured) depression called Long Valey Caldera. The largest eruption in the region came from this caldera and occured rougly 750,000 years ago. Estimated volumes of ejecta from this eruption peg the volume of the mega eruption at 625 cubic kilometers (by comparison, the eruptive volume of the Mt. St. Helens eruption back in 1980 had expelled roughly 1 cubic kilometer of material.)
I do have to remind you that currently, there is no indication that an eruption of the magnitude of the 750 ka event is poised to occur anytime soon. Smaller events, such as the quiet emplacement of lava domes are far more likely to occur, but even then, chances are still remote.
~CGK
thank you for the information, it is very interesting. I may have to start looking into geology again. This thread may have restarted my old interests in that subject. As it goes so well and shares so much with geography.
I used to be obsessed with collecting rocks as a kid... Now I mainly limit myself to fossils and rough opal. I don't like polished opal or opal jewelry because you can get lab-made stuff that will look exactly the same. When it's rough with the matrix and everything it looks natural, it looks like a rock (which is what it is).
Here are some pictures of some of my opal:
Opal in ironstone matrix from Lightning Ridge, Australia (I think, I got it a long time ago).
Opalized wood from Nevada (forget the specific locality).
White opal from Coober Pedy, Australia.
I just took the pictures now in the indoor light, so they don't look nearly as good as they do in real life. I might take some more pictures tomorrow in the sunlight and see how they come out.
I know this rock may not seem all that intresting like the ones Natalie posted. But this is one of the volcanic rocks from the Yosemite N.P. area i was talking about.
When I was 12 this was my favorite object, I would always carry it. I used to stare at it for hours. I even sleep with it. That went on for a year. It might have been an aspie thing I do not know.
3cl,
Based on the photo, your volcanic rock appears to have a disitnct porphyritic texture ( having large minerals in a fine-grained groundmass). Porphyritic textures record two distinct crystalization rates: the large crystals (called phenocrysts, and most likely feldspars) crystallized slowly within the magma chamber, as a function of slow cooling rates (up to years). The fine grained groundmass froze very quickly as a consenquence of either sudden temperature drop, or the sudden exolution of magmatic volitiles, (which also causes vesiculation).
If some of the "large pale spots" appear to be inclusions of smaller rock fragments, called 'lithics,' then the rock may be a sample of a pyroclastic flow. If that is the case, then the material MAY...MAY be a part of the Bishops Tuff, the ignimbrite that resulted from the pyroclastic flow of the 750 ka resurgent-dome eruption at Long Valley caldera. I will admit the rock appears a bit too dark for it to be Bishops tuff, but then again, it may very well be.
I hope that I have provided useful material for you to start describing your rock, but I will admit that I am bad at rock diagnosis from photos alone.
If you want more information about the Long Valley caldera and other nearby volcanic features, visit the Long Valley Observatory (LVO) at
http://lvo.wr.usgs.gov/.
~CGK
p.s., I also sleep with rocks on a regular basis.
I miss rocks. I used to really like them. It was sort of an obsession that got nipped in the bud when I was 7. Maybe when I go back to school in two years I'll take Geology. I've been considering that for a while.