Geology/Please ID this Rock?
QUESTION: I found this rock in the central cascades of Washington state. It is different from all others in the area. Is it a quartzite?
ANSWER: I certainly cannot tell what the rock is without AT LEAST a picture but in that area it very well could be quartz. Ouartzite is a metamorphic mineral formed from the metamorphism of sandstone and has many characteristics of quartz (hardness, specific gravity, streak, etc.) Quartzite is hard, 7 on the Mohs scale and has a specific gravity of 2.65. If your rock meets these criteria it may be quartz or maybe quartzite.
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QUESTION: The attach image link was missing in the original message. I see it here so here are the photos.
Can you identify the rock? Is it quartzite? Is it ruby?
The geology of the Central Cascade Mountains is very complex and without a detailed study (probably on-site visit) I cannot tell you specifics about the rocks; however, I will give you a background and try to tell you what I can. Rock 1 --- COULD be a quartzite, I cannot tell for sure but I can tell that it is significantly weather (changes from the interior outward of the mineral type (color)) and appears to be an alter (by metamorphism) sedimentary rock. The purple in the rock may be florite (CaF) but this is in question since florite is an evaporite and most probably would not be in direct contact with quartz; however could more likely be in direct contact with limestone (CaCO3). If the rock effervesces with HCl acid it is limestone.
Rock 2 --- appears to be different and may be the reason that Rock 1 is altered (contact metamorphism). Rock 2 (first guess from picture) is a rhyolite. Rhyolite is an igneous, volcanic rock, of felsic (silica-rich) composition (typically > 69% SiO2). It may have any texture from glassy to aphanitic to porphyritic. The mineral assemblage is usually quartz, alkali feldspar and plagioclase (in a ratio > 1:2). Biotite and hornblende are common accessory minerals.
Rhyolite can be considered as the extrusive equivalent to the plutonic granite rock, and consequently, outcrops of rhyolite may bear a resemblance to granite. Due to their high content of silica and low iron and magnesium contents, rhyolite melts are highly polymerized and form highly viscous lavas. They can also occur as breccias or in volcanic plugs and dikes. Rhyolites that cool too quickly to grow crystals form a natural glass or vitrophyre, also called obsidian. Slower cooling forms microscopic crystals in the lava and results in textures such as flow foliations, spherulitic, nodular, and lithophysal structures. Some rhyolite is highly vesicular pumice. Many eruptions of rhyolite are highly explosive and the deposits may consist of fallout tephra/tuff or of ignimbrites.
The reason that these rocks can not be identified from pictures is that the Central Cascade Mountains of Washington are composed largely of Tertiary continental and volcanic rocks. They lie between older metamorphic rocks to the north, Tertiary marine rocks to the west, and younger volcanic rocks to the south and east. An eastward-dipping reverse fault through Lake Kachess separates two Tertiary basins of the same general age. This fault cuts Eocene rocks and is older than the Snoqualmie granodiorite.
East of the fault the structures are open. The Swauk arkose unconformably covers pre-Tertiary peridotite and Easton schist. The Swauk is folded along northwest trends that become more east-west south of Mount Stuart. This change in trend and the local origin of some of the Swauk suggest that the Mount Stuart block was high during early Tertiary time. Locally the Silver Pass volcanic rocks overlie the Swauk. The conformable sequence of Teanaway basalt-Roslyn arkose lies unconformably above the Swauk and is deformed into a broad basin. The nearly horizontal Yakima basalt unconformably overlies the older rocks.
West of the Kachess fault, the structures are more complex. The oldest rocks are limy hornfels and marble of the Denny formation. They are overlain apparently unconformably by the sedimentary rocks of the Guye formation. Unconformably above the Guye is the extrusive Mount Catherine rhyolite; that is overlain by the tightly folded sedimentary rocks and basalt of the Naches formation. The mildly deformed Keechelus andesite overlies the Naches unconformably. The Snoqualmie granodiorite intrudes all the units in the western area.
The Cascades are part of the Pacific Ring of Fire, the ring of volcanoes and associated mountains around the Pacific Ocean. All of the eruptions in the contiguous United States over the last 200 years have been from Cascade volcanoes. The two most recent were Lassen Peak in 1914 to 1921 and a major eruption of Mount St. Helens in 1980. Minor eruptions of Mount St. Helens have also occurred since, most recently in 2006.