Volcanoes are mountains, but they are very different from other mountains; they are not formed by folding and crumpling or by uplift and erosion. Instead, volcanoes are built by the accumulation of their own eruptive products -- lava, bombs (crusted over lava blobs), ashflows, and tephra (airborne ash and dust). A volcano is most commonly a conical hill or mountain built around a vent that connects with reservoirs of molten rock below the surface of the Earth. The term volcano also refers to the opening or vent through which the molten rock and associated gases are expelled.Another USGS FAQ. There are parts of this answer that are good- in fact, I would say it's the order of those parts, rather than the answer itself, that's problematic. Let's try to rephrase:
The term volcano refers to an opening or vent through which molten rock and associated gases are expelled. A volcano is most commonly a conical hill or mountain built around a vent that connects with reservoirs of molten rock below the surface of the Earth. The built-up area, sometimes referred to as the volcanic edifice, is built by the accumulation of their own eruptive products -- lava, bombs (crusted over lava blobs), ashflows, and tephra (airborne ash and dust). In some cases, however, the actual volcano is a low place in the landscape. This can happen as a result of an explosion and excavation of a crater or maar, or from the emptying of a magma chamber, and subsequent collapse of the overlying landcape, forming a caldera. Additionally, flood basalts, the most voluminous volcanoes of all, probably do not create significantly elevated landforms in the vicinity of their vents, because the erupted lava is quite hot and fluid.
It may seem kind of nit-picky, but the point is, volcanoes should not first and foremost be described as mountains or hills, but as vents. Most are local elevation maxima, but many are not.
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3 comments:
Considering I didn't know volcanoes could be flat for, oh, the first 30 years of my life, I think your definition is far better! Definitions like the first one kept me ignorant for 3 damned decades.
And please don't tell me Peavine is a volcano simply because it's mostly underlain by volcanic rocks. Kthx.
@Dana- The thing I found frustrating about this is that most of the info is good, but the order emphasizes the part that's not really right, to be charitable. The leading half of my answer is cut and pasted directly from the original answer, and the second half is less substantive and more about nuance and exceptions to the rule of thumb. I just feel USGS should be able to do this better than me.
@SF- hate to admit it, but I don't know what or where Peavine is. I think you make an important point though: volcanic rocks are not the same thing as volcanoes.
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