Re: [Geology2] Re: Nyiragongo Crater by The Big Picture

There is something I want to clarify in this discussion.  I'm going to be picky about the terminology.  Bre's question was about the vent being circular.  The vent is the hole in the ground the lava comes out of.  I agree that this is generally circular because of flow dynamics. Then there is the crater around the vent.  This is the depression around the vent and it is generally formed by material coming out of the vent explosively.  Craters are generally deeper than they are wide. A caldera is caused by collapse as demonstrated in the video Lin found.  The collapse occurs as magma empties from the magma chamber and there is no longer support for the roof of the magma chamber.  Calderas are usually wider than they are deep and the shape of the caldera is similar to the shape of the magma chamber. Diorite --- On Sun, 3/27/11, scienceherman <hermanscience@sbcglobal.net> wrote: From: scienceherman <hermanscience@sbcglobal.net> Subject: [Geology2] Re: Nyiragongo Crater by The Big Picture To: geology2@yahoogroups.com Date: Sunday, March 27, 2011, 7:05 PM   s simple experiment will also show you why pits are normally circular in nature. dig sideways in a hill and then up so as to make a hole in the dirt above. As the hole naturally widens it widens in a circular manner as things fall into the hole. A caldera is where material has been blown out. When a volcano first erupts. (this is a guess but I am 95% sure its true) the caldera (volcanoes top where the eruption was) is not circular but as time goes on and erosion occurs it also becomes more circular over time also as it gets evened out by the natural forces of erosion and gravity. -Rex --- In geology2@yahoogroups.com, Rick Bates <HappyMoosePhoto@...> wrote: > > If I can offer a thought, the outer edges will cool at a greater rate than the core of the flow, which would make it denser and slower. After a period of distance, that vent would evolve into a circle because all the edges would have been slowed down. > > Does that make sense? > > Rick > > Tiny iPhone keypad + my fat thumbs = typos. Sorry. > > On Mar 27, 2011, at 10:28 AM, Lin Kerns <linkerns@...> wrote: > > > Hi Bre, > > > > I thought your question was an excellent one; in fact, I was just about bursting at the seams in order to find an answer. What I found was that the circular vent, the caldera, is a general form for most volcanoes, except for the shield volcanoes--like those in Hawaii. The shields are more unpredictable in their formation of the typical round caldera due to their particular type of eruption. > > > > Therefore, I will focus upon the factors present for the formation of the typical caldera: > > > > Magma moves upward at a speed that is dependent upon the combination of the viscosity of material (dacite, rhyolite, basalt, etc.) with the amount of gases present. Magma is thrust upward through the points of crustal deformation much like a projectile and the faster it moves, the more it behaves like any force moving through its surroundings. Think of an arrow shot from a bow; even though you cannot see it, the arrow will displace air in a conical manner due to the power of its thrust from the bow. Magma moving through the earth behaves in a like manner. It is what happens next that is exciting; the emergence of the magma and the formation of the caldera. And for that, I turn to a video I found that shows the exact process: > > > > http://pubs.usgs.gov/of/2010/1173/ > > > > There is a paper published that focuses upon the properties of caldera formation HERE > > I'm sure if you look around through various sources, you can find a copy of that paper without having to shell out the $41. for it. JSTOR is a good place to look, but if you can, search WORLDCAT and some library will have a copy of it. You can then request an inter-library loan. > > > > I hope my crude manner of explaining the physics of caldera formation helps. > > > > Lin > > > > > > On Sun, Mar 27, 2011 at 9:33 AM, Lin Kerns <linkerns@...> wrote: > > Looking for an answer for you, Bre.... I'll be back with one... > > > > Lin > > > > On Sat, Mar 26, 2011 at 6:28 PM, <LadyTozi@...> wrote: > > > > > > Okay, I'm late in reading my e-mails. Spectacular photographs. But I do have a question, it seems that in all of the photographs I have seen of central volcanic vents of various volcanoes Nyiragongo, Alta Erta, Shasta, Rainier, etc. the main vent is circular. Why? > > > > Bre > > > > In a message dated 3/1/2011 11:00:26 P.M. Central Daylight Time, linkerns@... writes: > > > > 28 Great Images of Nyiragongo--Crater Click Here > > > > > > > > > > -- > > Got Penguins? > > > > Penguin News Today > > The Science of Penguins > > The Gentoos are back! Come see them on live cam at: > > Gentoo Penguins of Gars O'Higgins Station, Antarctica > > > > > > > > > > > > > > > > > > -- > > Got Penguins? > > > > Penguin News Today > > The Science of Penguins > > The Gentoos are back! Come see them on live cam at: > > Gentoo Penguins of Gars O'Higgins Station, Antarctica > > > > > > > > > > > > > > -- > > Got Penguins? > > > > Penguin News Today > > The Science of Penguins > > The Gentoos are back! Come see them on live cam at: > > Gentoo Penguins of Gars O'Higgins Station, Antarctica > > > > > > > > >

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