kingwinner Posted October 28, 2005 Report Share Posted October 28, 2005 1) "Composite volcanoes have alternate periods of quiet eruption and explosive eruption." "Composite volcanoes eject viscous lava and pyroclastic materials alternately." These 2 quotes are both from reliable sources. But I don't understand how viscous lava flows can be quiet, and not explosive. I thought that only fluid (not viscous) lava can produce quiet eruptions...right? 2) "Pyroclastic materials are produced by (viscous) felsic lava and they can flow as fast as hundreds of km/h" Why? If they are produced by (viscous) felsic lava, they should be very sticky, how can they flow that fast? (even faster than (fluid) mafic lava flow...) Quote Link to comment Share on other sites More sharing options...
LOC Posted November 2, 2005 Report Share Posted November 2, 2005 1) "Composite volcanoes have alternate periods of quiet eruption and explosive eruption." "Composite volcanoes eject viscous lava and pyroclastic materials alternately." These 2 quotes are both from reliable sources. But I don't understand how viscous lava flows can be quiet, and not explosive. I thought that only fluid (not viscous) lava can produce quiet eruptions...right?I'm not a volcanologist but I can help a little. The reason volcanoes erupt explosively is because gases build up in the magma chamber, usually by solidification of magma higher up in the conduit/neck of the volcano or because the magma is so viscous, gases have a hard time escaping the melt. I do believe the two examples I've given result in disparate eruptive behavior, the latter likely being much more violent and explosive. Once the plug is breached, magma can flow freely onto the surface as lava, and this activity is generally non-violent. 2) "Pyroclastic materials are produced by (viscous) felsic lava and they can flow as fast as hundreds of km/h" Why? If they are produced by (viscous) felsic lava, they should be very sticky, how can they flow that fast? (even faster than (fluid) mafic lava flow...)A pyroclastic flow is generally a mixture of hot rock, ash, gas, and air that surges down the mountainside. They are not molten lava and therefore can flow at much higher speeds. I'm not positive, but I do not believe a pyroclastic flow has to be felsic and/or viscous - it can result from a collapse of any dome. Quote Link to comment Share on other sites More sharing options...
Turtle Posted November 2, 2005 Report Share Posted November 2, 2005 ___I watched a PBS special last night on a volcano in Africa which has effusive lava flows clocked at 60 mph.http://www.pbs.org/wgbh/nova/volcanocity/producer.html My neighbor Mt. St. Helens has a history of effusive lava flows (one of these formed the popular Ape Cave) as well as pyroclastic erruptions (1980), & is currently undergoing a dome-building event which consists of a rising column of very viscous lava through the crater floor. :confused: Quote Link to comment Share on other sites More sharing options...
LOC Posted November 2, 2005 Report Share Posted November 2, 2005 ___I watched a PBS special last night on a volcano in Africa which has effusive lava flows clocked at 60 mph.http://www.pbs.org/wgbh/nova/volcanocity/producer.html Are you sure it was 60? I managed to watch the first part of the show and I thought they said 40 mph. Either way, it's much too fast for anyone to outrun. Unfortunately, I didn't hear what type of lava it was nor what caused such low viscosity. Did you happen to catch that? My neighbor Mt. St. Helens has a history of effusive lava flows (one of these formed the popular Ape Cave) as well as pyroclastic erruptions (1980), & is currently undergoing a dome-building event which consists of a rising column of very viscous lava through the crater floor. :confused:Ape Cave was formed in basalt and I've always found it fascinating that some volcanoes, such as Mount St. Helens, can be the source of such a wide variety of lavas. Quote Link to comment Share on other sites More sharing options...
Turtle Posted November 2, 2005 Report Share Posted November 2, 2005 Are you sure it was 60? I managed to watch the first part of the show and I thought they said 40 mph. Either way, it's much too fast for anyone to outrun. Unfortunately, I didn't hear what type of lava it was nor what caused such low viscosity. Did you happen to catch that? ___I may have taped the show for review; the 60 mph figure they mentioned later in the show. I don't recall that they mentioned the type of lava & the unusually 'thin' consistancy is part of what the study is trying to elucidate. Damn daring geologists for sure. :confused: Quote Link to comment Share on other sites More sharing options...
LOC Posted November 2, 2005 Report Share Posted November 2, 2005 ___I may have taped the show for review; the 60 mph figure they mentioned later in the show. I don't recall that they mentioned the type of lava & the unusually 'thin' consistancy is part of what the study is trying to elucidate. Damn daring geologists for sure. :confused:Perhaps it was 60 km/hr?? I'll have to watch the show again. The type of lava was apparently a nephelinite. Quote Link to comment Share on other sites More sharing options...
Turtle Posted November 2, 2005 Report Share Posted November 2, 2005 Just watched the first half; they definately said the lava of Nyiragongo has been clocked at 60 mph. The 2002 eruption is the 40 mph figure given at the beginning. In the first half they used the phrase "fastest flowing lava in the world", & said the lava has a "unique composition". The main investigator has been there researching over 20 years. :confused: Caldera of lava; cup of java. :confused: Quote Link to comment Share on other sites More sharing options...
LOC Posted November 2, 2005 Report Share Posted November 2, 2005 Just watched the first half; they definately said the lava of Nyiragongo has been clocked at 60 mph. The 2002 eruption is the 40 mph figure given at the beginning. In the first half they used the phrase "fastest flowing lava in the world", & said the lava has a "unique composition". Ah, okay. Wow, that's fast. Guess I have to eat my words after watching the movie "Volcano" with Tommy Lee Jones. :confused: Quote Link to comment Share on other sites More sharing options...
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