Guest Post by Steven Goddard
The Guardian image below taken this week near Iceland has the caption “Smoke and ash billows from a volcano in Eyjafjallajokull, Iceland Photograph: Ingolfur Juliusson/Reuters”
The Guardian caption is for the most part incorrect. Note that the volcanic cloud is largely indistinguishable from the other clouds, except for it’s shape. The reason for the similarity is that the vast majority of the volcanic plume is water vapour, not ash and definitely not smoke. Where would smoke come from??? There aren’t any trees on Iceland to burn.
The abundance of gases varies considerably from volcano to volcano. However, water vapor is consistently the most common volcanic gas, normally comprising more than 60% of total emissions. Carbon dioxide typically accounts for 10 to 40% of emissions.
70% of the earth’s surface is covered with water. Where did that water come from? It is generally believed that most of it outgased from the interior of the earth during the first 700 million years of the earth’s existence.
Steam from the interiorToday most authors believe that early steam from the hot mantle but already cool atmosphere, caused the oceans in the very early stages of the planet. They reason from studies of chondrites (space rocks) in space that under compression, enough water could be released to form an ocean. Today one can observe the gases escaping from active volcanoes, and these too contain water. In this scenario, the oceans would still be increasing in size, a gradual process that would never really end.The amount of water stored in rocks of the primary lithosphere is estimated at 25E21kg (Hutchinson G E, 1957), whereas the water in all oceans is 1.35E21kg, so it is quite possible that all this water emerged slowly after rocks were compressed and heated while the atmosphere had cooled already.
We know that the oceans could not have condensed out of the early atmosphere, because even a 100% water vapour atmosphere would only contain 10 metres of liquid water. People have hypothesized that the oceans came from comets, but the hydrogen isotope ratios in the oceans are different than that seen in comets Halley, Hyakutake and Hale-Bopp.
The only plausible origin of the oceans is from the interior of the earth. So why don’t we see oceans on other planets and the moon? Liquid water only exists in a narrow range of temperatures and pressures. Other planets are too hot, too cold or too small to hold liquid water, though some of the moons of the giant planets may have liquid water.
Why is the relationship between volcanoes and water important? Because steam pressure is the primary driver of explosive volcanic eruptions.
Below are some images of potentially explosive eruptions :
Mt. St. Helens 1980 : Mostly steam, some ash, almost no smoke.
The video above shows the moment of the big eruption May 18, 1980
Mayon 1984 USGS photo : Steam rising, ash cloud falling down the sides of the mountain.
Fourpeaked Volcano, Alaska 2006 USGS photo : 100% steam
Tungurahua 2006 NASA EO image : Steam, ash and lava
Eyjafjallajökull 2010 NASA EO image : Steam, lava, ice
Below are USGS images of non-explosive eruptions at Mauna Loa, Hawaii
Note in the image above that there is some smoke on the left side – from burning trees, and a little steam at the summit. So what is the difference between explosive and non-explosive eruptions? The difference is mainly due to the presence or absence of water. Water mainly enters volcanoes from two primary sources.
- Subduction on the sea floor, and transport upwards into a magma chamber. (Mt. St. Helens)
- Melt from snow and ice above. (Eyjafjallajökull and Mt. St. Helens)
Mauna Loa on the other hand has very little water mixed in with the magma, as it is neither near a subduction zone nor is it covered with snow most of the time. So eruptions from Mauna Loa tend to produce lava rather than steam and ash.
Looking at the mechanics, it becomes clear that explosive volcanic eruptions can not occur in the absence of large amounts of steam. Liquids (like magma) have very low compressibility and can not store enough mechanical energy to cause an explosion. Gases on the other hand are extremely compressible and can store vast amounts of energy. Steam has the unique property that it is liquid until it comes in contact with the magma (or the overburden pressure becomes low enough to allow it to switch to vapour phase) – then it converts thermal energy into mechanical energy very efficiently. The world used to run off steam engines based on this principle.
Most modern power plants still use steam to convert thermal energy into mechanical energy. Same principle that makes volcanoes explode.
Paul Hildebrandt (20:32:18) :
If you heated up a high silica magma to 1200C (basalt temperature) it would lose most of it’s viscosity and volatiles.
Again, the reason why high silica magmas are viscous is because the eutectic melt with quartz, feldspar and water happens at a low temperature. Low temperature means more viscous. (You can’t have molten basalt at 800C) This allows it to retain volatiles which increase the viscosity further.
It is the presence of the volatile gases, like water, which makes high silica magma often explosive.
If you think carefully about what I am saying, it is consistent with the references you are providing. I am just adding a further dimension explaining what makes those lavas potentially explosive.
Iceland is a “hot spot”? I thought it was a mid ocean spreading ridge exposed above the surface.
See: http://vulcan.wr.usgs.gov/Glossary/PlateTectonics/description_plate_tectonics.html
Not that the two are mutually exclusive, I suppose . . . .
Mike Fox (22:55:37)
Iceland is definitely a hot spot. Do you see other similar places along the mid-Atlantic ridge?
“When there’s the right ratio of water to magma, volcanoes can have what are known as phreatomagmatic eruptions, where the water flash-steams, creating a huge plume of ash and steam. “Once the interaction with the (water in the) glacier stops, you go into dry mode,” says Shanaka deSilva, a professor of geoscience at Oregon State University in Corvallis.”
http://www.usatoday.com/tech/science/2010-04-19-volcano-eruption-lava_N.htm
RE: stevengoddard (23:21:42) : [Phreatomagmatic Eruptions]
Steve:
I assume this applies to the case where the volcano is already exploding with such violence that the overburden of ice is quickly boiled or melted away on direct contact with the expanding plume of erupting material. I suspect that a major eruption at the Crater Lake Volcano could vaporize the whole lake in short order.
Matt B (18:29:40) :
Orbit does not effect the build-up of pressure under the planet surface. Orbit is the mechanics of rotating and the enegy infused at the time of creation is slowly being used up. All planets orbits are slowly moving away from the sun and the suns corona explading slowly is the slowing of rotation.
Spector (21:42:48) :
I note that the undersea volcanoes near Hawaii have not produced any grand explosive eruptions.
Of course, it does have 3,000 or so meters of water over top of it. Here’s a video of one coming near the surface:
It seems to be rather violent.
Spector (21:42:48) :
I suspect that water entrained in marine rocks subducted from the sea floor might produce a much more explosive emerging magma than similar material subducted from a dry environment.
That water does not necessarily stay with those rocks. The water tends to be forced out into the overlying rock and helps with the partial melting of the overlying rock. This water is then incorporated into the magma. Depending on the degree of melting and the temperatures reached, the water either is incorporated into the mineralogy (plutonic rocks); is exsolved on the way up (basaltic to andesitic volcanics); or remains in the magma to help produce the explosive volcanics (andesitic to rhyolitic). If the dacitic to rhyolitc magma doesn’t quite make it to the surface, the gases (along with the other incompatible elements) are driven off as the magma body cools and crystallizes. As these gases and other elements cool on their way away from the plume, the elements eventually precipitate out forming those fantastic mineral deposits containing everything from molybdenum to silver and gold. Sometimes that water doesn’t quite make it to the surface.
Spector (21:42:48) :
I suspect that water entrained in marine rocks subducted from the sea floor might produce a much more explosive emerging magma than similar material subducted from a dry environment.
Forgot to ask, where would you find a dry subduction zone?
E.M.Smith (11:40:17) :
[…]
http://chiefio.wordpress.com/2009/06/02/of-trees-volcanos-and-pond-scum/
My botany is a bit rusty but as I recall, during the day while photosynthesis is the dominant process in leaves, carbon dioxide is taken up from the air and used in the process of making sugars. At night, when photosynthesis does not occur – no sunlight for energy – respiration occurs, which gives off carbon dioxide just as it does in animals.
Is this nighttime respiration from plants figured in to your CO2 calculations? Are your calculations “net” CO2 being scrubbed by plants?
janama (21:52:47) :
The late Professor Lance Endersbee, Emeritus Professor; former Dean of Engineering and Pro-Vice Chancellor of Monash University claimed that water was created internally.
Endersbee’s main focus was on the state of the world’s groundwater, the rapid consumption of which has put the world on the edge of a little understood catastrophe because contrary to popular belief groundwater reserves are not replenished from the surface.
http://www.abc.net.au/rn/latenightlive/stories/2006/1808528.htm
http://mpegmedia.abc.net.au/rn/podcast/2006/12/lnl_20061211.mp3
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There is a massive under ground reservoir lake in the American Mid-West
called, I think, the Magellan…, which due to high water extraction for farming
in Nebraska, Etc. is running dangerously low. That was from a National Geographic a few years ago. With the flooding out there, has this
Under ground reservoir returned to normal? Without replenishment, it’ll conk out in our life times and end the mid-west farming. it stretches from the Canadian border (?) to New Mexico(?). Anyone know anything about this issue and can provide an update??
You’re assuming the surface of the Earth became molten, that at one time all of Earth was molten, and that magma can’t contain water. The first two are difficult to prove, while it is known that there is water in magma and it is important to magma characteristics and chemistry. Also, even if early Earth was molten early on, was it in that condition when the Mars-sized impactor created the Moon — and was the impactor fully molten? That impact would have provided a lot of new material. There also is evidence that the resulting rock atmosphere may have lasted only a few hundred years because there was an ocean on Earth shortly after the Moon was created. There’s been a lot of water around.
Much of earth’s water is tied up in rocks. Almost all minerals are hydrates, with some rocks holding H2O as a clathrate, or complex, with as many as twenty molecules of water per molecule of dry mineral.
When rocks are heated, even moon or Martian rocks, they give off water. Also, water is subducted into the mantle at plates, and if they are in or adjacent to the sea, this material contains lots of free water as well.
RE: Paul Hildebrandt (06:07:02) : “Forgot to ask, where would you find a dry subduction zone?”
I think these may be found in places where continents from two different plates have come together forming a huge mountain chains such as under the Alps or Himalayas.
BTW, I believe the Cascadian volcanoes in the Pacific Northwest are thought to be the result of a stream of marine material from the Juan de Fuca Plate that is being subducted under the North American Plate.
Spector (14:32:56) :
I think these may be found in places where continents from two different plates have come together forming a huge mountain chains such as under the Alps or Himalayas.
No subduction there. Hence, the great height of the Himalayas. In fact, they are still going up. Also, try to find volcanoes in the Himalayas. Let me know if you find any.
BTW, I believe the Cascadian volcanoes in the Pacific Northwest are thought to be the result of a stream of marine material from the Juan de Fuca Plate that is being subducted under the North American Plate.
Correct.
johnythelowery (10:32:14) :
There is a massive under ground reservoir lake in the American Mid-West
called, I think, the Magellan…, which due to high water extraction for farming
in Nebraska, Etc. is running dangerously low. That was from a National Geographic a few years ago. With the flooding out there, has this under ground reservoir returned to normal? Without replenishment, it’ll conk out in our life times and end the mid-west farming. it stretches from the Canadian border (?) to New Mexico(?). Anyone know anything about this issue and can provide an update??
It’s called the Ogallala Aquifer. It was a problem back in the 80’s when I lived along the Front Range, Colorado.
http://www.waterencyclopedia.com/Oc-Po/Ogallala-Aquifer.html
http://www.iitap.iastate.edu/gccourse/issues/society/ogallala/ogallala.html
http://www.npwd.org/Ogallala.htm
http://www.washingtonpost.com/wp-dyn/content/article/2010/04/20/AR2010042005301.html?hpid=topnews
RE: Paul Hildebrandt (19:43:31) : [Himalayan Subduction Zone] No subduction there. Hence, the great height of the Himalayas. In fact, they are still going up. Also, try to find volcanoes in the Himalayas. Let me know if you find any.
This may depend on how you define ‘subduction zone.’ I believe there must be a zone of descending magma underneath these mountains that is pulling the continents together. This subduction process may be occurring at such depths that the subducted material can only come near the surface again after a complete convection cycle when it might rise again under a line of active sea-floor spreading.
Spector (06:43:39) :
This may depend on how you define ’subduction zone.’ I believe there must be a zone of descending magma underneath these mountains that is pulling the continents together. This subduction process may be occurring at such depths that the subducted material can only come near the surface again after a complete convection cycle when it might rise again under a line of active sea-floor spreading.
Let’s try this one, even though it’s from a wiki:
Definitions of Subduction zone on the Web:
* In geology, subduction is the process that takes place at convergent boundaries by which one tectonic plate moves under another tectonic plate, sinking into the Earth’s mantle, as the plates converge. …
en.wikipedia.org/wiki/Subduction_zone
My definition, above, says that one tectonic plate has to override another tectonic plate resulting in the subduction of the overridden plate into the mantle. Even if the subducted material is incorporated into the mantle, most of the water and/or volatiles will be driven off due to the high temperatures and pressures. Even if continental crust is subducted, the crust is too dry to initiate melting in the overriding crust. Hence, the lack of volcanics.
RE: Paul Hildebrandt (10:06:41) :
Ref: USGS site document entitled “Understanding Plate Motions”
http://pubs.usgs.gov/gip/dynamic/understanding.html
At this site I note that there is a diagram that appears to show the lithosphere of the Indian Plate being subducted under the Eurasian plate and dropping into the asthenosphere without producing any volcanic activity. When this subduction stops pulling the two land-masses together, I assume the Himalayan mountain building will also stop. This appears to be a classic example of one of the three types of convergent plate boundaries.
Another type of convergent plate boundary is the deep trench that forms at the convergence of two oceanic plates. I presume that material subducted there would be more likely to pull water and carbon compounds down into the interior of the earth.
Iceland, BTW, appears to be located on a divergent plate boundary.
Spector (14:51:26) :
At this site I note that there is a diagram that appears to show the lithosphere of the Indian Plate being subducted under the Eurasian plate and dropping into the asthenosphere without producing any volcanic activity. When this subduction stops pulling the two land-masses together, I assume the Himalayan mountain building will also stop. This appears to be a classic example of one of the three types of convergent plate boundaries.
I stand corrected on the subduction of continental crust. However, only thin slices of it actually get subducted. The majority of the crust is scraped off and piled up against the overriding plate. This brings us back to the original statement you made. Here it is below for your convenience.
Spector (21:42:48) :
Paul Hildebrandt (20:13:17) : [Surface Water Driven Volcanic Explosions]
I suspect that water entrained in marine rocks subducted from the sea floor might produce a much more explosive emerging magma than similar material subducted from a dry environment.
You state that magma from a dry subduction zone would produce less energetic eruptions than from a wet subduction zone. I’m not sure if any melting takes place except at extreme depths due to the absence of sufficient quantities of water. What little magma that is produced most likely never even comes close to the surface and therefore crystallizes over the millenia forming a pluton, stock, or sill.
Iceland, BTW, appears to be located on a divergent plate boundary.
Correct. There is some speculation in some circles that Iceland may have been the impact point of a meteor on a spreading center, thus the island and copious volcanic activity.
RE: Paul Hildebrandt (16:04:52) :
On reflection, I probably should have said that the overall dissolved gas content of the rising magma contributes to the rate or frequency of volcanic activity at any given site. This gas content, including water, probably depends on the overall history of the rising magma at each location.
I think the explosivity of these eruptions is primarily determined by the typical strength of the confinement structure that forms after each eruption to block the continued flow of magma. This strength would determine how high the magma pressure must increase to once more rupture the structure. I assume this blockage may contain cracks allowing some gas but not magma penetration as long as it remains in a general state of compression. I believe that strong confinement structures produce less frequent, but more violent eruptions.
I am not sure whether ice-loading on a volcano acts to delay eruptions by offsetting the pressure building up from below or if it might cause top-heavy torsional stresses that would weaken the magma confinement.
I note that the following site from the Department of Geological Sciences, San Diego State University entitled “How Volcanoes Work” seems to contain a lot of useful information:
http://www.geology.sdsu.edu/how_volcanoes_work/
“”” Paul Hildebrandt (06:07:02) :
Spector (21:42:48) :
I suspect that water entrained in marine rocks subducted from the sea floor might produce a much more explosive emerging magma than similar material subducted from a dry environment.
Forgot to ask, where would you find a dry subduction zone? “””
Isn’t India the Poster Child for that ? Supposedly it is still pushing northwards, and driving the Himalayas to new heights.
George E. Smith (10:38:34) :
Forgot to ask, where would you find a dry subduction zone? “””
Isn’t India the Poster Child for that ? Supposedly it is still pushing northwards, and driving the Himalayas to new heights.
It’s more of oceanic crust/continental crust subduction zone, followed by crustal collision with the mantle breaking off and the “subducted” crust underplating the overriding crust as can be seen in Figures 26 and 27 of this link:
http://www.geo.arizona.edu/~ozacar/models~1.htm