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Author Topic: Is there a hole left when moltern rock errupts?  (Read 5363 times)

Wim Spronk

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Is there a hole left when moltern rock errupts?
« on: 09/05/2011 07:01:02 »
Wim Spronk  asked the Naked Scientists:
   
Hello Chris
 
Recently there have been some very interesting programmes on BBC TV dealing with the Vulcanic Eruption that took place 250 Million Years ago when Lava covered an area the size of North America and even affected South Africa. My neighbour who also watched these programmes came up with an interesting question. If so much molten rock erupted from Siberia there must surely be a massive hole somewhere in that region? Can you enlighten us?

Kind regards

Wim Spronk   

What do you think?
« Last Edit: 09/05/2011 07:01:02 by _system »


 

Offline JimBob

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Is there a hole left when moltern rock errupts?
« Reply #1 on: 09/05/2011 21:59:47 »
It does not necessarily mean that there is a hole where the lava come from. The lava of this type of eruption is basaltic in composition. That s the same type of lava that forms ocean floor, erupting at the mid-ocean ridges.

Basaltic lava comes from deep within the earth - it is mantel material. The mantle is not like any substance you or I have ever seen. It is under such great pressure it exists as a solid within the earth. As pressure is released mantle material melts. My books go on at leingth about all of this (with 8,642 phase diagrams to explain it ;) - actually, only 6)  so here is a good synopsis of the process from Wikipedia.

Phase or "Decompression melting occurs because of a decrease in pressure. The solidus temperatures of most rocks (the temperatures below which they are completely solid) increase with increasing pressure in the absence of water. Peridotite at depth in the Earth's mantle may be hotter than its solidus temperature at some shallower level. If such rock rises during the convection of solid mantle, it will cool slightly as it expands in an adiabatic process, but the cooling is only about 0.3C per kilometer. Experimental studies of appropriate peridotite samples document that the solidus temperatures increase by 3C to 4C per kilometer. If the rock rises far enough, it will begin to melt. Melt droplets can coalesce into larger volumes and be intruded upwards. This process of melting from upward movement of solid mantle is critical in the evolution of the Earth.

"Decompression melting creates the ocean crust at mid-ocean ridges. It also causes volcanism in intraplate regions such as Europe, Africa and the Pacific sea floor. There, it is variously attributed either to the rise of mantle plumes (the "Plume hypothesis") or to intraplate extension (the "Plate hypothesis")"


What happens is that a part of the mantle starts to rise in the crust for some reason, resulting in this decompression melting.  As this liquid requires a bit more room, it erupts on the surface.

No hole is left to fill.


 

Offline CliffordK

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Is there a hole left when moltern rock errupts?
« Reply #2 on: 09/05/2011 22:50:33 »
With conservation of matter, the matter has to come from somewhere.

The movement of tectonic plates, land subsidence, something.  But, it could include movement of matter from a distant location, but nonetheless, it ultimately leads to someplace on the surface.

There would be different types of eruptions.  Lava/building eruptions will take subsurface matter and deposit it on the surface.  Ultimately the matter could be displaced from a distant location.

Ash/Explosive eruptions often blow the top off of a mountain, as well as leaving a crater.  Some theories about Crater Lake/Mt. Mazama are that it left a cavity behind following the eruption that later collapsed in on itself forming the lake.  However, such a cavity would mean vapor displacing the rock.  I don't know if these theories have been modified following the Mt. St. Helens eruption and other similar eruptions.
 

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Is there a hole left when moltern rock errupts?
« Reply #3 on: 09/05/2011 23:13:05 »
When ice is frozen, it expands. Everyone knows this.

Why must crustal material be different????? What is so difficult with the concept that a change in phase can cause a change in space occupied - whether that be an expanded or contracted space with decreased heat. With basaltic magma, it is an increase in space occupied.

There are such things as empty magma chambers but these are very few and far between. For the most part, vulcanism just moves material from one place to another. Even if things like craters look impressive in a few instance, for the most part this is just a passing impression. Even the largest volcano on earth is less (and prepare yourself for a rather earth comparison) than a SMALL pimple on the over-all face of the earth.

 

Offline CliffordK

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Is there a hole left when moltern rock errupts?
« Reply #4 on: 10/05/2011 03:57:00 »
When ice is frozen, it expands. Everyone knows this.

Why must crustal material be different????? What is so difficult with the concept that a change in phase can cause a change in space occupied - whether that be an expanded or contracted space with decreased heat. With basaltic magma, it is an increase in space occupied.
Ice expands when it freezes, but once frozen, it contracts when cooled, and expands when it is warmed.  Water also expands when warmed between 4C and 100C.

Most other materials contract when they cool.  The only expansion might be with materials such as pumice that incorporate gas with the solids.

There is some debate about what causes the interior of the earth to be warmer than the surface, but it is not getting warmer, and thus it is not expanding.
 

Offline Bass

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Is there a hole left when moltern rock errupts?
« Reply #5 on: 10/05/2011 18:04:08 »
Don't know about a hole, but there does appear to be subsidence after eruption of traps (large volumes of basaltic lava).  This probably has more to do with cooling as it does with eruption of material.  A recent example is the Snake River Plain- which has subsided several hundred meters over the past several million years. 
 

Offline JimBob

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Is there a hole left when moltern rock errupts?
« Reply #6 on: 10/05/2011 19:53:38 »
When ice is frozen, it expands. Everyone knows this.

Why must crustal material be different????? What is so difficult with the concept that a change in phase can cause a change in space occupied - whether that be an expanded or contracted space with decreased heat. With basaltic magma, it is an increase in space occupied.
Ice expands when it freezes, but once frozen, it contracts when cooled, and expands when it is warmed.  Water also expands when warmed between 4C and 100C.

Most other materials contract when they cool.  The only expansion might be with materials such as pumice that incorporate gas with the solids.

There is some debate about what causes the interior of the earth to be warmer than the surface, but it is not getting warmer, and thus it is not expanding.

We are not comparing oranges to oranges when comparing ice and magma. As stated above the materials of the magma (peridotite) EXPANDS when cooled and provides the pressure for eruption.


If such rock rises during the convection of solid mantle, it will cool slightly as it expands in an adiabatic process, but the cooling is only about 0.3C per kilometer. Experimental studies of appropriate peridotite samples document that the solidus temperatures increase by 3C to 4C per kilometer. If the rock rises far enough, it will begin to melt. Melt droplets can coalesce into larger volumes and be intruded upwards. This process of melting from upward movement of solid mantle is critical in the evolution of the Earth.
 

Offline Geoquest

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Is there a hole left when moltern rock errupts?
« Reply #7 on: 10/05/2011 20:45:30 »
Isn't this essentially what a caldera is? For instance the last time Yellowstone erupted it emptied a massive magma chamber which was then unable to support the land above it. This land subsided and this created an area of land which is now much lower in elevation then before it erupted.
 

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Is there a hole left when moltern rock errupts?
« Reply #8 on: 10/05/2011 23:08:23 »
A caldera is the result of a volcanic eruption. It is defined as the remains of the eruption. In Spanish, 'caldera" is a cauldron, a large cooking bowl. In geology, it is a bowl shaped depression - from a nice round one to a very elongated or irregular one.

The swelling of a magma chamber is very often part of the indication of eruptive potential. However, the explosive potential is more closely related to the amount of ground water in the total system that by becoming gaseous is the main driving force behind explosive eruptions.

As Bass lives on - well, very closely adjacent to - Yellowstone and is the world's foremost expert on Yellowstone, giving lectures to school children as far away from Montana as Georgia (USA - not Asia) I will let him answer this question.

But GASEOUS WATER VAPOR is the main reason for explosive volcanic eruptions.

 

Offline Bass

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Is there a hole left when moltern rock errupts?
« Reply #9 on: 11/05/2011 02:17:33 »
But GASEOUS WATER VAPOR is the main reason for explosive volcanic eruptions.

Just curious, but isn't water vapor GASEOUS by definition?
 

Offline Bass

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Is there a hole left when moltern rock errupts?
« Reply #10 on: 11/05/2011 02:53:04 »
Isn't this essentially what a caldera is? For instance the last time Yellowstone erupted it emptied a massive magma chamber which was then unable to support the land above it. This land subsided and this created an area of land which is now much lower in elevation then before it erupted.

Essentially correct.  Explosive calderas form from silicic volcanoes.  Silica-rich volcanic material is viscous- it doesn't flow or move well.  As a result, these magmas can sit for tens or even hundreds of thousands of years before finally getting close enough to the surface to erupt.  As the magma sits, it starts to crystallize, leaving the remaining melt more and more volatile rich.  Volatiles include GASEOUS WATER VAPOR, carbon dioxide, sulfur dioxide and anything else that doesn't mix well with the magma.  Keep in mind that so long as the magma is deeply buried, the volatiles, under immense pressure, are in solution and not in a gaseous state.  A good analogy is a can of beer- so long as it's under pressure the CO2 remains in solution.  Now shake it up and pop the top- the CO2 flashes into gas and the beer explodes as foam all over everything.

Same thing happens in explosive caldera eruptions.  Once the magma is close enough to the surface, and the cap is punctured (by a small volcanic eruption, landslide, earthquake, etc), the volatiles flash into gas, which increases the pressure in the system, causing an explosive eruption.  As the material erupts, it allows more volatiles to flash into gas- etc. etc. until the magma chamber is played out.  With the magma chamber now empty, the roof collapses and a caldera (collapse feature) forms.

Basaltic traps are very different- because basalt has relatively low viscosity.  Everyone has seen pictures of the Hawaiin eruptions- the lava oozes out and runs downhill.  There are very few accumaulated volatiles- which is why basalt volcanoes aren't explosive.

The big eruption 250 million years ago referred to in the first post was known as the Siberian Traps.  I suspect that the earth's surface domed up, mostly due to rising heat, before the Siberian Traps erupted, and probably subsided over millions of years as the crust cooled afterward.

Calderas do form in basalt terrains, but usually because the magma chambers find some other place to simply flow out, resulting in a collapse- rather than the explosive eruptions in more silicic rocks.



 

Offline JimBob

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Is there a hole left when moltern rock errupts?
« Reply #11 on: 12/05/2011 03:13:48 »
But GASEOUS WATER VAPOR is the main reason for explosive volcanic eruptions.

Just curious, but isn't water vapor GASEOUS by definition?

smart arse

And he doesn't even know what a "crevasse splay" is! 

Watch - he will look it up and post that "i do, too!"

« Last Edit: 12/05/2011 03:30:28 by JimBob »
 

Offline Bass

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Is there a hole left when moltern rock errupts?
« Reply #12 on: 12/05/2011 04:43:36 »
hey! I resemble that remark!
 

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Is there a hole left when moltern rock errupts?
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