1) Is the top of a volcano open or closed? I mean, is there a hole in the top that connects to the mantle? If you still don't get what I am saying, lets put it this way, when you walk to the top of a volcano, is there a "land" that you can stand on the top or is it a hole?

From the following definition of a volcano, what I understand is that the top of a volcano is open, or there is a hole! But I am not sure! When the volcano is erupting, certainly there is a hole, but what if it isn't erupting yet? I have never seen a volcano from the top...

"...a volcano is a mountain that opens downward to a pool of molten rock below the surface of the earth. It is a hole in the Earth from which molten rock (magma) and gas erupt."

2) Are the terms "crater" and "caldera" meaning the same thing? (interchangeable?)

Would somebody like to share their knowledge? :)

1) Depends on what kind of volcano.

If you're standing on top of an active shield volcano like Kilauea in Hawaii, then there's a hole with lava. If you're standing atop a stratovolcano like Mt. Rainier, then there's land.

2) No, they are not interchangable. A caldera is a giant volcanic crater that's usually formed from the collapse of a subsided active magma chamber or a depression on top of one, like Long Valley in California or Yellowstone.
A crater is applicable to a bowl-shaped depression that is more or less circular. It can also be applied to a much broader variety of causes besides volcanoes, such as a bomb or meteor.

Wait... I think I got the caldera definition wrong. Can someone check?

1) "a volcano is a mountain that opens downward to a pool of molten rock below the surface of the earth. It is a hole in the Earth from which molten rock (magma) and gas erupt"

Do you think the above definition is incorrect? (because a volcano doesn't always "open") I got it from http://www.woodlands-junior.kent.sch.uk/Homework/mountains/volcanoes.htm

3) Would explosive eruptions or quiet eruptions be more likely to increase the height of a volcano? I think exposive eruptions are more likely to increase the height because pyroclastic materials are ejected upward explosively. On the other hand, quiet eruptions allow lava to flow freely and quietly into wide areas. But how come cinder cone volcanoes, which only eject pyroclastic matierals, are not very tall? (sometimes even shield volcanoes are taller than them)

Thanks!

Wait... I think I got the caldera definition wrong. Can someone check?

You do. You have attributed size to the criteria for a caldera. I imagine you've done so because of the association with the Yellowstone caldera which is probably the most famous caldera in the world.

A caldera is simply a collapsed magma chamber. Wikipedia describes it as "a volcano that has collapsed in on itself." Thus, the vital aspect of a caldera is that the magma has been emptied out of it and thus the land over the magma chamber (or volcano) has collapsed.

http://en.wikipedia.org/wiki/Caldera

Would explosive eruptions or quiet eruptions be more likely to increase the height of a volcano?

I don't think 'eruptions' increase the height of a volcano at all. They usually decrease the height. A violent eruption would decrease it more than the gentle though. Look at Mt. St. Helens. That volcano went on a crash diet when it blew.

The building of volcano is generally not done through eruptions. I think. Although, if it were, then it would have to be the gentle eruption.

I think that volcanos generally form before an eruption. It's the underground pressure of the rising magma that pushes the ground up to form the volcanic cone.

Now. Don't take my word for that, but I do feel safe in saying that if anything the violent eruptions definitely wouldn't contribute to volcanic height.

1) "a volcano is a mountain that opens downward to a pool of molten rock below the surface of the earth. It is a hole in the Earth from which molten rock (magma) and gas erupt"

Yeah. But only after the magma has made its way to the surface. And, the opening doesn't have to remain open.

Chances are there'll at least be vents releasing some pressure. But there doesn't need to be a 'classic' lava filled crater.

A caldera is simply a collapsed magma chamber. Wikipedia describes it as "a volcano that has collapsed in on itself." Thus, the vital aspect of a caldera is that the magma has been emptied out of it and thus the land over the magma chamber (or volcano) has collapsed.
so at the place where Yellowstone caledra now is in the past there was a gigantic volcano?

Not a volcano (not the classic 'mountain' volcano, anyway), but a huge magma chamber. That's why they call it a super volcano.

I don't think 'eruptions' increase the height of a volcano at all. They usually decrease the height. A violent eruption would decrease it more than the gentle though. Look at Mt. St. Helens. That volcano went on a crash diet when it blew.

The building of volcano is generally not done through eruptions. I think. Although, if it were, then it would have to be the gentle eruption.

I think that volcanos generally form before an eruption. It's the underground pressure of the rising magma that pushes the ground up to form the volcanic cone.

Now. Don't take my word for that, but I do feel safe in saying that if anything the violent eruptions definitely wouldn't contribute to volcanic height.



That's mostly true but isn't the full story - it varies. :)

Eruptions raise the height of the surrounding area and subsequent eruptions will add to the height of the volcano itself, usually.

For good examples of the eruptions actually raising the height of the volcano itself, you need look no farther than Hawaii. And the upward-building is still going on today. ;)

I see.
By the way, if anybody here is interested about more volcanism, this is worth checking out:
http://en.wikipedia.org/wiki/Siberian_traps
A magma chamber larger than Europe.

Light,

For good examples of the eruptions actually raising the height of the volcano itself, you need look no farther than Hawaii. And the upward-building is still going on today.

I was actually thinking of Hawaii when I decided that I'd better qualify my answer with the fact that I really don't know for sure.

Hawaii does have a few things that are different, however. The magma from Hawaiian volcanos (at least some of them) are of a different sort than most volcanoes. The lava is silica poor and thus more liquid than your average volcano. Hawaiian volcanos tend to flow more than erupt.

Semantics, of course, as it is most likely still considered an eruption.

But. It would be a gentle eruption rather than a violent one.

There's no coincidence in the fact that violent eruptions are often termed as "blowing their top."


Avatar,

Ah. The Siberian Traps. You know that they're conjectured to be the cause of one of the 5 major extinction events, yes?

I always wondered why they call them 'traps'.

There are numerous types of volcanoes, and a stratovolcano builds its height by eruption. There are also cinder cones and mixed types. Hawaii's volcanoes are really heavy on flows of basaltic magma and light on ash, while Iceland's volcanoes are heavy on ash. The differences are caused by different chemical composition and water content.

Mt. St. Helens exploded because lava had congealed over its access to the magma below, which meant that it took a lot more pressure for the lava to force its way to the surface. You could say it's like soldering over a crack in a scuba tank versus leaving it open. The solder holds well enough to let the tank build up a few hundred PSI before it blows. When it blows, whoever was filling it cashes in his death benefits. An open crack simply vents when pressure is applied. If somehow cooled lava filed the tube from top to bottom, that plug might be stronger than the surrounding rock, so the lava would find another way up. When we're talking about a mountain that big, we're talking about something that can have a whole system of caves and cracks between it and Hell. The Hawaiian volcanoes don't blow apart because they are always open. Less active volcanoes are more dangerous that way than more active volcanoes. That's why whenever the Yellowstone volcano goes, it's like a Verne shot. Siss boom, to the moon!

Here's the USGS (http://volcanoes.usgs.gov/Hazards/What/hazards.html) word on volcano hazards.

More or less :) I "stumbled" upon them while I was researching Permian extinction.
I always wondered why they call them 'traps'.
Maybe because it generally was a huge hole->holes in the ground.

Light,



I was actually thinking of Hawaii when I decided that I'd better qualify my answer with the fact that I really don't know for sure.

Hawaii does have a few things that are different, however. The magma from Hawaiian volcanos (at least some of them) are of a different sort than most volcanoes. The lava is silica poor and thus more liquid than your average volcano. Hawaiian volcanos tend to flow more than erupt.

Semantics, of course, as it is most likely still considered an eruption.

But. It would be a gentle eruption rather than a violent one.

There's no coincidence in the fact that violent eruptions are often termed as "blowing their top."




Yes, all very true. :) And that's the reason I said it varies. And you're also correct about the semantics. While people tend to think of an eruption as an explosion, even a slow, gentle lava flow is still an eruption. (People focus in the spectacular.) ;)

I don't think 'eruptions' increase the height of a volcano at all. They usually decrease the height. A violent eruption would decrease it more than the gentle though. Look at Mt. St. Helens. That volcano went on a crash diet when it blew.

The building of volcano is generally not done through eruptions. I think. Although, if it were, then it would have to be the gentle eruption.

I think that volcanos generally form before an eruption. It's the underground pressure of the rising magma that pushes the ground up to form the volcanic cone.

Now. Don't take my word for that, but I do feel safe in saying that if anything the violent eruptions definitely wouldn't contribute to volcanic height.
There are four main types of volcanoes and they are all built up from the out pouring of lava that forms the cone:
1) Cindir cones: built from lava erupting out of a single vent.
2) Composite volcanoes: multiple vents.
3) Shield volcanoes: lava pours out in all directions forming a flatter cone.
4) Lava domes: the cone, or dome, is initial formed by a lava uprising but is the usually expanded upwards from lava within.

The Hawaiin island Volcanoes are shield valcanoes that originated from underwater cracks in the earth's Pacific plate creating outburst of lava that formed the cones. These lava flows continued and created some of the largest volcanoes in the world. Some in Hawaii are still very active. Also, if you were to measure a mountain from its base to its height, irrespective if there was water around it or not, then the Mauna Lau volcano is the highest and most massive mountain in the world at 32,800 ft. In comparison, Mount Everest is 29,035 ft.

Wow...there's a lot of people in this Earth Science forum!

2) For the definition of a "crater" on the top of a volcano, is it like a bowl-shaped depression and the top of the volcano is closed?

So does a "caldera" mean that the volcano has collapsed, levelled off to a flat land, and there is no longer any magma below it?

3) I was told in my text book and my notes that magma erupts to the surface, then cools and solidifies to form rocks. And this forms the volcano and when more eruptions happen, the volcano grows higher and/or wider. In general, would explosive eruptions or quiet eruptions be more likely to increase the height of a volcano? I can't figure this out...

The initial cone or vent of a volcanic has to be formed by an initial outburst uprising of lava. When the lava cools off, the cone is no longer entirely open, although if it where a dramatic explosive eruption, you might be able to see down into the cone for quite a depth. Then sometimes too, a cooled off volcano will form a lake within the cone.

Yes, a caldera is the collapse of a large wide volcano into itself usually caused by the collapse of the magma chamber beneath it. But there are three different types of caldera formations depending upon the pressue within, the silica content of the lava, and the erosion around it. The more silca content: the less likely the lava will be to flow out over a longer range. For an excellent explanation about this go to:
http://en.wikipedia.org/wiki/Caldera

Again, there are more than one type of volcano, and different volcanologists classify them in different ways. A shield volcano, like I was talking about above, usually involves massive amounts of lava that flow out around it and widens the cone. On the other hand, a sort-lived explosive eruption will generally make the cone higher, but not wider, as in a shield volcano, because the rocks, cinders and volcanic glass and ash will be thrust out at greater heights and fall all over the place. Again, for a more indepth discussion, with photos and diagrams, go to:
http://en.wikipedia.org/wiki/Volcanoes

get google earth, find Mopunt Helena

The initial cone or vent of a volcanic has to be formed by an initial outburst uprising of lava. When the lava cools off, the cone is no longer entirely open, although if it where a dramatic explosive eruption, you might be able to see down into the cone for quite a depth. Then sometimes too, a cooled off volcano will form a lake within the cone.

Yes, a caldera is the collapse of a large wide volcano into itself usually caused by the collapse of the magma chamber beneath it. But there are three different types of caldera formations depending upon the pressue within, the silica content of the lava, and the erosion around it. The more silca content: the less likely the lava will be to flow out over a longer range. For an excellent explanation about this go to:
http://en.wikipedia.org/wiki/Caldera

Again, there are more than one type of volcano, and different volcanologists classify them in different ways. A shield volcano, like I was talking about above, usually involves massive amounts of lava that flow out around it and widens the cone. On the other hand, a sort-lived explosive eruption will generally make the cone higher, but not wider, as in a shield volcano, because the rocks, cinders and volcanic glass and ash will be thrust out at greater heights and fall all over the place. Again, for a more indepth discussion, with photos and diagrams, go to:
http://en.wikipedia.org/wiki/Volcanoes
So explosive eruptions are more likely to add to the height of a volcano than do quiet eruptions! :)
Lava can make a volcano higher. But how can hot rocks and ash add height...won't they slide off the slope? How can they "stick" to the surface, making the volcano higher?

Well, as I said above, it would certainly also depend on the silica content: The more silica content: the less likely the lava will be to continously flow for a prolonged period of time. Intuitively, explosive eruptions shoot out straight up, compared to non-explosive lava that simply flows out over the side. In the later case there would definitely be more of a wide horizontal build up, as in shield vocanoes. In the former, there would bound to be some accumulation of "sticky" rock and debri that would accumulate on and above the surface of the cone while progressing upward.

You have an excellent counter questionary assumption! If you find any data to the contrary, please post it. I can say that I honestly don't know that for sure. That is why I posted the links for further research. In any case, an explosive eruption would not widen the crater (core-vein) as much as a non-explosive eruption would.

Also, to answer another of the poster's questions, most - but not all - volcanoes are totally made from the magma and lava eruptions. However, some fault zones, particulary those that occur in already formed mountainous geographic areas, may already have a mound of rock present. A new fault zone may break open and erupt in this area that will produce a volcanoe with a rock/soil base that results in a volcano that is not strictly formed from the lava.

The different type of magma present in the eruption determines the size and shape of a volcano. Shield volcanoes are the largest (widest and tallest) because they are formed with lava with a high silica content that solidifies very rapidly allowing it to build up. Lava with a low silica content are very fluid and will flow out for long distances without building up as much. It would be very difficult to produce a narrow, yet extremely high, volcano because our environment (rain, landslides, erosion), or a caldera collapse, could easily destroy it. Therefore, the highest volcanoes are also the most widest, as in the shield volcano like Mauna Lau.

Stratovolcanoes are the steepest and are formed from layers of lava, mudflows, and tephra that pile up. They have symmetrical cones with steep sides - more narrow. The steepness of their slopes are determined by the amounts of pyroclastic debris (fragments of the explosion), the type of lava erupted, and mudflows that accumulate. Mount Vesuvius, Mount Fuji, Mount Rainier (the most well-known volcanoes) are all stratovolcanoes.

How explosive a volcano is depends on the viscosity (thickness and fluidity) of the lava. Lava that is thick and sticky will not allow the gases to escape. Pressure builds up inside until the gases escape violenty and explode. The magma blasts into the air and breaks up into particles called tephra that range in size from ash-like particles to house-size boulders.

Some interesting facts:
1) The largest volcano in the world (height, volume, and width) Mauna Loa in Hawaii.
2) The highest "active" volcano in the world is Cotopaxi in the Andes Mountains in South America.
3) History's best-known volcano is Mount Vesuvius in southern Italy: its eruption buried the entire city of Pompeii in 79 A.D.
4) The country Iceland is a volcanic island entirely formed by cooled basaltic lava from volcanoes.
5) On Mars, the largest volcano is Olympus Mons: about 88,500 feet (27 km). A volcano of this height would be impossible on Earth because the force of gravity here would not allow it to form that high. It is possible on Mars because of much lower gravity.

"Some volcanoes consist of a crater alone, with scarcely any mountain at all; but in the majority of cases the crater is situated on top of a mountain (the volcano), which can tower to an enormous height. Volcanoes that terminate in a principal crater are usually of a conical form."
http://en.wikipedia.org/wiki/Volcanoes

"Shield volcanoes, with their gentle slopes and curved profile, are the largest of all volcanoes. They are built up from repeated basaltic flows, often beginning at the ocean floor. Basaltic magma has a relatively low silica content, allowing it to flow readily. As a result, shield volcanoes are characterized by lava flows rather than explosive pyroclastic activity. Shield volcanoes are most commonly formed above hot spots under basaltic oceanic crust. They are also formed in areas where the mid-ocean ridge intersects with land, as in Iceland, or in areas of active rifting, like east Africa. In these areas, as the magma is rising to the surface, it mixes with only basaltic rocks, allowing it to preserve its mafic composition and flow readily. Probably the most famous shield volcanoes, Mauna Loa and Mauna Kea, currently rest above the Hawaiian hotspot. Measured from its base on the ocean floor to its summit, Mauna Kea is 5.6 mi (9 km) tall—slightly taller than Mt. Everest.

Composite volcanoes, also known as stratovolcanoes, have steep sides and a characteristic cone shape. They are built up from alternating layers of lava and pyroclastic debris. Lava associated with composite volcanoes generally has an intermediate composition, and is more resistant to flow than basaltic lava. This results in the mixture of flows and explosions. Composite volcanoes occur above subduction zones, where rising magma mixes with both oceanic and continental crust raising the overall silica content. They are ubiquitous along the subduction zones of the Pacific Rim, and some famous examples include Mt. Fuji in Japan and Mt. Rainier in Washington. Their ability to erupt explosively, as demonstrated by Mt. St. Helens in 1980, makes these some of the most dangerous volcanoes on Earth.

Lava domes are steep-sided, rounded domes, formed because of pressure exerted by rising viscous magma. Rhyolite, a felsic magma, is usually associated with lava domes. Its felsic composition makes it highly viscous, forcing it to move slowly, building up pressure and deforming the ground surface above. Lava domes are generally associated with composite volcanoes, although they can occur on their own. They are capable of causing deadly eruptions as tremendous amounts of built-up pressure are suddenly released in giant explosions. Eruption of a lava dome was responsible for the death and destruction caused by the 1902 eruption of Mt. Pelée on Martinique.

Calderas are massive depressions created by rare, violent explosions. Also associated with rhyolitic magma, caldera eruptions are capable of expelling enormous amounts of ash and debris in a single explosion. Calderas form where hotspots occur under continental crust. As magma rises, it mixes with the felsic continental crust, resulting in a high silica content. As is the case with lava domes, the resultant viscous magma cannot flow, and explodes when sufficient pressure has built up. Although there have been none in recent geologic history, about 600,000 years ago a large caldera eruption occurred at what is presently the site of Yellowstone National Park in Wyoming and Montana. The famous hot springs and geysers of the area are the legacy of that eruption, and it is believed that the site has the potential to produce another eruption in the future.

Cinder cones are steep-sided, cone-shaped, relatively small volcanoes that are formed by the accumulation of pyroclastic debris. They are not associated with any one particular lava type, and occur in a number of settings. They are commonly found on the flanks or inside the summit craters of larger volcanoes, and form when pyroclastic debris ejected by the main volcano accumulates to form the smaller cone. Perhaps the most famous cinder cone, Parícutin volcano in Mexico, grew suddenly out of a farmer's cornfield and within one month had risen to a height of almost 1,000 ft (305 m). Cinder cones tend to have short life spans; lava flows released by Parícutin eventually covered an extensive area, but within 10 years the volcano became dormant."
http://www.bookrags.com/sciences/earthscience/volcano-woes-02.html

For a great introduction all about volcanoes, go to:
http://www2.nature.nps.gov/views/KCs/Volcanism/HTML/ET_01_Intro.htm

Paracutin in Mexico was what's known as a monogenetic (meaning one birth) volcano. That means it erupts once and falls silent, never erupting again. The Auckland volcanic field in New Zealand is like that--there are over 50 cones in the area of the city, and several phraetic depressions as well. A phraetic crater is caused by magma coming up from below and contacting the water-table, which creates a steam-blast eruption, like the bursting of an enormous boiler. Fortunately, they are comparitively rare events.

There are essentially three different types of volcanoes (and I know I'm going to smacked around the ears for saying that, because there's a lot more detail involved). Anyway, they are 1) shield or basaltic, such as Hawaii; 2) Dacite or andesite, because most of the volcanoes in the Andes are of that type; 3) rhyolitic, such as Yellowstone in the USA, and Taupo, Kaponga, Maroa and others in New Zealand.

You don't have to worry about getting killed by a basaltic eruption unless you're idiot enough to get in the way of a lava stream, and don't run away fast enough. They are very low in silica content. (Some Hawaiian magmas have been measured as low as 16% silica). They are called shield volcanoes because the mountain has a very slight slope, only about 2-3 degrees, and they look like shields lying on their sides when seen in cross-section.

Higher silca content makes the magma more sticky and more likely to hold onto gases, which makes the volcano more explosive, as they have to build up to greater pressures to escape. Hence the greater explosiveness of the andesite volcanoes, which build more steep-sided cones.

Rhyolite is extremely dangerous; There are two types of rhyolite volcanoes; the first type is where the magma comes out very slowly, extruded like toothpaste from a tube. This builds up dome volcanoes which have very steep sides, flat tops, and when you drill into them they have distinct layers like an onion because of seperate eruptions building up on each other. They tend to form along fracture zones, and when one vent is blocked the activity moves along the fracture and forms another dome. Eventually you end up with a multiple volcano that has several large domes all fused together.

The second type of rhyolitic volcano is the most dangerous thing you'll find in the world. They're so explosive they just blast enormous holes in the ground. About 26,000 years ago, Lake Taupo erupted in New Zealand and evacuated roughly 800km3 of material. In 232AD, it erupted about 110 km3 of material. That time, it was heard in China. :eek:

First of all, I would like to thank everyone for your time and consideration :)

Back to Question 3 -- Would explosive eruptions or quiet eruptions be more likely to increase the height of a volcano? (I was given this question in my text book, but there is nothing regarding this in the section...)

I did a research project today and found out that explosive eruptions of Mount St. Helens in 1980 and Mount Pinatubo in 1991 both REDUCED the height of the volcanoes because their tops were blown off (from Wikipedia), can I conclude that QUIET eruptions are more likely to increase the height of a volcano than do explosive eruptions?

http://www.janesoceania.com/png_rabaul_volcano/
http://www.whoi.edu/oceanus/viewArticle.do?id=7142

Definitely, kingwinner, a shield volcano such as Hawaii, which has very fluid lava (being very low in silica content) is much more likely to add height to itself during an eruption. However, because of the fluidity of the magma, the process takes an immense amount of time. The Hawaiian chain has been building up over millions of years. I'm not sure how old the main island of Hawaii is now, but it's presently the largest mountain on Earth, way bigger than Mount Everest.

That said, andesite volcanoes can get pretty big as well--but they tend to lose height in a major eruption. The largest volcanoes, as I pointed out earlier in this thread, are so big (in terms of crater size) that you have to look at them from space or at least from an ariel view in order to even recognise them as volcanoes. Rhyolite volcanoes tend to have a particular pattern of eruption--they will have a 'throat-clearing' eruption (which is basaltic) and then about 150 years later they will have a really major rhyolitic eruption from the same caldera. When I say really major, I mean an eruption of about 100 km3 in a single day. For a rhyolitic volcano, that's a relatively small outburst.

The last time there was a basaltic precedent eruption, such as the one I've just talked about above, was June 10th 1886, from Tarawera in the Bay of Plenty, in the North Island of New Zealand. That's coming up to 120 years ago. In about 30 years time, there's going to be a major rhyolic eruption in that area.

1) Is the top of a volcano open or closed? I mean, is there a hole in the top that connects to the mantle? If you still don't get what I am saying, lets put it this way, when you walk to the top of a volcano, is there a "land" that you can stand on the top or is it a hole?

From the following definition of a volcano, what I understand is that the top of a volcano is open, or there is a hole! But I am not sure! When the volcano is erupting, certainly there is a hole, but what if it isn't erupting yet? I have never seen a volcano from the top...

"...a volcano is a mountain that opens downward to a pool of molten rock below the surface of the earth. It is a hole in the Earth from which molten rock (magma) and gas erupt."

2) Are the terms "crater" and "caldera" meaning the same thing? (interchangeable?)

Would somebody like to share their knowledge? :)

If you can get your question answered in this thread, check out the possibility of drilling a hole into the magma chamber centered over Yellowstone, home of the world's largest supervolcano. You should get your question answered then.........

May_wentee :D

We can always drill a hole deep enough until we reach China but that doesn't answer the question. Yellowstone's volcanoe is not the largest volcano. How many times do you have to hear it: Mauna Loa is the Earth's largest and tallest volcano and the tallest mountain. Compare the physical size and facts.