Hi Lyn!
Talking about volcanoes always gets me charged up! WOOHOO!!!
First of all, the common definition of the word "volcano":
"A mountain or hill, typically conical, having a crater or vent through which lava, rock fragments, hot vapor, and gas
are being or
have been erupted from the earth's crust."
Lyn, I'm going to talk about the second part of your comment first:
SoCalGalcas wrote:I always thought that caldera meant the reservoir of magma. The way it was used here, it means the hole left in the volcano after it explodes. Lyn
A caldera forms in this way:
1. A reservoir of molten rock (called magma) is formed when molten rock (or semi-molten rock) escapes from the earth's mantle through fractures in the crust.
This commonly happens at the boundaries of the tectonic plates, where one plate is subducting under an adjacent plate. This can also occur at "hot spots" in the crust (Yellowstone).
2. This rising magma accumulates - generally over thousands of years - and eventually forming a reservoir within the earth's crust
3. The pressure on the crust from this expanding reservoir (expanding from the pressure of the rising magma, and other reasons) creates fracturing
above the reservoir.
4. The ever-increasing pressure of the filling magma chamber on the fractured rock above it causes the magma to migrate upwards, through these fractures, under very great pressure.
5. As the magma reaches nearer the surface, pressure is reduced, the gas trapped in the magma suddenly expands very
very rapidly, and, voila! We have a volcanic eruption! (As an illustration, think about what happens when you shake a bottle of soda really hard, keeping your finger on the opening, then take your finger off - the gas in the soda, which was under pressure, suddenly expands at the release of pressure - you took your finger off - and the soda explodes out of the bottle. Volcanic eruption is much the same principal)
6. The eruption will go on for a time, depending upon how much pressure needs to be released, and how much magma was in the magma chamber (reservoir) in the first place. Finally, the magma chamber has been emptied.
7. When the magma chamber is empty, it leaves a void - an empty space in the crust - where the magma used to be. A caldera forms when the now-empty magma chamber can't support the weight of the fractured rock above it. The rock collapses back into this void (the empty magma chamber), leaving a bowl-shaped depression in the earth.
This depression is called a caldera.
Here is a short video explaining caldera formation:
https://www.youtube.com/watch?v=gQRt2NCmEw8Lyn, I'm now going to talk about the first part of your comment.
SoCalGalcas wrote:Annie, reguarding your last quesrion...that Hugh volcano near Naples, is a volcano that has NOT erupted!!! The earth rose, there were many earthquakes, then the earth fell. . . .Thousands of people live on that caldera.
I believe that you are speaking of the Campi Flegrei volcanic area near Naples, Italy (also not too far from Vesuvius). The area is closely monitored by geologists, and the earth there has, indeed, been "rising and falling" for decades.
Yes, there have been volcanic eruptions of of the Campi Flegrei volcanic area. It has been established that Campi Flegrei had major eruptions 39,000 years ago, and also 15,000 years ago. There was a smaller ("smaller" being relative!) eruption there in 1538. The 1538 eruption being it's most recent eruption.
SoCalGalcas wrote:Thousands of people live on that caldera.
Yes, you are correct. People
do live in that
caldera. And, yes, this
is a volcanic caldera, a caldera that was formed by volcanic eruption.
If anyone reading this isn't sick of reading about volcanoes by now (LOL! I never am!), here is a fun map of the city of Naples, Italy, population 3,000,000 (three million). Note the location of Vesuvius in relation to Naples. Suburbs surround Vesuvius on all sides, many buildings even creep up the volcano's slopes.
Vesuvius last erupted in March, 1944 (a mere blink in geologic time).
Will it ever erupt again? Absolutely. Without doubt.
Naples is an ancient city, much of it comprised of narrow, one-lane streets that have been there since before the invention of the automobile. Imagine three million people trying to escape down narrow, twisting, streets, all running for their lives.
When the next eruption draws close, it will be a major task to evacuate all three million people before Vesuvius explodes.
Naples, Italy (be sure to zoom in on Vesuvius)
https://www.google.com/maps/place/Naples,+Metropolitan+City+of+Naples,+Italy/@40.8523054,14.3980633,21803m/data=!3m1!1e3!4m5!3m4!1s0x133b084f6a6c7e99:0x3df52cc13b78191d!8m2!3d40.8517983!4d14.26812Thanks, everyone!
Anne
