Naked Science Forum

Non Life Sciences => Geology, Palaeontology & Archaeology => Topic started by: paul.fr on 22/08/2007 18:42:34

Title: the forming of volcanoes - part 2
Post by: paul.fr on 22/08/2007 18:42:34

Thanks to bass, and an excelent answer by tony in this topic http://www.thenakedscientists.com/forum/index.php?topic=7842.0 . i know how volcanoes are formed.

But it does not really answer why they have a big hole down the middle of them. why does this hole not fill and set hard after an eruption?

Title: the forming of volcanoes - part 2
Post by: frethack on 22/08/2007 20:22:36

Generally, they actually do mostly harden on the top.  The giant hole that you see (for instance, on Mt St Helens) is the blast crater. 

(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fupload.wikimedia.org%2Fwikipedia%2Fcommons%2Fthumb%2F5%2F51%2FSthelens2.jpg%2F300px-Sthelens2.jpg&hash=90cdd48b57c14c4a8ceedf38f00936b8)

The opening of the volcano forms a lava dome inside the crater that expands and contracts, depending on the mood of the volcano.

(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fearthobservatory.nasa.gov%2FNewsroom%2FNewImages%2FImages%2FStHelens_MAS2004268.jpg&hash=c2790927da5c46d690eea62f3dbdc6b3)

This is a similar satellite photo, but with the landscape thrown in.

(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fearthobservatory.nasa.gov%2FNewsroom%2FNewImages%2FImages%2FStHelens_VIS_MAS2004268_lrg.jpg&hash=6bb786c47c0998bea1735b817f3faeb1)

Im not sure if this applies to very active shield volcanos like Mauna Loa and Kilauea (anyone who knows, please enlighten me!...is the magma too low in viscosity to create a proper dome?) Hope this tidbit helps!

Title: the forming of volcanoes - part 2
Post by: Bass on 27/08/2007 06:55:27

Frethack is spot-on with regards to Mount St. Helens.

However, the "hole in the middle" of many volcanoes are collapse features, often called craters or calderas.  The top of the mountain collapses because magma (or molten rock underground) is removed, so there is no longer any support on the roof or cap rock, which then collapses.

There are a couple of mechanisms for magma to be removed.  The simplest is for the magma to erupt somewhere else, typically out of vents or fissures further down the mountain.  This requires low viscosity lavas (so they can flow easily), like the basalt in shield volcanoes (Hawaii).  A fissure opens further down the mountain, the lava drains out, the magma chamber on top empties and the top of the volcano collapses.

Silica rich magmas are very high viscosity, but even so material can be removed from the magma chamber allowing the roof to collapse.  Silica rich magmas are usually full of volatile components, such as CO2, H2O, SO2 and other gasses.  The volatiles are similar to the CO2 in a can of soda pop.  If you shake the can hard, then flip open the lid, the dissolved CO2 "flashes" into gas and causes an explosion of pop and foam.  After the explosion, very little of the soda pop will be left in the can.
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The process is similar in silica rich magmas.  Once the magma gets close enough to the surface and starts to erupt, it reduces the pressure in the magma chamber.  The pressure reduction allows the volatile components to "flash" into gas- which then increases the pressure and leads to ever more explosive eruptions.  The explosive eruptions will continue out of cracks and small vents until the volatiles are spent.  Once spent, there is no internal pressure to hold up the roof of the magma chamber, so the roof collapses to create a hole in the volcano.
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Crater Lake, Oregon- Mount Mazama volcano

Supervolcanoes, like Yellowstone and Toba, don't build up classical volcanic cones- rather they erupt explosively and collapse into gigantic calderas.  If taken to the middle of the Yellowstone caldera and asked where the Yellowstone volcano was- most people wouldn't have a clue.  Even though they are standing in the center of the volcano, since it looks like a featureless, forested plain.

Title: the forming of volcanoes - part 2
Post by: frethack on 27/08/2007 14:35:34

hehehe *sheepish grin* should have remembered calderas.

Even though Crater Lake is a collapse feature, arent Wizard Island and Merriam Cone the lava dome(s) inside the caldera?

 

Title: the forming of volcanoes - part 2
Post by: Bass on 27/08/2007 16:31:39

Yes, they are resurgent domes inside the caldera.  Similarly, there are two resurgent domes in the Yellowstone Caldera, the Mallard Lake Dome (near Old Faithful) and the Sour Creek Dome (between Yellowstone Lake and Canyon).
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