|About a bag of flour or a similar amount of sand, sawdust etc||A small balloon|
|A pair of scissors||A box or something similar to contain the flour somewhat.|
Inflate the balloon to about the size of an adult's fist
Put the balloon in the box , knot upward and bury it in flour or sand, forming a mountain over the top of it.
Take your scissors and gently find the knot of the balloon and at arms length cut the knot off. If you are doing this with sand be particularly careful as it may blow sand towards your eyes.
You should find that some flour is blown upwards and then the mountain collapses to form a depression at the centre.
You have built a model of a very large volcanic eruption. Beneath most volcanoes there is a magma chamber, this is an area underground where magma - molten rock welling up from much deeper - accumulates. As the amount of magma increases, the pressure will increase and the chamber can push up the ground level to make space for this magma. In the experiment this rock stored under pressure is modelled by the balloon In between eruptions the magma chamber is blocked off by a plug of solid lava, or in your case your balloon is sealed by the knot.
When the pressure builds up to such an extent that the plug is pushed out or bypassed you can get an eruption. If the eruption is big enough the magma chamber will suddenly empty, and the pressure drops suddenly.
This means that there is a large unsupported void underground which can collapse forming a large depression which is known as a Caldera.
These calderas can be very large, for example Santorini in Greece is an island in the form of a huge bay that was formed in a gigantic volcanic eruption in about 1600BC and is a large caldera.
|The island of Santorini from the edge of the caldera||From a satellite|
There are other even larger examples such as the whole of Yellowstone National Park in the USA which is a large depression caused by the erruption of a
Why do volcanoes have a cone shape?
|In an eruption, material is thrown out from a vent. The further you are away from this vent the less material will reach you so the less the land is built up.||So after many eruptions you end up with a cone shaped volcano.|