As we know, when large scale volcanic eruptions occurs, large amounts of ashes and lava rushed out at short time, is it possible to drill some canels or bomb some holes in the crater so that the gas or lava can leak out slower in stead of rushing out quickly and hence reduce the scale of volcanic eruption?
I will be using for an example in this answer the collapse of the lava dome at Mount St Helens in 1980. The lava dome on top of the mountain grew between March 20 and May 18, and then a landslide caused by the angle of the top of the dome exceeding the angle of repose of the country rock caused the pressure on the magma chamber underneath to be released, causing it to instantly boil and explode. Your suggestion to relieve pressure by either drilling or bombing would have had the same effect. Drilling or bombing a hole in this dome would have caused the pressure to greatly reduce, making all of the dissolved gases in the chamber to exsolve out, causing the pressure to instantly greatly increase, over what the surrounding rock was capable of containing.
So by bombing or drilling a magma chamber that might be about to explode, you run the very real risk of guaranteeing that it will explode. The precursors earthquakes to the Mount St Helens eruption would have been the signal that something was wrong you would use to justify this intervention, and as a result you would have simply advanced the timetable of eruption.
A good analogy is that of a radiator cap on a hot car engine. The coolant inside the radiator might be sitting at 20 or 30°C over its boiling temperature, but because it is under higher pressure due to the heat and being pumped around, it has not boiled. Once that pressure is relieved, it will instantly boil out of the radiator because of the reduction in pressure.
The general answer is no.
There are two critical factors that govern how a volcanic eruption behaves; the pressure of the magma and the type of magma.
Regarding the types of magma, this was asked and answered in the question, Is the magma in one volcano different from the magma in every other volcano?
With current volcanoes there are three types of magma: basaltic, andesitic and rhyolitic.
- Basaltic magmas are hot, have low viscosity, low gas content and flow relatively smoothly.
- Andesite magmas are mid temperature magmas that have moderate viscosity and gas content and can be moderately explosive.
- Rhyolite magmas are lower temperature magmas with high viscosity and high gas content and they erupt explosively.
For magma to rise to the Earth's surface in a volcanic eruption it can only do so because of the pressure the magma is under. The higher the pressure, the more quickly the magma rises and the more energy it contains and the more explosive it can be.
The other factor affecting the explosive nature of magmas is the gas content they contain. Deep underground, the gases are also under the same pressure as the magma. As the magma reaches the surface the gas within it decompresses causes the gases to expand. The expansion of the gases makes the magma explosive.
Yes, but no.
It highly depends on the circumstances. As Fred said in his answer, there are several types of magma. Some might flow almost like water while others might fragment and explode. You really want to know what kind of magma is inside the magma chamber before you release it.
A possible thing to do, for example in Hawaii, is to force the magma to flow to the ocean or to a place where it will not flow over things we care about (houses, roads, people, etc). This might not always be possible, based on geometrical and topographical constraints. In areas where the magma is explosive, this might actually be a bad idea. It might not even work - I asked a friend of mine who deals with this kind of stuff and he said that if the hole is not wide enough (= at least several metres) magma will just flow in, solidify and plug the hole. Drill holes are usually centimetres wide, not metres. If you want to blow it up, you need quite a lot of stuff because this stuff might be deep: digging a hole severel metres wide 10s of metres deep is not something that's easy to do.
Another problem is that you don't always know what kind of magma is there. You can have a guess, you can estimate it based on geophysical techniques, but sometimes you just don't know until it erupts. Even if you do, we still don't entirely understand the physics of how and why volcanos erupt. This is an active area of research and you don't want to do something that's potentially hazardous without understanding how it works.
Layman speaking! I don't believe any of the above answers are correct. If answer one is correct that the amount of lava on top is causing increasing pressure on the gas underneath then releasing the lava on top by creating a flow should theoretically reduce the pressure on the gases resulting in a smaller explosion. Maybe drilling is not the answer due to type of magma filling the hole but certainly bunker bombs or some type of explosive could be used to control the flow and to relieve pressure? Most of these volcanos seem to be isolated from populations and therefore are open to any type of intervention. Why not blow the side of a volcano apart on the preferred side and see what happens?