Why Magma Rises From the Mantle to the Crust
The primary reason magma rises from deep in the mantle is buoyancy. If you were to drag an inflated balloon to the bottom of a swimming pool and release it, it would float towards the surface due to the fact that it is far less dense than the water around it, making it buoyant. This is essentially the same reason why magma rises: molten rock is less dense than the cooler non-molten rock around it.
As blobs of magma (known as Diapirs) push upward into the upper crust, the buoyancy will reach equilibrium, causing the magma to stop migrating upward.
In many instances, the magma will permanently stall in the crust at various depths, slowly hardening to form what is known as a batholith (a large intrusive body of igneous rock that never reaches the surface).
Why Magma Rises From the Crust to the Surface (Eruption)
As we just learned, magma tends to stall in the crustal layers, so what would cause it extrude onto the surface to form volcanoes? The secret lies in pressure.
- As the magma rises and accumulates, more and more magma will get pushed into the magma chamber. This alone will build up some pressure.
- As more magma is added into the magma chamber, more volatile gases that are saturated in that magma are also added to the magma chamber.
- Eventually, the magma in the magma chamber will slowly start to form hardened crystals as it cools. Those crystals form from various minerals (such as Olivene) and sink to the bottom, leaving behind the dissolved volatile gases. This process leaves a greater ratio of volatiles to liquid magma, increasing overall pressure.
- Once the magma chamber reaches a critical threshold where the pressure of the magma chamber is greater than the strength of the rock surrounding it, it will form conduits to try to expand and relieve that pressure.
- These conduits will often expand towards the surface. If the conduit is large enough and expands upward enough (as a result of a LOT of pressure), an extremely important change will take place.
Exsolution of Volatile Gases
The explosive shockwave in this small eruption video is created due to the explosive effect of gases coming out of solution, and expanding rapidly.
Once the magma moving upward in a conduit is shallow enough, the surrounding pressure will drop dramatically.
This dramatic drop in pressure changes the nature of the dissolved volatile gases, causing them to come out of solution. This is essentially where you reach the point of no-return for an eruption, as a the rapid exsolution causes dramatic expansion and pressure increase, often as much as 50x the volume increase or more. This increase in pressure and volume rapidly forms an explosive eruption, forcing magma to the surface in sometimes violent fashion.
The explosivity of the eruption will be dependent on how much volatile gases play a role. In volcanoes that produce less explosive eruptions such as Hawaii, there is not a high degree of volatile gases in the magma composition. On the other hand, volcanoes such as Krakatoa have a high volume of volatile gases, causing eruptions to be rather violent in nature.
The Role of Crustal Strength
Another less discussed factor is the role of crustal strength. The stronger and thicker the crust over a magma chamber, the greater the resistance will be to preventing an eruption. In simple terms, you need a lot more force to break through an extremely thick sheet of crust than you would a weak and thin sheet of crust.
As such, volcanoes in areas with thick strong crust or deep magma chambers sometimes require more force to reach the surface, resulting in stronger eruptions.
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