THE BEHAVIOR OF GAS MOLECULES AND BUBBLES IN MAGMA

Gas is the main driving force behind volcanic eruptions. The external pressure on a blob of magma has a great deal to do with how gas within the magma behaves. Here we follow a blob of magma as it moves from deep depths (high pressure) to shallow depths (low pressure), and eventually the surface:

1. At high pressure, gas molecules are floating around in the magma separately from each other. You say that the gas is "dissolved" in the magma.

2. As the pressure begins to drop (the magma begins to rise), the gas molecules start to glom together ("nucleate") into clusters.

3. When the concentration of molecules in each cluster becomes high enough, it becomes an actual bubble of gas, surrounded by the magma.

4. The bubbles continue to grow in size, making up an ever-increasing percentage of the total volume.

5.At the point where the bubbles comprise 75% of the total volume, the magma disrupts into a spray of magma blobs (which, when they are erupted become pyroclasts) surrounded by a stream of gas.