A gas slug is a conglomerate of high pressure

Partial pressure

In a mixture of ideal gases, each gas has a partial pressure which is the pressure which the gas would have if it alone occupied the volume. The total pressure of a gas mixture is the sum of the partial pressures of each individual gas in the mixture....

 gas bubbles that forms within certain volcano

Volcano

2. Bedrock3. Conduit 4. Base5. Sill6. Dike7. Layers of ash emitted by the volcano8. Flank| 9. Layers of lava emitted by the volcano10. Throat11. Parasitic cone12. Lava flow13. Vent14. Crater15...

es, the agittation of which is a key driving factor in Strombolian eruption

Strombolian eruption

Strombolian eruptions are relatively low-level volcanic eruptions, named after the Italian volcano Stromboli, where such eruptions consist of ejection of incandescent cinder, lapilli and lava bombs to altitudes of tens to hundreds of meters...

s. They start out as small bubbles of gas

Gas

Gas is one of the three classical states of matter . Near absolute zero, a substance exists as a solid. As heat is added to this substance it melts into a liquid at its melting point , boils into a gas at its boiling point, and if heated high enough would enter a plasma state in which the electrons...

 inside of volcanic magma

Magma

Magma is a mixture of molten rock, volatiles and solids that is found beneath the surface of the Earth, and is expected to exist on other terrestrial planets. Besides molten rock, magma may also contain suspended crystals and dissolved gas and sometimes also gas bubbles. Magma often collects in...

. These accumulate into one large bubble, which starts to rise through the lava plume

Magma chamber

A magma chamber is a large underground pool of molten rock found beneath the surface of the Earth. The molten rock in such a chamber is under great pressure, and given enough time, that pressure can gradually fracture the rock around it creating outlets for the magma...

. Once this accumulated slug reaches the top of the column and comes in contact with air, it bursts with a loud pop because of the lower air pressure, throwing magma into the air in the typical lava arc of a Strombolion eruption. This type of eruption is episodic, non-damaging to its source vent, and one of the slowest forms of activity, with the ability to sustain itself for thousands of years. Although the effect of lava slugs is well understood, how they form is not well understood, but recent research suggests that they can form as deep as 3 km (2 mi) under the surface.