After laying dormant for 12,000 years, the Hayli Gubbi in northeastern Ethiopia erupted on Sunday.
But instead of hot, molten lava flowing down the mountain side, thick plumes of ash and gaseous matter dominated the eruption.
The difference in whether a volcano erupts with flowing lava or violent bursts of ash is determined by these ----
Magma Viscosity ( thickness )
Viscosity is fluid's resistance to flow. It says how " sticky " it is.
It is primarily controlled by magma's silica ( SiO2 ) content.
Low viscosity gives " Runny magma " having less silica.
High viscosity gives " Sticky magma " having high silica.
Magma contains dissolved gases ( mostly water vapour and carbon dioxide ).
As magma rises, pressure decreases. These gases try to escape and form bubbles.
In low viscosity magma, magma flows out as lava.
But in high viscosity magma, sticky magma traps the gas bubbles, preventing them from escaping.
The pressure builds up immensely.
When the pressure exceeds the strength of surrounding rock, the eruption is violent and explosive.
It shatters the magma into tiny pieces of rock, mineral and glass called volcanic ash.
While Hayli Gubbi is built from low viscosity magma, it was dormant for 12,000 years !!!
This dormancy developed a high gas pressure and resulted in explosive eruption that produced a large ash plume.
Now the ash cloud has surpassed Himalayas and moving towards Japan !!!
