I was formed in a swamp near the sea. Sorry no molten rock, fire or ash -
just very very salty water!
I form when crusty salty sand overlays some swampy organic material.
When the sun comes out the surface becomes crusty and heats the underlying
organic material.
The hot salty water wants to burst out but is its blocked by the
surface"crust".
The underlyng organic mud gives of rotting methane gas and the underlying
very salty water expands as it gets hotter builds up pressure.
The combined pressures of the heated gas and expanding water squirt very
salty water through the crust forming a little volcanoe shape.
As a result methane gas is formed and hypersaline water squirts out
through the tin crust of salty sand making little salt volcanoes.
"Hypersaline" means very very salty. "Methane" is a gas created when
vegetation rots.
I "erupt" in little squirts, each time the underlying mud gets very hot.
So the volcanoe I make has layers in the crater.