The island of Nisyros is exclusively composed of volcanic rocks of which the large variety in its composition and forms renders the island of Nisyros an open- air geologic museum. Their kind and composition depend on the magma's composition feeding the effect generating these rocks. The initial magma feeding the volcanic effect in Nisyros is called by the petrologists alkaline limestone basalt. The surfacing of the magma in a brief period of time results in the appearance of basaltic laves.



In case it remains for a long period of time in the crust, the percentage of the silicon dioxide, the potassium and the sodium varies. Thus, the basalt becomes andesite, dacite and finally rhyolithe.



The structure of the volcanic rocks is composed of amorphous glass and by mineral crystals.

The mineral crystals residing in Nisyros' rocks are:



 Liparites (minerals of silicon dioxide, argyle, calcium, sodium and potassium in the form of white limpid crystals)

 Pyroxenes (minerals of silicon dioxide, calcium, ferrum magnesium in the form of dark green to black crystals)

 Hornblendes (minerals of silicon dioxide, calcium, ferrum magnesium, sodium and of titan in the form of dark green to grey crystals)

 Olivines (minerals of silicon dioxide, ferrum and magnesium in the form of honey to green- grey crystals).

The speed of the frigidity and the chemical composition of the rocks determine their colour (white- ash, grey, green to black).
The gases residing in the magma determine the final form of the volcanic rocks. Thus, we have the following kinds of rocks:
 Rust(spongy form and characteristic black or dark red colour)



 Pumice (=pumice rock, white rock composed of blown glass and some crystals).It does not exist in Nisyros but in the islet Gyali.

The deteriorated rocks created from an intense hydrothermal activity constitute a special group of rocks in Nisyros.



Finally, the scarn is a type of a particular kind of rock and fairly rare. Its colour can vary from light green to olive green or ruddy to black in relation to the crystals that compose it. These crystals are mainly pyroxenes, olivines, epidotes, spinels and garnet. The scarn is created by the time that the melted rock penetrates in limestone and marble.