What does 'carnal' mean in geography?

The study of the Earth.

Geo mean "Earth" Graphy means writing in a subject. Geo+Graphy means Earth writing. Thus, Geography means Earth's map, topography, etc...

write about the earth

A lengend is the box in a corner of a map that tells you the different colours and names relating to the colours and names on the map telling you what country is which colour.

Yes, volcanoes are part of physical geography. Volcanoes are natural landforms and are not man-made. Thus, physical geography includes volcanoes.