Complex sentence: They are situated above 400-500 metres and are mostly uninhabitable, because the water precipitating as rain or snow infiltrates quickly into the ground, which results largely in a desert surface of grey, black or brown earth, lava and volcanic ashes.
Split into simpler sentences: They are situated above 400-500 metres and are mostly uninhabitable, because the water coming down as rain or snow does not remain this high in the ground. It is therefore mostly a desert of grey, black or brown earth, lava and volcanic ashes.