The text 'СФЕРА — ТВЕРДАЯ' is visible. What does this translate to in English, and what is its relevance to the map?

The text 'СФЕРА — ТВЕРДАЯ' translates to 'SPHERE — SOLID' or 'SPHERE — HARD'. This likely refers to the Earth's lithosphere, which is the rigid outer part of the Earth, consisting of the crust and upper mantle. Its relevance to the map is that it's depicting tectonic plates, which are part of this solid sphere.