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This layer of hard, plate-like rock is called a plate.
Literal
This plate-shape [genitive-の] hard rock [genitive-の] layer [object-を] plate [quotative-と] call.
A geology-textbook definition. 板状 ('plate-shaped,' 'tabular') uses 状 ('shape, form') as a noun suffix to form '-shaped' adjectives — productive pattern: 円状 (circular), 棒状 (rod-shaped), 液体状 (liquid-state). The genitive の connects 板状 to the noun it modifies. ~を~と呼ぶ is the standard 'to call X Y' construction, using と as the marker of the name/label being applied. This sentence defines the concept of tectonic plates — 'plate' is a loanword used in the Japanese geology vocabulary.