The elemental plates raises the power of moves of the same type by 20%, not 50% as Gray Test said. So if a Dugtrio used Earthquake while holding the Earth Plate, it would do 120 Base Power (without STAB).
A Plate (Japanese: プレート Plate) is a type of held item introduced in Generation IV, primarily for use in changing the type and color of Arceus and the type of its signature move, Judgment to the corresponding type of Plate. However, it will also increase the attack power of moves of the Plate's type used by any Pokémon holding it by 20%. - Bulbapedia