The theory you are referring to is likely the theory of plate tectonics, which was developed in the 1960s and 1970s, building upon Alfred Wegener's earlier theory of continental drift from 1912. Wegener proposed that the continents were once part of a single supercontinent called Pangaea, which eventually broke apart and drifted to their current positions.
The modern theory of plate tectonics explains that the Earth's lithosphere (the rigid outer layer) is divided into several tectonic plates that float on the semi-fluid asthenosphere beneath. These plates interact at their boundaries, leading to geological activity such as earthquakes, volcanic eruptions, and the formation of mountain ranges.
In summary, the "blank" you are looking for is likely "plates," as the theory of plate tectonics involves the movement and interaction of these tectonic plates.
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