A sonnet (14 lines) but with an unconventional structure… the structure is normal until a turning point (a volta) at Line 9 (..these words appear). This reflects how human structures can be destroyed or decay.
The iambic pentameter rhyme scheme is also disrupted or decayed.
First eight lines (the octave) of the sonnet: the statue is described in parts to show its destruction.
Final two lines: the huge & immortal desert is described to emphasise insignificance of human power