"A geologist's song 03" joke

The Marginal Basin Song by Chris Stillman
(melody: Lead us on, thou Heavenly Father)

On a margin runs a canyon down into the ocean dark;
There's a basin slowly filling with detritus from the arc.

Refrain: For the drifting causes rifting,
Opens basins mighty fine
Which strike-slip will close in time.

With volcanics there's no problem; they're erupting all the time;
Fill the thin with pillow lavas, sheeted dikes and serpentine.

Rising slowly from the ocean filled with gritties coarse and fine,
Are you fore-arc? Are you anti-arc? Are you just a geosyncline?