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Webster 1913 Edition

(kr?st′al)

Adj.

Relating to a crust.

crustal ‎(comparative more crustal, superlative most crustal)

Of, pertaining to, or forming a crust, especially the crust of the Earth or other planet etc.
- 2003, Bill Bryson, A Short History of Nearly Everything, BCA, p. 189:
  The hope was to lower a drill through over 4,000 metres of Pacific Ocean water off the coast of Mexico and drill some 5,000 metres through relatively thin crustal rock.