Skip to main content

Concordant versus discordant igneous bodies

Short answer

Concordant igneous bodies like sills and laccoliths are parallel to bedding; discordant bodies like dykes and batholiths cut across it. Concordant are sheet-like and conformable; discordant are cross-cutting and unconformable.

Distinguish between concordant and discordant bodies of igneous rocks (forms of igneous rock).

Intrusive igneous bodies are classified by their relation to the surrounding country rock.

BasisConcordant bodiesDiscordant bodies
Relation with country rockParallel to the bedding or foliationCut across the bedding or foliation
Magma pathSpreads between layersForces its way through layers
ShapeSheet-like or lens-like, parallel to layersIrregular or vertical, cross-cutting
TypesSill, laccolith, lopolith, phacolithDyke, batholith, stock, volcanic neck
ContactConformableUnconformable
ExampleSill, laccolithDyke, batholith
 Concordant (sill)      Discordant (dyke)
 ==================      ===|##|=====
 ##################      ===|##|=====
 ==================      ===|##|=====
  • Sill: horizontal sheet between beds. Laccolith: lens with domed top. Dyke: vertical sheet cutting beds. Batholith: huge irregular mass, over 100 km², of granite. Stock: smaller version of batholith.

✓ Answer checked by Claude Opus 5.5 (high effort)

Related questions

All Engineering Geology I chapter 4 questions →