dyke

laccolith

lava flow

sill

trap

vent

volcanic neck

 

USGS diagram of the main structural landforms developed in igneous rocks

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Basalt lavas of the Garleton Hills

 

 

Structural Forms: Igneous Rocks

Definition: landforms adjusted to structures within igneous rocks

Sparks (1971) has provided a useful classification of igneous masses for geomorphologists.

 Major Intrusions  Concordant  Lopolith
 Discordant  Batholiths with associated bosses and stocks
 Minor Intrusions  Concordant  Sills, laccoliths, phacoliths
 Discordant  Dykes, ring complexes
 Major Extrusions  Lava, ignimbrite and ash plateaux
 Minor Extrusions

 Lava flows, cones, ash beds, volcanic necks and  plugs

The igneous rocks may give primary and secondary landforms. Primary landforms are a direct result of active or recent igneous activity, notably volcanism. Secondary landforms are a result of differential weathering and erosion of igneous rocks, often long after igneous activity has ceased.

In East Lothian and Edinburgh, volcanic activity ceased around 300 million years ago so all the igneous features give secondary landforms.