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 |
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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.
