Cross-section of Agathla volcanic neck, Navajo Volcanic Field, Arizona from Volcano World. The process of exhumation is similar to that for North Berwick Law except that here the volcano last erupted at ~340 Ma.

crag and tail

laccolith  lava flow  trap  dyke  sill  vent

Volcanic neck

Definition: a hill resulting from differential weathering and erosion between the former feeder tube of a volcano and its surrounding rocks.

A volcanic plug, also called a volcanic neck or lava neck, is a volcanic landform created when lava hardens within a vent on an active volcano. When forming, a plug can cause an extreme build-up of pressure if volatile-charged magma is trapped beneath it, and this can sometimes lead to an explosive eruption. If a plug is preserved, erosion may remove the surrounding rock while the erosion-resistant plug remains, producing a distinctive landform.

North Berwick Law, pictured above, is an exceptional example. The neck is composed of phonolitic trachyte, a red-mottled and fine-grained resistant igneous rock.  The original superstructure of the Carboniferous volcano has been entirely lost to erosion. Glacial erosion has created a magnificent crag and tail.