dyke

laccolith

lava flow

sill

trap

volcanic neck

Archibald Geikie's (1887) line drawing of a volcanic neck. The sketch shows the geology of the neck and and its relations to the original and current land surfaces

 

 

 

Partan Craig, North Berwick. A headland formed in a Carboniferous vent filled with agglomerate which stands proud of the weak tuffs of the shore platform

 

 

 

 

Detail of the vent aggom,erate at Partan Craig.

The field guide records that the agglomerate contains blocks of tuff, sandstone and siltstone, as well as bombs of dark basanite

Volcanic vents

Definition: steep, sub-volcanic tubes filled with fragments of both igneous rocks and wall rocks

Volcanic vents or diatremes form by explosion as a consequence of the release of contained carbon dioxide gas (CO2) and water vapour (H2O) near the surface. The vent may then be filled by ash or later sediments.

The Carboniferous rocks of East Lothian includes many vents, especially around North Berwick and Dunbar. The vents tend to form small hills but the vent exposed on the shore platform at Scoughall has no topographic expression. The vents represent the feeder tubes of volcanoes that erupted either though wet sediment or into shallow lagoons.