IGH11 - Igneous Intrusions Theme
Most volcanic rocks in Ireland are considered under the relevant stratigraphical theme in which they occur, so for example, the Ordovician volcanic rocks of Wexford and Waterford are assessed in the Cambrian to Silurian Theme, IGH4. However, some igneous rocks are intrusive, that is they penetrated existing rocks as molten magma, but were not erupted at the surface. Moreover, they may have occurred at a much later time than the rocks that host them. This theme examines these rocks, where they were not formed at the same time as the sedimentary rocks that host them.

There are two main types: the major bodies such as granites, and the minor features such as sills and dykes. When the expert panel for this theme meets they will be considering such rocks as the Wicklow Granite. This was formed 400 million years ago and is actually made up of several different phases of crustal melting and intrusion of massive bodies, which now form the Dublin and Wicklow Mountains.

They will also look at various dykes, narrow sheets of igneous lave which cut across any structural or sedimentary features of pre-existing host rocks. They usually relate to periods when the Earth's crust was under tension and large scale fractures were filled by molten lava from depth. In Northern Ireland the earliest rifting associated with the opening of the Atlantic Ocean caused major outpourings of lavas in County Antrim. Across other parts of Ireland, dykes of Tertiary age (around 60 million years ago) are known and these are the kind of geological feature to be examined in this theme.