by prof ulrich ofterdinger, QUB

Down to the core

Collecting rock samples from dolerite dykes across Northern Ireland

March 2026

Throughout the summer of 2025 a team from Queens University Belfast and GSNI, with the kind support from Northstone Materials and the Breedon Group, visited quarries across Northern Ireland to collect rock samples from dolerite dykes.   

what is dolerite?

Dolerite is a hard, dark grey, igneous rock, formed from magma. It is similar to basalt in composition but cooled more slowly beneath the Earth’s surface rather than being erupted above it. This  means you can see the individual grains and minerals, hence it is often used as a decorative stone. Along with its durable and heat resistant properties, this makes it a common material for countertops and fireplaces (sometimes called ‘black granite’). 

The dolerite forced its way into different host rocks (a process called ‘intrusion’) on its journey from a source of magma at depth towards the surface forming so-called ‘dykes’. These often sheet-like, and nearly vertical, dykes have cut across the Sherwood Sandstone Formation, which is one of the target rock formations in Northern Ireland with high geothermal potential.  Across County Antrim, the Sherwood Sandstone lies beneath the Antrim Basalts and the questions that we are trying to answer are:

(1) to what extent are dolerite dykes present in the sandstone? and

(2) are these nearly vertical intrusions likely to separate the sandstone into individual blocks? This could influence the movement of heat-bearing fluids through that rock volume.

To learn more about the location and geometry of the dolerite dykes, the QUB team together with colleagues at the University of Sao Paulo and the Campinas State University in Brazil, are analysing airborne geophysical data collected by the Tellus Project. The airborne measurements of the Earth’s magnetic field can provide insight into the nature of the rocks beneath the surface – we can identify the dolerite as it has different magnetic properties compared to its sandstone host.  

This magnetization was preserved in the dolerite as it cooled many millions of years ago. Physical core samples of the rock collected from quarries in Northern Ireland will be analysed in a laboratory in Sao Paulo to measure its magnetic properties and the resulting data will be used to ground-truth the airborne data. The Tellus data can then be interpreted more accurately across a wider regional area and provide more information about the structure and connectivity, and therefore geothermal potential, of the rock layers beneath the surface.

Image: Quarry fieldwork. The blue arrow indicates an 8-metre-wide dyke (seen as a band of dark rock); inset: Prof Ulrich Ofterdinger.

Queens University Belfast is a GEMINI partner.