Summary of site:
500 m north of Port Moon a mass of Lower Basalt 9 m high stand on the rock platform at high water mark. It contains the only certain example of a lava tube known in Northern Ireland.
Lava tubes are formed at a late stage of fissure eruption when the rate of lava flow can no longer sustain movement along the entire lava front. In these circumstances some of the lava slows to a near standstill and forms almost static “ponds” usually with a solid surface crust as heat is lost to the atmosphere. Fresh lava is then either confined to channels over the surface of the flow or, often, it flows beneath the crust in channels with oval to sub-circular cross sections. Fresh lava in this setting is capable of mobilising ponded lava and can establish sub-surface courses several kilometres in length in rare cases where the flow is strong. In the last phases of eruption the lava begins to cool and congeal onto the walls of the tube and eventually solidifies and obliterates it. Sometimes it can drain out of the distal end leaving an open tube in the form of a lava cave.
The tube at Port Moon is completely filled with solidified lava. At its seaward end half has been eroded away but the complete rod form, 9 m in diameter, can be seen trending west south west inland. The base of the tube coincides with the base of a flow, the normal relationship in an active lava field. A series of concentric joints at roughly 20 cm intervals is also a feature of this structure.
Lava tubes form more commonly in viscous lavas such as trachytes and rhyolites so this example in a basaltic “runny” lava is rare.
The feature was first recognised by an amateur geologist, Peter Millar of the Belfast Geologists’ Society and indicates the important contributions that can still be made at this level.
As the only known example of a lava tube in Northern Ireland, this site should be offered the highest level of protection possible on an exposed shoreline. Access is difficult and potentially dangerous.