Rockall is part of the Rockall Plateau, a 800 by 150 km fragment of the continental plate or a micro continent formed 300-270 million years ago when the supercontinent of Pangaea began to break apart. The pinnacle of Rockall islet is an eroded remnant of volcano most likely dating to 60-50 million years ago when North America and Greenland finally split from Europe. This period was characterized by extensive volcanic activity. The volcano was last active 55 million years ago. After the period of volcanism the plateau began to lower. The 3000m deep Rockall Though (a deepwater geological feature) separates the Plateau from the British Isles. A map of the underwater mountain ranges of Rockall plateau can be viewed here:http://bit.ly/15UtqEp.
Remarkably, geological features on the Rockall plateau are officially named after the geographic features of Middle Earth from Tolkien’s Lord of the Rings, for example the Edoras, Fangorn and Lorien banks, the Rohan and Gondor Seamounds and the Isengard Ridge.
In 1975 a new mineral called bazirite was found at Rockall which is chemically composed of barium and zirconium. Besides this Rockall is made up from peralkaline granite. Two km northeast of Rockall lies Helen’s Reef, a series of skerries which only submerge from the water sporadically. A skerry is considered a rocky island too small for habitation. As you can see there is a void in between the definition of a skerry and an islet (a very small island which is also too small for habitation).
Image: blog.geogarage.com. Rockall with a fishers boat and a whale in the left corner of the image.
British Geological Survey: http://www.bgs.ac.uk/
Sutherland D.S. 1982 Igneous rocks of the British Isles.
Holliday, N et al. 2006. Were extreme waves in the Rockall Trough the largest ever recorded? Geophysical Research Letters 33:5