Breiddalur central volcano is an ancient volcanic area above Breiddalur valley and Berufjordur, The southern side of the volcanic formation is highly colourful and the intrusions assume an irregular aspect, set with tephra-layers . The southern region of Breiddalur - the very centre of area - forms a basin where the excruciating heat has transformed the rock to such an extent that the basalt and the andestite have turned pale-green, making it problematic to tell them apart from the rhyolite.
This can be clearly detected at river Innri-Ljosa BlAgil ravine. The beforementioned chain of mountains was formed at a later date as the rhyolite wielded its way to the surface through the basalt layers, forming insertions on top of the massive tephra layers along the edges of the crater. Their remnants are visible in rhyolithic-rocks throughout the Breiddalur region. This central volcano is considered to be of later date than its Alfta- and Reydarfjordur counterparts.
The Reydarfjordur ignimbrite, a pyroclastic flow made as a burning hot mixture of particles and gases flew from the volcano, originated from Mt.Rondolfur, covering about 430 Sq kms with a diameter of about 6 m. thykkt. This layer is linked the Mt. Skessa south of Reydarfjordur, best known as the Skessulag. The immense volcanism of the area has resulted in a great variety of rare minerals and semi-precious stones on display at Petra's wonderful stone collection in Stodvarfjordur and The Breiddalur mineral museum in Breiddalsvik.