Hvitserkur mouintain icelandThe environs of Borgarfjörður are accounted Iceland's second largest rhyolite area. One extinct central volcano is located in Njarðvík and another at the landward end of Borgarfjörður; both have been carved apart by Ice-Age glaciers. Unique geological formations are frequent, and the area is famous for its attractive minerals; however, the taking of rocks is prohibited without special permission from the property owner.

North of Borgarfjörður by the inlet of Njarðvík, it is easy to view the marvellous rhyolite of Innra-Hvannagil canyon, where numerous dark basalt dikes crisscross the bright slopes. It is the mountain colours and forms which visitors first notice upon arrival in the area.

dyrfjoll mountainsWest of Borgarfjörður, most mountains are dark basalt, with Dyrfjöll ("the door mountains") highest and most splendid. East of the fjord, the mountains are much lighter, being mostly composed of rhyolite with dark basalt intrusions, although they change sharply in colours by weather, above all if wet, when their hues become significantly darker and more distinct.