Valahnúkur is composed of tuff layers, pillow lava and breccia. The mountain was formed in a single eruption and shows evidence of the different phases of the eruption. The tuff forms in explosive eruption, while the pillow lava forms when the lava erupts under water. The mixture of lava rocks and hardened volcanic ash found in Valahnúkur is called tuff. Tuff forms when 1200°C hot lava rapidly cools in water. This forms glass fragments, as crystals do not have time to grow. The fragments quickly transform into tuff. Lower down in Valahnúkur is a diagonal layer of breccia. Breccia forms when slag or ash form angled slopes. Individual pillows or parts thereof roll down the slope, are covered with ash and form the breccia layer.
Pillow lava is one of the most common lava types on Earth, as it is the most common lava that erupts from the oceanic crust. These strange pillows form in eruptions under water or under glaciers. Such eruptions are often where the pressure is too high to allow steam explosions to occur. They can also form when there is little or no gas in magma that rapidly pushes out from flowing lava. A glassy coating forms over the pillows as the magma cools rapidly. They are often several metres in length but only 10-30 cm in diameter. When examining a cliff wall with a cross-section of such lava, each bulbous formation looks like a ball or a pillow. The pillow lava in Valahnúkur probably formed in a sub-glacial eruption.