Dome mountains (Laccoliths) are formed by volcanism. However, unlike volcanoes that erupt to the surface, laccoliths form when magma is injected between two layers of sedimentary rock, causing the overlying strata to bulge upwards (much like a blister on your hand), forming the dome shape.
Eventually this magma cools and hardens, and if sufficient erosion takes place of the overlying strata the great igneous dome will be revealed. A great example of a dome mountain is Bear Butte in South Dakota.
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