Almost all volcanic activity involves magmas based on silicates, so the existence of igneous rocks containing over 50 per cent carbonates is surprising, When first discovered in the early 20th century, these carbonate rocks, called carbonatites, were thought by geologists to be simply limestones moulded by the heat of silicate magmas, especially as many carbonatites occur interleaved with silicate rocks in volcanic structures called complexes. In fact, carbonatites originate as magmas in the mantle, just like other
magmas.