igneous clusters are formed by conditions heat / viruses
heat_of_formation is heat. rubella_virus is rna_virus. rift_valley_fever_virus is viral_agent. heat_of_formation is not heat. bunyaviridae_virus is rna_virus. rna_virus is virus. rift_valley_fever_virus is bunyaviridae_virus. mumps_virus is rna_virus. groundmass is igneous_rock.