Mpyupyu is a small conical hill rising from recent sediments fringing the southwestern shores of Lake Chilwa. It consists of hornblende-biotite syenites, some of which contain a little quartz. There are rare pyroxene cores to the amphibole which is in turn mantled by biotite. Associated minor intrusions are microsyenite. Garson (1960) considered that a solvsbergite dyke outcropping near the foot of the hill is a ring dyke. Immediately south of the hill are several arcuate outcrops of pyrochlore-bearing aegirine-riebeckite syenite containing up to 1% pyrochlore, which is radio-active; Garson (1960) gives an analysis of the riebeckite.