The riverbed of the Porto river, approximately 2 km east of the town Ota, exposes beautiful examples of magma mingling. This term denotes the process of physical mixing of different magmas in a (partially) molten state. Magma mingling produces intriguing structures in the rocks, but also subordinate chemical mixing can be observed, e.g., via propagating reaction fronts.
The best outcrops are located below the old stone bridge, 300 m downstream from the new street bridge. Parts of the riverbed expose rock surfaces polished by the river. The first main lithology is a light gray to pinkish granite, the second is a dark gray, fine-grained gabbro, a dolerite. The contacts between the lithologies are irregular and lobate, yet sharp, which indicates that the two magmas were at least partially molten when they came in contact. The granite usually forms embayments or back-intrusions into the round dolerite bodies. The dolerite often becomes very fine-grained at the contact to the granite, which may be interpreted as chilled margins. This means that a hot gabbroic melt intruded into a cooler, but still-molten granite magma chamber.
Though immiscibility and the viscosity contrast of the two melts largely prevented their homogenization, chemical mixing can be observed: In some places, the dolerite and granite are separated by a thin layer of hornblende-bearing diorite, a rock of intermediate composition that indicates a mixing reaction between the two melts. Apart from these local zones of mixing, the two magmas kept their original chemical composition.
The assemblage is sharply cross-cut by fine-grained mafic dykes similar to those at Bussaglia beach. This late intrusion event clearly postdates the cooling and crystallization of the intermingled granite and dolerite.