The granitic gneiss rocks throughout the entire coastline are home to large populations of invertebrates, fish and seaweed. These gneiss and the dunes form rich ecosystems, but are threatened by waterfront construction, holiday-makers and dune sports. Short-beaked common dolphins occur in large numbers in Northern Portugal. Dolphins show themselves by porpoising and bowriding along with boats just off the city of Póvoa de Varzim, where fishing activities occur. Alive or dead discarded fish while fishing can be easily caught by dolphins, which could help explain this relationship in the city's waters. The OMARE Marine Observatory also identified the Risso's dolphin and the Atlantic bottlenose dolphin in local waters. Stranded whales and dolphins and found dead occur with some frequency, natural causes, rough seas events, incidents with ships or fishing boats can explain these events. The city core is situated in a region dominated by old Hercynian granites, including Póvoa de Varzim Granite and Santo André Granite. A continuous area dominated by schist exists to the Northeast and vernacular architecture of the rural areas highlight the local geological diversity. Oldest rocks, from the Pre-Ordovician epoch, are found in the Schist-greywacke Complex between Rates, Laundos, and Estela. Wandering even further Northeast, there is shale of micaceous clay nature, occasionally quartzite, dating to the Devonian period, that may contain fossils. Climate Póvoa's climate is classified as a Warm-summer Mediterranean climate (Csb in the Köppen climate classification system), with gentle summers and mild winters, influenced by the Atlantic ocean. Temperature extremes recorded at Sá Carneiro Airport, records started in 1967, range from to . In Póvoa de Varzim proper, average temperatures oscillate between in winter and in the summer. The city's beach area averages between and and diurnal temperature variation there is very low, just . The remaining territory, including the northwestern rural beaches, swings around daily. Using 1971–2000 data, there are 33 days a year above and 9 days above . Heat waves can occur for an average of 10 days, yet it tends to be balmier than inland Northwestern Portugal, and it is used by inland Minho Province residents seeking to escape the summer heat in the valleys. The so-called tropical nights with minimum temperature above occur three days per year. There are no full-days below freezing, yet temperatures below can occur at the height of winter during the night and early morning. The city possesses a microclimate and is considered the