The National Geographic special continues. Geological edition.  This was a day dominated by a 10 mile (16km) hike to Isabela Island’s Sierra Negra volcano. It probably comes as no surprise to say that the Galapagos archipelago was formed mostly as a result of volcanic activity, but what makes Isabela unique is that unlike every other island here, which only have one volcano each, Isabela has six. Three of them are still active. Sierra Negra last erupted in 2018.  It is the 2005 eruption however that nearly eradicated the giant tortoises from the island, but more specifically from the area around Sierra Negra. And while It’s estimated that some 800 of…