The Galapagos National Park says La Cumbre volcano began spewing lava, gas and smoke on uninhabited Fernandina Island on Saturday after four years of inactivity.
The park says in a statement the eruption is not a threat to people living on nearby Isabela Island.
But it says lava flowing to the sea will likely affect marine and terrestrial iguanas, sea lions and other fauna.
The Galapagos are home to unique animal and plant species that became the basis for Charles Darwin's theory of evolution.
Scientists say Fernandina is the island with the most volcanic activity in the archipelago. La Cumbre last erupted in May 2005.