In the Lesser Sunda Islands there are two geologically different groups of islands. The northern group is volcanic. It is made up of Bali, Lombok, Sumbawa, Flores and Wetar. Some of the volcanoes, like Mount Rinjani on Lombok, are still active. However, there are some other volcanoes, such as Kelimutu on Flores, which are extinct. The northern group of islands started forming during the Pliocene, about 15 million years ago. They formed because of the collision between the Australian and the Asian tectonic plates.