The Hadean eon was the first eon on Earth and lasted from about 4.6bya to 4.0bya. The Hadean eon contains the Cryptic, Basin Groups, Nectarian, and Lower Imbrian eras. The Moon formed 4.5bya during the Cryptic era and is the main focus of scientists studying that era. The Earth was extremely hot due to constant volcanic eruptions and collisions with other bodies in space. The first rocks and minerals also formed during this time after the constant lava flow on Earth cooled. The Earth is thought to have cooled by this time because zicons dating 4.0 billion years old have been found. Hadean comes from the Greek god Hades, since the planet was covered in lava and volcanoes.