Earth was formed and grew by colliding with space debris. Many small pieces stuck together one after another until a large body, Earth, was formed. Gravity pulled in more debris afterwards and some of the collisions released so much energy that part of the planet's surface could have been melted. Its surface was covered with lava and volcanoes. Heat was also being produced from radioactive materials in Earth's core. Earth's age was determined by studying sedementary rock in the Earth's crust.