main idea. A. Early Earth's lack of an atmosphere explains why it was bombarded with much more frequency and violence than other planetesimals. B. Continued bombardments and internal pressures made the growing Earth hotter, causing its interior to melt and the heavier elements to sink and form Earth's core. C. Lighter elements from Earth's interior rose and formed the mantle, a denser layer of silicates around the core, and the crust, a thinner layer of silicates at Earth's surface. D. The formation of Earth's crust protected the inner layers of Earth from the high-energy particles in space, reducing the temperatures of the mantle and the core. E. Once Earth had gone through the final stages of its formation, gases bubbled to the surface and were held by Earth's gravitational field to form the atmosphere. F. Volcanoes today are the result of gases that were trapped in Earth's interior during the planet's early stages of formation.