Final answer:
Iron and Nickel settled in the Earth's core as they are denser, while Aluminum and Silicon formed the crust because they are lighter. This separation occurred due to planetary differentiation during a molten phase of Earth's early formation.
Step-by-step explanation:
The reason Iron and Nickel settled in the core while Aluminum and Silicon settled in the crust is because Iron and Nickel are heavier and have a higher density compared to Aluminum and Silicon. During the early stages of Earth's formation, the planet was hot enough to melt, allowing for the process of planetary differentiation. This process caused denser materials, such as Iron and Nickel, to sink towards the core due to gravity, while lighter elements like Aluminum and Silicon rose towards the surface, consequently forming the crust. The Earth's core, which is 85% Iron with Nickel making up much of the remaining 15%, is more dense than the Earth's crust that predominantly consists of Silicon Oxide and Aluminum Oxide.
The outer layer of the Earth, or crust, is considerably less dense than the core and is made up of lighter silicates, Oxygen accounting for 94 percent of the crust by volume. In contrast, the core comprises a dense alloy of Iron and Nickel, with the mantle in between having an ultramafic composition containing more Iron and less Aluminum and Silicon than the crust. Both the inner core, composed of solid Iron and Nickel, and the semi-liquid outer core contribute to Earth's magnetic field while the mantle's composition affects its viscosity and behavior.