The letters following each Review Question refer to the corresponding Learning Objective from the Chapter Opener. How have the Earth's interior, crust, and atmosphere changed since the planet first formed? (A)

The Earth's interior consists of layers: core (inner core and outer core), mantle, and crust. When the Earth first formed, it was made up of a mixture of materials. Over millions of years, these materials began to separate based on their density through a process called differentiation. The denser iron and nickel sank to the center to form the core, and lighter elements like silicates rose to form the mantle and crust. Over time, the core has been cooling and solidifying, with the inner core being solid and the outer core remaining as a molten layer.

The Earth's crust has also undergone significant changes since its formation. It is divided into large slabs called tectonic plates that float on the semi-solid mantle below. These plates are in constant motion, driven by heat flow and convection currents within the Earth's mantle. Throughout Earth's history, the movement of tectonic plates has caused continents to collide, drift apart, and be reshaped. These processes have formed supercontinents like Pangaea and led to the present-day arrangement of continents. The crust has also changed due to volcanic activities and erosion/rebuilding processes. Oceanic crust is thinner and younger than continental crust, as it constantly being formed at mid-ocean ridges and destroyed at subduction zones.

The Earth's atmosphere has changed dramatically since its formation. Initially, it was composed of hydrogen and helium, which gradually escaped into space due to their light weight. Volcanic activity released gases trapped in the Earth's interior, including water vapor, carbon dioxide, and small amounts of nitrogen, eventually forming our current atmosphere. Over billions of years, living organisms, especially photosynthesizing cyanobacteria, began to produce oxygen through the process of photosynthesis. This increased the level of oxygen in the atmosphere, transforming it into an oxygen-rich environment, enabling the evolution of more complex life forms. The atmospheric composition has and continues to change due to natural processes (e.g., volcanic emissions) and human activities (e.g., burning fossil fuels) that influence the levels of greenhouse gases like carbon dioxide and methane, as well as pollutants like smog and aerosols.