Chapter 1
Earth Science
Course: Elementary Education Multiple Subjects 7001
Subject: Elementary Education Science Subtest 7005
Chapter: Earth Science
1. Origin of Earth
The Earth formed about 4.6 billion years ago through a process called accretion within the early solar system. This process began in a rotating solar nebula composed of hydrogen, helium, and heavier elements from older supernova explosions. Gravity caused particles in the nebula to collide and stick together, forming planetesimals. These gradually merged into protoplanets, including Earth. The kinetic energy from collisions, radioactive decay of unstable isotopes, and gravitational compression caused the early Earth to heat up significantly, becoming partially or completely molten.
As Earth cooled, it underwent chemical differentiation. Denser materials such as iron and nickel sank toward the center to form the metallic core, while lighter silicate minerals floated upward to form the mantle and crust. This process resulted in the Earth's layered internal structure: crust, mantle, outer core, and inner core.