Origin and Evolution of the Earth

The One Thing

The Earth formed about 4.6 billion years ago from material in the early solar nebula and developed through successive cosmic, geological and biological changes. Cooling, differentiation, atmospheric and oceanic development, and the evolution of life transformed an initially hot, nearly uniform body into a layered, water-rich planet capable of supporting a biosphere.

Chronology

Who and What

• Universe: The totality of space, time, matter, energy, galaxies, stars and other celestial bodies.
• Big Bang Theory: The theory that the universe began from an extremely hot and dense state and has been expanding ever since.
• Galaxy: A huge system of stars, gas, dust and other matter held together by gravity.
• Milky Way Galaxy: The spiral galaxy containing the Solar System and a very large number of stars, including the Sun.
• Solar System: The Sun and all objects revolving around it, including eight planets, their satellites, asteroids, comets and meteoroids.
• Nebular Hypothesis: The explanation that the Sun and planets formed from a rotating cloud of gas and dust called a nebula.
• Planetesimals: Small solid bodies formed when particles in the early solar nebula collided and joined together.
• Accretion: The gradual growth of a planet through the gravitational attraction and collision of smaller particles and bodies.
• Differentiation: The separation of Earth’s materials according to density, with heavier materials sinking toward the centre and lighter materials rising outward.
• Crust: The thin, outermost solid layer of Earth.
• Mantle: The thick layer between the crust and core, made mainly of dense silicate rocks.
• Core: The innermost layer of Earth, composed mainly of iron and nickel.
• Outer core: The liquid part of the core.
• Inner core: The solid, central part of the core, largely because pressure increases greatly toward Earth’s centre.
• Atmosphere: The envelope of gases surrounding Earth.
• Primitive Atmosphere: Earth’s early gaseous envelope, which differed from the present atmosphere and contained gases released mainly through volcanic activity.
• Hydrosphere: All water present on Earth, including oceans, rivers, lakes, groundwater, glaciers and water vapour.
• Lithosphere: The rigid outer part of Earth, consisting of the crust and uppermost mantle.
• Biosphere: The narrow zone where land, water and air interact to support life.
• Big Splash or Giant Impact Hypothesis: The widely accepted explanation that the Moon formed from debris produced when a large planetary body collided with the early Earth.
• Photosynthesis: The process by which green plants use sunlight, carbon dioxide and water to produce food and release oxygen.
• Radiometric dating: The method used to determine the approximate age of Earth through the dating of rocks and meteorites.
• Inner planets: Mercury, Venus, Earth and Mars; they are mainly rocky and metallic.
• Outer planets: The generally larger planets, which are rich in gases or ices.
• Astronomical unit: A unit equal to the approximate distance between Earth and the Sun: 149.6 million km.
• Earth’s dimensions: Earth has an average radius of about 6,371 km and an average diameter of about 12,742 km.

Causes and Consequences

• The Big Bang Theory explains the origin of the universe as an extremely hot and dense state followed by continuing expansion. This provides the wider cosmic setting in which galaxies, stars and planets developed.

• The Milky Way Galaxy contains the Solar System. The Solar System consists of the Sun, eight planets, their satellites and smaller bodies such as asteroids, comets and meteoroids.

• Under the Nebular Hypothesis, a rotating cloud of gas and dust collapsed to form the Sun and planets. Collisions among particles produced planetesimals, and gravitational attraction caused accretion, leading to the formation of Earth about 4.6 billion years ago.

• Collisions, compression, radioactive decay and gravitational energy made the early Earth extremely hot. As it cooled, differentiation caused iron and nickel to sink towards the centre, producing the core, while lighter materials formed the mantle and crust.

• Earth’s present internal structure consists of the crust, mantle, outer core and inner core. The outer core is liquid, whereas the inner core is solid, largely because pressure increases greatly towards the centre.

• Volcanic eruptions and the release of gases from Earth’s interior formed the primitive atmosphere. Impacts and later biological activity also contributed to atmospheric development.

• Cooling caused water vapour to condense into rainfall, helping to form oceans and other parts of the hydrosphere. The development of liquid water was essential for the emergence and continuation of life.

• Photosynthesis used sunlight, carbon dioxide and water to produce food and release oxygen. As photosynthetic organisms developed, free oxygen became abundant, changing Earth’s atmosphere and contributing to the later evolution of life.

• The interaction of land, water and air established the biosphere. Together with the lithosphere, atmosphere and hydrosphere, it created an integrated physical setting capable of supporting living organisms.

• Earth’s location approximately 149.6 million km from the Sun, or 1 astronomical unit, contributes to a suitable temperature range. Liquid water, a protective atmosphere, solar energy and the chemical elements required by living organisms together make Earth favourable for life.

• The Big Splash or Giant Impact Hypothesis explains the formation of the Moon as the result of debris produced when a large planetary body collided with the early Earth. This illustrates that major impacts contributed to the development of the Earth–Moon system.

What Gets Asked

• Explain how the Nebular Hypothesis, planetesimals and accretion account for the formation of the Solar System and Earth.
• Explain why the early Earth was hot and how differentiation produced the crust, mantle, outer core and inner core.
• Compare the inner planets—Mercury, Venus, Earth and Mars—with the larger outer planets rich in gases or ices.
• Describe the formation of the primitive atmosphere and hydrosphere, including the roles of volcanic activity, cooling, condensation, impacts and biological activity.
• Evaluate the importance of photosynthesis in changing Earth’s atmosphere and supporting the development of the biosphere.
• Explain why Earth’s position in the Solar System, liquid water, atmosphere, suitable temperature range, solar energy and chemical elements make it capable of supporting life.