Earth was born about 4.6 billion years ago. From its birth to the present, Earth has undergone dramatic evolution. The environment of early Earth was completely different from that of today.
Basic Knowledge
About 4.6 billion years ago, collisions and accretion of planetesimals from the solar nebula formed the proto-Earth. Heat from collisions melted the surface, forming a magma ocean (an ocean of molten rock).
Dense iron sank toward the center to form the core, while lighter materials formed the mantle. This process is called differentiation of the magma ocean.
Around 4.4 billion years ago, Earth began to cool, and water vapor condensed to form the primordial ocean. At the same time, the primitive atmosphere consisted mainly of water vapor, CO₂, and N₂, with very little oxygen like that in the present atmosphere.
The oldest life is thought to have appeared about 4.0〜3.8 billion years ago. The first life forms were anaerobic bacteria. Later, photosynthetic organisms (cyanobacteria) began releasing oxygen into the atmosphere around 2.7 billion years ago, leading to the Great Oxidation Event (about 2.4 billion years ago).
Proto-Earth (Early Earth formed by the accretion of planetesimals. Its surface was a magma ocean)
Magma ocean (A state in which the entire surface melted from heat generated by planetesimal collisions)
Differentiation of the core and mantle (Iron sank toward the center because of density differences, while lighter rock formed the mantle)
Primordial ocean (Formed when water vapor condensed. From about 4.4 billion years ago onward)
Great Oxidation Event (An increase in atmospheric oxygen caused by cyanobacterial photosynthesis about 2.4 billion years ago)
Snowball Earth (A state in which the entire Earth froze, occurring several times, including about 700 million years ago)
Further Study (Background and Significance)
Earth's internal structure was clarified through the study of seismic waves. It consists of layers: the inner core (solid iron), outer core (liquid iron and nickel), mantle (solid rock), and crust. Convection of liquid iron in the outer core produces Earth's magnetic field (the dynamo mechanism), protecting life from cosmic rays and solar wind.
The giant impact hypothesis is the leading explanation for the formation of the Moon. According to this hypothesis, about 4.5 billion years ago, a Mars-sized body (Theia) collided with the proto-Earth, and the ejected material accumulated to form the Moon. This collision produced Earth's axial tilt (about 23.5°), which caused seasonal changes.
After the Great Oxidation Event, oxygen formed an ozone layer high in the atmosphere and blocked harmful ultraviolet radiation. This gradually created an environment in which life could advance onto land. Earth's history is also a history of the coevolution of life and the environment.
- Birth of Earth = about 4.6 billion years ago, through planetesimal accretion
- Magma ocean → differentiation into core and mantle
- Primordial ocean = from about 4.4 billion years ago, through condensation of water vapor
- Primitive atmosphere = water vapor, CO₂, N₂ (almost no oxygen)
- Oldest life = about 4.0〜3.8 billion years ago
- Great Oxidation Event = about 2.4 billion years ago, caused by cyanobacteria
- Formation of the Moon = giant impact hypothesis
Points to Note (Common Confusions)
① There was almost no oxygen in the primitive atmosphere. An oxygen-rich atmosphere like today's formed through biological activity. ② The Great Oxidation Event was an event in which life (cyanobacteria) changed the atmosphere. ③ Do not confuse Earth's age of 4.6 billion years with the universe's age of 13.8 billion years. ④ The giant impact occurred soon after Earth's birth (about 4.5 billion years ago).
Practice
- What was the state called in which the entire surface was molten immediately after Earth's birth?
- Name the organisms that began supplying oxygen to the primitive atmosphere.
- What is the name of the leading hypothesis explaining the formation of the Moon?