Basics of geology
Definition of geology
Geology is a scientific discipline that studies the Earth and its crust: its history, structure, composition, and the processes involved in its formation and ongoing changes. Geological research draws upon physics, biology, and chemistry. (Geological Survey of Finland)
Geological eras
Geological time offers a window into Earth’s long and complex history, revealing how our planet has been shaped and transformed over billions of years. Since the Big Bang, Earth has undergone dramatic changes: the movement of continents, volcanic eruptions, ice ages, and the increasing diversity of life through evolution.
On this page, you will be able to explore the divisions of geological time and their significant events—from the Precambrian era to the present day. Join us as we travel back in time to better understand the uniqueness of our planet and its geological heritage, etched into landscapes and bedrock structures.
The divisions of geological time
Earth’s geological history is divided into time intervals based on significant events such as mass extinctions and climate changes. Naturally, the further back in time we go, the less certain the information becomes.
Eons are the longest intervals on the geological timescale, spanning billions or hundreds of millions of years. To put this into perspective, dinosaurs lived and went extinct within the same eon we are currently living in.
Eons are divided into eras, the second-longest intervals, which typically last hundreds of millions of years.
Eras are further divided into periods, which generally span tens of millions of years.
Periods are broken down into epochs, which last from a few million to tens of millions of years.
Finally, epochs are subdivided into stages (or ages), the shortest intervals, typically lasting a few million years.
This hierarchical structure provides a framework for understanding Earth’s dynamic history, from the earliest formations to the present day.
The Hadean eon (4,567–4,000 million years ago)
The Hadean eon marks the first chapter in Earth’s history, a time when the planet was formed and began to take shape as part of the solar system. The name “Hadean” derives from Hades, the Greek god of the underworld, aptly reflecting the extreme conditions of this era: intense heat, constant meteor impacts, and landscapes of molten lava.
During this period, Earth was a newborn planet composed of molten rock. As its surface gradually cooled, a solid crust began to form, and the condensation of water vapor likely led to the creation of the first oceans. Over time, Earth’s internal structure differentiated into the core, mantle, and crust, establishing the foundation of the planet’s modern composition.
A key event of the Hadean eon was the formation of the Moon. According to the most widely accepted hypothesis, the Moon originated when Earth collided with a Mars-sized planet called Theia. Although this impact theory cannot be definitively proven, it is broadly supported within the scientific community as the leading explanation for the Moon’s formation.
The Hadean eon was a time of chaos and transformation, laying the groundwork for Earth’s evolution into the habitable planet we know today.
Archean eon (4000 - 2500 million years ago)
Eoarchean Era (4,000–3,600 million years ago)
- During the Eoarchean Era, Earth’s primordial atmosphere began to form, consisting primarily of carbon dioxide, methane, ammonia, and water vapor.
- Oceans are believed to have partially formed during this era as the planet’s surface cooled and water vapor condensed into liquid.
Paleoarchean Era (3,600–3,200 million years ago)
- The first evidence of unicellular life, such as prokaryotes, dates back to this era, marking the earliest known life on Earth.
Mesoarchean Era (3,200–2,800 million years ago)
- The early continent of Ur may have formed during this time. It is thought to have included parts of present-day India, Australia, Madagascar, Zimbabwe, and the Kalahari. While Ur’s existence is not fully proven, it is considered a likely early tectonic plate.
Neoarchean Era (2,800–2,500 million years ago)
- Earth’s oxygen levels rose significantly during this period, a precursor to the Great Oxidation Event.
- The formation of possible supercontinents, such as Vaalbara or Kenorland, is thought to have occurred during this era, highlighting key developments in Earth’s tectonic history.
Proterozoic eon (2500 - 538.8 million years ago)
Paleoproterozoic Era (2,500–1,600 million years ago)
First known ice age: Earth’s earliest known glaciation occurred during this era.
Oxygen production: cyanobacteria (blue-green algae) used photosynthesis to produce oxygen, significantly increasing atmospheric oxygen levels.
Formation of Early Continents:
The continents Arctica and Nena formed around 2,500 million years ago. Arctica included present-day Canada, Siberia, and Greenland. Nena encompassed parts of Scandinavia, Greenland, Siberia, and North America.
- Columbia, Earth’s first supercontinent, assembled around 1,900 million years ago.
Mesoproterozoic Era (1,600–1,000 million years ago)
Evolution of multicellular algae: marking a significant step in the evolution of life.
Formation of supercontinent Rodinia, the second supercontinent, formed approximately 1,100 million years ago.
Formation of the Kurikka valley: Around 1,500 million years ago, the Kurikka Valley began to form.
Neoproterozoic Era (1,000–538.8 Million Years Ago)
Emergence of multicellular life: The first multicellular organisms appeared, with significant advancements in the complexity of life.
Development of early animals: simple animals and their ancestors evolved during this time.
Snowball earth events: Several significant glaciations, including the Sturtian and Marinoan ice ages, occurred. These events likely covered much of the planet with ice, profoundly affecting Earth’s climate and life.
Phanerozoic eon (538.8 millions years ago - Present day)
Paleozoic Era (538.8–252.17 million years ago)
Formation of Pangaea: The third confirmed supercontinent, Pangaea, formed, merging nearly all of today’s continents into one massive landmass.
Life’s expansion: Life flourished in the oceans and later moved onto land for the first time. Marine life included fish, mollusks, and trilobites, while land saw the emergence of reptiles, insects, and early plants.
Trilobites and extinctions: Trilobites, iconic marine arthropods, thrived but eventually went extinct.
Mass extinction event: The era ended with Earth’s most severe mass extinction (~252 million years ago), wiping out approximately 60 % of known genera.
Mesozoic Era (252.17–66.0 million years ago)
The Age of Dinosaurs: Dinosaurs dominated Earth, while birds and mammals began to diversify into distinct groups.Fishes and insects evolved into forms resembling modern species.
Breakup of Pangaea: The supercontinent Pangaea began to fragment, and continents drifted toward their current positions.
Warm Climate: The climate was predominantly warm, supporting the proliferation of diverse ecosystems.
Mass Extinction Event: This era ended with a catastrophic extinction event about 66 million years ago, likely caused by an asteroid impact, which led to the demise of the dinosaurs.
Cenozoic Era (66.0 million years ago – Present Day)
“Age of Mammals”: After the extinction of dinosaurs, mammals rapidly evolved and spread across the globe. Modern mammalian orders, including primates, emerged and diversified.
Continental Drift: Continents moved into their present positions due to plate tectonics.
Climate Evolution: Initially warm and humid, the climate gradually cooled, leading to the development of ice ages.
Rise of Human Civilizations: Following the ice ages, human civilizations emerged, transforming the planet through agriculture, industry, and urbanization.