Basics of Geology

Geologic time

Geologic time opens a window into the long and complex history of Earth. It reveals how our planet has formed and evolved over approximately 4.6 billion years (Gradstein et al., 2020). When the Earth was born, its surface was molten and conditions were extreme, but over time the planet cooled. After that, tectonic plates began to move, the atmosphere developed, and life emerged (Stanley, 1999).

Earth’s geological history is divided into periods based on layers of sedimentary rock and fossil discoveries (Gradstein et al., 2020). At the beginning of geological time, during the Precambrian era, the planet underwent major transformations: the first signs of life appeared about 3.5 billion years ago (Stanley, 1999), and atmospheric oxygen levels began to rise around 2.4 billion years ago as a result of the so-called Great Oxidation Event (Gradstein et al., 2020).

In the following eras, such as the Paleozoic, Mesozoic, and Cenozoic, life became more diverse, continents shifted, and the climate changed dramatically (Gradstein et al., 2020). Volcanic eruptions, ice ages, and asteroids have all influenced the development of life and caused mass extinctions, such as the K-Pg extinction 66 million years ago that led to the disappearance of the dinosaurs (Stanley, 1999).

On this page, you’ll be able to explore the periods of geological time and their significant events from the Precambrian to the present day. By understanding the past, we can better comprehend the future development of our planet. Its geological legacy is written in landscapes, rock layers, and fossils that tell the story of Earth (Geological Survey of Finland, 2023).

Geologic time periods

Earth’s geological history is divided into time intervals based on significant events such as mass extinctions and climate changes (Gradstein et al., 2020). Naturally, the further back in time we go, the more uncertain our knowledge becomes, as the oldest rock layers may have eroded or transformed due to geological processes (Stanley, 1999).

Eons are the longest units of geological time, lasting billions or hundreds of millions of years. To put their length into perspective: dinosaurs lived and went extinct during the same eon in which we are currently living.

Eons are divided into eras, which are the second-longest time units and typically span hundreds of millions of years.

Eras are divided into periods, which generally last tens of millions of years.

Periods are subdivided into epochs, lasting from a few million to tens of millions of years.

Finally, epochs are divided into stages (or ages), which usually span a few million years.

The Hadean eon (4,567–4,000 million years ago)

The Hadean eon was the first interval in Earth’s history, during which our planet formed and began to take shape as part of the solar system around 4.6-4.0 billion years ago (Gradstein et al., 2020). The name “Hadean” is derived from Hades, the Greek god of the underworld, and fits the extreme conditions of the time: intense heat, constant meteor impacts, and landscapes bubbling with lava (Dalrymple, 2001).

During this time, Earth was a newborn planet composed of molten rock, and its surface gradually began to cool. The first solid crust started to form, and as water vapor condensed, it is believed that the first oceans began to emerge (Wilde et al., 2001). Over the course of the Hadean, Earth’s internal structure: core, mantle, and crust differentiated, laying the foundation for the planet’s present-day composition (Jacobsen, 2001).

Archean eon (4000 - 2500 million years ago)

Eoarchean Era (4,000 – 3,600 million years ago)

Earth’s early atmosphere consisted mainly of carbon dioxide, methane, ammonia, and water vapor, with little to no oxygen.
The formation of oceans is believed to have occurred partly during this era, as Earth’s surface cooled and water vapor condensed into liquid water (Wilde et al., 2001).

Paleoarchean Era (3,600 – 3,200 million years ago)

The first evidence of unicellular life dates back to this era. The oldest known fossils are stromatolites, formed by the activity of cyanobacteria.

Mesoarchean Era (3,200 – 2,800 million years ago)

A primordial continent named Ur may have formed during this time, although its existence has not been fully proven (Dalrymple, 2001).

Neoarchean Era (2,800 – 2,500 million years ago)

The possible formation of early unconfirmed supercontinents such as Vaalbara or Kenorland is dated to this era.

Proterozoic eon (2500 - 538.8 million years ago)

Paleoproterozoic Era (2,500 – 1,600 million years ago)

The first known ice age occurred during this era, around 2.4 billion years ago (Jacobsen, 2001).
Cyanobacteria began producing oxygen through photosynthesis, leading to the Great Oxidation Event (Gradstein et al., 2020). Atmospheric oxygen levels rose significantly—an essential development for the emergence of modern life. However, it also caused a mass extinction of early anaerobic organisms for whom oxygen was toxic (Stanley, 1999).
The first supercontinent, Columbia, formed around 1.9 billion years ago (Dalrymple, 2001).

Mesoproterozoic Era (1,600 – 1,000 million years ago)

Multicellular algae evolved, marking a significant step in the evolution of life (Stanley, 1999).
The second supercontinent, Rodinia, formed around 1,100 million years ago (Gradstein et al., 2020).
The Kurikka valley formed approximately 1,500 million years ago as part of the geological development of the Fennoscandian Shield (Geological Survey of Finland, 2023).

Neoproterozoic Era (1,000 – 538.8 million years ago)

The first multicellular animals evolved (Stanley, 1999).
Several major ice ages occurred during the Neoproterozoic, including the Sturtian and Marinoan glaciations, which may have covered large parts of Earth in ice (Gradstein et al., 2020).

Phanerozoic eon (538.8 millions years ago - Present day)

Paleozoic Era (538.8 – 252.17 million years ago)

The third supercontinent, Pangaea, formed, uniting nearly all of today’s continents (Stanley, 1999). Life evolved rapidly in the oceans and emerged onto land for the first time, leading to the rise of reptiles, insects, and the first land plants (Gradstein et al., 2020).
The Paleozoic Era ended with the largest known mass extinction in Earth’s history around 252 million years ago, during which up to 60% of known animal genera disappeared (Stanley, 1999).

Mesozoic Era (252.17 – 66.0 million years ago)

The age of dinosaurs began, and birds and mammals diverged into separate groups (Gradstein et al., 2020).
Pangaea began to break apart, and the continents started drifting toward their current positions (Stanley, 1999).
The era ended with the mass extinction of the dinosaurs around 66 million years ago.

Cenozoic Era (66.0 million years ago – present)

After the extinction of the dinosaurs, mammals came to dominate the planet and diversified into many ecological niches.
The continents moved into their present positions due to tectonic plate movement.
The climate became cooler and drier, leading to the onset of ice ages (Geological Survey of Finland, 2023).
Following the ice ages, human civilizations developed and began to reshape the planet.