Geology is the science and study of the composition, structure, properties, and history of the Earth, as well as the processes by which it is formed, moved, and changed.

The field is a major academic discipline, and is commercially important for mineral and hydrocarbon extraction, as well as for predicting and understanding earthquakes and volcanic eruptions. It is also an important foundation for many other science and engineering disciplines, including evolutionary biology, paleoclimatology, and geotechnical engineering.

The word geology was first used by Jean-André Deluc in 1778 and introduced as a fixed term by Horace-Bénédict de Saussure in 1779. The word is derived from the Greek "gê", meaning "earth" and "logos", meaning "speech".

The study of the physical material of the Earth dates back at least to ancient Greece when Theophrastus (372-287 BC) wrote "Peri Lithon" (On Stones). In the Roman period, Pliny the Elder wrote in detail of the many minerals and metals then in practical use.

Modern geology began in the medieval Islamic world. Abu al-Rayhan al-Biruni (973–1048 AD) was one of the earliest Muslim geologists, whose works included the earliest writings on the geology of India, hypothesizing that the Indian subcontinent was once a sea. Islamic scholar Ibn Sina (Avicenna (981–1037) proposed detailed explanations for the formation of mountains, the origin of earthquakes, and other topics central to modern geology, which provided an essential foundation for the later development of the science.

In China, Shen Kua (1031–1095) formulated a hypothesis for the process of land formation. Based on his observation of fossil animal shells in a geological stratum in a mountain hundreds of miles from the ocean, he inferred that the land was formed by erosion of the mountains and by deposition of silt.

Nicolas Steno (1638–1686) is credited with the law of superposition, the principle of original horizontality, and the principle of lateral continuity: three defining principles of stratigraphy.

William Smith (1769–1839) drew some of the first geological maps and began the process of ordering rock strata (layers) by examining the fossils contained in them.

James Hutton is often viewed as the first modern geologist. In 1785, he presented a paper entitled "Theory of the Earth" to the Royal Society of Edinburgh. In his paper, he explained his theory that the Earth must be much older than had previously been supposed in order to allow enough time for mountains to be eroded and for sediments to form new rocks at the bottom of the sea, which in turn were raised up to become dry land. Hutton published a two-volume version of his ideas in 1795..

Sir Charles Lyell first published his famous book "Principles of Geology", in 1830. The book influenced the thought of Charles Darwin, and successfully promoted the doctrine of uniformitarianism. This theory states that slow geological processes have occurred throughout the Earth's history and are still occurring today. In contrast, catastrophism is the theory that Earth's features formed in single, catastrophic events and remained unchanged thereafter. Though Hutton believed in uniformitarianism, the idea was not widely accepted at the time, and is somewhat oversimplified compared to today's knowledge. Listen HERE to a radio debate that illustrates how little was understood about these processes as recently as 1965.


Much of 19th-century geology revolved around the question of the Earth's exact age. Estimates varied from a few thousand to billions of years. By the early 20th century, radiometric dating allowed the Earth's age to be estimated at two billion years. The awareness of this vast amount of time opened the door to new theories about the processes that shaped the planet.

The most significant advances in 20th century geology have been the development of the theory of plate tectonics in the 1960s, and the refinement of estimates of the planet's age. Plate tectonic theory arose out of two separate geological observations: seafloor spreading and continental drift. The concept was first mentioned by Antonio Snyder-Pelligreni in 1858, who attributed it to the Biblical flood. In 1912, Alfred Wegener, proposed a theory which accounted for the movement of the continents and the apparent wandering of the north and south poles. However, it was not until the mid-1960's, that Wegener's theory was widely accepted by the geological community, Tuzo Wilson of the University of Toronto being one of the significant contributors. The theory revolutionized the Earth sciences. 

Source: Wikipedia and others

The history of the Earth (also called the chronology or timeline) is inferred from the minerals found in older rocks and from the fossils found in younger rocks, as observed in outcrops and well bores drilled for oil and other minerals, as well as those drilled specifically to elucidate the issue of plate tectonics, for example the Ocean Drilling Program that discovered the mid-oceanic ridges and documented sea-floor spreading.

The earliest Solar System material is dated to 4567.2 ± 0.6 million years ago, and by 4540 million years ago, the Earth and the other planets in the Solar System had formed out of the solar nebula, a disk-shaped mass of dust and gas left over from the formation of the Sun. This assembly of the Earth through accretion was largely completed within 10 to 20 million years. Initially molten, the outer layer of planet Earth cooled to form a solid crust when water began accumulating in the atmosphere. The Moon formed shortly thereafter, 4530 million years ago, due to a collision between Earth and a Mars sized object.

The history of the Earth is subdivided into eons, eras, and periods, as illustrated below:

Terrestrial Time Scale (
Source: Wikipedia)

Other major events led to the precursors of life and finally to the Earth we know today.

Hadean Eon: "Black Earth"
   4500 Ma: surface solidifies with dark basalt
Archean Eon: "Red Earth"
   3800 Ma: end of Late Heavy Bombardment, first life
   3500 Ma: Start of photosynthesis
Proterozoic Eon: "White Earth"
   2600 Ma: Oxygenated atmosphere, first snowball Earth
   730–635 Ma: two snowball Earths, glaciation from poles to equator
Paleozoic Era: "Green Earth"
   542 Ma: Cambrian explosion of hard-bodied life; first abundant fossils
   380 Ma: First vertebrate land animals
Mesozoic Era:
   250 Ma: Permian-Triassic extinction – 90% of all land animals die
 Cenozoic Era:
   65 Ma: Cretaceous-Tertiary extinction, dinosaurs die off

About 7 million years ago, ancient predecessors to humans emerged, and look at the mess we are in now!

An older but simpler version of Earth HistorY

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