People have studied stones for practical or decorative usages since prehistoric times. The ancient Greek philosopher Theophrastus in his work On Stones described many minerals. There are those who claim that the history of geology begins in the eleventh century AD with the Persian polymath Avicenna, a view which is not convincing. In China, the polymath Shen Kuo upon noticing that there were seashells embedded in a sandstone cliff far above sea level inferred that the sandstone must have derived from an ancient beach that had somehow been compressed and elevated. While this insight was correct, it remained an isolated observation and was not followed up by other Chinese or Asian scholars. Geology, like modern science in general, was therefore born in Europe after the Scientific Revolution and the Enlightenment.

There were important mines in the mountainous regions of Germany and Eastern Europe. After the introduction of gunpowder from China during the Mongol conquests in the 1200s and the independent development of large cannon in Europe, the growing demand for copper for the manufacture of bronze cannon in the fifteenth century was a stimulus for advances such as the “liquation” process, used in ores containing silver to separate it from copper.

The German scholar Georgius Agricola (1494-1555) was a pioneer in mineralogy. He got a degree from the University of Leipzig and studied medicine in Italy. On his return to Saxony in 1526 he developed a life-long interest in mining and spent some time in Bohemia, the richest metal mining district in Europe. His work De Re Metallica, published posthumously in 1556, was a comprehensive summary of all aspects of mining and metal production then known. His work was highly regarded by contemporaries and has stood the test of time well.

Nicolas Steno, or Niels Stensen (1638-1686) from Copenhagen, Denmark, studied medicine and moved to Italy in 1665. In 1666, two fishermen caught a huge shark which Steno dissected. While examining its teeth he was struck by their resemblance to stony objects that were found in certain rocks. He argued that these objects had come from once-living sharks and come to be buried in mud or sand that was now dry land. His English contemporaries Robert Hooke and John Ray, too, argued that fossils were the geologically preserved remains of once-living organisms. Steno is also famous for his law of superposition. In 1669 he concluded that layers of rock (strata) are arranged in a time sequence with the oldest on the bottom and the youngest on the top, unless later processes have disturbed this arrangement.

The French naturalist Jean-Étienne Guettard (1715-1786) was the first person to recognize the volcanic nature of the Auvergne region in central France. In addition, he prepared early geological maps and identified heat as the causative factor of change in the Earth’s landforms. Nicolas Desmarest (1725-1815) in the 1760s studied the Auvergne region and found large basalt deposits and traces of flows of lava (magma, molten rock) from nearby now-extinct volcanoes. The German naturalist and explorer Alexander von Humboldt carried our major studies of volcanoes in the first part of the nineteenth century.

The word “geology” as a term for the study of the Earth was popularized in the late eighteenth century by the Swiss (Genevan) naturalists Horace-Bénédict de Saussure (1740-1799), the aristocrat and scholar who is famous for his voyages in the Alps and often considered the founder of alpinism, and Jean-André Deluc (1727-1817). Deluc was the son of a clockmaker and spent years climbing the Alps with his brother. He made accurate instruments to measure the height of mountains and in 1773 sought a place in England. He was elected a fellow of the Royal Society in London on the strength of his barometry and instrumentation skills.

The German scholar Abraham Gottlob Werner (1749-1817) studied law at the University of Leipzig and later got a teaching appointment at the Mining Academy of Freiberg in Saxony, where he stayed for many years. As a talented mineralogist he worked up simple descriptive standards of classification and discovered eight new minerals, but mineralogy gradually diminished from the overarching category for the study of the Earth to a mere subdiscipline. While sometimes wrong, Werner was an influential geologist and the first to work out a comprehensive theory for the history of the Earth’s formation. He believed that all rock was once sediment or precipitate in a universal ocean, a view which became known as Neptunism.

James Hutton (1726-1797) was the leading representative of the rival Plutonist theory. He was born and educated in Edinburgh, Scotland, during what has become known as the Scottish Enlightenment. He was a Newtonian in natural philosophy and counted among his friends the chemist Joseph Black, the economist Adam Smith and the inventor James Watt. Hutton proposed the uniformitarian view of geological history where all strata could be accounted for in terms of geological forces operating over very long periods of time, such as the slow erosion of rocks. His ideas were popularized by John Playfair (1748-1819) of the University of Edinburgh and picked up by the young Scottish geologist Charles Lyell (1797-1875).

Charles Lyell became fascinated with geology and took several field trips to Continental Europe. Sicily with the active stratovolcano Mount Etna in particular impressed him. As a member of the Geological Society he took part in lively debates and supported the uniformitarian theory. Contrary to catastrophism it indicated the past to have been an uninterrupted period of erosion, sediment deposition, volcanic action, earthquakes etc. These gradual processes, still going on today, could account for great changes when given enough time, which meant that the Earth had to be many millions of years old. Lyell’s Principles of Geology, first published in 1830, was very successful and accessible to a wider audience, something which Hutton’s work never had been. It went through many editions and brought the author a considerable income, which he used to travel and expand his ideas. Lyell greatly influenced a number of men of science, including the young Charles Darwin. Modern geology can be said to have been born with Charles Lyell’s extension of James Hutton’s theories.

The principles of stratigraphy, the study of the Earth’s strata or layers of sedimentary rock, had been created by Nicolas Steno in the seventeenth century and were rapidly extended between 1810 and 1840. Over the next century, geologists filled in the details of the stratigraphic column with ever-greater precision. By the turn of the nineteenth century, it was generally accepted among Western European scholars that fossils could be used to identify and correlate strata. The great naturalist Georges Cuvier (1769-1832), widely considered the founder of paleontology, together with fellow French scholar Alexandre Brongniart (1770-1847) produced a pioneering geological map of the Paris region in 1812. Brongniart had studied chemistry under the brilliant chemist Antoine Lavoisier. The fruitful collaboration between these two men established a scientific approach to stratigraphy and demonstrated that particular geological strata could be recognized by the fossils found within them.

The English surveyor, canal engineer and geologist William Smith (1769-1839) came from a family of small farmers. He received little formal education, but from an early age took an interest in exploring fossils. Based on stratigraphic investigations from canals and quarries he produced a complete geologic map of England and Wales in 1815, the first nationwide geological map. Partly due to his humble origins and limited education his great contributions were overlooked at first by the scientific community, and Smith suffered from severe financial difficulties. Not until the later part of his life was his careful work fully appreciated.

Although the marriage between geology and mining took a long time to yield practical results, the frequent claims that dynamic Britain during the Industrial Revolution was exhausting its coal supplies turned out to be false alarms. State-sponsored geological surveys were undertaken throughout Europe and North America after the mid-nineteenth century. This research would greatly benefit the mining industry as well as the emerging petroleum industry. Many geologists in the twentieth century found work in the oil industry, which joined geological surveys and mining as the main sources of non-academic employment.

Roderick Murchison (1792-1871) was born into a wealthy Scottish Highland family. He spent years in the army and became a very active member of the Geological Society of London, collaborating with Charles Lyell and the Englishman Adam Sedgwick (1785-1873). Murchison’s great work The Silurian System in 1839 established the Silurian geological time period of the Paleozoic Era, followed a year later by the Devonian while collaborating with Sedgwick. Murchison’s travels through Russia and Scandinavia after 1840 resulted in the establishment of the Permian period, which ended 250 million years ago with the greatest mass extinction of life on Earth, which wiped out perhaps 90% of all then-existing species.

Adam Sedgwick taught geology at the University of Cambridge, where Charles Darwin was one of his students. He proposed the Cambrian period, the first part of the Paleozoic, lasting from roughly 540 million to 490 million years ago. Judging from the fossil record this was an age of rapid development of complex life-forms which is called the Cambrian explosion.

Gideon Mantell (1790-1852) was an influential English paleontologist. In 1822 his wife noticed an object which he recognized as a fossil tooth but was unable to match to any known creature. The respected scholar Georges Cuvier in Paris in an uncharacteristic error suggested that the remains were from a rhinoceros. In London, Mantell was shown the skeleton of an iguana with teeth almost identical to the ancient teeth that he had just found, though much smaller. Mantell realized that he had discovered the remains of an extinct giant reptile which he called Iguanodon, making it one of the first dinosaurs to be formally named. Also in England, Mary Anning (1799-1847) was an early fossil collector who produced many remarkable finds. Perhaps the most important one was her discovery of the first plesiosaur.

The English paleontologist Richard Owen (1804-1892) coined the term “dinosaur” in 1842. The name means “terrible lizard” and is not very scientifically accurate, but it stuck. Owen was a quarrelsome man who claimed the discovery of the Iguanodon for himself when it had been done by Gideon Mantell, yet according to Bill Bryson in A Short History of Nearly Everything, he also contributed to the development of modern museums: “Owen’s plan was to welcome everyone, even to the point of encouraging working men to visit in the evening, and to devote most of the museum’s space to public displays. He even proposed, very radically, to put informative labels on each display so that people could appreciate what they were viewing. In this, somewhat unexpectedly, he was opposed by T. H. Huxley, who believed that museums should be primarily research institutions. By making the Natural History Museum an institution for everyone, Owen transformed our expectations of what museums are for.”

The Scottish geologist James Hall (1761-1832), a friend of James Hutton, founded experimental geology by artificially producing various rock types in the laboratory. He carried out dangerous experiments with limestone heated under pressure and lived to report that it did indeed consolidate under sufficient pressure. In the twentieth century Pentti Eskola (1883-1964), a professor of geology and mineralogy in Helsinki, Finland, applied chemical methods to the study of minerals and metamorphic facies (groups of mineral compositions in metamorphic rocks), thereby laying the foundations of studies in metamorphic petrology.

There are three main rock types: Igneous rocks are formed from the solidification of molten rock (magma). Intrusive igneous rocks such as diorite, gabbro and granite solidify below the Earth’s surface while extrusive igneous rocks such as basalt, obsidian and pumice solidify on or above the surface. Sedimentary rocks are formed by the accumulation of sediments. Some such as conglomerate and sandstone are formed from mechanical weathering debris. Organic sedimentary rocks such as coal form from the accumulation of plant or animal debris. Metamorphic rocks have been modified by heat, pressure and chemical processes, usually while buried deep below Earth’s surface. This has altered the mineralogy, texture and chemical composition of the rocks. Examples of this would be marble produced from the metamorphism of limestone or quartzite from the metamorphism of sandstone with quartz.

The nebular hypothesis was first proposed in 1734 by the Swedish philosopher and theologian Emanuel Swedenborg (1688-1772), who was born in Stockholm and studied at Uppsala University. He wrote on mathematics, chemistry, physics, mineralogy and astronomy and made a sketch of a glider-type aircraft. The German Enlightenment philosopher Immanuel Kant developed this theory further in 1755, and the French astronomer Pierre-Simon Laplace also advanced a nebular hypothesis in 1796. Laplace suggested that our Solar System was created from the cooling and condensation of a large and hot rotating “nebula,” a gassy cloud of particles and dust. This idea strongly influenced scientists in the nineteenth century, and central elements of it have survived to this day. For a long time, geologists preferred the hypothesis that the Earth had cooled and contracted. The work on rates of cooling made by the brilliant French mathematical physicist Joseph Fourier seemed to support this model.

In 1831 the French geologist Élie de Beaumont (1798-1874) suggested that the Earth had cooled from a molten body and that the crust at intervals had buckled under the strain, throwing up mountain ranges. Variants of this contraction theory flourished, culminating in the four-volume Face of the Earth (1883-1904) by Eduard Suess (1831-1914), a professor of geology at the University of Vienna. Throughout the twentieth century, geologists amassed a mass of new data from all corners of the planet and, crucially, from the bottom of the oceans.

Geologists knew that there was evidence of past upheavals, but many still believed these had been caused by the Biblical flood of Noah. There were a few individuals who believed that glaciation had been much more extensive in the past than it is today, for instance because of the presence of huge boulders dumped far away from the strata where they belonged. They included the Norwegian geologist Jens Esmark (1763-1839) writing in the 1820s and the German-Swiss mining engineer and naturalist Jean de Charpentier (1786-1855). In Norway and the Alps there are surviving glaciers, and much of the landscape was shaped by previous glaciers. The Norwegian fjords are valleys carved by glacial activity and now filled with seawater. The ideas of Charpentier and others in Switzerland were taken up and developed further by the Swiss paleontologist, geologist and glaciologist Louis Agassiz (1807-1873).

Louis Agassiz studied medicine at Zürich and Heidelberg before moving to Paris, where he was influenced by the ideas of Georges Cuvier. Agassiz was already interested in palaeontology and soon became a leading expert of fossil fishes. Despite initial skepticism, after personal studies he became an enthusiastic supporter of the glacial model. In 1840 Agassiz published a work in two volumes entitled Etudes sur les glaciers (Study on Glaciers), which can be considered the first mature scientific work on the existence of a previous Ice Age when glaciers had covered much larger land areas than they do now. Later scholars discovered evidence for several distinct ice ages, not just one. Last Glacial Maximum was about 20,000 years ago. Still, this left the unresolved issue of what could cause such ice ages.

The French mathematician Joseph Adhemar (1797-1862) suggested that ice ages were caused by astronomical forces. His theory was modified by the Scottish scientist James Croll (1821-1890) and above all by the gifted Serbian civil engineer and mathematician Milutin Milankovitch (1879-1958). Milankovitch studied at the Institute of Technology in Vienna in Austria-Hungary and later taught mechanics, theoretical physics and astronomy at the University of Belgrade in Serbia. During the turmoil in the Balkans following the collapse of the Ottoman and Austro-Hungarian Empires he served in the Serbian army during World War I. He picked up an obsession with climate and a determination to set up a detailed mathematical explanation of how temperatures change as a result of changes in the eccentricity, axial tilt and precession of the Earth’s orbit around the Sun. His complex work on what has now became known as Milankovitch cycles took him many years and was carried out only with brain power. It was published in a 1920 book that met with widespread acclaim.

The reasons for the periodic ice ages we know of from the geological record are not fully understood, but are believed to be at least partly related to cyclic changes in the Earth’s orbit and tilt. Other factors such as the composition of the atmosphere, the changing position of the continents, eruptions of supervolcanoes or cometary impacts may contribute as well.

We currently know a lot more about the surface of other planets such as Mars than about the interior of our own planet, but what little we think we know to a large extent derives from the study of seismic waves. The Dutch mathematician Willebrord Snell in the seventeenth century in what has become known as Snell’s Law described the bending of light, or refraction, which takes place when light travels from one medium to a medium with a different composition and density, for instance from air to water. This effect can be seen by anybody in a small boat who puts an oar into the water and observes how it appears to be “bent.” This phenomenon is caused by the change in velocity that occurs when light waves pass from one medium to another. The same principle applies to other waves, too, for example seismic waves, the shock waves generated by earthquakes or explosions that travel through the Earth’s interior.

The Irish geophysicist Richard Dixon Oldham (1858-1936) discovered that seismic waves travel through the interior of the Earth in different directions and at different speeds. This insight was used by the Croatian seismologist Andrija Mohorovicic (1857-1936), who had studied physics in Prague and taught geophysics at the University of Zagreb. By analyzing the data from a 1909 earthquake, Mohorovicic realized that the velocity of a seismic wave is related to the density of the material that it is moving through. He interpreted the acceleration of seismic waves observed within Earth's outer shell as a compositional change within the Earth itself. This Mohorovicic Discontinuity, or “Moho” for short, is believed to constitute the boundary between the Earth’s crust and mantle. It can be found at an average depth of 8 kilometers beneath the ocean basin and as much as 32 kilometers beneath the continents.

The Danish seismologist Inge Lehmann (1888-1993) studied at the University of Copenhagen in Denmark and later worked on cataloging seismograms from Denmark and the Danish-ruled island of Greenland. In 1929 a large earthquake occurred near New Zealand. Lehmann studied the recorded shock waves, and in a 1936 paper she theorized that the Earth’s center consists of two parts: a solid inner core surrounded by a liquid outer core. The outer core boundary lies below the mantle almost 2,900 km beneath the Earth’s surface. The inner core begins about 5150 kilometers beneath the Earth’s surface where the temperature is estimated to be close to 6000 °C, similar to the temperature at the Sun’s surface. In total, the Earth’s core is about 7,000 kilometers in diameter, making it roughly comparable in size to the planet Mars.

Beno Gutenberg (1889-1960) was a German-born seismologist educated at the University of Göttingen. In 1930 Gutenberg became a professor of geophysics at the California Institute of Technology in the USA. His colleague there was the American seismologist Charles Francis Richter (1900-1985). They collaborated on the development of various scales using seismic waves so that observers could assign magnitudes to earthquakes. In 1935 this work resulted in the creation of a logarithmic magnitude scale that came to be named after Richter alone. Earthquakes below 2.5 on the Richter scale are too weak to be noticed by humans. Earthquakes with an intensity of 10.0 or more have so far never been measured, the strongest being one of 9.5 in Chile in 1960. The Great Lisbon Earthquake which destroyed the Portuguese capital city in 1755 happened more than a century before modern seismographs had been invented, but based on descriptions it may have approached a magnitude of 9.0.

The age of the Earth was a subject of great interest to both theologians and naturalists. The French naturalist Georges-Louis Leclerc, Comte de Buffon in the 1770s made one of the first scientific attempts to establish the age of the Earth. He assumed that it had gradually cooled from a much hotter state in its early history. Based on experiments with heating balls of iron, Comte de Buffon estimated that the Earth was at least 75,000 years old. While this is far too young it was nevertheless a lot older than a literal reading of the Bible would indicate.

In 1859 Charles Darwin published an estimate of 300 million years for a piece of rock. Lord Kelvin calculated that it would take the Earth about 100 million years to cool from an assumed primordial molten condition to its present state. These calculations changed soon after the realization that radioactive elements constantly emit heat. The birth of geophysics as distinct from geology depended upon the discovery of radioactivity by Henri Becquerel and Pierre and Marie Curie in France in the late 1890s. This provided a source of heat that could prevent the interior of the Earth from cooling into a solid, inert lump over long timescales. The physicist Ernest Rutherford suggested that the decay of radioactive elements could be used to measure the age of various rocks. This idea was followed up by Arthur Holmes (1890-1965) in Britain and Clair Cameron Patterson (1922-1995) in the United States.

According to The Oxford Guide to the History of Physics and Astronomy, “Robert John Strutt and his student, the geologist Arthur Holmes, pursued Rutherford’s idea. By 1911, Holmes had used uranium/lead ratios to estimate the ages of several rocks from the ancient Precambrian period. One appeared to be 1,600 million years old. Many geologists were initially skeptical, but by 1930, largely as a result of the work of Holmes, most accepted radioactive dating as the only reliable means to determine the ages of rocks and of the earth itself. The discovery of isotopes in 1913, and the development of the modern mass spectrometer in the 1930s, greatly facilitated radioactive dating. By the late 1940s, the method produced an estimate of between 4,000 and 5,000 million years for the age of the earth. In 1956, the American geochemist Clair Cameron Patterson compared the isotopes of the earth’s crust with those of five meteorites. On this basis, he decided that the earth and its meteorites had an age of about 4,550 million years. All subsequent estimates of the age of the earth have tended to confirm Patterson’s conclusion.”