Origins
The word 'geology' entered English in the eighteenth century from Modern Latin 'geologia,' composed of Greek 'gē' (earth, land) and '-logia' (study of, discourse about), from 'logos' (word, reason, study). The Greek 'gē' traces to PIE *dʰéǵʰōm (earth), one of the fundamental roots of the language family, which also gave Latin 'humus' (soil, ground) and, through Germanic, the English word 'humble' (literally 'of the ground, lowly').
The word 'geology' existed in Medieval Latin, but with a different meaning. Richard de Bury used 'geologia' in his 1344 work 'Philobiblon' to mean 'the study of earthly things' as opposed to theology (the study of divine things). The modern scientific sense — the systematic study of the earth's physical structure, composition, and history — was established in the eighteenth century, particularly through the work of James Hutton, whose 'Theory of the Earth' (1788) laid the foundations of modern geological thought.
Hutton's revolutionary insight was 'uniformitarianism' — the principle that the geological processes observable in the present (erosion, sedimentation, volcanic activity, uplift) are the same processes that shaped the earth throughout its history. This seemingly modest principle had radical implications: if present-day erosion rates are typical of past rates, then the earth must be vastly older than the few thousand years suggested by literal readings of the Bible. Hutton concluded that the earth showed 'no vestige of a beginning, no prospect of an end' — a statement that implied geological time measured in millions or billions of years.
Development
Charles Lyell's 'Principles of Geology' (1830-1833), which expanded and popularized Hutton's ideas, was among the books Charles Darwin carried aboard HMS Beagle. Darwin's theory of evolution by natural selection depended on geological time: the slow accumulation of small changes required vast stretches of time that only geology could provide. Biology and geology were thus interdependent — each needed the other's discoveries to make sense.
The Greek root 'gē' (earth) is the foundation of a large family of English scientific and geographical terms. 'Geography' (earth-writing, the description of the earth's surface), 'geometry' (earth-measurement, originally the practical art of measuring land in Egypt), 'geophysics' (the physics of the earth), 'geothermal' (earth-heat), 'geocentric' (earth-centered), and 'geopolitics' (the politics of geography) all begin with 'gē.' The prefix 'geo-' has become so productive that it is freely combined with modern English words: geolocation, geotagging, geofencing.
The mythological personification of the earth in Greek religion was Gaia (or Gē) — the primal goddess of the earth, mother of the Titans. In the 1970s, the scientist James Lovelock named his hypothesis about the earth as a self-regulating system the 'Gaia hypothesis,' reviving the Greek goddess's name for a scientific concept: the idea that the biosphere, atmosphere, oceans, and geology interact as a complex system that tends to maintain conditions suitable for life.
Proto-Indo-European Roots
The PIE root *dʰéǵʰōm (earth) had remarkably diverse reflexes across the language family. In Latin, it produced 'humus' (soil, earth) — giving English 'humus,' 'humble,' 'humility,' and 'exhume' (to dig from the earth). In Germanic, it produced words for 'earth' and 'ground.' In Slavic, it gave Russian 'zemlya' (earth, land). In Indo-Iranian, it gave Sanskrit 'kṣam' (earth). The diversity of descendants reflects the centrality of the concept: every Indo-European people needed a word for the ground they stood on, and each branch shaped the inherited root in its own way.
Modern geology is divided into numerous sub-disciplines: mineralogy (the study of minerals), petrology (the study of rocks), paleontology (the study of ancient life through fossils), seismology (the study of earthquakes), volcanology (the study of volcanoes), stratigraphy (the study of rock layers), and plate tectonics (the theory of the earth's moving crustal plates). Each sub-discipline represents a different way of reading the earth — of extracting information from rock, mineral, fossil, and landscape.
The word 'geology' is, in the end, a precise description of what geologists do: they read the earth. 'Gē' + 'logos' — earth-discourse, earth-reasoning. The ground beneath our feet is a text written in stone, sediment, and crystal, and geology is the discipline that has learned to read it.