Geology: History of the Earth

Sections on this page: Beginnings of GeologyThe Age of the EarthStructure of the Earth: Plate TectonicsEarth's MagnetismWeather and Climate: MeteorologyEnd.

Beginnings of Geology

Conrad Gesner (1516-1565) published the first book about fossils (De Rerum Fossilium 1565). Niels Stensen, latinised as Nicolaus Steno, (1638-1686) in his Prodromus 1669 recognised that fossil shells in rock strata must have been laid down on the seabed, even though they are now found inland and high above sea-level, and that lower levels without fossils must predate the existence of life.

James Hutton & & & (1726-1797), in his Theory of the Earth 1785, maintained there were cycles of geological activity, rocks being worn down and raised up, and that Earth had 'no vestige of a beginning, no prospect of an end'. Hutton's unconformity at Siccar Point is now a Scottish national heritage site.

James Hall (1761-1832) advocated the importance of volcanic activity and conducted experiments on rock in a foundry. William Smith & & (1769-1839) a canal surveyor investigated how the strata could be identified by fossils in them and in 1815 published his Geological Map of England and Wales. Georges Cuvier (1769-1832) did similar work in the Paris area.

Other early geologists were Alexandre Brongniart (1770-1847), William Buckland & (1784-1856), Adam Sedgwick & & (1785-1873), Roderick Murchison (1792-1871). For further information on history of geology see:


The Age of the Earth

All this new knowledge was synthesised by Charles Lyell & & & & (1797-1875) in his Principles of Geology (3 vols 1830-1833) with many later editions; 'an attempt to explain the former changes at the Earth's surface, by reference to causes now in operation'.

The researchers introduced terms to describe the types of rock formations, which we now associate with eras (jurassic, silurian, devonian, cambrian ...). This work also implied that Earth was much older than had been deduced by biblical chronologists like James Ussher & & & (1581-1656) who dated creation at 4004 BC.

The discovery of radioactivity led to new methods for dating rocks. Bertram Boltwood (1870-1927) in 1911 measured the ratio of radioactive isotopes of uranium and lead in a mineral and calculated that it formed several hundred million years ago. From methods such as this we now know that the Earth formed 4,570,000,000 years ago, although it took 70 or 75 million of those years to reach its present mass. Earth's oldest known rock material is a zircon grain from Australia dated at 4.4 billion years by the uranium-lead method in 2001.

Here are some charts of geological chronology:
British Geological Survey
Rockman Carl W. Haywood
Fossil Mall
Dinosauria Jeff Poling
Geological Ages Nick Shanks
Proctor Museum Houston Texas
Connexions Jack E. Maxfield
Heavy Oil Science Centre Alan Hagen


Structure of the Earth: Plate Tectonics

John Milne & (1859-1913), studied earthquakes and developed a seismograph for recording shock waves. R. D. Oldham & (1858-1936) analysed the compressional and shear waves and concluded that Earth had a dense core 7000km in diameter. Andrija Mohorovicic (1857-1936) identified an outer crust over a denser, hotter mantle 2900km thick.

Eduard Suess & (1831-1914) and Alfred Wegener & & & (1880-1930) formulated theories of continental drift, after the break-up of an earlier supercontinent.

Maurice Ewing & & (1906 - 1974) used marine seismic techniques that showed the thinness, 7km, of the crust under the ocean, 20-80km under continents. In 1953 he and Bruce Heezen & & (1924 - 1977) discovered the global extent of mid-ocean ridge systems, and a central rift indicating they are being pulled apart.

In 1962 Harry Hess & & & (1906 - 1969) described mid-ocean ridges as forming over rising convection currents of molten rock in the hot mantle, to extrude from the rift valleys, spreading to form new ocean-floor material. In 1967 Dan Mckenzie & (1942-) synthesised continental drift and sea-floor spreading into the theory of plate tectonics &, in which the Earth's crust is formed of large mobile plates. Most volcanoes and earthquakes are at the plate boundaries, and where the plates collide mountain ranges form.


Earth's Magnetism

Natural magnetism in the form of lodestone (the iron mineral magnetite) was known to the Chinese 300BC, it was they who devised the first navigational compass. William Gilbert & & & & & & (1540-1603) in his De Magnete & 1600 thought that Earth itself was a magnet. By 1635 it was known that the magnetic field varies across the globe.

Reverse magnetization in rocks was first noticed by Bernard Brunhes (1867 - 1910) in 1906. In 1929 Motonori Matuyama & (1884 - 1958) showed that Earth's magnetic field had reversed polarity over the past 2 million years. He measured switches in polarity through a stratified succession of basalt lavas. In the 1960s geologists measured more than 20 reversals in the past 5 million years. The source of the magnetic field is believed to be due to currents in the liquid outer core.

Drummond Matthews (1931-1997) and Frederick Vine & (1939 -) in 1963 found symmetrical patterns of polarity reversals in magnetised lavas on either side of the mid-ocean ridges, recording the history of reversals in Earth's magnetic field.


Weather and Climate: Meteorology

Alexander von Humboldt & & & & & & (1769-1859) was the first to study nature from a global perspective, introducing isobars and isotherms to mark equal pressure and temperature levels on the world map.

Louis Agassiz & & & & (1807 - 1873) developed the theory of a recent ice age, and of the power of glaciers, based on the evidence of rock debris and scratched rock surfaces.

John Tyndall & & & & (1820 - 1893) in 1863 reported experiments on the abilities of oxygen, nitrogen, carbon dioxide, water vapour and ozone to absorb and transmit heat, the latter three being nearly opaque. He described how variations in the levels of these gases could lead to climate change; the now famous 'greenhouse effect'. He also explained why the sky is blue and the sun is red at sunset: light of different wavelengths being scattered differently by large molecules in the atmosphere, and travelling further through the atmosphere to the observer when low in the sky.

Vilhelm Bjerknes & & (1862 - 1951) in 1904 combined hydrodynamics and thermodynamics to found numerical weather prediction, and with his son Jacob Bjerknes (1897 - 1975) in 1920 developed the 'polar front' theory of the way warm and cold masses of air interact to form cyclones in mid-latitudes, the basis of modern weather forecasting.

Milutin Milankovitch & (1879 - 1958) in 1918 recognised the influence of three cycles & & on the Earth's climate: (1) the shape of the earth's orbit varies from circular to more elliptical and back again with a period of about 100,000 years, (2) the Earth's axis precesses, making a complete loop in 26,000 years, (3) the angle between Earth's equatorial and orbital planes varies by a few degrees every 40,000 years. Milankovich realised that the resulting changes in sunshine in some zones echoed the climate changes between ice ages and temperate periods. In the 1970s this work was continued by Anandu Vernekar &. Modern knowledge of geomagnetic reversals allows us to date past glaciation periods more accurately.

Other more recent developments to be added includes:

James Lovelock (1919-) Gaia hypothesis.

Mario Molina (1943-) and Sherwood F. Rowland (1927-) Ozone hole.

Greenland rocks show plate tectonics in action 3.8 million years ago:

Climate change:
IPCC - intergovernmental panel on climate change

The Science Book edited by Peter Tallack has been a very useful guide in compiling the above information.