Early theories of the Earth

Mankind has long taken interest in the nature and origin of the rocks forming the Earth's surface. Among the ancient Greeks, Herodotus (c 485-425 вс) recorded the presence of marine shells far inland in Egypt and concluded that they had been left there by the retreating waters of some earlier sea. Pythagoras (sixth century вс) is said to have taken the presence of fossil marine creatures high in the mountains of Greece as evidence of the elevation of a former sea bed, and Strabo (c 63 bc-ad 23) certainly accepted such a view.

As the Mediterranean lies in one of the world's volcanic aid seismic regions many Greek and Roman authors were fascinated by the phenomena of volcanoes and earthquakes. Strabo, in common with many later writers, saw volcanoes as natural safety valves designed to permit the escape of dangerous terrestrial vapours, and the Roman, Seneca (c ad 3-65), regarded earthquakes as the result of those same vapours becoming turbulent within the Earth’s supposedly cavernous interior. In China, in 132, Chang Heng 178-139) invented an ‘earthquake weathercock’, the first seismograph ever constructed. Rather later, Avicenna (980- 1037). the father of the Earth sciences in the Arab world, discussed not only earthquakes but a wide variety of other geological phenomena.

He also displayed a sound understanding of many Earth processes at a time when, in western Europe, Christian scholars were more interested in saints and seraphim than in sandstones and sapphires. Some of the ideas then current in western Europe must now strike us as bizarre. Gems, for instance, were commonly believed to form as a result of some celestial energy penetrating into the Earth, the energy impinging less obliquely upon the tropics than upon higher latitudes with a result that precious stones were thought to be most abundant in the Earth’s warmest climes.

Yet, despite such fanciful early notions, it was in western Europe rather than in the Middle or Far East that, in the centuries following 1500, the modem Earth sciences originated.

To some extent Europeans were drawn to the Earth sciences in the aftermath of the Renaissance because of the obvious practical applications of those sciences during an age in which the demand for minerals increased. Not least, the replacement of the medieval feudal economy by capitalism resulted in a pressing need for various precious metals for coinage purposes. De re metallica by Georgius Agricola (1494-1555). published in Basle in 1556, offers among much else a detailed discussion of the field occurrence of mineral veins and is a superb early example of applied geology (Figure 1.1).

1.1: One of the many illustrations by Georgius Agricola depicting mineral veins: a ‘principal vein' (A), a 'transverse vein (B) and a ‘vein cutting principal one obliquely’ (C). From Agricola's De re metallica (Basle, 1556)

Important though such practical matters undoubtedly were, it was theoretical questions that mostly engaged the attention of scholars interested in the Earth sciences. Here one problem above all others commanded their attention: what was the origin of the ‘figured stones’, or fossils, found so widely in the Earth’s rocks ? Some scholars dismissed fossils as meaningless sports of nature. Others saw them as growths formed within rocks as a result of the Earth’s vis plastica (moulding force). Yet others, such as Leonardo da Vinci (1452-1519), correctly interpreted fossils as the remains of former marine creatures which had become entombed within the rocks forming on some ancient sea bed, the strata and their fossils then being elevated into their present terrestrial environments (Figure 1.2).

1.2: Early representations of fossils, from Robert Plots The Natural History of Oxfordshire. Being an Essay towards the Natural History of England (second edition. Oxford. 1705)

During the seventeenth century it was accepted increasingly that fossils could only be organic in origin and there developed the parallel notion that the necessary exchange of ancient sea bed into modern dry land must have occurred during that universal catastrophe described ш the Bible—the Noachian Deluge. In the Christian world it was generally accepted that; the Bible offered a divinely inspired account of the Barth’s early history, and as a result the Flood became a favourite device with most students of the Barth sciences. Even down to the early years of the nineteenth century the Flood was still being invoked by thoroughly reputable scientists in explanation of phenomena ranging from the drift mantle of the continents to the configuration of the north Atlantic. Belief in the Flood was by no means the only scripturally based idea to influence the Barth sciences; there was the even more important belief in the Barth's recent origin. The various genealogical tables of the Old Testament were studied by chronologers in order to arrive at a date for the Creation. The most renowned of these chronologers was the Irish prelate James Ussher (1581 -1656), who in 1650 published his conclusion that the Creation had taken place in the year 4004 вс, with the Flood following in the year 2349 вс. Such dates obtained wide currency, but they posed a problem. How could so recent a date for the origin of the Earth be reconciled with the increasing evidence, derived from field observation, that since the time of its formation the Barth’s surface had been the scene of many vicissitudes. How, for instance, in the short period that seemed to be available, could great thicknesses of sedimentary strata accumulate, mountain ranges upheave, and valleys be excavated? Clearly, it seemed, reconciliation could be effected only by supposing all those many changes to have been the result of almost instantaneous diluvial, seismic and volcanic catastrophes. In this manner originated the interpretation of the history of the Barth as having consisted of periods of quiescence interrupted by violent cataclysms—an interpretation that came to be known as Catastrophism.

During the seventeenth century scholars began to formulate what were then termed ‘theories of the Barth’. These were attempts to devise convincing and fairly detailed histories of the globe: basically their authors sought to employ science in the amplification of the somewhat bald Genesis account of the Barth's origin and early development. The theorists all made extensive use of catastrophic events, but while their efforts now seem naive they do deserve our attention as pioneer essays in historical geology. The French philosophei Rene Descartes (1596 1650) was an early influence in this new genre, but geologically the most important of the theories was that published in Florence in 1669 by Nicolaus Steno (1638-86), a Dane resident in Tuscany. Steno illustrated his theory by means of a series of diagrams (Figure 1.3) representing successive stages in the evolution of the Barth. He displays an understanding of many important geological concepts. He accepts the organic origin of fossils; he perceives the true nature of sedimentary strata; he appreciates the work of a marine transgression: he implicitly formulates the law of order of superposition (in any normal sequence the upper strata are younger than the lower); and he illustrates what we now know as a geological unconformity. His theory thus stands as one of the great classics of geological literature.

1.3: Nicolaus Steno's interpretation of the history of the Tuscan landscapes. Redrawn from Nicolai Stenonis de solido intra solidum naturaliter contento dissertationis prodromus (Florence, 1669). 1. During the Creation the globe is submerged beneath a universal fluid front which precipitation takes place to form horizontal, superimposed beds of sedimentary rock (F-G). In the absence of continents, plants and animals the individual strata are uniform in composition and free from fossils and all other heterogeneous material. 2. The newly formed strata have emerged from the fluid to become the Earth's pristine continents. Beneath the continents water is sapping the continental foundations, eating away the strata, and opening up great caverns.

3. Large areas of the continents have now been undermined, and the roofs of the great caverns collapse (1). The resulting valleys are drowned by the sea and this explains the Flood. 4. In the Flood's waters new sedimentary strata are formed (B-A-C). These younger rocks incorporate plant and animal remains from the antediluvial world and are therefore fossiliferous. 5. These new rocks emerge at the close of the Flood and are themselves undermined as a second generation of caverns develops. 6. A period of renewed crustal collapse reduces the youngest strata to their present disordered state (A-D) and forms the Earth's modern topography.