A Very Short History of the Earth

An understanding of the eons of geological time - deep time - is essential to understanding how the earth and the Tectonic models work.

Some regularly repeating (or cyclic) changes, like the advance and retreat of glaciers during the ice ages, are reflected in the stratigraphic record. Catastrophic irregular or episodic changes are more common, however, and they have interrupted the record with large scale unconformities which bound the major rock sequences. The stratigraphic record is thus punctuated, rather than being either continuous or gradual in character.

Unconformity bounded sequences of strata, comprising many formations and covering large areas provide insights into the histories of plates, continental margins and cratonic interiors. Recognition of the same unconformities on different continents show a considerable degree of global synchronisation which suggests world-wide causes of, for example, sea level fluctuations.

The following relates to the stratigraphic column showing the larger divisions of geological time given in Table 2.1, and the gross distribution of tectonic elements on the Earth's surface in Figure 2.1.

The Prephanerozoic or Precambrian Time

This includes some 80% of the earth's history from its birth about 4.6 billion years ago (i.e. 4600 x 106 y, or 4600 Ma years ago). The record in the rocks of the Prephanerozoic is much more obscure than for Phanerozoic times because the lack of datable fossil assemblages in rocks older than 600 to 700 Ma. Correlation must, therefore, be based on physical field criteria and isotopic dating.

Prephanerozoic time may be divided into the oldest rocks, the Archean, followed by the Proterozoic.

The Archean, from 4600 to 2500 Ma, was dominated during its early part by a massive heat flux which was so great that little permanent crust could survive; an oxygen rich atmosphere had not yet developed. It has two main assemblages of rocks:

By Proterozoic times, from 2500 Ma to 545 Ma, heat generation had apparently declined sufficiently to allow much larger masses of continental crust to survive. Importantly, clearly recognisable cratons bordered by well-defined orogenic belts suggested that plate tectonic processes may have been operating much as today.

Early Proterozoic sediments differed from the Archean ones in that they were texturally and compositionally more mature. They included terrigenous (land derived) clastic material, with well sorted and well rounded quartz grains, and non-terrigenous chemical carbonate and evaporate strata. Stromatolites formed by algae or bacteria occurred quite widely in carbonate rocks all of which were deposited in broad shallow seas on stable cratonic areas.

By late Proterozoic times, important large-scale rifting accompanied by eruption of widespread flood basalts was occurring. Climates, as far as they can be deduced, seem not to have been too dramatically different from Phanerozoic ones except that more carbon dioxide may have caused warmer global temperatures, due to the greenhouse effect. There is clear evidence of extremes of both glaciation and aridity. The more the later Prephanerozoic sedimentary record is studied, the more it seems to resemble that of Phanerozoic times, but with the absence of animal remains.

Phanerozoic Time - the Palaeozoic and Mesozoic

These eras (which started some 545 Ma ago) are dominated by the presence of two supercontinents, Gondwanaland mainly in the south and Laurasia mainly in the north. Gondwanaland came into existence through several continental plate collisions associated with Cambro-Ordovician orogenesis: shallow inland seas covered parts of Gondwanaland until the Devonian and there was a distinct southern marine assemblage of fossils. Laurasia still comprised scattered small continents in the warm Tethyan sea until the late Palaeozoic, when they began to merge.

Soon after the Permo-Triassic Gondwana orogeny the new super-supercontinent, Pangea, was formed by the collision of Gondwanaland with the newly assembled Laurasia.

The palaeoclimate of Gondwanaland, and later, of early Pangea, was clearly zoned since glacial centres can be shown to have shifted as different parts of the continent drifted across the South Pole. When the Gondwanaland part finally drifted away from the South Pole and the landmass became larger with the formation of Pangea, the overall climate became warmer and drier. It was not until the break-up of Pangea at the end of the Permo-Triassic period some 250 million years ago and the eruption of basalts associated with rifts, that the formation of the Atlantic rift began at the margins of the present central Atlantic basin.

During the Cretaceous, new Atlantic and Indian oceanic zones opened and rifting of micro continents from northern Africa developed with the widening of the Tethyan Sea. The destruction of the former Gondwanaland/ Pangea was completed about 100 million years ago: the last continental breaks were between Australia and Antarctica (in the late Cretaceous/early Tertiary) and South America and Antarctica (in the Miocene).

Young rifts between the newly separated continents first received non-marine clastic and evaporite sediments followed by deep marine sediments as the ocean basins widened. Shallow marine transgressions then flooded the adjacent cooling and subsiding passive continental margins.

Phanerozoic Time - the Cenozoic

During this period (the last 65 million years), the world began to take on broadly the landforms and landscapes more or less as we know them today, i.e. the basic engineering geology environment of the regions began to form.

In the Americas the Cordilleran orogenesis was complete during Palaeocene and Eocene times, when the Rocky and Andes Mountains were formed by compression of Palaeozoic or Mesozoic strata. Inter-mountain basins were then filled with river and lake deposits, scattered granites and ore deposits were also produced during tectonic episodes.

Regional up-warping during the late Cenozoic (mainly Neogene) times rejuvenated rivers in the Rocky Mountain-Colorado Plateau region, resulting in deep canyon cutting. Extension and transform movements of the crust of the westernmost parts of North America characterised Neogene times in contrast with the compressional tectonics of Palaeogene times and caused:

A passive continental margin coastal plain and the deposition of continental shelf strata succeeded many of the former tectonically active Palaeozoic margin structures.

The Gulf of Mexico Province is now underlain by the thickest sequence of any passive margin and is characterised by abundant rising salt domes, which produced important petroleum traps: the Arctic Province also possesses evaporite domes reflecting the dramatically different hot climate that existed during sediment deposition, in contrast to today's cold one. The Atlantic Coast Province of Northern America also experienced a rejuvenation of the Appalachian Mountains by crustal up-warping and river down-cutting.

The Pacific Ocean region underwent major changes from Palaeogene to Neogene times.

The distribution of equatorial sediments and the track of the Hawaiian mantle plume indicate a change of Pacific plate motion about the same time that California collided with an ancient ocean ridge.

Australia is currently on a collision course with the Indonesian Arc and ultimately with south east Asia, converging at the rate of some 6 cm/year.

2.3.4 The Quaternary

The Quaternary is the last and shortest period of the Cenozoic, but it deserves special treatment, as the recent past is within this period and the top few tens of metres of the Earth's surface, i.e. the part of the earth in which engineering geologists, geomorphologists and civil engineers work, was predominantly shaped, certainly in detail, by the events of the Quaternary. These events were mainly climatically controlled and typified by glacial and interglacial periods. Details of the Quaternary Ice Age are given in the accompanying text box.