1.2 The dynamic earth
1.2.1 Historical introduction
The Earth is a dynamic planet, perpetually changing both externally and internally. Its surface is constantly being altered by endogenic processes (i.e., of internal origin) resulting in volcanism and tectonism, as well as by exo-genic processes (i.e. of external origin) such as erosion and deposition. These processes have been active throughout geological history. Volcanic explosions like the 1980 eruption of Mt. St. Helens in the northwestern United States can transform the surrounding landscape virtually instantaneously. Earthquakes also cause sudden changes in the landscape, sometimes producing faults with displacements of several meters in seconds. Weather-related erosion of surface features occasionally occurs at dramatic rates, especially if rivers overflow or landslides are triggered. The Earth's surface is also being changed constantly by less spectacular geological processes at rates that are extremely slow in human terms. Regions that have been depressed by the loads of past ice-sheets are still rebounding vertically at rates of up' to several mm yr-1. Tectonic forces cause mountains to rise at similar uplift rates, while the long-term average effects of erosion on a regional scale occur at rates of cm yr-1. On a larger scale the continents move relative to each other at speeds of up to several cm yr-1 for time intervals lasting millions of years. Extremely long times are represented in geological processes. This is reflected in the derivation of a geological timescale (Section 4.1.1.3). The subdivisions used below are identified in Fig. 4.2.
Fig. 4.2. A simplified version of the geological timescale of Harland et al., 1990
The Earth's interior is also in motion. The mantle appears hard and solid to seismic waves, but is believed to exhibit a softer, plastic behavior over long geological time intervals, flowing (or "creeping") at rates of several cm yr-1. Deeper inside the Earth, the liquid core probably flows at a geologically rapid rate of a few tenths of a millimeter per second.
Geologists have long been aware of the Earth's dynamic condition. Several hypotheses have attempted to explain the underlying mechanisms. In the late nineteenth and early twentieth centuries geological orthodoxy favored the hypothesis of a contracting Earth. Mountain ranges were thought to have formed on its shrinking surface like wrinkles on a desiccating apple. Horizontal tectonic displacements were known, but were considered to be a by-product of more important vertical motions. The realization that large overthrusts played an important role in the formation of nappe structures in the Alps implied amounts of horizontal shortening that were difficult to accommodate in the contraction hypothesis. A new school of thought emerged in which mountain-building was depicted as a consequence of horizontal displacements.
A key observation in this context was the congruity between the opposing coasts of the South Atlantic, especially the similar shapes of the coastlines of Brazil and Africa. As early as 1620, despite the inaccuracy and incompleteness of early seventeenth century maps, Francis Bacon drew attention to the parallelism of the Atlantic-bordering coastlines. In 1858 Antonio Snider constructed a map showing relative movements of the circum-Atlantic continents, although he did not maintain the shapes of the coastlines. In the late nineteenth century the Austrian geologist Eduard Suess coined the name Gondwanaland for a proposed great southern continent that existed during late Paleozoic times. It embodied Africa, Antarctica, Arabia, Australia, India and South America, and lay predominantly in the southern hemisphere. The Gondwana continents are now individual entities and some (e.g., India, Arabia) no longer lie in the southern hemisphere, but they are often still called the "southern continents". In the Paleozoic, the "northern continents" of North America (including Greenland), Europe and most of Asia also formed a single continent, called Laurasia. Laurasia and Gondwanaland split apart in the Early Mesozoic. The Alpine-Himalayan mountain belt was thought to have developed from a system of geo-synclines that formed in the intervening sea, which Suess called the Tethys ocean to distinguish it from the present Mediterranean Sea. Implicit in these reconstructions is the idea that the continents subsequently reached their present positions by slow horizontal displacements across the surface of the globe.