EARTH, OCEANS AND SOLAR SYSTEM
Over 361,000,000 square kilometres of Earth's total surface of 510,000,000 square kilometres are covered by 1,370,000 cubic kilometres of water or 0.02% of its total weight. 0ne cubic kilometre of sea water weighs more than 1,000,000,000 tones and, because standard sea water contains about 3.5% additives, contains35,000,000 tons of salt, minerals and other substances. With ample justification, the ocean is said to be the largest resources of readily accessible minerals . Many of the mineral deposits mined on land are fallouts from the sea, i.e., were originally dissolved in the ocean.
The mean height of the dry land is 875 metres, the mean depth of the ocean 3,794 metres. Mount Everest to the north of India is 8,882 metres high, the Marianas Trench East of Japan 11,022 metres deep. The shallow water of the continental shelf, with depths of up to 200 metres covers 27,000,000 square kilometres. The deep water of the abyssal depths covers 295,000,000 square kilometres, and the continental slope, where water depths vary over short distances between 200 and 4000 metres, accounts for 39,000,000 square kilometres.
The Earth as well as the ocean respond to the forces balancing the solar system. However, the strength of the solid Earth is such that her tides can only be detected by precision instruments. In contrast, the water of the ocean moves relatively freely in its basins on the surface of the Earth and gives rise to the oceanic tides, detected readily in coastal regions.
These tides are generated by the centrifugal forces, caused by the rotation of the Earth about its own axis and the forces of gravitational attraction of Earth, Moon and Sun. The balance of these forces on the surface of the Earth is very delicate. At any one instant, the water masses in the oceans tend to move towards two antipodal points at which gravitational and centrifugal forces, respectively, predominate. As the Earth rotates about its axis, two tidal bulges travel around the Earth, giving rise twice daily to semi-diurnal tides at most coastal locations. Daily or diurnal tides are observed in relatively small regions such as Australia's south-west; they are caused by interaction between the tidal bulges and the configuration of the continental shelf.
Other coastal phenomena such as wind waves, storm surges, tsunamis caused by submarine earthquakes, and cyclone surges are superimposed on the astronomical tides, the magnitude of which they rarely exceed for short periods. The only exception to this rule along Australia's coast occurs in the South-West, where meteorological tides overshadow the astronomical tides most of the time and the seasonal effects of monsoons and dry seasons.
Tidal and meteorological phenomena are the best-studied geophysical processes on the Earth, but their history of intense recording is relatively short. The first recording tide gauge was installed on the River Thames in 1828. Daily weather maps were first compiled systematically at the Smithsonian Institute in Washington, D.C. in 1851 after the construction of the telegraphic system linking Europe and America. While observations of meteorological and tidal phenomena occurred much earlier, sufficient accuracy of measurements on a routine basis was only achieved in the Nineteenth Century.
In Australia, in connection with the establishment of ports, recording of tides began in 1850.