Finally, the well-known difference between the Pacific and Atlantic types of coast may be considered. The Atlantic coasts display fractures of a plateau-land, whilst those of the Pacific are distinguished by marginal mountain chains and the presence of ocean deeps in front of them. The coasts with the Atlantic structure also include those of East Africa with Madagascar, India, West and South Australia, as well as East Antarctica, whilst with the Pacific type there are those of the west coast of Further India and of the Sunda Archipelago, the east coast of Australia with New Guinea and New Zealand, and West Antarctica. The West Indies, including the Antilles, have a Pacific structure. A different behaviour of the force of gravity also corresponds to the tectonic distinctions between these types.[1] The Atlantic coasts are, apart from the disturbance of the continental margin described above, isostatically compensated; that is, the floating blocks are in equilibrium. On the other hand, deviations from isostasy prevail on the Pacific coasts. Further, it is known that the Atlantic coasts are relatively free from earthquakes and vulcanicity also, whilst the Pacific are rich in both. Where a volcano does occur on a coast of Atlantic type, then the lava, as pointed out by Becke, has systematic mineralogical differences compared with the Pacific lavas, and is notably heavier and more ferruginous, thus appearing to originate from greater depths.[2]
According to our ideas, the Atlantic coasts are in all cases those that have been formed through the
- ↑ Otto Meissner, “Isostasie und Küstentypus,” Peterm. Mitt., 64, p. 221, 1918.
- ↑ Walther Penck distinguishes a third still heavier kind of magma, which he calls the Arctic magma, and the place of origin of which he places at a still greater depth (“Die Entstehung der Gebirge der Erde,” Deutsche Revue, 1921).
