In one dictionary islands are defined as “tracts of land entirely surrounded by water”. In another the definition is qualified as “pieces of land of less than continental size, entirely surrounded by water”. Thus Australia is not an island, but Greenland is, although one could walk to Greenland from Canada across the northern ice.

Geologists are very well aware that water level, be it ocean, lake or river, is never at any time more than a temporary feature. Islands must therefore be also temporary features for this reason alone, and many instances can be found of places that must have been islands in the past but are no longer so, such as the Mendips and Glastonbury Tor which now stand above the plains of Somerset, or the Goodwin Sands which are said to have carried a small permanent population in the early Middle Ages. But such exclusions still leave many thousands of islands of many different geological types that can be of rewarding interest both to the traveller and to the armchair geologist.

In lakes and lochs, and sometimes river estuaries, islands are often the remnants of harder materials that have resisted erosion as the present water level developed, but piles of debris or moraine left behind by glaciers in a past ice-age can also become islands. There are many examples of such features in Scotland and the Lake District of England.

Low, sandy islands often arise in estuaries, when sand or silt is brought in and deposited on a rising tide, and then left to dry out as the tide falls, whereupon it is blown into heaps by on-shore winds. Once a coarse grass cover is developed, such islands tend to become larger by trapping more wind-blown sand, and finally rise permanently above spring tides, although they usually unite with the shore in time and cease to be islands by definition. There are examples in many places round the coasts of Britain, some in the Severn estuary. Related processes have also given rise to sandy islands off Spurn Head, in the Wash, and in the outermost parts of the Thames estuary, some quite large enough to be built upon.

Offshore islands can be simple extensions of a mainland geological feature where resistant materials have managed to withstand erosion by the sea, the Isles of Scilly being a good example, but many other factors are sometimes involved. The Channel Islands exist because granite bosses were intruded under the country rock such as Bodmin the surrounding land, now inundated by the sea, at a slightly Moor and Dartmoor have arisen in Cornwall and in isolation of higher level. But more is required to explain the folding all having played a part, together with rising sea levels and after the last ice-age. The Isle of Wight deserves special mention because, although it is clearly an extension of the geology of southern Hampshire, it owes its isolation to the erosion of large river valleys in softer rock when sea level was lower than at present, valleys now flooded and further widened by the sea, but could the present rivers of Hants and Dorset have been responsible for so much erosion? It seems unlikely, unless the climatic regime was very different from the present, noting that the erosional agent appears to have been water rather than ice.

Moving away from the British coast, completely different types of islands can be found in other parts of the world. The Falkland Islands are attributed to wrinkling and folding of the continental shelf hundreds of miles from the South American mainland, but Mauritius, Réunion, and the Seychelle Islands present special geological problems. They are detached from the African mainland by hundreds of miles of deep ocean which do not contain a spreading ridge, yet the plateau on which they stand contains granite, continental material that can be seen best in the Seychelles. Madagascar further complicates the picture. It appears geologically to be a separate mini-continent having its own continental shelf, with a deep trough separating it from Africa. These circumstances are difficult to reconcile with current theories of ocean floor spreading.

There are surprisingly many islands in the deepest parts of all the world’s oceans, but the Pacific Ocean probably has more than all the others put together, most of them arising in isolation from each other, from average depths of 16000 ft. These were recognised by Charles Darwin as volcanic peaks, many active but many others extinct, rising from the sea bed until their tops emerge from the surface of the sea. Some form large islands such as those of the Hawaiian group, but there are innumerable examples no bigger than a football field rising only a few feet above the water, and there are others that are not quite islands, with their tops only a few feet below the surface, known as Guyots or Sea Mounts, a danger to mariners who happen to pass that way. It was Darwin who first suggested that a volcanic island of this type in the tropics would acquire a fringing reef of coral, which would grow upwards as fast as the island would slump downwards under its own weight as the volcanic base became extinct, thus giving rise to volcanic coral islands, where all signs of the supporting volcanic and coral is obscured. The large number of these geological features the ocean floors are comparatively to geologists. Recognising that it is difficult to resist the idea that the pattern of world volcanism must have been different in the recent past from its pattern today.

There are many islands all over the world associated with the moving edges of tectonic plates, the holiday islands of the Mediterranean being largely of this type. Some are examples of country rock edges have been lifted, folded and tilted into position, but vulcanism that has been type altogether from that of the deep oceans is also a factor in shaping some of them. Sicily has Mount Etna, which is almost continuously active though not explosively so, and the Lipari Islands which contain Vulcano and Stromboli, are entirely volcanic. The Greek islands too, owe their existence to tectonic movement and vulcanism which fortunately is almost quiescent at present, although in the historic past there have been terrifying eruptions within the area. The island of Thira (Santorini) is thought to be the remnant of a caldera formed in a dramatic eruption nearly three thousand five hundred years ago, which brought to an end the dominance of the Minoans in the Eastern Mediterranean, and probably gave rise to the Biblical Plagues of Egypt by virtue of its tsunami and the red dust that coloured heavy rain over thousands of square miles. It is also thought by many that the legends of Atlantis originated in this catastrophe, a view that is receiving some support from excavations at present being carried out below the red volcanic horizon in Thira and in Crete.

In other parts of the world, tectonic movement is creating much larger islands than any in the Mediterranean. There are the islands of Japan known as The Island Arc, spanning the East China Sea, the East Indies, and in the western Pacific Ocean in the West and their greatest size in Indonesia accounts loud where they reach of Japan. These islands have a young but complex geology, often dramatic scenery, and usually active vulcanism which is sometimes completely destructive. Mount Pelee, in the island of Martinique, destroyed the town of St Pierre in 1902, and Krakatoa, between Java and Sumatra, killed 36,000 people in 1883, the dust of its eruption colouring sunsets all over the world for the next two years. Island Arcs appear to be features of tropical and some temperate latitudes, which suggests that tectonic movements are somehow related to the earth’s spin, although there is no justification for this point of view in current literature.

Physical comparisons are sometimes made between Japan and Great Britain, both being islands at the edge of an ocean, separated by narrow channels from a large continent, but the quest for similarities cannot be taken much further except along philosophical lines. The British Isles are stable regions of an old continent, separated by a geosyncline that happens to be flooded while the sea is at its present level. The Japanese Isles, however, are fairly new lands geologically being raised by tectonic forces, causing islands that are still being raised by tectonic forces, causing islands that are still being raised by tectonic forces, causing frequent earthquakes. The Mediterranean Sea, where Britain is at the same latitude as and active vulcanism. Japan are at opposite sides of the Aleutian Islands, and the two islands at the latitude of the respective oceans relative to the direction of the earth’s rotation, so that ocean currents and weather patterns are by no means the same. Comparisons like this can be of much geological interest.

This paper is already longer than was intended without mentioning many fascinating islands such as New Zealand, Iceland, Tasmania, St Helena and Madeira, each of which deserves a special paragraph, together with thousands of others. Ceylon, The Nicobars, Sakhalin, where should I stop. Perhaps this is as good a place as any, hoping that this glimpse of the geology of islands has proved to be of interest.