Cabo Girao in Madeira, Portugal.
At about noon on Christmas day 2002 I looked down at my heels and beyond them to a shining blue sea, some 70 metres (220 feet) directly below. Only my toes, inside rubber climbing shoes, and finger tips kept me from falling off the near vertical, slightly mottled limestone into the water. Not being a very good or experienced climber, and climbing at what seemed to me to be close to the limit of my abilities, I was pretty scared.
But then the fear passed. I could fall back and the endless sea would catch me. What a good way to die, I thought before thinking better of it. I kept on climbing. Those few minutes of intense effort and concentration, holding fast to the living rock on a cliff on the Mediterranean coast of Spain, brought me to a threshold just one of the many that shorelines present.
They are always there. Even in the age of mass urbanisation and air travel, shorelines are essential to deepest human cognition and to our survival.
Around three and a half billion people more than half the worlds population live within 60 kilometres (40 miles) of the shoreline, and this is predicted to rise to three-quarters (about 5.6 billion out of about 7.5 billion) by the year 2020.
Once a small fishing village inhabited by the Bani Yas tribe, Dubai is now the commercial capital of the UAE with a surging population of over 1 million people.
Shorelines: origin, myth and evolution
A photograph of the earth from space or a map of the world in a school atlas is, first of all, a picture of shorelines. The shapes of the continents are almost as familiar to many of us as the faces of people we know. But how did they come into being? How long have they been there?
Shorelines are the border between the two worlds of land and sea. Which came first?
Many origin myths imagine the earth and so its shorelines emerging from an infinite, primeval ocean (notes Philip Ball in his excellent Biography of Water). Right across central and northern Asia, North America, India and Russia, a recurring motif is the Earth Diver: an animal or god who plunges to the bottom of a primordial ocean to bring up the seed of earth. In the Polynesian cosmogony the supreme being Io says Let the waters be separated, let the heavens be formed, let the earth be! For the Omaha of North America, all creatures once floated above a wholly submerged earth until a great boulder rose from the deep.
In Hindu mythology, the sound that embodied Brahma became first water and wind, from which was woven the web of the world. Darkness was there, all wrapped around by darkness, and all Water was indiscriminate says the Rig Veda, probably the oldest sacred text in existence. For the Maya of Central America, the deity Hurakan called forth the land from a universe of darkness and water. Stories from the Fertile Crescent follow the same line.
But not all traditions imagine the seas as antecedent. In Norse mythology, for example, the land is the flesh and bones of Ymir, the first giant slain by Odin. His salty blood, gushing from the spear wound in his heart, becomes the oceans. And in Chinese myth, land and sea are coeval aspects of the primal being Pan-Ku, the sculptor, whose medium was his own body.
Top: Prince Dhurva at the centre of the world Mural on Peir-Maal Pagoda, Madura, India, c.1800 Creating the World from Tui Bei Quan Tu, 1820
As it turns out, the second view is closer to what actually happened. About 4,500,000,000 (4.5 billion) years ago the earth collided with a planet about the size of Mars. The impact sheared off the material that became our moon, leaving behind a ball of molten rock awash with a fiery ocean of magma (liquid rock) from pole to pole. Water only existed as steam in the skies, and it took between one and five hundred million years for things to cool down enough for the water to fall as rain.
When it did, the scene must have been almost as wet as the English Lake District in summer. Enough water fell to cover 71% of the earths surface to a mean depth of 3.8km or 2.37 miles (the oceans cover more than 330 million square kilometres, or 130 million square miles; the deepest point the Mariana Trench is just over 11km 6.87 miles down; and the total volume is 1,347,000,000 cubic kilometres or 322,300,000 cubic miles).
Rain storm over the sea, John Constable.
Ever since, on the edges of this immensity, there have been shorelines. But they have always been changing: the outline of land we see in atlases today is one frame from a movie that so far has run for four billion years without a break for popcorn (Paleomaps).
At one point just about all the land was joined in a super-continent called Pangea. This means that just about the entire world had one continuous shoreline. A long-lived and tough-footed Robinson Crusoe could have walked around the whole thing.
But even the rockiest, most indestructible seeming shores eventually disappear washed away, submerged, left high and dry, buried or crunched together with other rock. And new shores are continuously created too. The Great Rift Valley in East Africa, where Man evolved, is hundreds of kilometres from the sea, but one day it will split apart and fill with seawater (as its northern extensions, the Red Sea and Gulf of Aden, have already done), creating new shorelines which, over the course of a few hundred million years, could separate to form an ocean as wide as the Atlantic.
This process, known as the Wilson cycle of ocean basin evolution, is continuous. Continental plates the topmost layer of a crust known as the lithosphere some 35 50 km (20 30 miles) thick float gradually over a relatively flexible, hotter layer known as the asthenosphere, grinding past each other apart to create new oceans where none had been before, subducting, crushing, colliding and uplifting to form mountain chains.
Many places that are now shorelines were once deep in the sea or high in mountain ranges. Marine fossils can be found in the highest parts of the Himalayas, record of a time when the rocks that are now these mountains were sediments in an ocean basin that existed between India and mainland Asia. Mount Everest (in Tibetan, Chomolungma Mother Goddess of the Universe), the highest point in the world, was once at the very bottom of an ocean basin. Ive stood on a pass at 5,700 metres, looking up at the face of Lhotse beneath Everest, and noticed in a rock the imprint of sea shell such as you might find on a beach.
Gradients and tides
Whether you climb, stumble or squish along a shore depends on its gradient. And its gradient is largely determined by what its made of. Large rock faces may be vertical. Shingle beaches are steep sloped (around 12 degrees for pebbles 7cm, or 2.5 inches, across). Sandy shores have a much shallower gradient around 2 degrees for medium sand. Muddy shores, made up of fine silt, tend to be just about flat.
The horizontal range of tide over a shoreline depends on several factors, but the main one is its gradient. A tide will cover many miles in the case of a mud flat or just a few inches in the case of a steep pebble bank like Chesil Beach in Dorset, England.
pebbles at Chesil Beach
Leonardo da Vinci (who was one of the first people in Europe to note sea fossils in high mountains, in his case the Apennines) likened the tides to the breathing of the entire ocean, just as humans and animals breath. Its a pleasing analogy but is more poetry than science.
Tide is a generic term for the alternating rise and fall in sea level with respect to the land. It is produced by the gravitational pull of the moon and the sun. In their simplest form, tides are single waves that stretch across entire oceans, causing their water to move up the front of its crest on one side of the ocean basin and down its back into the trough on the opposite side.
The height of the tide and its period (the time between its top and bottom) vary depending on where you are in the world. The two most common patterns are twice a day, with a period of 12 hours 25 minutes between high tides, and once a day, with a period of 24 hours 50 minutes. No two shorelines have tides completely alike. On some coasts the tides vary irregularly twice daily and on others there is close to no tide at all.
From Collins Pocket Book of the Seashore by John Barrett and C.M.Yonge, 1958
In De Temporum Ratione the English monk Bede (673 735) discussed the lunar control of tides, and recognised monthly tidal variations and the effect of wind drag on tidal height. Chinese records go back much further, but what neither the Chinese nor Bede knew was that the forces that affect tides operate far from the seashore.
Tides are just one effect of the passing moon: contemplating them on the shore reminds of other phenomena. The surface of the land itself may rise and fall by as much as fifty centimetres (twenty inches) in response to its gravitational pull. In her book High Tide in Tucson Barbara Kingsolver describes a hermit crab that finds itself living in southern Arizona, several hundreds of kilometres from the nearest sea. Despite the distance, the crab regularly becomes much more active at times of day that correspond to when the high tide would be, were there a sea in the southern Arizona desert.
Kingsolver also notes that in their natural state that is, sleeping out of doors as they did for more than 99% of the species existence human females ovulate with the full moon.
Drowning Man published by William Blake, 1793
Shorelines overwhelmed and stranded
For much of human history, the shore has marked a limit of safety the haven of dry land from the threat of drowning that is always present at sea. But there are times when shores cease to be the limit of the sea and safety becomes an illusion.
In a big storm the seawater surface rises in response to the low atmospheric pressure of the storm. Strong winds blowing onshore drive the water landward and cause it to stack against the shore. The combined effect of these two processes can result in something called a storm surge, which can flood far inland.
The most devastating storm surges come with unusually high tides and hurricane winds. One day in 1900 a storm surge raised the water level 5 metres (17 feet) higher than predicted high tide in Galveston, Texas. Over 5,000 people were drowned. In 1970 more than 500,000 people died in floods created by a 6 metre high (20 feet) storm surge in the northern Bay of Bengal.
Another, vastly more gradual, process of worldwide inundation has been taking place since the end of the last glaciation. The Holocene sea level rise, as it is called, is caused by the gradual melting of huge ice caps across much of the northern hemisphere land mass over the last few thousand years. If all the remaining ice on land were to melt, sea levels could rise by tens of meters. Some scientists think this is unlikely, as climate change may lead to more snow and ice forming in, for example, Antarctica.
The effects of climate change on sea level and coastal regions are the subject of intense study by the Intergovernmental Panel on Climate Change, among others. Peer reviewed research predicts increases in sea level by perhaps 65cm this century, together with increased sea-surface temperature; decreases in sea-ice cover (which does not itself raise sea level just as the level of a drink in a glass does not rise when an ice cube in the glass melts); and changes in salinity, alkalinity, wave patterns and ocean circulation. All of these factors could have profound significance for shorelines and the organisms that inhabit them.
In this century, Britain is likely to lose many beaches and coastal marshes, according to latest reports. In Bangladesh, tens of millions of people could be driven off low-lying land that is flooded.
Mozambique during the 1999 floods
In the very long term, things can go down as well as up. In the distant past entire seas have dried up altogether, leaving what were once shorelines high and dry. Six million years ago, a long period of exceptionally dry climate lowered the sea level in the Mediterranean and cut it off from the Atlantic at the shallow straight of Gibraltar. Without replenishment from the global oceans, the Mediterranean slowly evaporated. It became a desert, two thousand meters below global mean sea level, filled with huge tracts of gypsum and other evaporite salts. (Think Dead Sea, only five times as deep, many thousands of times larger, and dried out altogether.) The rivers feeding into it from Europe and Africa plunged over the dried up coasts, carving great gorges into the rock.
About a million years later the dam at Gibraltar breached, and the resulting waterfall as the Atlantic fed back into the dry basin was one hundred times larger than the Victoria Falls (the largest falls in the world today). One hundred cubic kilometres of water poured into the Mediterranean every day, filling it up again over the course of about a hundred years.
Victoria Falls, photographed by Chris Johns, 1997.