A careful look at any landscape reveals a kaleidoscope of colour. The colours of soils, plants, water and sky, all picked out in subtly differing hues as the light changes over the course of the day.
The colours are produced in many ways. Those of rocks and soil tend to be dominated by the minerals they contain; iron stains red and brown; copper tints rock green and Sulphur makes them yellow. When the sun is low on the horizon at dawn and dusk and its light passes through more of the atmosphere, dust particles scatter the light and makes red and oranges appear more intense.
At midday, full sunlight can turn a red rock amber. Desert rocks are coated with blackish or brownish stains coming from iron and manganese oxides known as “desert varnish”. The coating is also found on the rocks in waterfalls along great rivers such as the Nile in Africa and the Orinoco in South America, suggesting that these deserts were once much wetter than they are now.
Rocks are also coloured by living organisms, such as algae and lichens that live on the surface and in cracks. In the Antarctic, organisms live inside the rock. Sandstones in the so-called “dry valleys “of Victoria Land, south of New Zealand, have black streaks of lichen, white layers of fungus and green veins of blue-green algae living in pores inside the rock. Even Antarctic snow need not be white. In the southern spring, the snow can be coloured pink by snow algae. Similarly, oceans, seas, lakes and ponds are coloured by the sediments that pour into them from rivers and streams. Other colouring agents are the “blooms” of plant plankton, such as red dinoflagellates (red algae) and green algae.
Solitary splendour
One of the Southern Hemisphere’s most spectacular sights is Ayers Rock, the largest sandstone monolith in the world rising from the desert of Australia’s Northern Territory. As the sunlight plays across it, the rock undergoes radiant transitions from bright orange to a darkening crimson. At its richest, it appears to be incandescent, as if lit from inside by molten lava. Ayers Rock is a sacred site to the country’s Aboriginal people, who call it Uluru.
It was officially restored to them in 1985. At places such as Ayers Rock, the different colours of the rock are the result of the sun shining on different stones within it – stones that are variable as marble and granite. Marble alone can range in colour from the purest white to the deepest black, passing through the spectrum of pink, yellow, green, blue, violet and red on the way.
The colour of a stone is determined as much by its physical nature as its chemical composition, since it is the geometrical structure that controls how the rock absorbs or reflects particular wave-lengths of light. Any variation in its structure, no matter how small will often yield different colours. As the quality of the light changes with the approach of dawn or dusk, so the colour of rock also seems to change. One result of this is that the geologist can never rely on colour as a foolproof means of identifying a rock. The Earth, with its single sun and relatively few minerals, would be a monotonous place if sunlight was constant. Fortunately, climate, season and location, as well as different types of rocks and their response to light, are all part of the paintbox that combines with the sun to create nature’s colourful canvases.
Despite its name, Ayers Rock is not one gigantic stone, but an inselberg. “It is an island mountain” that has been shaped by wind-blown sands. It is all that remains of sandstone mountains that were pushed up 500 million years ago. The surface is not smooth, but covered with parallel grooves, pockmarked with caves and fissures and streaked with black runnels where water runs away after infrequent rainstorms.
Arizona’s Painted Desert
Red rocks ‘glow’ most intensely at sunset; green and blue rocks are seen to best advantage at dawn. Noonday fades out colour contrasts, while the moon enhances light colours. Every rock has its ideal light. However, by the same token, some scenery can have no ideal light, thanks to its variety.
The multicoloured sandstone landscapes of the Painted Desert of Arizona are a case in point. Here, every change in the colour of sandstone is the result of some subtle alteration in the climate, with centuries of erosion and sedimentation forever revealing different layers of stone.
Sometimes the desert air glows pink or purple with dusts derived from brightly coloured shales, marls and sandstones. The desert is banded with vividly coloured rock strata shaded red, yellow, blue, white and lavender.
Coloured lagoon in South America
The Navaho and Hopi Indians, who have reservations there, use the multicoloured sands for ceremonial sand paintings. The Laguna Colorado, or ‘coloured lagoon’, in the Bolivian Andes looks like a vast bowl of tomato soup, with remnants of cream around the rim. The tomato red of the ‘soup’ is due to a rich concentration of microscopic plants and animals in the water; the ‘cream’ is a sludge of salt and gypsum. Other deposits are derived from the streams and rivers that enter the lake after eroding and dissolving rocks in the nearby mountains. For much of the time, the colour is non-existent, as the lake evaporates to become a dazzling basin of salt.
The same phenomenon, but less spectacular, gave the Red Sea its name. In other red lagoons, particularly in Lake Natron in the Great Rift Valley of East Africa, the algae provide a rich source of food for flamingos. Their specially adapted beaks enable them to filter the vegetable matter from the sludge. The presence of beta-carotenoids in the algae also gives the birds their pink plumage, adding colour to their strutting elegance. On the Indonesian island of Flores, three lakes nestling in the craters of the extinct volcano Keli Mutu, change colour and nobody knows why. Today one is black, another dark green and the third light green, but in the 1960s, one was coffee coloured, the second red-brown and the other blue. The local people believe in the souls respectively of magicians, of criminals and of virgins and infants live in the lakes.
White sands of New Mexico
Most sand dunes are formed from tiny, wind-blowing particles of the hard, crystalline mineral quartz. However, on the broad, flat floor of the Tularosa Valley in New Mexico, they are made of gypsum, a mineral that is almost pure white in colour. Elsewhere in the world, deposits of gypsum have been mined, removed and used by the building trade in plaster, plaster of Paris and plasterboard. But White Sands, New Mexico is remote and for a long time remained nearly inaccessible; it thus escaped exploitation. Since 1933, this vast gypsum desert has been a US national monument. It is now a protected area.
The gypsum was washed from rocks of the nearby San Andres Mountains and accumulated on the bed of the now waterless Lake Lucero. Evaporation, aided by sun and wind, caused the lake to dry up. The coarse fragments of gypsum were ground into finer particles as they were blown about by the swirling winds. Vast quantities of gypsum sand lie in the valley and has been pushed up into dunes over 100 feet high. Blown by the prevailing winds, they are moving steadily to the North-East.
Some of the creatures living in the desert have adapted to their background of white. The bleached earless lizard and the Apache pocket mouse are both coloured white. Their camouflage helps protect them from the attention of passing hawks and eagles.
Black tropical sands
Most holiday brochures draw attention to a resort’s miles of golden sands’, but beaches can be almost any colour, depending on the underlying bedrock. Some, indeed, are pure black or white. In temperate climates, beaches tend to be the traditional golden colour, consisting of ground-down crystalline quartz containing various iron oxide ‘impurities. Shell beaches, coral beaches and those at the edge of warm shallow seas, by contrast, are composed mainly of the skeletons of dead marine organisms and being made of calcium carbonate, appear pure white.
On volcanic islands, the sand is sometimes black, betraying its origin as volcanic ash or the ground-up debris from the dark-coloured basaltic rocks and lava. The Canary Islands, for example, are cones that were formed by extinct volcanoes.
Salt deposits in Ethiopia
There are comparatively few beaches on these rocky outposts in the ocean, but among them, some have black sand. Similarly, surf and wind action have reduced the dark basaltic lavas of Hawaii to black sands, as at Kalpana, on Hawaii Island’s South-East coast.The Danakil Depression, in the north-east corner of Ethiopia, was once part of the nearby Red Sea. Major movements of the Earth’s surface pushed up the nearby Danakil Highlands.
The depression was cut off to form Lake Karum, a salt-water lake. The water in the lake evaporated, leaving a layer of salt over 3km thick in certain places. For most of the time, the blinding white salt surface is dry, but rainwater does occasionally wash down from the plateau to the West, forming a brine-rich lake below sea level.
The rain also percolates down to pockets of molten magma below the Earth’s surface.
The superheated water re-erupts at the surface as searing hot springs that stain the rocks red, brown or yellow according to the minerals contained in them. The local people, who are referred to as the ‘Afars’, mine the salt, which is transported by camel train to Makale and then to towns and villages all over Africa.