The one thing we have already found out about mountains is that at the Earths surface, nothing is for ever. Eventually they are all worn down to the sea, a truth that poets have often found useful when on the hunt for a gloomy metaphor.
This process is erosion. This scientilic term has had the rare luck to enter popular usage with its technical meaning more or less intact, unlike, say, a quantum leap, or schizophrenia.
Some erosion is simply caused by gravitation. Material is forever looking for ways to get downhill. Soft material in particular tends to “creep” downslope and will eventually find its way to a river or stream as it does so. In soil, creep can produce hillside terraces that are easy to mistake for the relics of human agriculture. But this unaided creep is a slow process. The main factor responsible for more appreciable erosion at the Earths surface is water, and it comes in several forms including ice, rain, rivers and the sea.
Each of these forms of water is a hazard to the solid Earth, but in areas where it freezes in winter, ice is the biggest threat. When water runs into a tiny crack in a rock and freezes, it also expands. The effect is just like putting a bottle of wine in the freezer to cool it fast and then forgetting about it – the solid glass or rock cannot resist the expanding ice and ends up cracking. This can speed up the comparatively gradual process of water-based erosion massively.
Rain falling straight from clouds to the land is perhaps the least significant cause of erosion. However, it may be the principal erosional factor at work on the steep slopes of a high mountain where there are no rivers or glaciers at work.
When rain gets loose on a soft surface, it removes matter and shifts it downhill, a process called rainwash. Once the rain has lormed into surface water, the forces involved increase and the phenomenon becomes known as soilwash.
At some point, rain that tails on land will find its way to a river. Rivers are often described as “cutting” the valleys through which they flow', but this is misleading. The river directly cuts the central channel in which it flows, but the rest of the valley is cut by smaller tributary streams or by water running across the land to reach them.
Even the way a river erodes a landscape is not a straightforward process. Thus, the Niagara Falls have been eroding their way backwards up the Niagara River since the end of the last ice age. In 12,300 years they have covered 11.4km, or 93cm a year.
Charles Lyell, the British geologist who determined the rate of recession of the Niagara Falls in 1841, would not agree completely with the current model of how rivers perform erosion. He was the father of uniformitarianism, the idea that the processes we see in action on Earth now are the ones that have shaped it through time. He was completely right about this and uniformitarianism is one of the great concepts of science. But we have a growing awareness that most of the erosion that a river carries out is not a slowr, creeping effect but occurs in a few short episodes when the river is in spate. This is when it has the most erosion power, partly because the water itself is more abundant and is moving faster. In addition, especially in mountain streams, water that moves faster sets rocks and pebbles in the river moving and they erode the riverbed even faster.
Chemical weathering can speed the process of erosion. Its most notorious form is acid rain. Loaded with oxides of sulphur and nitrogen. this polluted water dissolves rocks such as limestone, as well as killing trees and harming fish and other wildlife. But the most common form of chemical weathering is carbonation. Here, carbon dioxide combines with rainwater to form an acidic solution that reacts with calcium carbonate in rock such as limestone to produce calcium bicarbonate, which is soluble in water. The result, in time, is caves – wherever there is limestone, there are caves, and people with wetsuits and head lanterns to make the most of them.
The presence of limestone caves also reminds us that not all rivers are on the surface. Indeed underground rivers carry large volumes of water and perform significant erosion. My favourite place to see them emerging into daylight is the Jurassic coast of southern England, whose soft cliffs are in almost constant motion because of streams making their way through the mud and clay. The water eases the material onto the beach only for the next high tide to make short work of it. This is a sight not to be missed, if only because of the fossils it brings to light.
The sea and the erosion it carries out are perhaps the most striking reminder for many people that Earth forces are not something gradual and far-off. Most people never see a volcano, but if you holiday at the seaside at the same place each year, you are almost bound to notice change. Many coastal areas of the Earth are plainly deposition or erosion coasts. Thus the British Isles are gradually tilting, with the west rising while the east falls. In the west, rivers need to be dredged and new land appears. But to the east, it vanishes, and as with rivers, most of the erosion happens during a few violent storms, not at a smooth, gradual rate.
Often the effects of marine erosion are spectacularly obvious. The erosion itself can occur as a slow sliding process or via unmissable avalanches or rock falls, with the more high-profile processes concentrated in areas of harder rock. Look at a cliff and you rarely see a smooth rock face. There are commonly steps in and out where layers of harder or softer rock have eroded at different speeds, or marking old sea levels where erosion was once severe but which have now been left far above high water. High cliffs, offshore stacks that were once part of the land, and arches connecting isolated rocks to the land are all signs of erosion. Come back in a few hundred years and some will have collapsed.
On softer and less rocky terrain, such as Chesapeake Bay near Washington DC, the signs are different but the result is the same. Here land is being submerged, partly because of tectonically driven sinking, so islands vanish and coastlines shift inland.
The other Earth process that shifts material is the wind, which is a major contributor to erosion in dry areas such as deserts and the Arctic and Antarctic. Wrhere wind speeds in a desert rise above a certain threshold level, the wind can pick up small, loose particles and carry them along. These particles in turn act as an abrasive, wearing away at rock surfaces. Over time wind erosion produces astonishing desert landscapes, consisting of large, scoured areas. While sand and sand dunes are the characteristic landform of deserts, most of the land area of most deserts is not sandy. Instead it is pebbly, because the wind has removed all the smaller particles of material from the rocks.
However, water, in brief flash floods, is also a major factor in forming dry landscapes. Although such floods occur only once every several years, they alter the landscape very severely when they do occur because the water runs over large areas of loose, erosion-prone material.