IELTS Academic Reading Practice Test 416

For centuries the Sahara was less a barrier than a highway. Caravans crossed it carrying two commodities that the desert could not supply in the same place: salt and gold. The desert itself contains beds of rock salt, laid down by ancient seas, which were quarried long before the first camel reached North Africa. The people who moved that salt shaped the fortunes of the towns on the desert's edges, and the arrival of a caravan could set the calendar of an entire region.

Before camels, trade across the Sahara was conducted on a modest scale, using horses, oxen and donkeys, which could carry only limited loads and needed water frequently. The dromedary camel changed that calculation. Domesticated in Arabia, it was introduced into North Africa in the early centuries of the Christian era and had become the standard transport animal of the desert by late antiquity. A camel can go for days without drinking, and it can drink water too salty for other animals to tolerate. Its broad, padded feet do not sink into soft sand, and its load, in the hands of an experienced handler, may exceed a hundred kilograms.

The most celebrated of the Saharan salt mines was Taghaza, in what is now northern Mali. Salt was not mined there in the modern sense but cut. Labourers hacked slabs from a bed of rock salt and trimmed them into blocks of a standard size, which were then loaded onto camels.

Ibn Battuta, the Moroccan traveller who passed through in the 1350s on his way south, left a description of the settlement. Its houses, he reported, were built of salt blocks and roofed with camel skins, and its wells produced only brackish water. The work was done by labourers brought to the site, and conditions were notoriously harsh.

A caravan was an expensive undertaking. Merchants who could not afford animals of their own hired them from pastoral families who supplied the camels and the men to manage them. Routes were followed less by landmarks than by knowledge. A guide who knew the wells, the passes between dunes and the patterns of the stars could lead a caravan to an oasis that remained invisible until the last hour. Caravans travelled at night and in the cool before dawn, resting during the day, when the heat was most dangerous.

The salt trade was never a single route but a network of them. At Bilma, in what is now Niger, salt was produced by evaporating brine in shallow pits and moulded into blocks of a standard shape, which were loaded onto camels. From there caravans went south-west towards Agadez and the savanna beyond, a journey that took several weeks. In the markets of the south a load of salt was exchanged for grain, for cloth and for livestock, and the blocks were broken up and sold in smaller pieces. Because they were uniform and recognised almost everywhere, they could also be used as money.

Salt was precious in the interior for reasons that had nothing to do with taste. People who work in tropical heat lose salt through their skin and must replace it, and cattle cannot thrive without it. Salt was also the principal means of preserving meat and fish in a region where there was no ice. European and Arab writers later repeated a story that in some markets salt was exchanged for gold of equal weight. Such tales are probably exaggerated, but they convey how scarce the mineral was in the interior.

Southward caravans carried salt; northward they carried gold, copper and ivory. The gold came from the fields of West Africa, south of the savanna, and passed through market towns such as Timbuktu. In 1324 Mansa Musa, the ruler of Mali, made the pilgrimage to Mecca with an enormous caravan. The gold he distributed was so plentiful that its value in Cairo fell, and according to contemporary reports it took years to recover. The wealth of Mali rested on this exchange: salt from the desert, gold from the forests, and the merchants who moved both.

From the sixteenth century the trade declined. Ships from Portugal and other European countries began to bring goods, including sea salt, to the Atlantic coast, where they were cheaper than goods carried across the desert. In 1591 a Moroccan army crossed the Sahara and defeated the Songhai empire, disrupting the political order that had protected the caravan routes. The network did not collapse at once, but it contracted, and many of the routes that had flourished for centuries became minor paths. Caravan traffic shifted towards the coast, and the old crossings were gradually abandoned.

The trade never vanished altogether. In northern Mali, slabs of salt are still cut by hand at Taoudenni, a remote site far north of Timbuktu, and carried south, though nowadays the load usually goes by truck. In Niger the salt caravan from Agadez to Bilma, known as the azalai, still sets out, and now draws visitors as well as traders. For historians, the caravans are a reminder that a desert is not necessarily empty space: it can be the centre of an economy, so long as it holds something that people elsewhere cannot do without.

Of all the manifestations of volcanic heat, the geyser is the most spectacular and the rarest. A geyser is a hot spring that throws water and steam into the air at intervals, sometimes for several minutes and to a height of tens of metres. Thousands of hot springs exist around the world, yet only about a thousand are geysers, and about half of those are found in Yellowstone National Park in the United States. Geysers occur in only a handful of countries, among them Iceland, New Zealand, Chile and Russia. The reason is that a geyser needs three things at once: a supply of water, a source of heat, and a particular arrangement of rock through which the water must pass.

The heat comes from magma, or from hot rock, lying at no great depth. Rain and snowmelt seep down through cracks and are heated well above the boiling point of water at the surface. They do not turn to steam at once, because the weight of water above them raises the pressure, and under pressure water boils at a higher temperature. A few hundred metres down, water may exceed 200 °C and still be liquid. This is the first essential: a body of water far hotter than a kettle, but held in check by the weight of the column above it.

The second essential is the plumbing. In a geyser the channel through which the water rises is not a simple pipe. Near the surface it narrows into a constriction, sometimes only a few centimetres across, and it is this bottleneck that makes an eruption possible. Heated water and steam bubbles rising from below are trapped beneath the narrow section, unable to escape freely. Many geysers also have channels that bend or branch, and such shapes make the trapping of water even more effective. Pressure builds, and with it the temperature of the water in the reservoir beneath.

The eruption itself is usually set off by a small disturbance. Steam bubbles grow as the hottest water rises, pushing some of the water out of the vent at the top. That spillage lowers the level of water in the channel, which reduces the pressure on the water below. The effect is sudden and self-reinforcing. Water that was liquid at 200 °C is now far above its boiling point at the new pressure, and it flashes into steam, expanding to many hundreds of times its original volume. The expansion drives the column of water and steam out of the vent in a violent jet.

An eruption ends when the supply of hot water is exhausted or when the channel cools. Between eruptions the geyser refills slowly from below, and the cycle begins again. The interval varies from a few minutes in small geysers to months or years in the largest. Strokkur, in Iceland, erupts every few minutes. Steamboat, in Yellowstone, is the tallest active geyser in the world, with jets that have reached about ninety metres, but its eruptions are unpredictable: they may be separated by days or by years. The length of an eruption depends on how much water is available and on the size of the opening.

The third essential is the chemistry of the water, which must carry dissolved silica. As the hot water reaches the surface and cools, the silica precipitates out and is deposited around the vent. Over centuries this produces a sinter cone, a mound of pale, brittle mineral, and it also lines the channel, sealing the walls and narrowing the constriction still further. The lining matters because, without it, water would escape through fissures in the surrounding rock and the pressure needed for an eruption could not build up. A geyser, in other words, partly builds its own plumbing.

Exactly what triggers individual eruptions remains debated. The most widely accepted account is the bubble-trap model, in which rising bubbles collect beneath the constriction until the water above them is pushed out. But how bubbles form in the first place is not fully understood. Water in a confined space can remain liquid well above its boiling point, and the moment at which steam first appears may depend on tiny irregularities in the rock or on gases dissolved in the water. Instruments lowered into geysers have shown that the process is more complicated than any single model suggests.

Geysers are also fragile. Because they depend on a delicate balance of pressure and flow, drilling nearby for geothermal power, or extracting minerals, can silence them permanently. Geysers at Wairakei in New Zealand and at Beowawe and Steamboat Springs in Nevada fell silent after geothermal wells were drilled nearby, which is one reason Yellowstone is protected from such development. Earthquakes can change a geyser's rhythm as well: after large tremors, intervals and durations at Yellowstone have altered for months or years before returning to their previous pattern.

Modern study relies on instruments placed inside the channels. Temperature and pressure sensors, and microphones that record the rumble of rising water, allow researchers to follow an eruption from the inside. That work matters beyond curiosity. Geothermal engineers want to know how much heat can be extracted without destroying the surface features. Park managers, meanwhile, need to know whether a change in a geyser's behaviour is natural or the result of something they have done.

Governments that wish to punish or coerce another state without going to war have, for most of the past century, reached for economic sanctions. These take many forms, from restrictions on trade to controls on finance and technology. Their appeal is obvious. Sanctions can be announced by a minister, imposed without the loss of a single soldier and reversed when circumstances change. Yet the record is modest. A large body of cases, assembled by economists since the 1980s, suggests that sanctions achieve what their authors sought in roughly one case in three.

The first difficulty is deciding what success means. Sanctions may be imposed to change a government's behaviour, to weaken its military capacity or to deter it from acting. They may also be used to signal disapproval to domestic and foreign audiences, or simply because a government is under pressure to do something. A measure judged a failure by its stated aim may be counted a success by another standard. When Western states imposed sanctions on Russia after 2014, few expected them to reverse the annexation of Crimea; advocates argued instead that they raised the cost of further action.

Sanctions are supposed to operate through two distinct channels. The first is economic: by denying a target state access to markets and finance, they raise the cost of the policy the sender dislikes. The second is political: sanctions signal that resistance is futile and are meant to encourage those around the leadership to change course. This second channel is the weaker of the two. Governments under sanctions frequently tighten their grip, and populations often respond to outside pressure by rallying behind the leadership rather than turning against it. An instrument designed to divide a ruling group can, if used clumsily, unite it.

Where sanctions have been credited with contributing to change, certain conditions tend to be present. The measures have usually been broad and multilateral, backed by most of the target's trading partners. The purpose is to ensure that goods and finance cannot be routed through a friendly third country. The demands have been specific and achievable, and relief has been offered in return. The cost to the target's economy has been substantial and has fallen on the groups with political influence. Studies of the casebook find that sanctions are more often associated with the outcome sought when the sender's economy is much larger than the target's.

The most frequently cited success is South Africa. Economic pressure, an arms embargo and, above all, the costs of maintaining a system that most of the world rejected are widely held to have helped bring the government to the negotiating table. Even here the attribution is uncertain. Sanctions were only one of a number of factors, alongside internal resistance and the collapse of the political consensus among white voters. The government that negotiated the transition was not forced to its knees by economic pressure alone; it chose to bargain.

The most frequently cited failure is Iraq. Comprehensive sanctions imposed after 1990 were intended to disarm the government, and they did impose severe hardship on an economy that depended on oil exports. Critics argued that the measures punished the population while leaving the leadership untouched, and that they had been allowed to continue long after their humanitarian cost became known. The episode discredited comprehensive embargoes and accelerated a shift towards what were called targeted sanctions: asset freezes, travel bans and restrictions aimed at named individuals and companies.

Assessing sanctions also requires a comparison with something that did not happen. Would the target have behaved differently if no measures had been taken? A government that is sanctioned and then moderates its behaviour may be said to have responded to pressure; one that does not has evidently ignored it. Neither conclusion follows without knowing what the alternative would have been, and the alternative is unknowable. This is not a merely technical difficulty: it determines whether sanctions are judged by the damage they cause or by the change they produce, and the two can point in opposite directions.

The measures imposed on Russia after 2022 tested these arguments on an unprecedented scale. Central bank reserves were frozen, major banks were cut off from international payment systems, and a price cap was placed on oil exports. The Russian economy contracted far less than many commentators had predicted, partly because oil continued to flow to new buyers in Asia and partly because the government spent heavily to cushion the impact. Whether this demonstrates the power of sanctions or the limits of economic pressure is disputed.

The question posed at the start is, I believe, badly framed. Sanctions are not a switch that produces a change of policy; they are one instrument among many, and their record is neither as impressive as their advocates claim nor as worthless as their critics maintain. They are most defensible when the goal is narrow and the coalition behind them is broad. They work best, too, when relief is genuinely available for a government that complies, and when the costs imposed on ordinary people are limited as far as possible. Judged by those standards, many sanctions programmes fail. Judged as a substitute for war, the comparison is less damning.