My Science School

Seen from space, the majority of the Earth’s surface is covered by oceans – that makes up 71% of the surface of the Earth, with the remaining 29% for land. But what percentage of the Earth’s land surface is desert? Deserts actually make up 33%, or 1/3rd of the land’s surface area.

That might sound like a surprisingly large amount, but that’s based on the official definition of a desert. Deserts are any region on Earth that can have a moisture deficit over the course of a year. In other words, they can have less rainfall in a year than they give up through evaporation.

You would think that deserts are hot, but there are cold deserts too. In fact, the largest cold desert in the world is the continent of Antarctica. There are barren rock fields in Antarctica that never receive snow, even though they’re incredibly cold. The largest hot desert is the Sahara desert, in northern Africa, covering 9 million square kilometers.

Credit: UNIVERSE TODAY

Picture Credit : Google

A wadi is a freshwater ecosystem and a type of fluvial landform, which is considered any type of geological feature that is related to rivers or streams. The term wadi comes from both the Arabic and Hebrew languages. Specifically, it refers to a dry riverbed that contains water during rainy seasons and is located in a valley-like area of the desert.

Characteristics Of a Wadi

Wadis are generally located in the flat or slightly rolling areas of deserts and often leads to dry lakes as well. A wadi can be recognized by its braided appearance, which is caused by a lack of constant water flow and an excess of sediment build-up. Sometimes, this sediment may collect in significant amounts, blocking water flow and effectively changing the direction of seasonal rivers. In addition to the low water levels, wind also affects sediment buildup by bringing in dry sands that collect on top of moist sand.

The bottoms of wadis are often covered in sand and loose gravel. The lower levels of this sediment are often packed very densely. This means that during rainy season, water is not quickly absorbed by the ground and has nowhere to drain, resulting in rapid flooding. Flash flooding in wadis is made even more dangerous because of the previously mentioned directional changes, which can send waters into communities unexpectedly.

Credit: World Atlas

Picture Credit : Google

An oasis is where water from a source deep underground comes to the surface in a desert, supporting life and vegetation.

An oasis is a lush green area in the middle of a desert, centered around a natural spring or a well. It is almost a reverse island, in a sense, because it is a tiny area of water surrounded by a sea of sand or rock.

Oases can be fairly easy to spot—at least in deserts that do not have towering sand dunes. In many cases, the oasis will be the only place where trees such as date palms grow for miles around. For centuries, the sight of an oasis on the horizon has been a very welcome one for desert travelers.

Scientific Explanation

It seems amazing that trees could sprout in an oasis. Where do the seeds come from? As it happens, scientists believe that migrating birds spot the glint of water from the air and swoop down for a drink. Any seeds that they happen to have swallowed earlier will be deposited in the damp sand around the waterhole, and those seeds that are hard enough will sprout, providing the oasis with its tell-tale splash of color in the sand.

Caravans in desert areas such as Africa's Sahara or the dry regions of Central Asia have long depended on such oases for food and water, both for their camels and their drivers, during difficult desert crossings. Today, some pastoral peoples in western Africa still depend on oases to keep themselves and their livestock alive as they travel through deserts between different grazing areas. In addition, many kinds of desert-adapted wildlife will seek water and also take shelter from the blazing sun in local oases.

Credit: ThoughtCo

Picture Credit : Google

While snow is present in Africa at very high elevations, snow in the Sahara Desert is a very rare event. 

The Sahara Desert is an extremely hot and arid region.  An area about 3,600,000 square miles (9,200,000 square kilometers), the desert is about the size of the United States and stretches across much of North Africa. 

Winter is the season when any rainfall might fall in this area.  The air above the desert is so dry, that often rain doesn’t reach the ground or arrives in very small quantities. 

There have been three recorded episodes of significant snowfall.  The first was recorded in 1979, the second in December of 2016, and the third in January 7, 2018. 

On January 7, 2018, about 10 to 30 centimeters (4 to 12 inches) of snow covered the desert higher elevations above 1000 meters. The snow lasted for less than a day thanks to warming temperatures.

Credit: GEOGRAPHYREALM

Picture Credit : Google

The Aralkum Desert, in Uzbekistan and Kazakhstan. It was once a water body called the Aral Sea, but water from the two rivers that fed it was diverted for agriculture and, gradually, by 2000, most of the sea had become a desert.

The white salt terrain left behind by the desiccation of the southern Aral Sea is now known as the Aralkum Desert. 

At around 17,000 square miles (45,000 square kilometers), the Aralkum Desert is the world’s youngest desert, created entirely due to man-made disturbances. The desolate area has replaced a once vibrant fishing and tourist industry. With the climate mitigating effects of the Aral Sea diminished, winters are now colder and summers hotter.

The Aralkum Desert lies in the path of a powerful east-west airstream and these pollutants have been carried as far away as Antarctica.  Known as Black Blizzards, these powerful wind sorts carry dust pollutants from the Aral Sea over thousands of miles away; Aral dust has been found in the bloodstream of penguins in Antarctica, in the glaciers of Greenland, and in Norway’s forests.

Credit: GEOGRAPHYREALM

Picture Credit : Google

When grains of sand pile up to form a mound or ridge, it is called a sand dune. Dunes are usually formed by wind blowing the sand in one direction. They can be of various shapes and sizes, including crescents, stars, and long ridges called seifs.

Sand dunes are some of nature’s most scintillating creations. These eye-catching land masses are found around the globe in different climatic conditions. They are not only limited to deserts, but can form in any landscape on the earth’s surface provided the conditions are right. Every sand dune is formed as a result of the interaction between the wind and soil in the form of sand grains. There are many ways thrill seekers can explore sand dunes, for example, sliding down the dunes, Bird viewing in wetlands, skiing, sand boarding or just sledging on the sand slopes. Sky divers or those on airplanes experience the true beauty of sand dunes.

A Sand Dune is a small ridge of hill of sand found in a desert or on top of a beach. When they form on a beach, they are typically above the normal maximum reach of the waves. They form from millions of finely divided sand particles that are blown by the wind and get deposited against some obstacle such as a piece of drift wood, bush or rock.

Credit: EARTH ECLIPSE

Picture Credit : Google

Today's deserts were not always arid lands, and their soil was held in place by plants and trees. But when vegetation dies out, the soil is exposed to erosion. Gradually, the lighter clay and dried organic particles are blown away by the wind, leaving behind grains of sand made up of small particles from eroded rocks.

Sand consists of small particles of larger rock that’s been eroded. But erosion doesn’t happen fast enough in arid environments to be the only cause of desert sand.

Nearly all sand in deserts came from somewhere else – sometimes hundreds of kilometers away. This sand was washed in by rivers or streams in distant, less arid times – often before the area became a desert.

Once a region becomes arid, there’s no vegetation or water to hold the soil down. Then the wind takes over and blows away the finer particles of clay and dried organic matter. What’s left is desert sand.

Finding the exact origin – the source rock – of a desert’s sand can be difficult. Scientists might look for the origin by following dried riverbeds upstream or by tracking the “footprints” that sand left as it traveled – for example, streaks on the faces of boulders left behind by blowing sand in centuries past.

Sometimes an entire desert has migrated due to movement of Earth’s huge overlying land plates. When that’s happened, pieces of the same source rock are sometimes discovered on both sides of a fault line. When scientists identify a potential source rock, they match it to sand grains by its age and composition.

Credit: Earth Sky

Picture Credit : Google 

Deserts are dry for different reasons depending on where they are. Winds in sub-tropical deserts, such as the Sahara in northern Africa, prevent rain clouds from forming. Coastal deserts, like the Atacama in Chile, get no rain, just a little moisture from fog. Death Valley, in California, USA, is a rain shadow desert on mountainsides that face away from rain-filled winds. The Gobi, in Mongolia, is an interior desert; rain-bearing winds cannot reach so far inland. Polar deserts, such as the Arctic and Antarctic, are dry because the water is locked as ice.

Areas that receive less than 25 centimeters (10 inches) of rain annually are called deserts. Deserts are dry with sparse vegetation. Landforms tend to have angular features because the lack of rain results in minimal chemical weathering, and flash floods create steep?walled scarps and gullies. There are few plants to protect the soil from the wind, so the soil is blown away to expose the rocky surface. Even in such a dry climate, most of the landforms are carved by the rare periods of heavy rainfall that result in flash floods, erosion, and sediment deposition.

Hot air rises at the equator, where the land receives the greatest amount of the sun's radiation. Most of the world's deserts are located near 30 degrees north latitude and 30 degrees south latitude, where the heated equatorial air begins to descend. The descending air is dense and begins to warm again, evaporating large amounts of water from the land surface. The resulting climate is very dry.

Other deserts are located in the rain shadows of mountain ranges. As moist air passes over a mountain range, it expands and cools, precipitating most of its moisture as it rises. As it sweeps down the other side of the mountain range, it warms and compresses, causing high evaporation rates and shedding little rain. Many of the deserts in the southwestern United States are the result of rain shadows.

Credit: Cliffs Notes

Picture Credit : Google 

Deserts can have extremes in temperature. Daytime may get as hot as 54°C in hot deserts, while at night, dryness and lack of cloud cover cause a sharp drop in temperature, and it can get as cold as 4°C.

As extremes of nature go, the marked contrast in temperature of a desert during the day and at night is one of the most impressive. Searing heat can torture human occupants. After dark, the problem is reversed, and a winter jacket might prove useful. On average, temperatures in Africa's Sahara Desert can swing an astounding 75 degrees in 24 hours, rising to an average 100°F with the sun out and plummeting to 25°F after it sets.

When the sun is out, sand proves to be an effective distributor of heat, reflecting it back into the air. But it’s not very good at retaining heat. Once the sun goes down, heat from the sand is released quickly.

The thing that could help retain warm air overnight is humidity, but deserts don’t have much of that. Water vapor in the air traps heat: Think of it like an insulating blanket, preventing either heat or cold from dispersing into the air. When the heat source is taken away, that vapor will retain it for long periods. Without sun or humidity, daytime heat isn’t being held anywhere, and the desert will cool rapidly.

Humidity is also why deserts can feel hotter than other places even though the temperature is the same. The water vapor needs lots of solar energy to heat up, while a dry climate takes that energy head-on.

All of this happens quickly because of that lack of humidity. Just as warm air escapes when night falls, there’s no humidity to trap the chilly evening weather. When the sun rises, it’s back to scorching.

Other factors can come into play. Clouds that help moderate temperature and wind can both help keep temperatures from dropping. But generally, you’ll roast in a desert and then freeze because the combination of sand and low humidity isn’t really suited for comfortable and consistent climates.

Credit: MENTAL FLOSS

Picture Credit : Google

Deserts are vast and extremely dry lands, which receive very little or no rainfall. They can be both hot and cold - in both cases, the amount of evaporation is higher than the precipitation received, and the land remains very dry.

A desert is a barren area of land where little precipitation occurs and consequently living conditions are hostile for plant and animal life. The lack of vegetation exposes the unprotected surface of the ground to the processes of denudation. About one third of the land surface of the world is arid or semi-arid. This includes much of the Polar Regions where little precipitation occurs and which are sometimes called “cold deserts”. Deserts can be classified by the amount of precipitation that falls, by the temperature that prevails, by the causes of desertification or by their geographical location.

Deserts are formed by weathering processes as large variations in temperature between day and night put strains on the rocks which consequently break in pieces. Although rain seldom occurs in deserts, there are occasional downpours that can result in flash floods. Rain falling on hot rocks can cause them to shatter and the resulting fragments and rubble strewn over the desert floor is further eroded by the wind. This picks up particles of sand and dust and wafts them aloft in sand or dust storms. Wind-blown sand grains striking any solid object in their path can abrade the surface. Rocks are smoothed down, and the wind sorts sand into uniform deposits. The grains end up as level sheets of sand or are piled high in billowing sand dunes. Other deserts are flat, stony plains where all the fine material has been blown away and the surface consists of a mosaic of smooth stones. These areas are known as desert pavements and little further erosion takes place. Other desert features include rock outcrops, exposed bedrock and clays once deposited by flowing water. Temporary lakes may form and salt pans may be left when waters evaporate. There may be underground sources of water in the form of springs and seepages from aquifers. Where these are found, oases can occur.

Credit: Earth Science

Picture credit: Google

Many types of stones are available such as basalt, marble, limestone, sandstone, quartzite, travertine, slate, gneiss, laterite, and granite, which can be used as construction materials. The stones used for building construction should be hard, durable, tough, and should be free from weathered soft patches of material, cracks, and other defects that are responsible for the reduction of strength and durability. Stones for construction purposes are obtained by quarrying from massive solid rocks.

Each type of stone lends itself to various construction applications based on its properties. For instance, certain types like basalt and granite have superior characteristics like high compressive strength and durability and hence employed in major construction works. However, there are stones that their characteristic makes them suitable for minor construction works, for example, gneiss. So, stones are used as a building material and also for decorative purposes.

TYPES OF BUILDING STONES

Some of the common building stones which are used for different purposes in India are as follows:

GRANITE

It is a deep-seated igneous rock, which is hard, durable and available in various colours. It has a high value of crushing strength and is capable of bearing high weathering.

BASALT AND TRAP

They are originated from igneous rocks in the absence of pressure by the rapid cooling of the magma.

TRAP STONE

They have the same uses as granite. Deccan trap is a popular stone of this group in South India.

LIMESTONE

Limestone is used for flooring, roofing, pavements and as a base material for cement. It is found in Maharashtra, Andhra Pradesh, Punjab, Himachal Pradesh and Tamil Nadu.

SANDSTONE

This stone is another form of sedimentary rock formed by the action of mechanical sediments. It has a sandy structure which is low in strength and easy to dress.

Credit: cmpstone.com

Picture credit: Google

A mixture of different-sized pebbles cemented by sand, formed in river channels over thousands of years. It looks a bit like a Christmas pudding.

Puddingstone is a nonscientific name for a conglomerate in which subrounded to rounded pebbles occur in a matrix of sharply contrasting color.

The name "puddingstone" was first used in Great Britain where the rocks were said to "look like a plum pudding". A well-known example is the Hertfordshire Puddingstone, from the lower Eocene of the London Basin. It consists of colorful flint pebbles in a white to brown silicate matrix. It is a rock found at many locations in Hertfordshire County, England.

Puddingstones immediately catch the eye of the geologist and the eyes of people who otherwise have no special interest in rocks. People have an immediate interest in the rocks and carry them home from beaches, streams, and wherever they are found.

Many particularly nice specimens find a place on desks, bookshelves, window sills and other locations where they will be seen by and delight even more people. Their popularity greatly exceeds their abundance.

Credit: Geology.com

Picture credit: Google

Limestone is a sedimentary rock composed mainly of the mineral calcite, which is a crystalline form of Calcium Carbonate (CaCO3). Limestone often contains variable amounts of silica in the form of Jasper or Flint, as well as amounts of clay, silt, and sand as disseminations, nodules, or layers within the rock unit. The main source of this calcite in limestone is speleothems such as stalagmites and stalactites. The secondary source of calcite is the shells of sea animals and corals.

Limestone makes up about 10 percent of the total volume of all sedimentary rocks. Most Limestone starts as the floor of shallow tropical seas and can be seen in parts of the tropics that are only 30 to 40 years old. The stone has the unique property of retrograde solubility, meaning that the stone is less soluble in water as the temperature increases.

Limestone is also classified as a young marble formed from the consolidation of seashells and sediment. Shells of sea animals form grains in limestone that promotes the growth of cement crystals around themselves.

Limestone is a popular building material because of its availability and the relative ease with which it can be worked with and cut. The stone can have a very diverse chemical composition, which can result in a variety of different colours of limestone or even within a single cut of the stone.

Credit: Banas stones

Picture credit: Wikipedia

About 300 million years ago, in the Carboniferous period, there were huge tropical swamps filled with giant tree-like ferns. As the remains of the plants were buried and compacted in these huge, warm Swamps, they formed peat. As they sank deeper, heat and pressure changed the peat into brown coal. Further pressure changed it to black bituminous coal.

Coal swamps are the classical terrestrial (land-based) ecosystems of the Carboniferous and Permian periods. They are forests that grew during the Palaeozoic Era (encompassing the Carboniferous and Permian) in which the volume of plant biomass dying and being deposited in the ground was greater than the volume of clastic (grains of pre-existing rock) material, resulting in a build-up of peat. This was subsequently buried, and eventually turned into coal over geological time. These swamps gave rise to most of the major, industrial-grade coal reserves that are mined today. The palaeontology of these coal-forming ecosystems is well known from the Carboniferous rocks of Euramerica (modern day Europe and North America), owing to the history of coal exploitation in these regions. However, extensive swamp areas that produced thick coal reserves have also formed at other times in the Earth’s history, most notably in the Permian. During the Early Permian, the coal swamps of Euramerica continued to flourish in Cathaysia (the tectonic blocks that formed modern day China), and throughout the Permian, coal swamps dominated by seed plants called glossopterids were found on the Southern Hemisphere supercontinent Gondwana (formed from modern day India, Australia, Antarctica, Africa, Madagascar and South America). The coal swamps of the Carboniferous and Early Permian formed primarily in tropical regions, whereas the Gondwanan coal swamps of the later Permian formed in higher-latitude temperate regions. Coal forests developed primarily in lowland areas such as river deltas, but there is a bias in the plant fossil record because fossilization is most likely to occur in these waterlogged habitats, meaning that fossils of drier, upland plant communities are much less common, so little is known of the plants that grew there.

Credit: palaeontologyonline.com

Picture credit: Google

Rocks are the hard mass of which the ground is made. Though we usually see them exposed in places such as cliffs, mountain crags and quarries, rocks are everywhere, even deep beneath the ground. Rocks can be as old as Earth itself, and are made of tiny crystals or grains of naturally occurring chemicals called minerals.

A rock is a solid mass of geological materials. Geological materials include individual mineral crystals, inorganic non-mineral solids like glass, pieces broken from other rocks, and even fossils. The geological materials in rocks may be inorganic, but they can also include organic materials such as the partially decomposed plant matter preserved in coal. A rock can be composed of only one type of geological material or mineral, but many are composed of several types. Figure 6.2 shows a rock made of three different kinds of minerals.

Rocks are grouped into three main categories based on how they form. Igneous rocks form when melted rock cools and solidifies. Sedimentary rocks form when fragments of other rocks are buried, compressed, and cemented together; or when minerals precipitate from solution, either directly or with the help of an organism. Metamorphic rocks form when heat and pressure alter a pre-existing rock. Although temperatures can be very high, metamorphism does not involve melting of the rock.

Credit: University of Saskatchewan

Picture credit: Google