Chalk is a soft, white, porous sedimentary stone, a signifier of limestone composed of the mineral calcite. Calcite is calcium carbonate or CaCO3. It forms under moderately deep marine conditions from the gradual accretion of minute calcite home bases shed from microorganisms called coccolithophores. It is common to find chert or flint nodules embedded in chalk.
Chalk can also refer to other compounds, including Mg silicate and Ca sulfate. Chalk has greater resistance to enduring and slouching than the clays with which it is normally associated, therefore forming tall steep drops where chalk ridges meet the sea. Chalk hills, known as chalk downland, normally form where sets of chalk reach the surface at an angle, thus forming a scarp incline.
Because chalk is porous, it can hold a large volume of groundwater, providing a natural reservoir that releases water easily through dry seasons. Chalk is composed mostly of calcium carbonate with minor amounts of silt and clay. It usually forms underwater, typically on the seabed, then consolidates and compresses during diagenesis into the form commonly seen today.
During diagenesis, silica accumulates to form chert or flint nodules within the carbonate stone.
Uses:
- Blackboard chalk is a substance used for drawing on rough surfaces, as it readily crumbles, leaving particles that stick loosely to these surfaces. Although traditionally composed of natural chalk, modern chalkboard chalk is generally made from the mineral gypsum, often supplied in sticks of tight powder about 4 in (10 cm) long.
- Sidewalk chalk is similar to blackboard chalk, except that it is formed into larger sticks and often colored. It is used to draw on pavements, streets, and driveways, mostly by children but also by adult artists.
- In agriculture, chalk is used for raising pH in soils with high acidity. The most common forms are CaCO3 and CaO.
- Tailor’s chalk is traditionally a hard chalk used to make temporary markers on fabric, primarily by seamstresses. Nowadays, it is commonly made from talc.
- Chalk is a source of calcium oxide by thermal decomposition or slaked lime following slaking with water.
- Polishing chalk is chalk prepared with a carefully controlled grain size for very fine shining of metals.
- Woodworking joints may be fitted by chalking one of the coupling surfaces. A test fit will leave a chalk mark on the high spots of the corresponding surface. Chalk transferring to cover the complete surface indicates a good fit.
A seashell or sea shell, also known simply as a shell, is a hard, protective outer layer created by an animal that lives in the sea. The shell is part of the body of the animal. Empty seashells are often found washed up on beaches by beachcombers. The shells are empty because the animal has died and the soft parts have been eaten by another animal or have rotted out.
The term seashell normally refers to the exoskeleton of an invertebrate. Most shells that are found on beaches are the shells of marine mollusks, partially because many of these shells endure better than other seashells. Apart from mollusk shells, other shells that can be found on beaches are those of cirripeds, horseshoe crabs, and lamp shells.
Marine annelid worms in the family Serpulidae create shells made of chalky tubings cemented onto other surfaces. The shells of sea urchins are called tests, and the moulted shells of crabs and lobsters are called exuviae. While most seashells are external, some cephalopods have internal shells.
Seashells have been used by humans for many different purposes throughout history and pre-history. However, seashells are not the only type of shells; in various habitats, it is possible to find shells from freshwater animals such as freshwater mussels and freshwater snails, and it is also possible to find the shells of land snails.
When the word “seashells” is used to refer only to the shells of marine mollusks, then studying seashells is part of shell collecting. Conchologists or serious collectors who have a scientific bias are generally careful not to upset life populations and habitats: even though they may collect a few live animals, most responsible collectors do not often over-collect or otherwise disturb ecosystems.
When studying the whole molluscan animal is included as well as studying the shell, then the study is known as malacology; a person who studies mollusks is known as a malacologist.
Seashells are normally found in beach drift, which is natural debris deposited along strandlines on beaches by the waves and the tides. Shells are very often washed up onto a beach empty and clean, the animal having already died, and the soft parts having rotted off or having been eaten by either predators or scavengers.
Empty seashells are frequently picked up by beachcombers. However, the majority of seashells that are offered for sale commercially have been collected alive (often in bulk) and then killed and cleaned specifically for the commercial trade. This type of large-scale harvesting can sometimes have a strong negative impact on local ecosystems and can significantly reduce the distribution of rare species.
The word “seashell” is frequently used to mean merely the shell of a marine mollusk. Marine mollusk shells that are familiar to beachcombers and therefore most likely to be called “seashells” are the shells of marine species of pelecypods (or bivalves), univalves (or snails), scaphopods (or tusk shells), polyplacophorans, and cephalopods.
These shells are very frequently the most commonly encountered, both in the natural state and for sale as decorative objects. Marine species of univalves and pelecypods are more numerous than land and freshwater species, and the shells are frequently larger and more robust.
The shells of marine species also frequently have more sculpture and more color, although this is by no means always the case. In the tropical and subtropical regions of the planet, there are far more species of colorful, large, shallow-water-shelled marine mollusks than there are in the temperate zones and the regions closer to the poles.
Although there are a number of species of shelled mollusks that are quite large, there are also huge numbers of extremely small species, such as micro mollusks. Not all mollusks are marine, however; there are numerous land and freshwater mollusks, such as snails and freshwater bivalves. And not all mollusks have an external shell: some mollusks, such as some cephalopods (calamari and octopuses), have an internal shell, and many mollusks have no shell at all, such as nudibranchs and sea slugs.
Bivalves are frequently the most common seashells that wash up on large sandy beaches or in sheltered lagoons. They can sometimes be extremely numerous. Very frequently, the two valves become detached. There are more than 15,000 species of bivalves that live in both marine and freshwater environments.
Examples of bivalves are clams, scallops, mussels, and oysters. The majority of bivalves consist of two identical shells that are held together by a flexible hinge. Inside the shells lies the animal’s body. Bivalves that do not have two shells either have one shell or lack a shell altogether. The shells are made of calcium carbonate and are secreted by the mantle.
Bivalves, also known as pelecypods, are mostly filter feeders; they draw in water through their gills, which then trap tiny food particles. Some bivalves have eyes and even an open circulatory system. Bivalves are used all over the world as a source of food and as a way of obtaining pearls.
But in the water, the larvae of some freshwater mussels can be dangerous to fish and can even bore through wood. Shell Beach, Western Australia, is a beach that is entirely made up of the shells of the cockle Fragum erugatum, as shown here.
Certain species of univalve seashells, the shells of sea snails, can sometimes be commonly washed up on sandy beaches, as well as on beaches that are surrounded by rocky marine habitats.
Only a few species of cephalopods have shells, either internal or external, that are sometimes found washed up on beaches. Some cephalopods, such as Sepia (the cuttlefish), have a large internal shell, the cuttlefish bone, which often washes up on beaches in areas where cuttlefish are common.
Spirula peronii is a deep-water squid-like cephalopod that has a small (about 1 inch or 24 millimeters), but very light and floaty internal shell. This chambered shell floats very well and therefore, washes up easily, and is well known to beachcombers in the tropical zones.
Nautilus is the only genus of cephalopod that has a well-developed external shell. Females of the cephalopod genus Argonauta create a papery egg case which sometimes washes up on tropical beaches and is referred to as a “paper nautilus”. The largest group of shelled cephalopods, the ammonoids, is extinct, but their shells are very common in certain areas as fossils.
There are numerous popular books and field guides on the topic of shell-collecting. Although there are a number of books about land and freshwater mollusks, the majority of popular books emphasize, or focus solely on, the shells of marine mollusks. Both the science of studying mollusk shells and the hobby of collecting and sorting them are known as shell-collecting.
The line between professionals and recreational enthusiasts is often not well defined in this field because many amateurs have contributed to, and continue to contribute to, shell-collecting and the larger science of malacology. Many shell collectors belong to “shell clubs” where they can meet others who share their interests.
A large number of amateurs collect the shells of marine mollusks, and this is partly because many shells wash up empty on beaches or live in the intertidal or sub-tidal zones and are therefore easily found and preserved without much need for specialized equipment or expensive supplies.
Some shell collectors find their own materials and keep careful records or buy only “specimen shells”, which means shells that have complete collecting data, including information such as how, when, where, in what habitat, and by whom the shells were collected. On the other hand, some collectors buy the more widely available commercially-imported exotic shells, most of which have very little information, or none at all.
To museum scientists, having complete collecting data (when, where, and by whom it was collected) with a specimen is far more important than having the shell correctly identified. Some owners of shell collections hope to be able to donate their collections to a major natural history or fauna museum at some point.
However, shells with little or no collection data are usually of no value to science and are likely not to be accepted by a major museum. Apart from any damage to the shell that may have occurred before it was collected, shells can also suffer damage when they are stored or displayed.
For an illustration of one instead serious sort of harm, see Byné’s disease. Cassava, besides called yuca, mogo, bitter cassava, tapioca, and kamoteng kahoy, is a woody bush of the Euphorbiaceae (spurge household) indigenous to South America.
It is extensively cultivated as an annual harvest in tropical and semitropical parts for its comestible starchy, tuberous root, a major beginning of saccharides. It differs from the similarly spelled yucca, an unrelated fruit-bearing bush in the Asparagaceae household.
Cassava, when dried to a starchy, powdery (or pearly) infusion, is called tapioca, while its fermented, flaky version is named garri. Cassava is the third-largest beginning of nutrient saccharides in the Torrid Zones. Cassava is a major basic nutrient in the underdeveloped universe, supplying a basic diet for around 500 million people.
Cassava is one of the most drought-tolerant harvests, capable of turning on fringy dirts. Nigeria is the world’s largest manufacturer of manioc. Cassava root is a good beginning of saccharides but a hapless beginning of protein. A preponderantly cassava root diet can do protein-energy malnutrition. Cassava is classified as Sweet or bitter.
Like other roots and tubers, Cassava contains anti-nutrition factors and toxins. It must be decently prepared before ingestion. Improper readying of cassava can go forth adequate residuary nitrile to do acute nitrile poisoning and goiters and may even do ataxia or partial paralysis.
Nevertheless, husbandmans frequently prefer the acrimonious assortments because they deter plagues, animate beings, and stealers. The more toxic assortments of Cassava are a fallback resource in times of dearth in some topographic points.
Wild populations of M. esculenta races flabellifolia, shown to be the primogenitor of domesticated manioc, are centered in west-central Brazil, where it was likely first domesticated more than 10,000 old ages BP. By 6,600 BC, cassava pollen appears in the Gulf of Mexico Lowlandss, at the San Andres archeological site.
The oldest direct grounds of cassava cultivation come from a 1,400-year-old Maya site, Joya de Ceren, in El Salvador. Still, the species Manihot esculenta likely originated farther south in Brazil and Paraguay.
With its high nutrient potency, it had become a staple nutrient of the native populations of northern South America, southern Mesoamerica, and the Caribbean by the clip of the Spanish conquering, and its cultivation was continued by the colonial Portuguese and Spanish. Forms of the modern domesticated species can be found turning in the natural state in the South of Brazil.
While several Manihot species are wild, all varieties of M. esculenta are cultigens. Cassava was a staple food for pre-Columbian peoples in the Americas and is frequently portrayed in indigenous art. The Moche people often depicted yuca in their ceramics.
Since being introduced by Portuguese traders from Brazil in the sixteenth century, maize and manioc have replaced traditional African crops as the continent’s most important staple food crops. Cassava is sometimes described as the ‘bread of the tropics,’ but it should not be confused with the tropical and equatorial B.
World production of cassava root was estimated to be 184 million tons in 2002, rising to 230 million metric tons in 2008 (FAO). The majority of production in 2002 was in Africa, where 99.1 million tons were grown; 51.5 million tons were grown in Asia, and 33.2 million tons in Latin America and the Caribbean.
Nigeria is the world’s largest producer of manioc. However, based on statistics from the FAO of the United Nations, Thailand is the largest exporting country of dried manioc, with a total of 77% of world exports in 2005. The second-largest exporting country is Vietnam, with 13.6%, followed by Indonesia (5.8%) and Costa Rica (2.1%).
Worldwide cassava production increased by 12.5% between 1988 and 1990. In 2010, the average yield of cassava crops worldwide was 12.5 tons per hectare. The most productive manioc farms in the world were in India, with a nationwide average yield of 34.8 tons per hectare in 2010. Cassava, yams, and sweet potatoes are important sources of food in the tropics.
The manioc plant gives the highest yield of carbohydrates per cultivated area among crop plants, except for sugar cane and sugar beets. Cassava plays a particularly important role in agriculture in developing countries, especially in sub-Saharan Africa, because it performs well on poor soils and with low rainfall, and because it is a perennial that can be harvested as required.
Its broad harvesting window allows it to act as a drought reserve and is invaluable in managing labor schedules. It also offers flexibility to resource-poor farmers because it serves as either a subsistence or a cash crop. No continent depends as much on root and tuber crops to feed its population as does Africa.
In the humid and subhumid countries of tropical Africa, cassava is either a primary basic nutrient or a secondary staple. In Ghana, for illustration, manioc and yams occupy an important place in the agricultural economy and contribute approximately 46% of the agricultural gross domestic product.
Cassava accounts for a daily thermal consumption of 30% in Ghana and is grown by almost every farming family. The importance of cassava to many Africans is epitomized in the Ewe (a language spoken in Ghana, Togo, and Benin) name for the plant, agbeli, which means “there is life”.
The price of cassava has risen significantly in the last half-decade, and lower-income people have turned to other carbohydrate-rich foods such as rice. In Tamil Nadu, India, the National Highway 68 between Thalaivasal and Attur has many cassava-processing mills alongside it, indicating an abundance of it locally.
Cassava is widely cultivated and eaten as a basic food in Andhra Pradesh and in Kerala. In the subtropical region of southern China, cassava is the fifth-largest crop in terms of production, after rice, sweet potato, sugar cane, and corn. China is also the largest export market for cassava produced in Vietnam and Thailand. Over 60% of cassava production in China is concentrated in a single province, Guangxi.
Cassava-based dishes are widely consumed wherever the plant is cultivated, some having regional, national, or cultural importance. Cassava must be cooked properly to detoxify it before it is eaten. Cassava can be cooked in various ways.
The soft-hearted root has a delicate flavor and can replace boiled potatoes in many uses, such as an accompaniment for meat dishes or made into purees, dumplings, soups, fries, gravies, etc. This plant is also used in cholent, in some families, as well. Deep-fried, it can replace fried potatoes with a unique flavor.
In Brazil, detoxified cassava is ground and cooked to a dry, often hard or crunchy meal which is used as a condiment, toasted in butter, or eaten alone as a side dish.
Cassava root is essentially a carbohydrate source. Its composition shows 60–65% moisture, 20–31% carbohydrate, 1–2% crude protein, and a relatively low content of vitamins and minerals. However, the roots are rich in calcium and vitamin C and contain a nutritionally significant amount of vitamin B1, vitamin B2, and nicotinic acid.
Cassava starch contains 70% amylopectin and 20% amylose. Cooked cassava starch has a digestibility of over 75%. Cassava root is a poor source of protein. Despite the very low amount, the quality of cassava root protein is reasonably good in terms of essential amino acids. Methionine, cysteine, and cystine are, however, limiting amino acids in cassava root.
Cassava is attractive as a nutrition source in certain ecosystems because it is one of the most drought-tolerant crops, can be successfully grown on marginal soils, and gives reasonable yields where many other crops do not grow well.
Cassava is well adapted within latitudes 30° north and south of the equator, at elevations between sea level and 2000 meters above sea level, in equatorial temperatures, with rainfall of 50 millimeters to five meters annually, and to poor soils with a pH ranging from acidic to alkaline.
These conditions are common in certain parts of Africa and South America. Cassava is a highly productive crop in terms of food calories produced per unit land area per unit of time, significantly higher than other staple crops. Cassava can produce food calories at rates exceeding 250,000 cal/hectare/day, compared with 176,000 for rice, 110,000 for wheat, and 200,000 for corn (maize).
Cassava, like other foods, also has antinutritional and toxic factors. Of particular concern are the cyanogenic glucosides of cassava (linamarin and lotaustralin). These, on hydrolysis, release hydrocyanic acid (HCN). The presence of cyanide in cassava is of concern for human and animal consumption.
The concentration of these antinutritional and unsafe glycosides varies significantly between varieties and also with climatic and cultural conditions. Choice of cassava species to be grown, therefore, is quite important. Once harvested, cassava must be treated and prepared properly prior to human or animal consumption.
