Paper 2000 Question: 1 (a) Al-Beruni
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I INTRODUCTION Igneous Rock, rock formed when molten or partially molten material, called magma, cools and solidifies. Igneous rocks are one of the three main types of rocks; the other types are sedimentary rocks and metamorphic rocks. Of the three types of rocks, only igneous rocks are formed from melted material. The two most common types of igneous rocks are granite and basalt. Granite is light colored and is composed of large crystals of the minerals quartz, feldspar, and mica. Basalt is dark and contains minute crystals of the minerals olivine, pyroxene, and feldspar. II TYPES OF IGNEOUS ROCKS Geologists classify igneous rocks according to the depth at which they formed in the earth’s crust. Using this principle, they divide igneous rocks into two broad categories: those that formed beneath the earth’s surface, and those that formed at the surface. Igneous rocks may also be classified according to the minerals they contain. A Classification by Depth of Formation Rocks formed within the earth are called intrusive or plutonic rocks because the magma from which they form often intrudes into the neighboring rock. Rocks formed at the surface of the earth are called extrusive rocks. In extrusive rocks, the magma has extruded, or erupted, through a volcano or fissure. Geologists can tell the difference between intrusive and extrusive rocks by the size of their crystals: crystals in intrusive rocks are larger than those in extrusive rocks. The crystals in intrusive rocks are larger because the magma that forms them is insulated by the surrounding rock and therefore cools slowly. This slow cooling gives the crystals time to grow larger. Extrusive rocks cool rapidly, so the crystals are very small. In some cases, the magma cools so rapidly that crystals have no time to form, and the magma hardens in an amorphous glass, such as obsidian. One special type of rock, called porphyry, is partly intrusive and partly extrusive. Porphyry has large crystals embedded in a mass of much smaller crystals. The large crystals formed underground and only melt at extremely high temperatures. They were carried in lava when it erupted. The mass of much smaller crystals formed around the large crystals when the lava cooled quickly above ground. B Classification by Composition Geologists also classify igneous rocks based on the minerals the rocks contain. If the mineral grains in the rocks are large enough, geologists can identify specific minerals by eye and easily classify the rocks by their mineral composition. However, extrusive rocks are generally too fine-grained to identify their minerals by eye. Geologists must classify these rocks by determining their chemical composition in the laboratory. Most magmas are composed primarily of the same elements that make up the crust and the mantle of the earth: oxygen (O), silicon (Si), aluminum (Al), iron (Fe), magnesium (Mg), calcium (Ca), sodium (Na), and potassium (K). These elements make up the rock-forming minerals quartz, feldspar, mica, amphibole, pyroxene, and olivine. Rocks and minerals rich in silicon are called silica-rich or felsic (rich in feldspar and silica). Rocks and minerals low in silicon are rich in magnesium and iron. They are called mafic (rich in magnesium and ferrum, the Latin term for iron). Rocks very low in silicon are called ultramafic. Rocks with a composition between felsic and mafic are called intermediate. B1 Felsic Rocks The most felsic, or silicon-rich, mineral is quartz. It is pure silicon dioxide and contains no aluminum, iron, magnesium, calcium, sodium, or potassium. The other important felsic mineral is feldspar. In feldspar, a quarter or a half of the silicon has been replaced by aluminum. Feldspar also contains potassium, sodium, or calcium but no magnesium or iron. Felsic intrusive rocks are classified as either granite or granodiorite, depending on how much potassium they contain. Both are light-colored rocks that have large crystals of quartz and feldspar. Extrusive rocks that have the same chemical composition as granite are called rhyolite and those with the same chemical composition as granodiorite are called dacite. Both rhyolite and dacite are fine-grained light-colored rocks. B2 Intermediate Rocks Rocks intermediate in composition between felsic and mafic rocks are termed syenite, monzonite, or monzodiorite if they are intrusive and trachyte, latite, and andesite if they are extrusive. Syenite and trachyte are rich in potassium while monzodiorite and andesite contain little potassium. B3 Mafic Rocks The mafic rock-forming minerals are olivine, pyroxene, and amphibole. All three contain silicon and a lot of either magnesium or iron or both. All three of these minerals are often dark colored. Mafic intrusive rocks are termed diorite or gabbro. Both are dark rocks with large, dark, mafic crystals as well as crystals of light-colored feldspar. Neither contains quartz. Diorite contains amphibole and pyroxene, while gabbro contains pyroxene and olivine. The feldspar in diorite tends to be sodium-rich, while the feldspar in gabbro is calcium-rich. Extrusive rocks that have the same chemical composition as diorite or gabbro are called basalt. Basalt is a fine-grained dark rock. Ultramafic rocks are composed almost exclusively of mafic minerals. Dunite is composed of more than 90 percent olivine; peridotites have between 90 and 40 percent olivine with pyroxene and amphibole as the other two principal minerals. Pyroxenite is composed primarily of pyroxene, and hornblendite is composed primarily of hornblende, which is a type of amphibole. III FORMATION OF IGNEOUS ROCKS The magmas that form igneous rock are hot, chemical soups containing a complex mixture of many different elements. As they cool, many different minerals could form. Indeed, two magmas with identical composition could form quite distinct sets of minerals, depending on the conditions of crystallization. As a magma cools, the first crystals to form will be of minerals that become solid at relatively high temperatures (usually olivine and a type of feldspar known as anorthite). The composition of these early-formed mineral crystals will be different from the initial composition of the magma. Consequently, as these growing crystals take certain elements out of the magma in certain proportions, the composition of the remaining liquid changes. This process is known as magmatic differentiation. Sometimes, the early-formed crystals are separated from the rest of the magma, either by settling to the floor of the magma chamber, or by compression that expels the liquid, leaving the crystals behind. As the magma cools to temperatures below the point where other minerals begin to crystallize (such as pyroxene and another type of feldspar known as bytownite), their crystals will start to form as well. However, early-formed minerals often cannot coexist in magma with the later-formed mineral crystals. If the early-formed minerals are not separated from the magma, they will react with or dissolve back into the magma over time. This process repeats through several cycles as the temperature of the magma continues to cool to the point where the remaining minerals become solid. The final mix of minerals formed from a cooling magma depends on three factors: the initial composition of the magma, the degree to which already-formed crystals separate from the magma, and the speed of cooling. IV INTRUSIONS When magma intrudes a region of the crust and cools, the resulting mass of igneous rock is called an intrusion. Geologists describe intrusions by their size, their shape, and whether they are concordant, meaning they run parallel to the structure of neighboring rocks, or discordant, meaning they cut across the structure of neighboring rocks. An example of a concordant intrusion is a horizontal bed formed when magma flows between horizontal beds of neighboring rock. A discordant intrusion would form when magma flows into cracks in neighboring rock, and the cracks lie at an angle to the neighboring beds of rock. A batholith is an intrusion with a cross-sectional area of more than 100 sq km (39 sq mi), usually consisting of granite, granodiorite, and diorite. Deep batholiths are often concordant, while shallow batholiths are usually discordant. Deep batholiths can be extremely large; the Coast Range batholith of North America is 100 to 200 km (60 to 120 mi) wide and extends 600 km (370 mi) through Alaska and British Columbia, Canada. Lopoliths are saucer-shaped concordant intrusions. They may be up to 100 km (60 mi) in diameter and 8 km (5 mi) thick. Lopoliths, which are usually basaltic in composition, are frequently called layered intrusions because they are strongly layered. Well-known examples are the Bushveld complex in South Africa and the Muskox intrusion in the Northwest Territories, Canada. Laccoliths have a flat base and a domed ceiling, and are concordant with the neighboring rocks; they are usually small. The classic area from which they were first described is the Henry Mountains in the state of Utah. Dikes and sills are sheetlike intrusions that are very thin relative to their length; sills are concordant and dikes are discordant. They are commonly fairly small features (a few meters thick) but can be larger. The Palisades Sill in the state of New York is 300 m (1000 ft) thick and 80 km (50 mi) long. V EXTRUSIVE BODIES Many different types of extrusive bodies occur throughout the world. The physical characteristics of these bodies depend on their chemical composition and on how the magma from which they formed erupted. The chemical composition of the parent magma affects its viscosity, or its resistance to flow, which in turn affects how the magma erupts. Felsic magma tends to be thick and viscous, while mafic magma tends to be fluid. (See also Volcano) Flood basalts are the most common type of extrusive rock. They form when highly fluid basaltic lava erupts from long fissures and many vents. The lava coalesces and floods large areas to considerable depths (up to 100 m/300 ft). Repeated eruptions can result in accumulated deposits up to 5 km (3 mi) thick. Typical examples are the Columbia River basalts in Washington and the Deccan trap of western India; the latter covers an area of more than 500,000 sq km (200,000 sq mi). When basalt erupts underwater, the rapid cooling causes it to form a characteristic texture known as pillow basalt. Pillow basalts are lava flows made up of interconnected pillow-shaped and pillow-sized rocks. Much of the ocean floor is made up of pillow basalt. Extrusive rocks that erupt from a main central vent form volcanoes, and these are classified according to their physical form and the type of volcanic activity. Mafic, or basaltic, lava is highly fluid and erupts nonexplosively. The fluid lava quickly spreads out, forming large volcanoes with shallow slopes called shield volcanoes. Mauna Loa (Hawaii) is the best-known example. Intermediate, or andesitic, magmas have a higher viscosity and so they erupt more explosively. They form steep-sided composite volcanoes. A composite volcano, or stratovolcano, is made up of layers of lava and volcanic ash. Well-known examples of composite volcanoes include Mount Rainier (Washington), Mount Vesuvius (Italy), and Mount Fuji (Japan). Felsic (rhyolitic) magmas are so viscous that they do not flow very far at all; instead, they form a dome above their central vent. This dome can give rise to very explosive eruptions when pressure builds up in a blocked vent, as happened with Mount Saint Helens (Washington) in 1983, Krakatau (Indonesia) in 1883, and Vesuvius (Italy) in AD 79. This type of explosive behavior can eject enormous amounts of ash and rock fragments, referred to as pyroclastic material, which form pyroclastic deposits (See also Pyroclastic Flow) VI PLATE TECTONICS AND IGNEOUS ROCKS The advent of the theory of plate tectonics in the 1960s provided a theoretical framework for understanding the worldwide distribution of different types of igneous rocks. According to the theory of plate tectonics, the surface of the earth is covered by about a dozen large plates. Some of these plates are composed primarily of basalt and are called oceanic plates, since most of the ocean floor is covered with basalt. Other plates, called continental plates because they contain the continents, are composed of a wide range of rocks, including sedimentary and metamorphic rocks, and large amounts of granite. Where two plates diverge (move apart), such as along a mid-ocean ridge, magma rises from the mantle to fill the gap. This material is mafic in composition and forms basalt. Where this divergence occurs on land, such as in Iceland, flood basalts are formed. When an oceanic plate collides with a continental plate, the heavier oceanic plate subducts, or slides, under the lighter continental plate. Some of the subducted material melts and rises. As it travels through the overriding continental plate, it melts and mixes with the continental material. Since continental material, on average, is more felsic than the mafic basalt of the oceanic plate, this mixing causes the composition of the magma to become more mafic. The magma may become intermediate in composition and form andesitic volcanoes. The Andes Mountains of South America are a long chain of andesitic volcanoes formed from the subduction of the Pacific Plate under the South American plate. If the magma becomes mafic, it may form rhyolitic volcanoes like Mount Saint Helens. Magma that is too viscous to rise to the surface may instead form granitic batholiths. VII ECONOMIC IMPORTANCE OF IGNEOUS ROCKS Many types of igneous rocks are used as building stone, facing stone, and decorative material, such as that used for tabletops, cutting boards, and carved figures. For example, polished granite facing stone is exported all over the world from countries such as Italy, Brazil, and India. Igneous rocks may also contain many important ores as accessory or trace minerals. Certain mafic intrusives are sources of chromium, titanium, platinum, and palladium. Some felsic rocks, called granitic pegmatites, contain a wealth of rare elements, such as lithium, tantalum, tin, and niobium, which are of economic importance. Kimberlites, formed from magmas from deep within the earth, are the primary source of diamonds. Many magmas release large amounts of metal-rich hot fluids that migrate through nearby rock, forming veins rich in metallic ores. Newly formed igneous rocks are also hot and can be an important source of geothermal energy. Contributed By: Frank Christopher Hawthorne Microsoft ® Encarta ® 2006. © 1993-2005 Microsoft Corporation. All rights reserved. (e)producers A more useful way of looking at the terrestrial and aquatic landscapes is to view them as ecosystems, a word coined in 1935 by the British plant ecologist Sir Arthur George Tansley to stress the concept of each locale or habitat as an integrated whole. A system is a collection of interdependent parts that function as a unit and involve inputs and outputs. The major parts of an ecosystem are the producers (green plants), the consumers (herbivores and carnivores), the decomposers (fungi and bacteria), and the nonliving, or abiotic, component, consisting of dead organic matter and nutrients in the soil and water. Inputs into the ecosystem are solar energy, water, oxygen, carbon dioxide, nitrogen, and other elements and compounds. Outputs from the ecosystem include water, oxygen, carbon dioxide, nutrient losses, and the heat released in cellular respiration, or heat of respiration. The major driving force is solar energy. Plants are primary producers. All life in an ecosystem depends on primary producers to capture energy from the Sun, convert it to food that is stored in plant cells, and pass this energy on to organisms that eat plants. Microsoft ® Encarta ® 2006. © 1993-2005 Microsoft Corporation. All rights reserved. Consumers Primary Primary consumers are animals that feed on plants. This group includes some insects, seed- and fruit-eating birds, rodents, and larger animals that graze on vegetation, such as deer. When primary consumers eat primary producers (plants), the energy in plant cells changes into a form that can be stored in animal cells. Secondary Secondary consumers are a diverse group of animals—some eat primary consumers and some eat other secondary consumers. Those animals that eat smaller primary consumers include frogs, snakes, foxes, and spiders. Animals that eat secondary consumers include hawks, wolves, and lions. Decomposers Decomposers include worms, mushrooms, and microscopic bacteria. These organisms break down dead plants and animals into the nutrients needed by plants to survive. Question:8 Control Unit A CPU is similar to a calculator, only much more powerful. The main function of the CPU is to perform arithmetic and logical operations on data taken from memory or on information entered through some device, such as a keyboard, scanner, or joystick. The CPU is controlled by a list of software instructions, called a computer program. Software instructions entering the CPU originate in some form of memory storage device such as a hard disk, floppy disk, CD-ROM, or magnetic tape. These instructions then pass into the computer’s main random access memory (RAM), where each instruction is given a unique address, or memory location. The CPU can access specific pieces of data in RAM by specifying the address of the data that it wants. As a program is executed, data flow from RAM through an interface unit of wires called the bus, which connects the CPU to RAM. The data are then decoded by a processing unit called the instruction decoder that interprets and implements software instructions. From the instruction decoder the data pass to the arithmetic/logic unit (ALU), which performs calculations and comparisons. Data may be stored by the ALU in temporary memory locations called registers where it may be retrieved quickly. The ALU performs specific operations such as addition, multiplication, and conditional tests on the data in its registers, sending the resulting data back to RAM or storing it in another register for further use. During this process, a unit called the program counter keeps track of each successive instruction to make sure that the program instructions are followed by the CPU in the correct order. Microsoft ® Encarta ® 2006. © 1993-2005 Microsoft Corporation. All rights reserved. Question:9 Cell (biology) I INTRODUCTION Cell (biology), basic unit of life. Cells are the smallest structures capable of basic life processes, such as taking in nutrients, expelling waste, and reproducing. All living things are composed of cells. Some microscopic organisms, such as bacteria and protozoa, are unicellular, meaning they consist of a single cell. Plants, animals, and fungi are multicellular; that is, they are composed of a great many cells working in concert. But whether it makes up an entire bacterium or is just one of trillions in a human being, the cell is a marvel of design and efficiency. Cells carry out thousands of biochemical reactions each minute and reproduce new cells that perpetuate life. Cells vary considerably in size. The smallest cell, a type of bacterium known as a mycoplasma, measures 0.0001 mm (0.000004 in) in diameter; 10,000 mycoplasmas in a row are only as wide as the diameter of a human hair. Among the largest cells are the nerve cells that run down a giraffe’s neck; these cells can exceed 3 m (9.7 ft) in length. Human cells also display a variety of sizes, from small red blood cells that measure 0.00076 mm (0.00003 in) to liver cells that may be ten times larger. About 10,000 average-sized human cells can fit on the head of a pin. Along with their differences in size, cells present an array of shapes. Some, such as the bacterium Escherichia coli, resemble rods. The paramecium, a type of protozoan, is slipper shaped; and the amoeba, another protozoan, has an irregular form that changes shape as it moves around. Plant cells typically resemble boxes or cubes. In humans, the outermost layers of skin cells are flat, while muscle cells are long and thin. Some nerve cells, with their elongated, tentacle-like extensions, suggest an octopus. In multicellular organisms, shape is typically tailored to the cell’s job. For example, flat skin cells pack tightly into a layer that protects the underlying tissues from invasion by bacteria. Long, thin muscle cells contract readily to move bones. The numerous extensions from a nerve cell enable it to connect to several other nerve cells in order to send and receive messages rapidly and efficiently. By itself, each cell is a model of independence and self-containment. Like some miniature, walled city in perpetual rush hour, the cell constantly bustles with traffic, shuttling essential molecules from place to place to carry out the business of living. Despite their individuality, however, cells also display a remarkable ability to join, communicate, and coordinate with other cells. The human body, for example, consists of an estimated 20 to 30 trillion cells. Dozens of different kinds of cells are organized into specialized groups called tissues. Tendons and bones, for example, are composed of connective tissue, whereas skin and mucous membranes are built from epithelial tissue. Different tissue types are assembled into organs, which are structures specialized to perform particular functions. Examples of organs include the heart, stomach, and brain. Organs, in turn, are organized into systems such as the circulatory, digestive, or nervous systems. All together, these assembled organ systems form the human body. The components of cells are molecules, nonliving structures formed by the union of atoms. Small molecules serve as building blocks for larger molecules. Proteins, nucleic acids, carbohydrates, and lipids, which include fats and oils, are the four major molecules that underlie cell structure and also participate in cell functions. For example, a tightly organized arrangement of lipids, proteins, and protein-sugar compounds forms the plasma membrane, or outer boundary, of certain cells. The organelles, membrane-bound compartments in cells, are built largely from proteins. Biochemical reactions in cells are guided by enzymes, specialized proteins that speed up chemical reactions. The nucleic acid deoxyribonucleic acid (DNA) contains the hereditary information for cells, and another nucleic acid, ribonucleic acid(RNA), works with DNA to build the thousands of proteins the cell needs. II CELL STRUCTURE Cells fall into one of two categories: prokaryotic or eukaryotic (see Prokaryote). In a prokaryotic cell, found only in bacteria and archaebacteria, all the components, including the DNA, mingle freely in the cell’s interior, a single compartment. Eukaryotic cells, which make up plants, animals, fungi, and all other life forms, contain numerous compartments, or organelles, within each cell. The DNA in eukaryotic cells is enclosed in a special organelle called the nucleus, which serves as the cell’s command center and information library. The term prokaryote comes from Greek words that mean “before nucleus” or “prenucleus,” while eukaryote means “true nucleus.” A Prokaryotic Cells Prokaryotic cells are among the tiniest of all cells, ranging in size from 0.0001 to 0.003 mm (0.000004 to 0.0001 in) in diameter. About a hundred typical prokaryotic cells lined up in a row would match the thickness of a book page. These cells, which can be rodlike, spherical, or spiral in shape, are surrounded by a protective cell wall. Like most cells, prokaryotic cells live in a watery environment, whether it is soil moisture, a pond, or the fluid surrounding cells in the human body. Tiny pores in the cell wall enable water and the substances dissolved in it, such as oxygen, to flow into the cell; these pores also allow wastes to flow out. Pushed up against the inner surface of the prokaryotic cell wall is a thin membrane called the plasma membrane. The plasma membrane, composed of two layers of flexible lipid molecules and interspersed with durable proteins, is both supple and strong. Unlike the cell wall, whose open pores allow the unregulated traffic of materials in and out of the cell, the plasma membrane is selectively permeable, meaning it allows only certain substances to pass through. Thus, the plasma membrane actively separates the cell’s contents from its surrounding fluids. 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