التعرية بفعل الماء المزيد من العلوم للصفوف من 9 إلى 12

Water is an amazing substance. It is the basis of all life. Water quenches your thirst. It cools you off on hot day. Water can be used as tool or chemical. Water can dissolve almost every other substance. High pressure water will clean surfaces such as separating dirt and mold from brick. In nature, water is powerful force. By the process of erosion, it can carve out solid rock and wash away sandy beach. Let's take closer look at erosion by water. Erosion is natural geological process by which surface soil and rock are worn away through the action of moving water. Erosion by water occurs initially with the bombardment of raindrops on Earth's surface. During rainstorm, the impact of raindrops may be powerful enough to dislodge soil particles in damaged soil structure. As the rain water accumulates, it erodess the landscape by transporting rock and soil away from their original location. Through the erosion of rock and soil, water carries billions of tons of silt to the oceans every year. Agricultural areas where land is cleared and plowed for crops are especially prone to loss of soil due to water erosion. Water runoff flows down slopes of mountains and hills, transporting soil and small rock particles. The runoff gathers into small streams. As the water volume increases, so does erosional ability. Solid materials carried by the stream such as grit and stones further increases the erosional power of moving water. The velocity of the water flow also affect its erosional power. Faster moving water can carry more sediment. The size, shape, and steepness of water channel determines stream's velocity. The greater the velocity of the stream, the larger the particles it can carry. This relationship is one reason why fasting water causes more erosion than the same volume of slowmoving water. Some particles are made from soluble substances and become dissolved in water. However, heavy insoluble materials are transported slowly along stream floor. These stones and rocks comprise the stream's bed load. The heaviest particles in the bed load move through traction during which particles roll along the riverbed. Fine particles in suspension are carried by the water. Medium-sized particles move by saltation when they bounce along the riverbed. The materials are suspended for short distance then fall to the bed. Saltation may loosen more sediment increasing erosion. Abrasion occurs when suspended particles wear away rock surface. Saltation, abrasion, and traction scour the riverbed, making it deeper. Erosion also carves away the river banks, which is why rivers meander. When the movement of the water and sediment impacts surface, it causes mechanical weathering. Besides stream transport, mechanical weathering can occur by hydraulic action. Hydraulic action results when wave strikes cliff, compressing air in rock cracks. The pressure can splinter surrounding rock, detaching rock particles. When the wave retreats, the air decompresses suddenly, stressing and further weakening the rock. Hydraulic action is major cause of coastal erosion, particularly of rocky shores. Coastal erosion breaks up and wears away rock. Waves erode and shape the coastline. Hydraulic action of waves on cliff face cause cracks to widen, breaking off rock. Abrasion occurs along coastlines because of sand suspended in the water. Repeated wave action causes constant abrasion, eroding entire coastlines over time. Chemical weathering due to water occurs when acids in water dissolves certain types of rocks and minerals. The best known example is the dissolution of limestone by carbonic acid in rainwater. The acid forms when carbon dioxide in the atmosphere dissolves in water. This process results in the erosion of rocks to form cave systems. It's also responsible for the slow destruction of many man-made monuments. Although it can be destructive, water erosion is also responsible for creating varied and beautiful landforms. The running water of rivers and streams over time forms ravines, valleys, canyons, and meandering rivers. Tidal action has carved out tremendous cliffs and other coastal formations such as sea stacks and arches. It's no exaggeration to say our world is shaped by water.